kaish_kernel/

lexer.rs

//! Lexer for kaish source code.
//!
//! Converts source text into a stream of tokens using the logos lexer generator.
//! The lexer is designed to be unambiguous: every valid input produces exactly
//! one token sequence, and invalid input produces clear errors.
//!
//! # Token Categories
//!
//! - **Keywords**: `set`, `tool`, `if`, `then`, `else`, `fi`, `for`, `in`, `do`, `done`
//! - **Literals**: strings, integers, floats, booleans (`true`/`false`)
//! - **Operators**: `=`, `|`, `&`, `>`, `>>`, `<`, `2>`, `&>`, `&&`, `||`
//! - **Punctuation**: `;`, `:`, `,`, `.`, `{`, `}`, `[`, `]`
//! - **Variable references**: `${...}` with nested path access
//! - **Identifiers**: command names, variable names, parameter names

use logos::{Logos, Span};
use std::fmt;
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

/// Global counter for generating unique markers across all tokenize calls.
static MARKER_COUNTER: AtomicU64 = AtomicU64::new(0);

/// Maximum nesting depth for parentheses in arithmetic expressions.
/// Prevents stack overflow from pathologically nested inputs like $((((((...
const MAX_PAREN_DEPTH: usize = 256;

/// Tracks a text replacement for span correction.
/// When preprocessing replaces text (like `$((1+2))` with a marker),
/// we need to adjust subsequent spans to account for the length change.
#[derive(Debug, Clone)]
struct SpanReplacement {
    /// Position in the preprocessed text where the marker starts.
    preprocessed_pos: usize,
    /// Length of the marker in preprocessed text.
    marker_len: usize,
    /// Length of the original text that was replaced.
    original_len: usize,
}

/// Corrects a span from preprocessed-text coordinates back to original-text coordinates.
fn correct_span(span: Span, replacements: &[SpanReplacement]) -> Span {
    let mut start_adjustment: isize = 0;
    let mut end_adjustment: isize = 0;

    for r in replacements {
        // Calculate the length difference (positive = original was longer, negative = marker is longer)
        let delta = r.original_len as isize - r.marker_len as isize;

        // If the span starts after this replacement, adjust the start
        if span.start > r.preprocessed_pos + r.marker_len {
            start_adjustment += delta;
        } else if span.start > r.preprocessed_pos {
            // Span starts inside the marker - map to original position
            // (this shouldn't happen often, but handle it gracefully)
            start_adjustment += delta;
        }

        // If the span ends after this replacement, adjust the end
        if span.end > r.preprocessed_pos + r.marker_len {
            end_adjustment += delta;
        } else if span.end > r.preprocessed_pos {
            // Span ends inside the marker - map to end of original
            end_adjustment += delta;
        }
    }

    let new_start = (span.start as isize + start_adjustment).max(0) as usize;
    let new_end = (span.end as isize + end_adjustment).max(new_start as isize) as usize;
    new_start..new_end
}

/// Generate a unique marker ID that's extremely unlikely to collide with user code.
/// Uses a combination of timestamp, counter, and process ID.
fn unique_marker_id() -> String {
    let timestamp = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|d| d.as_nanos())
        .unwrap_or(0);
    let counter = MARKER_COUNTER.fetch_add(1, Ordering::Relaxed);
    #[cfg(target_os = "wasi")]
    let pid = 0u32;
    #[cfg(not(target_os = "wasi"))]
    let pid = std::process::id();
    format!("{:x}_{:x}_{:x}", timestamp, counter, pid)
}

/// A token with its span in the source text.
#[derive(Debug, Clone, PartialEq)]
pub struct Spanned<T> {
    pub token: T,
    pub span: Span,
}

impl<T> Spanned<T> {
    pub fn new(token: T, span: Span) -> Self {
        Self { token, span }
    }
}

/// Lexer error types.
#[derive(Debug, Clone, PartialEq, Default)]
pub enum LexerError {
    #[default]
    UnexpectedCharacter,
    UnterminatedString,
    UnterminatedVarRef,
    InvalidEscape,
    InvalidNumber,
    AmbiguousBoolean(String),
    AmbiguousBooleanLike(String),
    InvalidFloatNoLeading,
    InvalidFloatNoTrailing,
    /// Nesting depth exceeded (too many nested parentheses in arithmetic).
    NestingTooDeep,
    /// Arithmetic expansion `$((` reached end of input without a closing `))`.
    /// Silently evaluating the partial expression would mask a typo (`$(( 1 + 2`
    /// would compute `3`), so we surface it loudly instead.
    UnterminatedArithmetic,
    /// Heredoc body ended without seeing the closing delimiter on its own line.
    /// The user almost certainly meant to type the delimiter — silently using
    /// whatever was collected up to EOF would mask missing data.
    UnterminatedHeredoc { delimiter: String },
    /// Backtick command substitution. Kaish drops backticks intentionally —
    /// they're listed in `docs/LANGUAGE.md` and the help system as not supported.
    /// We surface this as a dedicated error (rather than `UnexpectedCharacter`)
    /// so the message can point users at the `$(cmd)` replacement.
    BackticksNotSupported,
}

impl fmt::Display for LexerError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            LexerError::UnexpectedCharacter => write!(f, "unexpected character"),
            LexerError::UnterminatedString => write!(f, "unterminated string"),
            LexerError::UnterminatedVarRef => write!(f, "unterminated variable reference"),
            LexerError::InvalidEscape => write!(f, "invalid escape sequence"),
            LexerError::InvalidNumber => write!(f, "invalid number"),
            LexerError::AmbiguousBoolean(s) => {
                write!(f, "ambiguous boolean, use lowercase '{}'", s.to_lowercase())
            }
            LexerError::AmbiguousBooleanLike(s) => {
                let suggest = if s.eq_ignore_ascii_case("yes") { "true" } else { "false" };
                write!(f, "ambiguous boolean-like '{}', use '{}' or '\"{}\"'", s, suggest, s)
            }
            LexerError::InvalidFloatNoLeading => write!(f, "float must have leading digit"),
            LexerError::InvalidFloatNoTrailing => write!(f, "float must have trailing digit"),
            LexerError::NestingTooDeep => write!(f, "nesting depth exceeded (max {})", MAX_PAREN_DEPTH),
            LexerError::UnterminatedArithmetic => {
                write!(f, "unterminated arithmetic expansion, expected closing `))`")
            }
            LexerError::UnterminatedHeredoc { delimiter } => {
                write!(f, "unterminated heredoc, expected closing delimiter `{}` on its own line", delimiter)
            }
            LexerError::BackticksNotSupported => {
                write!(f, "backticks are not supported in kaish; use $(cmd) instead")
            }
        }
    }
}

/// Tokens produced by the kaish lexer.
///
/// The order of variants matters for logos priority. More specific patterns
/// (like keywords) should come before more general ones (like identifiers).
///
/// Tokens that carry semantic values (strings, numbers, identifiers) include
/// the parsed value directly. This ensures the parser has access to actual
/// data, not just token types.
/// Here-doc content data.
///
/// - `literal` is true when the delimiter was quoted (`<<'EOF'` or `<<"EOF"`),
///   meaning no variable expansion should occur.
/// - `strip_tabs` is true for the `<<-EOF` form. Per POSIX, leading tabs on
///   each body line are stripped at materialization time. Stripping happens
///   downstream of the parser so byte offsets in `content` stay aligned with
///   their original-source positions for span-tracking purposes.
/// - `body_start_offset` is the byte offset of the first character of `content`
///   in the source string fed into the lexer's `tokenize`. This lets the parser
///   compute absolute spans for parts found inside the body during interpolation.
///   In sources without arithmetic preprocessing rewrites, this equals the
///   original-source offset; with arithmetic before the heredoc, line numbers
///   may shift slightly until full preprocessing-layer composition lands.
#[derive(Debug, Clone, PartialEq)]
pub struct HereDocData {
    pub content: String,
    pub literal: bool,
    pub strip_tabs: bool,
    pub body_start_offset: usize,
}

#[derive(Logos, Debug, Clone, PartialEq)]
#[logos(error = LexerError)]
#[logos(skip r"[ \t]+")]
pub enum Token {
    // ═══════════════════════════════════════════════════════════════════
    // Keywords (must come before Ident for priority)
    // ═══════════════════════════════════════════════════════════════════
    #[token("set")]
    Set,

    #[token("local")]
    Local,

    #[token("if")]
    If,

    #[token("then")]
    Then,

    #[token("else")]
    Else,

    #[token("elif")]
    Elif,

    #[token("fi")]
    Fi,

    #[token("for")]
    For,

    #[token("while")]
    While,

    #[token("in")]
    In,

    #[token("do")]
    Do,

    #[token("done")]
    Done,

    #[token("case")]
    Case,

    #[token("esac")]
    Esac,

    #[token("function")]
    Function,

    #[token("break")]
    Break,

    #[token("continue")]
    Continue,

    #[token("return")]
    Return,

    #[token("exit")]
    Exit,

    #[token("true")]
    True,

    #[token("false")]
    False,

    // ═══════════════════════════════════════════════════════════════════
    // Type keywords (for tool parameters)
    // ═══════════════════════════════════════════════════════════════════
    #[token("string")]
    TypeString,

    #[token("int")]
    TypeInt,

    #[token("float")]
    TypeFloat,

    #[token("bool")]
    TypeBool,

    // ═══════════════════════════════════════════════════════════════════
    // Multi-character operators (must come before single-char versions)
    // ═══════════════════════════════════════════════════════════════════
    #[token("&&")]
    And,

    #[token("||")]
    Or,

    #[token("==")]
    EqEq,

    #[token("!=")]
    NotEq,

    #[token("=~")]
    Match,

    #[token("!~")]
    NotMatch,

    #[token(">=")]
    GtEq,

    #[token("<=")]
    LtEq,

    #[token(">>")]
    GtGt,

    #[token("2>&1")]
    StderrToStdout,

    #[token("1>&2")]
    StdoutToStderr,

    #[token(">&2")]
    StdoutToStderr2,

    #[token("2>")]
    Stderr,

    #[token("&>")]
    Both,

    #[token("<<<")]
    HereString,

    #[token("<<")]
    HereDocStart,

    #[token(";;")]
    DoubleSemi,

    // ═══════════════════════════════════════════════════════════════════
    // Single-character operators and punctuation
    // ═══════════════════════════════════════════════════════════════════
    #[token("=")]
    Eq,

    #[token("|")]
    Pipe,

    #[token("&")]
    Amp,

    #[token(">")]
    Gt,

    #[token("<")]
    Lt,

    #[token(";")]
    Semi,

    #[token(":")]
    Colon,

    #[token(",")]
    Comma,

    /// Spread operator: `[...$xs date]`. Only meaningful inside a list literal
    /// (value context); inert everywhere else. logos resolves the `"..."` vs
    /// `".."` (`DotDot`) ambiguity by longest match, so no explicit priority
    /// is needed here.
    #[token("...")]
    DotDotDot,

    #[token("..")]
    DotDot,

    #[token(".")]
    Dot,

    /// Tilde path: `~/foo`, `~user/bar` - value includes the full string
    #[regex(r"~[a-zA-Z0-9_./+-]+", lex_tilde_path, priority = 3)]
    TildePath(String),

    /// Bare tilde: `~` alone (expands to $HOME)
    #[token("~")]
    Tilde,

    /// Relative path: `../foo/bar`, bare `src/kaish` (ident containing `/`),
    /// or a directory reference with a trailing slash like `dest/`. The
    /// trailing-slash form uses `*` (not `+`) after the slash so `dest/`
    /// lexes as one token instead of `Ident("dest")` + `Path("/")` — the
    /// latter split silently turned `cp a b dest/` into a 4-operand command.
    #[regex(r"\.\./[a-zA-Z0-9_./-]+", lex_relative_path, priority = 3)]
    #[regex(r"[a-zA-Z_][a-zA-Z0-9_.-]*/[a-zA-Z0-9_./-]*", lex_relative_path, priority = 3)]
    RelativePath(String),

    /// Dot-slash path: `./foo`, `./script.sh`
    #[regex(r"\./[a-zA-Z0-9_./-]+", lex_dot_slash_path, priority = 3)]
    DotSlashPath(String),

    /// Dot-prefixed bareword: `.parent`, `.gitignore`, `.foo.bar`.
    /// Treated as an opaque string in argv position. Distinct from `Token::Dot`
    /// (the POSIX `.` source alias) which only matches a bare `.` — the source
    /// alias requires whitespace before its file argument (`. script`), so
    /// `.parent` (no space) is unambiguously a single bareword.
    #[regex(r"\.[a-zA-Z_][a-zA-Z0-9_.-]*", lex_dotted_ident, priority = 3)]
    DottedIdent(String),

    #[token("{")]
    LBrace,

    #[token("}")]
    RBrace,

    #[token("[")]
    LBracket,

    #[token("]")]
    RBracket,

    #[token("(")]
    LParen,

    #[token(")")]
    RParen,

    #[token("*")]
    Star,

    #[token("!")]
    Bang,

    #[token("?")]
    Question,

    /// Merged glob word: span-adjacent tokens containing `*`, `?`, or `[...]`.
    /// Synthesized by `merge_glob_adjacent()`, never produced by logos directly.
    GlobWord(String),

    // ═══════════════════════════════════════════════════════════════════
    // Command substitution
    // ═══════════════════════════════════════════════════════════════════

    /// Arithmetic expression content: synthesized by preprocessing.
    /// Contains the expression string between `$((` and `))`.
    Arithmetic(String),

    /// Command substitution start: `$(` - begins a command substitution
    #[token("$(")]
    CmdSubstStart,

    // ═══════════════════════════════════════════════════════════════════
    // Flags (must come before Int to win over negative numbers)
    // ═══════════════════════════════════════════════════════════════════

    /// Long flag: `--name` or `--foo-bar`
    #[regex(r"--[a-zA-Z][a-zA-Z0-9-]*", lex_long_flag, priority = 3)]
    LongFlag(String),

    /// Short flag: `-l`, `-la` (combined short flags), or a dash-word with
    /// internal hyphens like `-not-a-flag`. Internal hyphens are part of the
    /// single shell word — without them the word fragments into separate flag
    /// tokens, which breaks `echo -- -not-a-flag` and the like. A leading `--`
    /// is still `DoubleDash` (the second char must be a letter here) unless
    /// the third char isn't a letter either, in which case it's
    /// `DoubleDashBare` — see below — and whether the word is a flag or a
    /// literal is the binding layer's call.
    #[regex(r"-[a-zA-Z][a-zA-Z0-9-]*", lex_short_flag, priority = 3)]
    ShortFlag(String),

    /// Plus flag: `+e` or `+x` (for set +e to disable options)
    #[regex(r"\+[a-zA-Z][a-zA-Z0-9]*", lex_plus_flag, priority = 3)]
    PlusFlag(String),

    /// Double dash: `--` alone marks end of flags. Only matches when nothing
    /// else follows (a longer match always wins) — a `--`-prefixed word with
    /// more characters after it either lexes as `LongFlag` (3rd char is a
    /// letter) or `DoubleDashBare` (3rd char is anything else).
    #[token("--")]
    DoubleDash,

    /// Bare word starting with `--` whose continuation isn't a valid
    /// long-flag name: `---`, `----`, `--=x`, `--1`, etc. Without this, the
    /// plain `--` literal above always won the length tie against a lone
    /// `--`, silently truncating a dash-only operand to its trailing
    /// remainder (`echo ---` printed `-` instead of `---` — GH #137). Mirrors
    /// `MinusBare`/`PlusBare` (bare-word fallback for an unrecognized
    /// flag-shaped prefix), just generalized to the `--` prefix. A standalone
    /// `--` (followed by whitespace/EOF) still lexes as `DoubleDash` — this
    /// regex requires at least one more non-whitespace character, so the two
    /// never tie in match length and no priority tiebreak is load-bearing;
    /// `priority = 2` is set for consistency with `PlusBare`'s tier.
    #[regex(r"--[^a-zA-Z\s][^\s]*", lex_double_dash_bare, priority = 2)]
    DoubleDashBare(String),

    /// Bare word starting with + followed by non-letter: `+%s`, `+%Y-%m-%d`
    /// For date format strings and similar. Lower priority than PlusFlag.
    #[regex(r"\+[^a-zA-Z\s][^\s]*", lex_plus_bare, priority = 2)]
    PlusBare(String),

    /// Bare word starting with - followed by non-letter/digit/dash: `-%`, etc.
    /// For rare cases. Lower priority than ShortFlag, Int, and DoubleDash.
    /// Excludes - after first - to avoid matching --name patterns.
    #[regex(r"-[^a-zA-Z0-9\s\-][^\s]*", lex_minus_bare, priority = 1)]
    MinusBare(String),

    /// Job specifier: `%1`, `%2` — the bash idiom for `wait`/`kill` targets.
    /// Keeps the leading `%` (kill uses it to distinguish a job from a PID;
    /// wait strips it). Without this token a bare `%1` is a lexer error.
    #[regex(r"%[0-9]+", lex_job_spec)]
    JobSpec(String),

    /// Standalone - (stdin indicator for cat -, diff - -, etc.)
    /// Only matches when followed by whitespace or end.
    /// This is handled specially in the parser as a positional arg.
    #[token("-")]
    MinusAlone,

    // ═══════════════════════════════════════════════════════════════════
    // Literals (with values)
    // ═══════════════════════════════════════════════════════════════════

    /// Double-quoted string: `"..."` - value is the parsed content (quotes removed, escapes processed)
    #[regex(r#""([^"\\]|\\.)*""#, lex_string)]
    String(String),

    /// Single-quoted string: `'...'` - literal content, no escape processing
    #[regex(r"'[^']*'", lex_single_string)]
    SingleString(String),

    /// Braced variable reference: `${VAR}` or `${VAR.field}` - value is the raw inner content
    #[regex(r"\$\{[^}]+\}", lex_varref)]
    VarRef(String),

    /// Simple variable reference: `$NAME` - just the identifier
    #[regex(r"\$[a-zA-Z_][a-zA-Z0-9_]*", lex_simple_varref)]
    SimpleVarRef(String),

    /// Positional parameter: `$0` through `$9`
    #[regex(r"\$[0-9]", lex_positional)]
    Positional(usize),

    /// All positional parameters: `$@`
    #[token("$@")]
    AllArgs,

    /// Number of positional parameters: `$#`
    #[token("$#")]
    ArgCount,

    /// Last exit code: `$?`
    #[token("$?")]
    LastExitCode,

    /// Current shell PID: `$$`
    #[token("$$")]
    CurrentPid,

    /// Variable string length: `${#VAR}` or a subscripted path `${#u[tags]}`.
    /// The trailing `(\[[^\]]*\])*` admits chained bracket subscripts so a
    /// length-of-path lexes in expression position, not just inside strings; the
    /// parser turns the captured inner into a `VarPath`.
    #[regex(r"\$\{#[a-zA-Z_][a-zA-Z0-9_]*(\[[^\]]*\])*\}", lex_var_length)]
    VarLength(String),

    /// Here-doc content: synthesized by preprocessing, not directly lexed.
    /// Contains the full content of the here-doc (without the delimiter lines).
    HereDoc(HereDocData),

    /// Integer literal - value is the parsed i64
    #[regex(r"-?[0-9]+", lex_int, priority = 2)]
    Int(i64),

    /// Float literal - value is the parsed f64
    #[regex(r"-?[0-9]+\.[0-9]+", lex_float)]
    Float(f64),

    // ═══════════════════════════════════════════════════════════════════
    // Invalid patterns (caught before valid tokens for better errors)
    // ═══════════════════════════════════════════════════════════════════

    /// Digit-leading bareword: `019dda1c` (SHA prefix), UUIDs, version-ish
    /// strings. Distinguished from `Int` because at least one alpha character
    /// follows the leading digits — the lexer commits to "this is a string,
    /// not a number." Treated as a bareword string in expression position.
    #[regex(r"[0-9]+[a-zA-Z_][a-zA-Z0-9_.-]*", lex_number_ident, priority = 3)]
    NumberIdent(String),

    /// Numeric word containing an embedded hyphen run, or a minus-led numeric
    /// word with a non-numeric suffix. These are single contiguous shell words
    /// the user typed — ISO dates (`2024-01-02`), `N-M` ranges (`10-20`,
    /// `cut -f 1-3`, `tr -d 0-9`), float-dash forms (`1.5-2`), and `find`
    /// predicate values like `-1k` (smaller than 1k). Without this token they
    /// fragment into adjacent `Int`/`Float`/flag tokens and trip the
    /// no-token-pasting guard. The raw slice is preserved verbatim (so leading
    /// zeros survive). A plain `2024`/`1.5`/`-1` stays `Int`/`Float` — the
    /// digit-hyphen form requires a `-segment`, and the minus-led form requires
    /// an alpha after the digits.
    #[regex(r"[0-9]+(\.[0-9]+)?(-[0-9a-zA-Z._]+)+", lex_slice_word, priority = 3)]
    #[regex(r"-[0-9]+[a-zA-Z_][0-9a-zA-Z._-]*", lex_slice_word, priority = 3)]
    DashNumWord(String),

    /// Leading-`@` bareword: `@scope/pkg` (scoped package), `@0` (epoch in
    /// `date -d @0`), or bare `@`. Mid-word `@` (`user@host`) is handled by
    /// `Ident`; this covers the leading-`@` cases that would otherwise be an
    /// "unexpected character" lexer error.
    #[regex(r"@[a-zA-Z0-9_./@-]*", lex_slice_word, priority = 3)]
    AtWord(String),

    /// Invalid: float without leading digit (like .5)
    #[regex(r"\.[0-9]+", lex_invalid_float_no_leading, priority = 3)]
    InvalidFloatNoLeading,

    /// Invalid: float without trailing digit (like 5.)
    /// Logos uses longest-match, so valid floats like 5.5 will match Float pattern instead
    #[regex(r"[0-9]+\.", lex_invalid_float_no_trailing, priority = 2)]
    InvalidFloatNoTrailing,

    // ═══════════════════════════════════════════════════════════════════
    // Paths (absolute paths starting with /)
    // ═══════════════════════════════════════════════════════════════════

    /// Absolute path: `/tmp/out`, `/etc/hosts`, etc.
    #[regex(r"/[a-zA-Z0-9_./+-]*", lex_path)]
    Path(String),

    // ═══════════════════════════════════════════════════════════════════
    // Identifiers (command names, variable names, etc.)
    // ═══════════════════════════════════════════════════════════════════

    /// Identifier - value is the identifier string
    /// Allows dots for filenames like `script.kai` and `@` for `user@host`,
    /// `a@b.com` (bare `@` is an ordinary word character, as in bash).
    #[regex(r"[a-zA-Z_][a-zA-Z0-9_.@-]*", lex_ident)]
    Ident(String),

    // ═══════════════════════════════════════════════════════════════════
    // Structural tokens
    // ═══════════════════════════════════════════════════════════════════

    /// Comment: `# ...` to end of line
    #[regex(r"#[^\n\r]*", allow_greedy = true)]
    Comment,

    /// Newline (significant in kaish - ends statements)
    #[regex(r"\n|\r\n")]
    Newline,

    /// Line continuation: backslash at end of line
    #[regex(r"\\[ \t]*(\n|\r\n)")]
    LineContinuation,

    /// Backtick command substitution — explicitly rejected. Kaish drops
    /// backticks; the callback always errors so users get a dedicated
    /// `BackticksNotSupported` message instead of the generic
    /// `UnexpectedCharacter` they would have hit before. Backticks inside
    /// single/double-quoted strings, heredoc bodies, and comments don't
    /// reach this match — those tokens are matched as a single unit
    /// (strings) or extracted before logos runs (heredocs) or skipped to
    /// EOL (comments).
    #[token("`", reject_backtick)]
    BacktickRejected,
}

/// Semantic category for syntax highlighting.
///
/// Stable enum that groups tokens by purpose. Consumers match on categories
/// instead of individual tokens, insulating them from lexer evolution.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum TokenCategory {
    /// Keywords: if, then, else, for, while, function, return, etc.
    Keyword,
    /// Operators: |, &&, ||, >, >>, 2>&1, =, ==, etc.
    Operator,
    /// String literals: "...", '...', heredocs
    String,
    /// Numeric literals: 123, 3.14, arithmetic expressions
    Number,
    /// Variable references: $foo, ${bar}, $1, $@, $#, $?, $$
    Variable,
    /// Comments: # ...
    Comment,
    /// Punctuation: ; , . ( ) { } [ ]
    Punctuation,
    /// Identifiers in command position
    Command,
    /// Absolute paths: /foo/bar
    Path,
    /// Flags: --long, -s, +x
    Flag,
    /// Invalid tokens
    Error,
}

impl Token {
    /// Returns the semantic category for syntax highlighting.
    pub fn category(&self) -> TokenCategory {
        match self {
            // Keywords
            Token::If
            | Token::Then
            | Token::Else
            | Token::Elif
            | Token::Fi
            | Token::For
            | Token::In
            | Token::Do
            | Token::Done
            | Token::While
            | Token::Case
            | Token::Esac
            | Token::Function
            | Token::Return
            | Token::Break
            | Token::Continue
            | Token::Exit
            | Token::Set
            | Token::Local
            | Token::True
            | Token::False
            | Token::TypeString
            | Token::TypeInt
            | Token::TypeFloat
            | Token::TypeBool => TokenCategory::Keyword,

            // Operators and redirections
            Token::Pipe
            | Token::And
            | Token::Or
            | Token::Amp
            | Token::Eq
            | Token::EqEq
            | Token::NotEq
            | Token::Match
            | Token::NotMatch
            | Token::Lt
            | Token::Gt
            | Token::LtEq
            | Token::GtEq
            | Token::GtGt
            | Token::Stderr
            | Token::Both
            | Token::HereDocStart
            | Token::HereString
            | Token::StderrToStdout
            | Token::StdoutToStderr
            | Token::StdoutToStderr2 => TokenCategory::Operator,

            // Strings
            Token::String(_) | Token::SingleString(_) | Token::HereDoc(_) => TokenCategory::String,

            // Numbers
            Token::Int(_) | Token::Float(_) | Token::Arithmetic(_) => TokenCategory::Number,

            // Variables
            Token::VarRef(_)
            | Token::SimpleVarRef(_)
            | Token::Positional(_)
            | Token::AllArgs
            | Token::ArgCount
            | Token::VarLength(_)
            | Token::LastExitCode
            | Token::CurrentPid => TokenCategory::Variable,

            // Flags
            Token::LongFlag(_)
            | Token::ShortFlag(_)
            | Token::PlusFlag(_)
            | Token::DoubleDash => TokenCategory::Flag,

            // Punctuation
            Token::Semi
            | Token::DoubleSemi
            | Token::Colon
            | Token::Comma
            | Token::Dot
            | Token::LParen
            | Token::RParen
            | Token::LBrace
            | Token::RBrace
            | Token::LBracket
            | Token::RBracket
            | Token::Bang
            | Token::Question
            | Token::Star
            | Token::Newline
            | Token::LineContinuation
            | Token::CmdSubstStart
            | Token::DotDotDot => TokenCategory::Punctuation,

            // Glob words (merged tokens containing wildcards)
            Token::GlobWord(_) => TokenCategory::Path,

            // Comments
            Token::Comment => TokenCategory::Comment,

            // Paths
            Token::Path(_)
            | Token::TildePath(_)
            | Token::RelativePath(_)
            | Token::Tilde
            | Token::DotDot
            | Token::DotSlashPath(_) => TokenCategory::Path,

            // Commands/identifiers (and bare words)
            Token::Ident(_)
            | Token::PlusBare(_)
            | Token::MinusBare(_)
            | Token::DoubleDashBare(_)
            | Token::MinusAlone
            | Token::NumberIdent(_)
            | Token::DashNumWord(_)
            | Token::AtWord(_)
            | Token::DottedIdent(_)
            | Token::JobSpec(_) => TokenCategory::Command,

            // Errors
            Token::InvalidFloatNoLeading
            | Token::InvalidFloatNoTrailing
            | Token::BacktickRejected => TokenCategory::Error,
        }
    }
}

/// Lex a double-quoted string literal, processing escape sequences.
fn lex_string(lex: &mut logos::Lexer<Token>) -> Result<String, LexerError> {
    parse_string_literal(lex.slice())
}

/// Lex a single-quoted string literal (no escape processing).
fn lex_single_string(lex: &mut logos::Lexer<Token>) -> String {
    let s = lex.slice();
    // Strip the surrounding single quotes
    s[1..s.len() - 1].to_string()
}

/// Lex a braced variable reference, extracting the inner content.
fn lex_varref(lex: &mut logos::Lexer<Token>) -> String {
    // Keep the full ${...} for later parsing of path segments
    lex.slice().to_string()
}

/// Lex a simple variable reference: `$NAME` → `NAME`
fn lex_simple_varref(lex: &mut logos::Lexer<Token>) -> String {
    // Strip the leading `$`
    lex.slice()[1..].to_string()
}

/// Lex a positional parameter: `$1` → 1
fn lex_positional(lex: &mut logos::Lexer<Token>) -> usize {
    // Strip the leading `$` and parse the digit
    lex.slice()[1..].parse().unwrap_or(0)
}

/// Lex a variable length: `${#VAR}` → "VAR"
fn lex_var_length(lex: &mut logos::Lexer<Token>) -> String {
    // Strip the leading `${#` and trailing `}`
    let s = lex.slice();
    s[3..s.len() - 1].to_string()
}

/// Lex an integer literal.
fn lex_int(lex: &mut logos::Lexer<Token>) -> Result<i64, LexerError> {
    lex.slice().parse().map_err(|_| LexerError::InvalidNumber)
}

/// Lex a float literal.
fn lex_float(lex: &mut logos::Lexer<Token>) -> Result<f64, LexerError> {
    lex.slice().parse().map_err(|_| LexerError::InvalidNumber)
}

/// Lex a digit-leading bareword like `019dda1c` or `019dda1c-5b3f-7000`.
/// Distinguished from `Int` because at least one alpha character follows the
/// leading digits — the slice is treated as a string, not a number.
fn lex_number_ident(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a dot-prefixed bareword like `.gitignore` or `.parent.parent`.
fn lex_dotted_ident(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a bareword by capturing its raw slice verbatim (used by `DashNumWord`
/// and `AtWord`, where exact characters — e.g. leading zeros — must survive).
fn lex_slice_word(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex an invalid float without leading digit (like .5).
/// Always returns Err to produce a lexer error instead of a token.
fn lex_invalid_float_no_leading(_lex: &mut logos::Lexer<Token>) -> Result<(), LexerError> {
    Err(LexerError::InvalidFloatNoLeading)
}

/// Reject a backtick — kaish doesn't support backtick command substitution.
/// The dedicated error gives the user a `$(cmd)` hint instead of the generic
/// `UnexpectedCharacter` they would have hit otherwise.
fn reject_backtick(_lex: &mut logos::Lexer<Token>) -> Result<(), LexerError> {
    Err(LexerError::BackticksNotSupported)
}

/// Lex an invalid float without trailing digit (like 5.).
/// Always returns Err to produce a lexer error instead of a token.
fn lex_invalid_float_no_trailing(_lex: &mut logos::Lexer<Token>) -> Result<(), LexerError> {
    Err(LexerError::InvalidFloatNoTrailing)
}

/// Lex an identifier, rejecting ambiguous boolean-like values.
fn lex_ident(lex: &mut logos::Lexer<Token>) -> Result<String, LexerError> {
    let s = lex.slice();

    // Reject ambiguous boolean variants (TRUE, FALSE, True, etc.)
    // Only lowercase 'true' and 'false' are valid booleans (handled by Token::True/False)
    match s.to_lowercase().as_str() {
        "true" | "false" if s != "true" && s != "false" => {
            return Err(LexerError::AmbiguousBoolean(s.to_string()));
        }
        _ => {}
    }

    // Reject yes/no/YES/NO/Yes/No as ambiguous boolean-like values
    if s.eq_ignore_ascii_case("yes") || s.eq_ignore_ascii_case("no") {
        return Err(LexerError::AmbiguousBooleanLike(s.to_string()));
    }

    Ok(s.to_string())
}

/// Lex a long flag: `--name` → `name`
fn lex_long_flag(lex: &mut logos::Lexer<Token>) -> String {
    // Strip the leading `--`
    lex.slice()[2..].to_string()
}

/// Lex a short flag: `-l` → `l`, `-la` → `la`
fn lex_short_flag(lex: &mut logos::Lexer<Token>) -> String {
    // Strip the leading `-`
    lex.slice()[1..].to_string()
}

/// Lex a plus flag: `+e` → `e`, `+ex` → `ex`
fn lex_plus_flag(lex: &mut logos::Lexer<Token>) -> String {
    // Strip the leading `+`
    lex.slice()[1..].to_string()
}

/// Lex a plus bare word: `+%s` → `+%s` (keep the full string)
fn lex_plus_bare(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a minus bare word: `-%` → `-%` (keep the full string)
fn lex_minus_bare(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a double-dash bare word: `---` → `---`, `--=x` → `--=x` (keep the
/// full string; see GH #137).
fn lex_double_dash_bare(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a job specifier: `%1` → `%1` (keep the leading `%`).
fn lex_job_spec(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex an absolute path: `/tmp/out` → `/tmp/out`
fn lex_path(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a tilde path: `~/foo` → `~/foo`
fn lex_tilde_path(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a relative path: `../foo` → `../foo`
fn lex_relative_path(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

/// Lex a dot-slash path: `./foo` → `./foo`
fn lex_dot_slash_path(lex: &mut logos::Lexer<Token>) -> String {
    lex.slice().to_string()
}

impl fmt::Display for Token {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Token::Set => write!(f, "set"),
            Token::Local => write!(f, "local"),
            Token::If => write!(f, "if"),
            Token::Then => write!(f, "then"),
            Token::Else => write!(f, "else"),
            Token::Elif => write!(f, "elif"),
            Token::Fi => write!(f, "fi"),
            Token::For => write!(f, "for"),
            Token::While => write!(f, "while"),
            Token::In => write!(f, "in"),
            Token::Do => write!(f, "do"),
            Token::Done => write!(f, "done"),
            Token::Case => write!(f, "case"),
            Token::Esac => write!(f, "esac"),
            Token::Function => write!(f, "function"),
            Token::Break => write!(f, "break"),
            Token::Continue => write!(f, "continue"),
            Token::Return => write!(f, "return"),
            Token::Exit => write!(f, "exit"),
            Token::True => write!(f, "true"),
            Token::False => write!(f, "false"),
            Token::TypeString => write!(f, "string"),
            Token::TypeInt => write!(f, "int"),
            Token::TypeFloat => write!(f, "float"),
            Token::TypeBool => write!(f, "bool"),
            Token::And => write!(f, "&&"),
            Token::Or => write!(f, "||"),
            Token::EqEq => write!(f, "=="),
            Token::NotEq => write!(f, "!="),
            Token::Match => write!(f, "=~"),
            Token::NotMatch => write!(f, "!~"),
            Token::GtEq => write!(f, ">="),
            Token::LtEq => write!(f, "<="),
            Token::GtGt => write!(f, ">>"),
            Token::StderrToStdout => write!(f, "2>&1"),
            Token::StdoutToStderr => write!(f, "1>&2"),
            Token::StdoutToStderr2 => write!(f, ">&2"),
            Token::Stderr => write!(f, "2>"),
            Token::Both => write!(f, "&>"),
            Token::HereDocStart => write!(f, "<<"),
            Token::HereString => write!(f, "<<<"),
            Token::DoubleSemi => write!(f, ";;"),
            Token::Eq => write!(f, "="),
            Token::Pipe => write!(f, "|"),
            Token::Amp => write!(f, "&"),
            Token::Gt => write!(f, ">"),
            Token::Lt => write!(f, "<"),
            Token::Semi => write!(f, ";"),
            Token::Colon => write!(f, ":"),
            Token::Comma => write!(f, ","),
            Token::Dot => write!(f, "."),
            Token::DotDot => write!(f, ".."),
            Token::DotDotDot => write!(f, "..."),
            Token::Tilde => write!(f, "~"),
            Token::TildePath(s) => write!(f, "{}", s),
            Token::RelativePath(s) => write!(f, "{}", s),
            Token::DotSlashPath(s) => write!(f, "{}", s),
            Token::LBrace => write!(f, "{{"),
            Token::RBrace => write!(f, "}}"),
            Token::LBracket => write!(f, "["),
            Token::RBracket => write!(f, "]"),
            Token::LParen => write!(f, "("),
            Token::RParen => write!(f, ")"),
            Token::Star => write!(f, "*"),
            Token::Bang => write!(f, "!"),
            Token::Question => write!(f, "?"),
            Token::GlobWord(s) => write!(f, "GLOB({})", s),
            Token::Arithmetic(s) => write!(f, "ARITHMETIC({})", s),
            Token::CmdSubstStart => write!(f, "$("),
            Token::LongFlag(s) => write!(f, "--{}", s),
            Token::ShortFlag(s) => write!(f, "-{}", s),
            Token::PlusFlag(s) => write!(f, "+{}", s),
            Token::DoubleDash => write!(f, "--"),
            Token::DoubleDashBare(s) => write!(f, "{}", s),
            Token::PlusBare(s) => write!(f, "{}", s),
            Token::MinusBare(s) => write!(f, "{}", s),
            Token::JobSpec(s) => write!(f, "{}", s),
            Token::MinusAlone => write!(f, "-"),
            Token::String(s) => write!(f, "STRING({:?})", s),
            Token::SingleString(s) => write!(f, "SINGLESTRING({:?})", s),
            Token::HereDoc(d) => write!(f, "HEREDOC({:?}, literal={})", d.content, d.literal),
            Token::VarRef(v) => write!(f, "VARREF({})", v),
            Token::SimpleVarRef(v) => write!(f, "SIMPLEVARREF({})", v),
            Token::Positional(n) => write!(f, "${}", n),
            Token::AllArgs => write!(f, "$@"),
            Token::ArgCount => write!(f, "$#"),
            Token::LastExitCode => write!(f, "$?"),
            Token::CurrentPid => write!(f, "$$"),
            Token::VarLength(v) => write!(f, "${{#{}}}", v),
            Token::Int(n) => write!(f, "INT({})", n),
            Token::Float(n) => write!(f, "FLOAT({})", n),
            Token::Path(s) => write!(f, "PATH({})", s),
            Token::Ident(s) => write!(f, "IDENT({})", s),
            Token::NumberIdent(s) => write!(f, "NUMIDENT({})", s),
            Token::DashNumWord(s) => write!(f, "DASHNUM({})", s),
            Token::AtWord(s) => write!(f, "ATWORD({})", s),
            Token::DottedIdent(s) => write!(f, "DOTIDENT({})", s),
            Token::Comment => write!(f, "COMMENT"),
            Token::Newline => write!(f, "NEWLINE"),
            Token::LineContinuation => write!(f, "LINECONT"),
            // These variants should never be produced — their callbacks always return errors
            Token::InvalidFloatNoLeading => write!(f, "INVALID_FLOAT_NO_LEADING"),
            Token::InvalidFloatNoTrailing => write!(f, "INVALID_FLOAT_NO_TRAILING"),
            Token::BacktickRejected => write!(f, "BACKTICK_REJECTED"),
        }
    }
}

impl Token {
    /// Returns true if this token is a keyword.
    // Must match the Keyword variants in `Token::category()` (minus the
    // TypeX variants, which `is_type()` covers separately). Currently
    // uncalled — kept exhaustive so future callers don't get wrong answers.
    pub fn is_keyword(&self) -> bool {
        matches!(
            self,
            Token::Set
                | Token::Local
                | Token::If
                | Token::Then
                | Token::Else
                | Token::Elif
                | Token::Fi
                | Token::For
                | Token::In
                | Token::Do
                | Token::Done
                | Token::While
                | Token::Case
                | Token::Esac
                | Token::Function
                | Token::Return
                | Token::Break
                | Token::Continue
                | Token::Exit
                | Token::True
                | Token::False
        )
    }

    /// Returns true if this token is a type keyword.
    pub fn is_type(&self) -> bool {
        matches!(
            self,
            Token::TypeString
                | Token::TypeInt
                | Token::TypeFloat
                | Token::TypeBool
        )
    }

    /// Returns true if this token starts a statement.
    // Currently uncalled — kept exhaustive so future callers don't get wrong answers.
    pub fn starts_statement(&self) -> bool {
        matches!(
            self,
            Token::Set
                | Token::Local
                | Token::Function
                | Token::If
                | Token::For
                | Token::While
                | Token::Case
                | Token::Ident(_)
                | Token::LBracket
        )
    }

    /// Returns true if this token can appear in an expression.
    pub fn is_value(&self) -> bool {
        matches!(
            self,
            Token::String(_)
                | Token::SingleString(_)
                | Token::HereDoc(_)
                | Token::Arithmetic(_)
                | Token::Int(_)
                | Token::Float(_)
                | Token::True
                | Token::False
                | Token::VarRef(_)
                | Token::SimpleVarRef(_)
                | Token::CmdSubstStart
                | Token::Path(_)
                | Token::GlobWord(_)
                | Token::LastExitCode
                | Token::CurrentPid
        )
    }
}

/// Result of preprocessing arithmetic expressions.
struct ArithmeticPreprocessResult {
    /// Preprocessed source with markers replacing $((expr)).
    text: String,
    /// Vector of (marker, expression_content) pairs.
    arithmetics: Vec<(String, String)>,
    /// Span replacements for correcting token positions.
    replacements: Vec<SpanReplacement>,
}

/// Skip a `$(...)` command substitution with quote-aware paren matching.
///
/// Copies the entire command substitution verbatim to `result`, handling
/// single quotes, double quotes, and backslash escapes inside the sub so
/// that parentheses within strings don't confuse the depth counter.
///
/// On entry, `i` points to the `$` of `$(`. On exit, `i` points past the
/// closing `)`.
fn skip_command_substitution(
    chars: &[char],
    i: &mut usize,
    source_pos: &mut usize,
    result: &mut String,
) {
    // Copy $(
    result.push('$');
    result.push('(');
    *i += 2;
    *source_pos += 2;

    let mut depth: usize = 1;
    let mut in_single_quote = false;
    let mut in_double_quote = false;

    while *i < chars.len() && depth > 0 {
        let c = chars[*i];

        if in_single_quote {
            result.push(c);
            *source_pos += c.len_utf8();
            *i += 1;
            if c == '\'' {
                in_single_quote = false;
            }
            continue;
        }

        if in_double_quote {
            if c == '\\' && *i + 1 < chars.len() {
                let next = chars[*i + 1];
                if next == '"' || next == '\\' || next == '$' || next == '`' {
                    result.push(c);
                    result.push(next);
                    *source_pos += c.len_utf8() + next.len_utf8();
                    *i += 2;
                    continue;
                }
            }
            if c == '"' {
                in_double_quote = false;
            }
            result.push(c);
            *source_pos += c.len_utf8();
            *i += 1;
            continue;
        }

        // Outside quotes
        match c {
            '\'' => {
                in_single_quote = true;
                result.push(c);
                *source_pos += c.len_utf8();
                *i += 1;
            }
            '"' => {
                in_double_quote = true;
                result.push(c);
                *source_pos += c.len_utf8();
                *i += 1;
            }
            '\\' if *i + 1 < chars.len() => {
                result.push(c);
                result.push(chars[*i + 1]);
                *source_pos += c.len_utf8() + chars[*i + 1].len_utf8();
                *i += 2;
            }
            '(' => {
                depth += 1;
                result.push(c);
                *source_pos += c.len_utf8();
                *i += 1;
            }
            ')' => {
                depth -= 1;
                result.push(c);
                *source_pos += c.len_utf8();
                *i += 1;
            }
            _ => {
                result.push(c);
                *source_pos += c.len_utf8();
                *i += 1;
            }
        }
    }
}

/// Preprocess arithmetic expressions in source code.
///
/// Finds `$((expr))` patterns and replaces them with markers.
/// Returns the preprocessed source, arithmetic contents, and span replacement info.
///
/// Example:
///   `X=$((1 + 2))`
/// Becomes:
///   `X=__KAISH_ARITH_{id}__`
/// With arithmetics[0] = ("__KAISH_ARITH_{id}__", "1 + 2")
///
/// # Errors
/// Returns `LexerError::NestingTooDeep` if parentheses are nested beyond MAX_PAREN_DEPTH.
fn preprocess_arithmetic(source: &str) -> Result<ArithmeticPreprocessResult, LexerError> {
    let mut result = String::with_capacity(source.len());
    let mut arithmetics: Vec<(String, String)> = Vec::new();
    let mut replacements: Vec<SpanReplacement> = Vec::new();
    let mut source_pos: usize = 0;
    let chars_vec: Vec<char> = source.chars().collect();
    let mut i = 0;

    // Whether we're currently inside double quotes. Single quotes inside
    // double quotes are literal characters, not quote delimiters.
    let mut in_double_quote = false;

    while i < chars_vec.len() {
        let ch = chars_vec[i];

        // Backslash escape outside quotes — skip both chars verbatim
        if !in_double_quote && ch == '\\' && i + 1 < chars_vec.len() {
            result.push(ch);
            result.push(chars_vec[i + 1]);
            source_pos += ch.len_utf8() + chars_vec[i + 1].len_utf8();
            i += 2;
            continue;
        }

        // Single quote — only starts quote mode when NOT inside double quotes
        if ch == '\'' && !in_double_quote {
            result.push(ch);
            i += 1;
            source_pos += 1;
            while i < chars_vec.len() && chars_vec[i] != '\'' {
                result.push(chars_vec[i]);
                source_pos += chars_vec[i].len_utf8();
                i += 1;
            }
            if i < chars_vec.len() {
                result.push(chars_vec[i]); // closing quote
                source_pos += 1;
                i += 1;
            }
            continue;
        }

        // Double quote — toggle state (arithmetic is still expanded inside)
        if ch == '"' {
            in_double_quote = !in_double_quote;
            result.push(ch);
            i += 1;
            source_pos += 1;
            continue;
        }

        // Backslash escape inside double quotes — only \" and \\ are special
        if in_double_quote && ch == '\\' && i + 1 < chars_vec.len() {
            let next = chars_vec[i + 1];
            if next == '"' || next == '\\' || next == '$' || next == '`' {
                result.push(ch);
                result.push(next);
                source_pos += ch.len_utf8() + next.len_utf8();
                i += 2;
                continue;
            }
        }

        // Comment — copy verbatim from `#` through end-of-line so apostrophes
        // and `$((..))` inside the comment body don't get processed. Logos's
        // own comment regex `#[^\n\r]*` doesn't require a word boundary, so
        // we match that: any `#` outside double quotes (and outside single
        // quotes — those are consumed above as a single run) starts a comment.
        // The newline is left for the next iteration so newline-significance
        // and span tracking are preserved.
        if ch == '#' && !in_double_quote {
            while i < chars_vec.len() && chars_vec[i] != '\n' && chars_vec[i] != '\r' {
                result.push(chars_vec[i]);
                source_pos += chars_vec[i].len_utf8();
                i += 1;
            }
            continue;
        }

        // Skip $(...) command substitutions — inner arithmetic belongs to the subcommand
        if ch == '$' && i + 1 < chars_vec.len() && chars_vec[i + 1] == '('
            && !(i + 2 < chars_vec.len() && chars_vec[i + 2] == '(')
        {
            skip_command_substitution(&chars_vec, &mut i, &mut source_pos, &mut result);
            continue;
        }

        // Look for $(( (potential arithmetic)
        if ch == '$' && i + 2 < chars_vec.len() && chars_vec[i + 1] == '(' && chars_vec[i + 2] == '(' {
            let arith_start_pos = result.len();
            let original_start = source_pos;

            // Skip $((
            i += 3;
            source_pos += 3;

            // Collect expression until matching ))
            let mut expr = String::new();
            let mut paren_depth: usize = 0;
            let mut closed = false;

            while i < chars_vec.len() {
                let c = chars_vec[i];
                match c {
                    '(' => {
                        paren_depth += 1;
                        if paren_depth > MAX_PAREN_DEPTH {
                            return Err(LexerError::NestingTooDeep);
                        }
                        expr.push('(');
                        i += 1;
                        source_pos += c.len_utf8();
                    }
                    ')' => {
                        if paren_depth > 0 {
                            paren_depth -= 1;
                            expr.push(')');
                            i += 1;
                            source_pos += 1;
                        } else if i + 1 < chars_vec.len() && chars_vec[i + 1] == ')' {
                            // Found closing ))
                            i += 2;
                            source_pos += 2;
                            closed = true;
                            break;
                        } else {
                            // Single ) inside - keep going
                            expr.push(')');
                            i += 1;
                            source_pos += 1;
                        }
                    }
                    _ => {
                        expr.push(c);
                        i += 1;
                        source_pos += c.len_utf8();
                    }
                }
            }

            // Reached EOF without the closing `))` — don't silently evaluate the
            // partial expression (`$(( 1 + 2` must not become `3`).
            if !closed {
                return Err(LexerError::UnterminatedArithmetic);
            }

            // Calculate original length: from $$(( to ))
            let original_len = source_pos - original_start;

            // Create a unique marker for this arithmetic (collision-resistant)
            let marker = format!("__KAISH_ARITH_{}__", unique_marker_id());
            let marker_len = marker.len();

            // Record the replacement for span correction
            replacements.push(SpanReplacement {
                preprocessed_pos: arith_start_pos,
                marker_len,
                original_len,
            });

            arithmetics.push((marker.clone(), expr));
            result.push_str(&marker);
        } else {
            result.push(ch);
            i += 1;
            source_pos += ch.len_utf8();
        }
    }

    Ok(ArithmeticPreprocessResult {
        text: result,
        arithmetics,
        replacements,
    })
}

/// Per-heredoc metadata collected during preprocessing.
///
/// Stored verbatim alongside the substituted marker so the parser, validator,
/// and interpreter can reconstitute the body with correct semantics:
/// - `body` is the raw body bytes; tab stripping for `<<-` is applied later
///   (at materialization), so byte offsets stay aligned with the original
///   source for span tracking.
/// - `strip_tabs` records whether the `<<-` form was used.
/// - `literal` records whether the delimiter was quoted (no interpolation).
/// - `body_start_offset` is the byte offset of the first body character in
///   the source string passed to `preprocess_heredocs`. When heredocs are
///   preprocessed AFTER arithmetic, this is in arith-preprocessed coordinates;
///   in the common case (no arithmetic before the heredoc) this equals the
///   original-source offset. See span-correction notes in `tokenize`.
#[derive(Debug, Clone)]
struct HeredocReplacement {
    marker: String,
    body: String,
    literal: bool,
    strip_tabs: bool,
    body_start_offset: usize,
}

/// Preprocess here-docs in source code.
///
/// Finds `<<WORD` patterns and collects content until the delimiter line.
/// Returns the preprocessed source and a vector of replacement records.
///
/// Example:
///   `cat <<EOF\nhello\nworld\nEOF`
/// Becomes:
///   `cat <<__HEREDOC_0__`
/// With heredocs[0] = HeredocReplacement { marker: "__HEREDOC_0__",
/// body: "hello\nworld", literal: false, strip_tabs: false }
fn preprocess_heredocs(source: &str) -> Result<(String, Vec<HeredocReplacement>), Spanned<LexerError>> {
    let mut result = String::with_capacity(source.len());
    let mut heredocs: Vec<HeredocReplacement> = Vec::new();
    let chars_vec: Vec<char> = source.chars().collect();
    let mut i = 0;
    // `pos` tracks the byte offset into `source` corresponding to chars_vec[i].
    // `result` accumulates output; we record body offsets in `pos` (input-side)
    // and emit positions via `result.len()` (output-side) where needed.
    let mut pos: usize = 0;

    while i < chars_vec.len() {
        let ch = chars_vec[i];

        // Pass <<< through verbatim so the logos tokenizer sees the here-string
        // operator. If we fell through naively, the next iteration would see
        // the remaining `<<` and misfire heredoc preprocessing.
        if ch == '<'
            && chars_vec.get(i + 1) == Some(&'<')
            && chars_vec.get(i + 2) == Some(&'<')
        {
            result.push_str("<<<");
            i += 3;
            pos += 3;
            continue;
        }

        // Look for << (potential here-doc).
        if ch == '<' && chars_vec.get(i + 1) == Some(&'<') {
            // Remember where the `<<` started so an unterminated-heredoc
            // error can point back at the introducer rather than at EOF.
            let introducer_start = pos;
            i += 2; // consume both '<'
            pos += 2;

            // Check for optional - (strip leading tabs)
            let strip_tabs = chars_vec.get(i) == Some(&'-');
            if strip_tabs {
                i += 1;
                pos += 1;
            }

            // Skip whitespace before delimiter
            while let Some(&c) = chars_vec.get(i) {
                if c == ' ' || c == '\t' {
                    i += 1;
                    pos += 1;
                } else {
                    break;
                }
            }

            // Collect the delimiter word
            let mut delimiter = String::new();
            let quoted = chars_vec.get(i) == Some(&'\'') || chars_vec.get(i) == Some(&'"');
            let quote_char = if quoted {
                let q = chars_vec.get(i).copied();
                i += 1;
                pos += 1;
                q
            } else {
                None
            };

            while let Some(&c) = chars_vec.get(i) {
                if quoted {
                    if Some(c) == quote_char {
                        i += 1; // consume closing quote
                        pos += 1;
                        break;
                    }
                } else if c.is_whitespace() || c == '\n' || c == '\r' {
                    break;
                }
                delimiter.push(c);
                i += 1;
                pos += c.len_utf8();
            }

            if delimiter.is_empty() {
                // Not a valid here-doc, output << literally
                result.push_str("<<");
                if strip_tabs {
                    result.push('-');
                }
                continue;
            }

            // Buffer text after delimiter word (e.g., " | jq" in "cat <<EOF | jq")
            // This must be emitted AFTER the heredoc marker, not before.
            let mut after_delimiter = String::new();
            while let Some(&c) = chars_vec.get(i) {
                if c == '\n' {
                    i += 1;
                    pos += 1;
                    break;
                } else if c == '\r' {
                    i += 1;
                    pos += 1;
                    if chars_vec.get(i) == Some(&'\n') {
                        i += 1;
                        pos += 1;
                    }
                    break;
                }
                after_delimiter.push(c);
                i += 1;
                pos += c.len_utf8();
            }

            // Collect content until delimiter on its own line.
            // `body_start_offset` is the byte position of the first char of
            // the body in the source — first char after the newline that
            // ended the delimiter line. See HeredocReplacement docs for
            // coordinate-system caveat (arith-preprocessed, not original).
            let body_start_offset = pos;
            let mut content = String::new();
            let mut current_line = String::new();

            loop {
                let next = chars_vec.get(i).copied();
                match next {
                    Some('\n') => {
                        i += 1;
                        pos += 1;
                        // Check if this line is the delimiter
                        let trimmed = if strip_tabs {
                            current_line.trim_start_matches('\t')
                        } else {
                            &current_line
                        };
                        if trimmed == delimiter {
                            // Found end of here-doc
                            break;
                        }
                        // Add line to content (including empty lines)
                        content.push_str(&current_line);
                        content.push('\n');
                        current_line.clear();
                    }
                    Some('\r') => {
                        i += 1;
                        pos += 1;
                        // Detect CRLF vs bare CR. We strip the line ending
                        // for delimiter matching (so `EOF\r` still matches
                        // `EOF`) but preserve the original byte sequence in
                        // the body content — the user's input is honored
                        // verbatim.
                        let crlf = chars_vec.get(i) == Some(&'\n');
                        if crlf {
                            i += 1;
                            pos += 1;
                        }
                        let trimmed = if strip_tabs {
                            current_line.trim_start_matches('\t')
                        } else {
                            &current_line
                        };
                        if trimmed == delimiter {
                            break;
                        }
                        content.push_str(&current_line);
                        content.push_str(if crlf { "\r\n" } else { "\r" });
                        current_line.clear();
                    }
                    Some(c) => {
                        current_line.push(c);
                        i += 1;
                        pos += c.len_utf8();
                    }
                    None => {
                        // EOF — check if current line is the delimiter (matches
                        // when the source ends without a trailing newline).
                        let trimmed = if strip_tabs {
                            current_line.trim_start_matches('\t')
                        } else {
                            &current_line
                        };
                        if trimmed == delimiter {
                            break;
                        }
                        // Not a delimiter — the heredoc was never closed.
                        // Crash rather than silently using whatever we
                        // collected: missing data is exactly the failure
                        // mode where silent fallback masks the bug.
                        let span_end = introducer_start
                            + 2
                            + if strip_tabs { 1 } else { 0 }
                            + delimiter.len();
                        return Err(Spanned::new(
                            LexerError::UnterminatedHeredoc {
                                delimiter: delimiter.clone(),
                            },
                            introducer_start..span_end,
                        ));
                    }
                }
            }

            // Create a unique marker for this here-doc (collision-resistant)
            let marker = format!("__KAISH_HEREDOC_{}__", unique_marker_id());
            heredocs.push(HeredocReplacement {
                marker: marker.clone(),
                body: content,
                literal: quoted,
                strip_tabs,
                body_start_offset,
            });

            // Output <<marker first, then any text that followed the delimiter
            // (e.g., " | jq") so the heredoc attaches to the correct command.
            result.push_str("<<");
            result.push_str(&marker);
            result.push_str(&after_delimiter);
            result.push('\n');
        } else {
            result.push(ch);
            i += 1;
            pos += ch.len_utf8();
        }
    }

    Ok((result, heredocs))
}

/// Extract the text contribution of a token for colon-adjacent merging.
///
/// Returns `Some(text)` for token types that can participate in word-like
/// merging, `None` for everything else.
fn mergeable_text(token: &Token) -> Option<String> {
    match token {
        Token::Ident(s) => Some(s.clone()),
        Token::NumberIdent(s) => Some(s.clone()),
        Token::DashNumWord(s) => Some(s.clone()),
        Token::AtWord(s) => Some(s.clone()),
        Token::DottedIdent(s) => Some(s.clone()),
        Token::Colon => Some(":".to_string()),
        Token::Int(n) => Some(n.to_string()),
        Token::Path(p) => Some(p.clone()),
        Token::Float(f) => Some(f.to_string()),
        _ => None,
    }
}

/// Per-token context computed ahead of the colon-merge and glob-merge passes:
/// is this token part of a *value*-position collection literal (assignment
/// RHS, `in`/`not in` RHS)? Both merge passes suppress their fusion there so
/// `x=[dog]` / `{port:8080}` reach the parser as primitive tokens instead of
/// a fused `GlobWord`/`Ident` — see docs/arrays-and-hashes.md
/// ("Implementation notes: Glob-merge is the big lexer collision" and the
/// colon-fusion note right after it).
///
/// A run is "at value position" when it opens immediately after a token that
/// *grammatically* introduces a value — an **assignment** `=` or a
/// **membership** `in`. The trap is that both of those tokens are reused for a
/// non-value role: `Token::Eq` is also the `[[ ]]` comparison `=`
/// (`[[ $x = [0-9]* ]]`), and `Token::In` is also the `for`/`case` statement
/// head (`for f in *.txt`, `case 5 in [0-9]*)`). In every one of those
/// non-value uses the following word is argv-shaped (a glob / char-class
/// pattern) and must keep fusing, so getting this discrimination wrong breaks
/// real syntax.
///
/// The discrimination is **grammar-exact by `[[ ]]` test depth**, which needs
/// no per-keyword special-casing:
/// - membership `in` occurs ONLY inside `[[ ]]`; a `for`/`case` head `in`
///   occurs ONLY outside. So `Token::In` opens value position **iff
///   `test_depth > 0`**.
/// - an assignment `=` occurs outside `[[ ]]`; a comparison `=` occurs inside.
///   So `Token::Eq` opens value position **iff `test_depth == 0`**.
///
/// Tracking `test_depth` in the same forward pass also dissolves two `$()`
/// leaks a hand-rolled keyword counter suffered: a bareword `in`/`for`/`case`
/// inside `$(…)` sits at `test_depth == 0` and never opens value position (nor
/// leaks any state past `RParen`), while a real sub-shell assignment
/// `$(x=[a b])` is still `test_depth == 0` and correctly does. This replaces
/// the earlier pending-`for`/`case` counter entirely.
///
/// `bracket_depth`/`brace_depth` are simple counters, not a real stack, but
/// that's sufficient here: the `!currently_value` guard on `[[` detection
/// above means a glued nested literal (`x=[[a] [b]]`) never gets mistaken for
/// a `[[ ]]` test opener once bracket_depth is already > 0, so deeply-nested
/// *glued* literals parse identically to spaced nesting (`[ [a] [b] ]`) at any
/// depth — see `deeply_nested_glued_list_literal_matches_spaced_nesting` in
/// `collection_literals_tests.rs`.
#[derive(Clone, Copy, Default)]
struct ValueContext {
    /// Inside (or opening) a value-position `[`/`{` literal — suppresses
    /// glob-merge bracket-pair fusion.
    in_literal: bool,
    /// Inside a value-position `{` literal specifically — suppresses
    /// colon-merge fusion. Narrower than `in_literal` on purpose: a plain
    /// scalar assignment like `x=foo:bar` must keep fusing into one `Ident`.
    in_brace: bool,
}

fn compute_value_context(tokens: &[Spanned<Token>]) -> Vec<ValueContext> {
    let mut ctx = vec![ValueContext::default(); tokens.len()];
    let mut bracket_depth: usize = 0;
    let mut brace_depth: usize = 0;
    // `[[ ]]` nesting depth. `[[` is two `Token::LBracket`, `]]` two
    // `Token::RBracket` (kaish has no compound bracket token). This is what
    // tells an assignment `=` from a comparison `=`, and a membership `in`
    // from a `for`/`case` head `in`.
    let mut test_depth: usize = 0;
    let mut expect_value = false;
    // Set after consuming the first `[`/`]` of a `[[`/`]]` pair so the paired
    // bracket is skipped for depth accounting (it's a pure test delimiter).
    let mut skip_paired_bracket = false;

    for i in 0..tokens.len() {
        let tok = &tokens[i].token;

        let currently_value = expect_value || bracket_depth > 0 || brace_depth > 0;
        ctx[i] = ValueContext {
            in_literal: currently_value,
            in_brace: brace_depth > 0,
        };

        if skip_paired_bracket {
            // Second `[` of `[[` or second `]` of `]]`: already accounted for
            // by its partner, no depth effect.
            skip_paired_bracket = false;
        } else {
            match tok {
                Token::LBracket => {
                    let next_is_lbracket =
                        matches!(tokens.get(i + 1).map(|t| &t.token), Some(Token::LBracket));
                    if next_is_lbracket && !currently_value {
                        // `[[` opens a test — NOT a value literal. The
                        // `!currently_value` guard keeps a glued nested value
                        // list (`x=[[a] [b]]`) as literal brackets, not a bogus
                        // test.
                        test_depth += 1;
                        skip_paired_bracket = true;
                    } else if currently_value {
                        bracket_depth += 1;
                    }
                    // A lone `[` outside any value literal (argv `ls [dog]`,
                    // a `[0-9]` char-class) neither opens a test nor a literal.
                }
                Token::RBracket => {
                    let next_is_rbracket =
                        matches!(tokens.get(i + 1).map(|t| &t.token), Some(Token::RBracket));
                    if bracket_depth > 0 {
                        // Close an open value-literal bracket first — in
                        // `[[ x in [a b] ]]` the `[a b]`'s own `]` closes here
                        // (bracket_depth 1→0) before the trailing `]]`.
                        bracket_depth -= 1;
                    } else if test_depth > 0 && next_is_rbracket {
                        test_depth -= 1;
                        skip_paired_bracket = true;
                    }
                }
                Token::LBrace if currently_value => brace_depth += 1,
                Token::RBrace if brace_depth > 0 => brace_depth -= 1,
                _ => {}
            }
        }

        // Grammar-exact: assignment `=` (outside `[[ ]]`) and membership `in`
        // (inside `[[ ]]`) open value position; comparison `=` and for/case
        // head `in` do not. `Token::EqEq` (`==`) is a distinct token and never
        // opens value position at all.
        expect_value = match tok {
            Token::Eq => test_depth == 0,
            Token::In => test_depth > 0,
            _ => false,
        };
    }

    ctx
}

/// Merge span-adjacent token runs containing `Token::Colon` into single `Ident` tokens.
///
/// In bash, `:` is a regular character in unquoted words. kaish tokenizes it
/// separately, which breaks Rust paths (`foo::bar`), URLs (`host:8080`), etc.
///
/// This pass fuses span-adjacent mergeable tokens (Ident, Colon, Int, Path, Float)
/// into a single `Ident` when the run contains at least one `Colon`. Runs without
/// colons or standalone tokens pass through unchanged. A run that opens inside a
/// value-position record literal (`{port:8080}`) is exempted — see
/// `ValueContext`/`compute_value_context` above — so the record parser sees the
/// `Colon` as its own token.
fn merge_colon_adjacent(tokens: Vec<Spanned<Token>>) -> Vec<Spanned<Token>> {
    if tokens.is_empty() {
        return tokens;
    }

    let value_ctx = compute_value_context(&tokens);
    let mut result = Vec::with_capacity(tokens.len());
    let mut run: Vec<&Spanned<Token>> = Vec::new();
    let mut run_start = 0usize;

    for (idx, token) in tokens.iter().enumerate() {
        if run.is_empty() {
            if mergeable_text(&token.token).is_some() {
                run.push(token);
                run_start = idx;
            } else {
                result.push(token.clone());
            }
            continue;
        }

        // Check span adjacency: previous run's last token ends where this one starts
        // Safety: run is non-empty (checked above)
        let Some(last) = run.last() else { unreachable!() };
        let adjacent = last.span.end == token.span.start;

        if adjacent && mergeable_text(&token.token).is_some() {
            run.push(token);
        } else {
            flush_colon_run(&mut run, &mut result, value_ctx[run_start].in_brace);
            if mergeable_text(&token.token).is_some() {
                run.push(token);
                run_start = idx;
            } else {
                result.push(token.clone());
            }
        }
    }

    flush_colon_run(&mut run, &mut result, value_ctx[run_start].in_brace);

    result
}

/// Flush a run of mergeable tokens: merge if it contains a colon, otherwise emit
/// individually. `suppress` (true when the run opened inside a value-position
/// record literal) forces individual emission even when `has_colon` would
/// otherwise fuse — see `ValueContext`.
fn flush_colon_run(run: &mut Vec<&Spanned<Token>>, result: &mut Vec<Spanned<Token>>, suppress: bool) {
    if run.is_empty() {
        return;
    }

    let has_colon = run.iter().any(|t| matches!(t.token, Token::Colon));

    if !suppress && run.len() >= 2 && has_colon {
        let text: String = run
            .iter()
            .filter_map(|t| mergeable_text(&t.token))
            .collect();
        // Safety: run.len() >= 2 so first/last exist
        let start = run.first().map(|t| t.span.start).unwrap_or(0);
        let end = run.last().map(|t| t.span.end).unwrap_or(0);
        result.push(Spanned::new(Token::Ident(text), start..end));
    } else {
        for t in run.iter() {
            result.push((*t).clone());
        }
    }

    run.clear();
}

/// Extract the text contribution of a token that can participate in a glob word.
///
/// Returns `Some(text)` for tokens that can be part of a glob pattern (identifiers,
/// wildcard chars, brackets, paths, etc.), `None` for structural tokens.
fn glob_mergeable_text(token: &Token) -> Option<String> {
    match token {
        Token::Star => Some("*".to_string()),
        Token::Question => Some("?".to_string()),
        Token::Dot => Some(".".to_string()),
        Token::DotDot => Some("..".to_string()),
        Token::Ident(s) => Some(s.clone()),
        Token::NumberIdent(s) => Some(s.clone()),
        Token::DashNumWord(s) => Some(s.clone()),
        Token::AtWord(s) => Some(s.clone()),
        Token::DottedIdent(s) => Some(s.clone()),
        Token::Path(s) => Some(s.clone()),
        Token::Int(n) => Some(n.to_string()),
        Token::LBracket => Some("[".to_string()),
        Token::RBracket => Some("]".to_string()),
        Token::Bang => Some("!".to_string()),
        Token::DotSlashPath(s) => Some(s.clone()),
        Token::RelativePath(s) => Some(s.clone()),
        Token::TildePath(s) => Some(s.clone()),
        Token::Tilde => Some("~".to_string()),
        Token::LBrace => Some("{".to_string()),
        Token::RBrace => Some("}".to_string()),
        Token::Comma => Some(",".to_string()),
        _ => None,
    }
}

/// Merge a span-adjacent metacharacter onto a flag token.
///
/// Handles the `awk -F:` idiom: the kaish lexer emits `-F` as `ShortFlag("F")` and
/// `:` as `Token::Colon` (an operator). When the two tokens are span-adjacent (no
/// whitespace between them), the `:` is part of the flag value, not a shell operator.
/// This pass fuses them so `ShortFlag("F:")` reaches the arg-binding layer, which
/// already handles the glued-value form (the same mechanism used for `cut -f1`).
///
/// Metachars handled: `:` (Colon) only.
///
/// `;` (Semi) and `|` (Pipe) are shell operators and must NOT be fused even when
/// span-adjacent — `cmd -p; cmd2` and `ls -l|cat` must produce real Semi/Pipe
/// tokens so the shell grammar can treat them as statement separators and pipes.
/// In bash, `-F;` and `-F|` require quoting (`-F';'`), so kaish matches that
/// contract.
///
/// **Safety**: the fuse is guarded by span adjacency (`last.span.end == token.span.start`),
/// which is only true when there is no whitespace between the flag and the metachar.
/// A space-separated `cmd -F :` leaves a gap and never reaches this merge.
///
/// **Colon-run fusion**: consecutive span-adjacent colons after the flag are all
/// absorbed in one pass, so `-F::` becomes `ShortFlag("F::")` rather than
/// `ShortFlag("F:") + Colon`.
///
/// Only `ShortFlag` is handled here. `LongFlag` with a bare `=:` form (e.g.
/// `--field-separator=:`) is handled by the parser's `long_flag_with_value` rule
/// via `primary_expr_parser`, which accepts the merged `Ident` that
/// `merge_colon_adjacent` already produces from `=:` runs.
fn merge_flag_metachar_adjacent(tokens: Vec<Spanned<Token>>) -> Vec<Spanned<Token>> {
    if tokens.len() < 2 {
        return tokens;
    }

    let mut result = Vec::with_capacity(tokens.len());
    let mut i = 0;

    while i < tokens.len() {
        let token = &tokens[i];

        // Only short flags can be followed by a glued colon.
        if let Token::ShortFlag(flag_name) = &token.token {
            // Absorb a run of span-adjacent colons into the flag name.
            let mut fused = flag_name.clone();
            let mut end_span = token.span.end;
            let mut j = i + 1;

            while let Some(next) = tokens.get(j) {
                if next.span.start == end_span {
                    if let Token::Colon = &next.token {
                        fused.push(':');
                        end_span = next.span.end;
                        j += 1;
                        continue;
                    }
                }
                break;
            }

            if j > i + 1 {
                // At least one colon was absorbed.
                let span = token.span.start..end_span;
                result.push(Spanned::new(Token::ShortFlag(fused), span));
                i = j;
                continue;
            }
        }

        result.push(token.clone());
        i += 1;
    }

    result
}

/// Merge span-adjacent token runs containing glob metacharacters into `GlobWord` tokens.
///
/// A run is merged into `GlobWord` when it contains at least one `Star`, `Question`,
/// or a `LBracket`+`RBracket` pair. Runs without glob chars pass through unchanged.
///
/// Runs after colon merge: `foo::bar` stays as `Ident("foo::bar")` because colon merge
/// already fused it before this pass sees it.
///
/// A run that opens at *value position* (`x=[dog]`, `[[ $a in [dog] ]]`) is exempted
/// from the `[`/`]` bracket-pair fusion — see `ValueContext`/`compute_value_context`
/// above — so the list-literal parser sees primitive `LBracket`/`RBracket` tokens
/// instead of a fused `GlobWord`. Argv-position brackets (`ls [dog]`, `for x in [a]`)
/// are untouched.
///
/// A SEPARATE trigger suppresses fusion for an **assignment lvalue**:
/// `fruits[0]=kiwi`, `user[email]=x`, `services[web][port]=9090`. This run sits
/// at *argv/LHS position* (before `=`), not value position, so
/// `ValueContext::in_literal` is false for it — the lvalue grammar needs its
/// own peek-ahead: a bracket-pair run (no `*`/`?`) immediately followed by
/// `Token::Eq` is a subscripted assignment target, not a glob, so it is
/// suppressed the same way regardless of whitespace before the `=` (see
/// `flush_glob_run`'s `followed_by_eq` parameter). See
/// `docs/arrays-and-hashes.md` ("Assignment lvalues").
fn merge_glob_adjacent(tokens: Vec<Spanned<Token>>) -> Vec<Spanned<Token>> {
    if tokens.is_empty() {
        return tokens;
    }

    let value_ctx = compute_value_context(&tokens);
    let mut result = Vec::with_capacity(tokens.len());
    let mut run: Vec<&Spanned<Token>> = Vec::new();
    let mut run_start = 0usize;

    for (idx, token) in tokens.iter().enumerate() {
        if run.is_empty() {
            if glob_mergeable_text(&token.token).is_some() {
                run.push(token);
                run_start = idx;
            } else {
                result.push(token.clone());
            }
            continue;
        }

        // Safety: run is non-empty (checked at top of loop)
        let Some(last) = run.last() else { unreachable!() };
        let adjacent = last.span.end == token.span.start;

        if adjacent && glob_mergeable_text(&token.token).is_some() {
            run.push(token);
        } else {
            // `token` is whatever broke the run — an lvalue's `=` is never
            // glob-mergeable, so it always lands here (not in the `if` arm
            // above) regardless of whether whitespace separates it from the
            // run (`fruits[0]=kiwi` and `fruits[0] = kiwi` both see it).
            let followed_by_eq = matches!(token.token, Token::Eq);
            flush_glob_run(
                &mut run,
                &mut result,
                value_ctx[run_start].in_literal,
                followed_by_eq,
            );
            if glob_mergeable_text(&token.token).is_some() {
                run.push(token);
                run_start = idx;
            } else {
                result.push(token.clone());
            }
        }
    }

    // End of input: no token follows the final run, so it can't be an lvalue
    // (an assignment always has a value after `=`).
    flush_glob_run(&mut run, &mut result, value_ctx[run_start].in_literal, false);

    result
}

/// Flush a run of glob-mergeable tokens: merge if it contains glob metacharacters.
///
/// `value_position_suppress` (true when the run opened at value position — see
/// `ValueContext`) forces individual emission for a run that contains an
/// `LBracket`, so a `[`-leading run at value position always reaches the
/// parser as primitive tokens for the list-literal grammar, never a
/// `GlobWord`. A pure `Star`/`Question` glob with no brackets (`X=*.txt`) is
/// untouched by `value_position_suppress` — it keeps fusing to a `GlobWord`,
/// which evaluates to a literal string at value position exactly as it did
/// before collection literals existed (see `test_glob_in_assignment_is_literal`);
/// only bracket-shaped runs are reinterpreted as list-literal syntax.
///
/// `followed_by_eq` (true when the token right after this run — in stream
/// order, whitespace or not — is `Token::Eq`) is the SEPARATE lvalue trigger:
/// a bracket-pair run with no `*`/`?` immediately before `=` is a subscripted
/// assignment target (`fruits[0]=kiwi`), not a glob, so it is suppressed too
/// — even though it sits at argv/LHS position, not value position. The
/// `has_star_or_question` guard keeps a real glob pattern that happens to
/// precede an `=` (vanishingly rare, but not this run's job to reinterpret)
/// out of the suppression.
fn flush_glob_run(
    run: &mut Vec<&Spanned<Token>>,
    result: &mut Vec<Spanned<Token>>,
    value_position_suppress: bool,
    followed_by_eq: bool,
) {
    if run.is_empty() {
        return;
    }

    let has_bracket_pair = run.iter().any(|t| matches!(t.token, Token::LBracket))
        && run.iter().any(|t| matches!(t.token, Token::RBracket));
    let has_star_or_question = run
        .iter()
        .any(|t| matches!(t.token, Token::Star | Token::Question));
    let has_glob = has_star_or_question || has_bracket_pair;

    // An lvalue subscript run is a ROOT IDENTIFIER followed by brackets, so the
    // run STARTS with an `Ident` (`arr[0]=` → run is `arr [ 0 ]`, glued, so
    // `arr` is the first token OF the run — a lvalue root parses via
    // `ident_parser()`, i.e. `Token::Ident`). A bare char-class comparison
    // operand starts with `[` instead (`[[ [a] = b ]]` → the suppressible run
    // is `[ a ]`, since the leading `[[` was flushed separately), so requiring
    // an `Ident`-led run keeps `[[ [a] = b ]]` fusing-and-comparing as before
    // while still catching every real lvalue.
    let run_starts_with_ident = matches!(
        run.first().map(|t| &t.token),
        Some(Token::Ident(_))
    );
    let lvalue_suppress =
        followed_by_eq && has_bracket_pair && !has_star_or_question && run_starts_with_ident;
    let suppress = (value_position_suppress && has_bracket_pair) || lvalue_suppress;

    if !suppress && run.len() >= 2 && has_glob {
        let text: String = run
            .iter()
            .filter_map(|t| glob_mergeable_text(&t.token))
            .collect();
        let start = run.first().map(|t| t.span.start).unwrap_or(0);
        let end = run.last().map(|t| t.span.end).unwrap_or(0);
        result.push(Spanned::new(Token::GlobWord(text), start..end));
    } else {
        for t in run.iter() {
            result.push((*t).clone());
        }
    }

    run.clear();
}

/// Tokenize source code into a vector of spanned tokens.
///
/// Skips whitespace and comments (unless you need them for formatting).
/// Returns errors with their positions for nice error messages.
///
/// Handles:
/// - Arithmetic: `$((expr))` becomes `Arithmetic("expr")`
/// - Here-docs: `<<EOF\nhello\nEOF` becomes `HereDocStart` + `HereDoc("hello")`
/// - Colon merge: span-adjacent `foo::bar` becomes `Ident("foo::bar")`
pub fn tokenize(source: &str) -> Result<Vec<Spanned<Token>>, Vec<Spanned<LexerError>>> {
    // Preprocess arithmetic first (before heredocs because heredoc content might contain $((
    let arith_result = preprocess_arithmetic(source)
        .map_err(|e| vec![Spanned::new(e, 0..source.len())])?;

    // Then preprocess here-docs. Spans inside the heredoc preprocessor are in
    // arith-preprocessed coords; correct back to original-source coords before
    // surfacing the error to keep parser diagnostics aligned with source.
    let span_replacements = arith_result.replacements;
    let (preprocessed, heredocs) = preprocess_heredocs(&arith_result.text)
        .map_err(|e| {
            let span = correct_span(e.span, &span_replacements);
            vec![Spanned::new(e.token, span)]
        })?;

    let lexer = Token::lexer(&preprocessed);
    let mut tokens = Vec::new();
    let mut errors = Vec::new();

    for (result, span) in lexer.spanned() {
        // Correct the span from preprocessed coordinates to original coordinates
        let corrected_span = correct_span(span, &span_replacements);
        match result {
            Ok(token) => {
                // Skip comments and line continuations - they're not needed for parsing
                if !matches!(token, Token::Comment | Token::LineContinuation) {
                    tokens.push(Spanned::new(token, corrected_span));
                }
            }
            Err(err) => {
                errors.push(Spanned::new(err, corrected_span));
            }
        }
    }

    if !errors.is_empty() {
        return Err(errors);
    }

    // Post-process: replace markers with actual token content
    let mut final_tokens = Vec::with_capacity(tokens.len());
    let mut i = 0;

    while i < tokens.len() {
        // Check for arithmetic marker (unique format: __KAISH_ARITH_{id}__)
        if let Token::Ident(ref name) = tokens[i].token
            && name.starts_with("__KAISH_ARITH_") && name.ends_with("__")
                && let Some((_, expr)) = arith_result.arithmetics.iter().find(|(marker, _)| marker == name) {
                    final_tokens.push(Spanned::new(Token::Arithmetic(expr.clone()), tokens[i].span.clone()));
                    i += 1;
                    continue;
                }

        // Check for heredoc (unique format: __KAISH_HEREDOC_{id}__)
        if matches!(tokens[i].token, Token::HereDocStart) {
            // Check if next token is a heredoc marker
            if i + 1 < tokens.len()
                && let Token::Ident(ref name) = tokens[i + 1].token
                    && name.starts_with("__KAISH_HEREDOC_") && name.ends_with("__") {
                        // Find the corresponding content
                        if let Some(hd) = heredocs.iter().find(|h| h.marker == *name) {
                            // Re-thread arithmetic markers that the arith
                            // preprocessor planted in the source — without
                            // this, `<<EOF\n$((1+2))\nEOF` materializes the
                            // marker text instead of `3`. Mirrors the
                            // String-content translation a few lines below.
                            // - Literal heredocs (no expansion): restore the
                            //   original `$((expr))` text verbatim.
                            // - Interpolated heredocs: wrap as
                            //   `${__ARITH:expr__}` so the spanned
                            //   interpolation parser turns it into a
                            //   StringPart::Arithmetic.
                            let mut content = hd.body.clone();
                            for (marker, expr) in &arith_result.arithmetics {
                                if content.contains(marker) {
                                    let replacement = if hd.literal {
                                        format!("$(({}))", expr)
                                    } else {
                                        format!("${{__ARITH:{}__}}", expr)
                                    };
                                    content = content.replace(marker, &replacement);
                                }
                            }
                            final_tokens.push(Spanned::new(Token::HereDocStart, tokens[i].span.clone()));
                            final_tokens.push(Spanned::new(
                                Token::HereDoc(HereDocData {
                                    content,
                                    literal: hd.literal,
                                    strip_tabs: hd.strip_tabs,
                                    body_start_offset: hd.body_start_offset,
                                }),
                                tokens[i + 1].span.clone(),
                            ));
                            i += 2;
                            continue;
                        }
                    }
        }

        // Check for arithmetic markers inside string content
        let token = if let Token::String(ref s) = tokens[i].token {
            // Check if string contains any arithmetic markers
            let mut new_content = s.clone();
            for (marker, expr) in &arith_result.arithmetics {
                if new_content.contains(marker) {
                    // Replace marker with the special format that parse_interpolated_string can detect
                    // Use ${__ARITH:expr__} format so it gets parsed as StringPart::Arithmetic
                    new_content = new_content.replace(marker, &format!("${{__ARITH:{}__}}", expr));
                }
            }
            if new_content != *s {
                Spanned::new(Token::String(new_content), tokens[i].span.clone())
            } else {
                tokens[i].clone()
            }
        } else {
            tokens[i].clone()
        };
        final_tokens.push(token);
        i += 1;
    }

    Ok(merge_glob_adjacent(merge_colon_adjacent(
        merge_flag_metachar_adjacent(final_tokens),
    )))
}

/// Tokenize source code, preserving comments.
///
/// Useful for pretty-printing or formatting tools that need to preserve comments.
pub fn tokenize_with_comments(source: &str) -> Result<Vec<Spanned<Token>>, Vec<Spanned<LexerError>>> {
    let lexer = Token::lexer(source);
    let mut tokens = Vec::new();
    let mut errors = Vec::new();

    for (result, span) in lexer.spanned() {
        match result {
            Ok(token) => {
                tokens.push(Spanned::new(token, span));
            }
            Err(err) => {
                errors.push(Spanned::new(err, span));
            }
        }
    }

    if errors.is_empty() {
        Ok(tokens)
    } else {
        Err(errors)
    }
}

/// Extract the string content from a string token (removes quotes, processes escapes).
pub fn parse_string_literal(source: &str) -> Result<String, LexerError> {
    // Remove surrounding quotes
    if source.len() < 2 || !source.starts_with('"') || !source.ends_with('"') {
        return Err(LexerError::UnterminatedString);
    }

    let inner = &source[1..source.len() - 1];
    let mut result = String::with_capacity(inner.len());
    let mut chars = inner.chars().peekable();

    while let Some(ch) = chars.next() {
        if ch == '\\' {
            match chars.next() {
                Some('n') => result.push('\n'),
                Some('t') => result.push('\t'),
                Some('r') => result.push('\r'),
                Some('\\') => result.push('\\'),
                Some('"') => result.push('"'),
                // Use a unique marker for escaped dollar that won't be re-interpreted
                // parse_interpolated_string will convert this back to $
                Some('$') => result.push_str("__KAISH_ESCAPED_DOLLAR__"),
                Some('u') => {
                    // Unicode escape: \uXXXX
                    let mut hex = String::with_capacity(4);
                    for _ in 0..4 {
                        match chars.next() {
                            Some(h) if h.is_ascii_hexdigit() => hex.push(h),
                            _ => return Err(LexerError::InvalidEscape),
                        }
                    }
                    let codepoint = u32::from_str_radix(&hex, 16)
                        .map_err(|_| LexerError::InvalidEscape)?;
                    let ch = char::from_u32(codepoint)
                        .ok_or(LexerError::InvalidEscape)?;
                    result.push(ch);
                }
                // Unknown escapes: preserve the backslash (for regex patterns like `\.`)
                Some(next) => {
                    result.push('\\');
                    result.push(next);
                }
                None => return Err(LexerError::InvalidEscape),
            }
        } else {
            result.push(ch);
        }
    }

    Ok(result)
}

/// Parse a variable reference, extracting the path segments.
/// Input: "${VAR.field[0].nested}" → ["VAR", "field", "[0]", "nested"]
pub fn parse_var_ref(source: &str) -> Result<Vec<String>, LexerError> {
    // Remove ${ and }
    if source.len() < 4 || !source.starts_with("${") || !source.ends_with('}') {
        return Err(LexerError::UnterminatedVarRef);
    }

    let inner = &source[2..source.len() - 1];

    // Special case: $? (last result)
    if inner == "?" {
        return Ok(vec!["?".to_string()]);
    }

    let mut segments = Vec::new();
    let mut current = String::new();
    let mut chars = inner.chars().peekable();

    while let Some(ch) = chars.next() {
        match ch {
            '.' => {
                if !current.is_empty() {
                    segments.push(current.clone());
                    current.clear();
                }
            }
            '[' => {
                if !current.is_empty() {
                    segments.push(current.clone());
                    current.clear();
                }
                // Collect the index
                let mut index = String::from("[");
                while let Some(&c) = chars.peek() {
                    if let Some(c) = chars.next() {
                        index.push(c);
                    }
                    if c == ']' {
                        break;
                    }
                }
                segments.push(index);
            }
            _ => {
                current.push(ch);
            }
        }
    }

    if !current.is_empty() {
        segments.push(current);
    }

    Ok(segments)
}

/// Parse an integer literal.
pub fn parse_int(source: &str) -> Result<i64, LexerError> {
    source.parse().map_err(|_| LexerError::InvalidNumber)
}

/// Parse a float literal.
pub fn parse_float(source: &str) -> Result<f64, LexerError> {
    source.parse().map_err(|_| LexerError::InvalidNumber)
}

#[cfg(test)]
#[allow(clippy::approx_constant)]
mod tests {
    use super::*;

    fn lex(source: &str) -> Vec<Token> {
        tokenize(source)
            .expect("lexer should succeed")
            .into_iter()
            .map(|s| s.token)
            .collect()
    }

    // ═══════════════════════════════════════════════════════════════════
    // Keyword tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn keywords() {
        assert_eq!(lex("set"), vec![Token::Set]);
        assert_eq!(lex("if"), vec![Token::If]);
        assert_eq!(lex("then"), vec![Token::Then]);
        assert_eq!(lex("else"), vec![Token::Else]);
        assert_eq!(lex("elif"), vec![Token::Elif]);
        assert_eq!(lex("fi"), vec![Token::Fi]);
        assert_eq!(lex("for"), vec![Token::For]);
        assert_eq!(lex("in"), vec![Token::In]);
        assert_eq!(lex("do"), vec![Token::Do]);
        assert_eq!(lex("done"), vec![Token::Done]);
        assert_eq!(lex("case"), vec![Token::Case]);
        assert_eq!(lex("esac"), vec![Token::Esac]);
        assert_eq!(lex("function"), vec![Token::Function]);
        assert_eq!(lex("true"), vec![Token::True]);
        assert_eq!(lex("false"), vec![Token::False]);
    }

    #[test]
    fn double_semicolon() {
        assert_eq!(lex(";;"), vec![Token::DoubleSemi]);
        // In case pattern context
        assert_eq!(lex("echo \"hi\";;"), vec![
            Token::Ident("echo".to_string()),
            Token::String("hi".to_string()),
            Token::DoubleSemi,
        ]);
    }

    #[test]
    fn type_keywords() {
        assert_eq!(lex("string"), vec![Token::TypeString]);
        assert_eq!(lex("int"), vec![Token::TypeInt]);
        assert_eq!(lex("float"), vec![Token::TypeFloat]);
        assert_eq!(lex("bool"), vec![Token::TypeBool]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Operator tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn single_char_operators() {
        assert_eq!(lex("="), vec![Token::Eq]);
        assert_eq!(lex("|"), vec![Token::Pipe]);
        assert_eq!(lex("&"), vec![Token::Amp]);
        assert_eq!(lex(">"), vec![Token::Gt]);
        assert_eq!(lex("<"), vec![Token::Lt]);
        assert_eq!(lex(";"), vec![Token::Semi]);
        assert_eq!(lex(":"), vec![Token::Colon]);
        assert_eq!(lex(","), vec![Token::Comma]);
        assert_eq!(lex("."), vec![Token::Dot]);
    }

    #[test]
    fn multi_char_operators() {
        assert_eq!(lex("&&"), vec![Token::And]);
        assert_eq!(lex("||"), vec![Token::Or]);
        assert_eq!(lex("=="), vec![Token::EqEq]);
        assert_eq!(lex("!="), vec![Token::NotEq]);
        assert_eq!(lex("=~"), vec![Token::Match]);
        assert_eq!(lex("!~"), vec![Token::NotMatch]);
        assert_eq!(lex(">="), vec![Token::GtEq]);
        assert_eq!(lex("<="), vec![Token::LtEq]);
        assert_eq!(lex(">>"), vec![Token::GtGt]);
        assert_eq!(lex("2>"), vec![Token::Stderr]);
        assert_eq!(lex("&>"), vec![Token::Both]);
    }

    #[test]
    fn brackets() {
        assert_eq!(lex("{"), vec![Token::LBrace]);
        assert_eq!(lex("}"), vec![Token::RBrace]);
        assert_eq!(lex("["), vec![Token::LBracket]);
        assert_eq!(lex("]"), vec![Token::RBracket]);
        assert_eq!(lex("("), vec![Token::LParen]);
        assert_eq!(lex(")"), vec![Token::RParen]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Literal tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn integers() {
        assert_eq!(lex("0"), vec![Token::Int(0)]);
        assert_eq!(lex("42"), vec![Token::Int(42)]);
        assert_eq!(lex("-1"), vec![Token::Int(-1)]);
        assert_eq!(lex("999999"), vec![Token::Int(999999)]);
    }

    #[test]
    fn floats() {
        assert_eq!(lex("3.14"), vec![Token::Float(3.14)]);
        assert_eq!(lex("-0.5"), vec![Token::Float(-0.5)]);
        assert_eq!(lex("123.456"), vec![Token::Float(123.456)]);
    }

    #[test]
    fn strings() {
        assert_eq!(lex(r#""hello""#), vec![Token::String("hello".to_string())]);
        assert_eq!(lex(r#""hello world""#), vec![Token::String("hello world".to_string())]);
        assert_eq!(lex(r#""""#), vec![Token::String("".to_string())]); // empty string
        assert_eq!(lex(r#""with \"quotes\"""#), vec![Token::String("with \"quotes\"".to_string())]);
        assert_eq!(lex(r#""with\nnewline""#), vec![Token::String("with\nnewline".to_string())]);
    }

    #[test]
    fn var_refs() {
        assert_eq!(lex("${X}"), vec![Token::VarRef("${X}".to_string())]);
        assert_eq!(lex("${VAR}"), vec![Token::VarRef("${VAR}".to_string())]);
        assert_eq!(lex("${VAR.field}"), vec![Token::VarRef("${VAR.field}".to_string())]);
        assert_eq!(lex("${VAR[0]}"), vec![Token::VarRef("${VAR[0]}".to_string())]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Identifier tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn identifiers() {
        assert_eq!(lex("foo"), vec![Token::Ident("foo".to_string())]);
        assert_eq!(lex("foo_bar"), vec![Token::Ident("foo_bar".to_string())]);
        assert_eq!(lex("foo-bar"), vec![Token::Ident("foo-bar".to_string())]);
        assert_eq!(lex("_private"), vec![Token::Ident("_private".to_string())]);
        assert_eq!(lex("cmd123"), vec![Token::Ident("cmd123".to_string())]);
    }

    #[test]
    fn keyword_prefix_identifiers() {
        // Identifiers that start with keywords but aren't keywords
        assert_eq!(lex("setup"), vec![Token::Ident("setup".to_string())]);
        assert_eq!(lex("kaish-tools"), vec![Token::Ident("kaish-tools".to_string())]);
        assert_eq!(lex("iffy"), vec![Token::Ident("iffy".to_string())]);
        assert_eq!(lex("forked"), vec![Token::Ident("forked".to_string())]);
        assert_eq!(lex("done-with-it"), vec![Token::Ident("done-with-it".to_string())]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Statement tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn assignment() {
        assert_eq!(
            lex("set X = 5"),
            vec![Token::Set, Token::Ident("X".to_string()), Token::Eq, Token::Int(5)]
        );
    }

    #[test]
    fn command_simple() {
        assert_eq!(lex("echo"), vec![Token::Ident("echo".to_string())]);
        assert_eq!(
            lex(r#"echo "hello""#),
            vec![Token::Ident("echo".to_string()), Token::String("hello".to_string())]
        );
    }

    #[test]
    fn command_with_args() {
        assert_eq!(
            lex("cmd arg1 arg2"),
            vec![Token::Ident("cmd".to_string()), Token::Ident("arg1".to_string()), Token::Ident("arg2".to_string())]
        );
    }

    #[test]
    fn command_with_named_args() {
        assert_eq!(
            lex("cmd key=value"),
            vec![Token::Ident("cmd".to_string()), Token::Ident("key".to_string()), Token::Eq, Token::Ident("value".to_string())]
        );
    }

    #[test]
    fn pipeline() {
        assert_eq!(
            lex("a | b | c"),
            vec![Token::Ident("a".to_string()), Token::Pipe, Token::Ident("b".to_string()), Token::Pipe, Token::Ident("c".to_string())]
        );
    }

    #[test]
    fn if_statement() {
        assert_eq!(
            lex("if true; then echo; fi"),
            vec![
                Token::If,
                Token::True,
                Token::Semi,
                Token::Then,
                Token::Ident("echo".to_string()),
                Token::Semi,
                Token::Fi
            ]
        );
    }

    #[test]
    fn for_loop() {
        assert_eq!(
            lex("for X in items; do echo; done"),
            vec![
                Token::For,
                Token::Ident("X".to_string()),
                Token::In,
                Token::Ident("items".to_string()),
                Token::Semi,
                Token::Do,
                Token::Ident("echo".to_string()),
                Token::Semi,
                Token::Done
            ]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Whitespace and newlines
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn whitespace_ignored() {
        assert_eq!(lex("   set   X   =   5   "), lex("set X = 5"));
    }

    #[test]
    fn newlines_preserved() {
        let tokens = lex("a\nb");
        assert_eq!(
            tokens,
            vec![Token::Ident("a".to_string()), Token::Newline, Token::Ident("b".to_string())]
        );
    }

    #[test]
    fn multiple_newlines() {
        let tokens = lex("a\n\n\nb");
        assert_eq!(
            tokens,
            vec![Token::Ident("a".to_string()), Token::Newline, Token::Newline, Token::Newline, Token::Ident("b".to_string())]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Comments
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn comments_skipped() {
        assert_eq!(lex("# comment"), vec![]);
        assert_eq!(lex("a # comment"), vec![Token::Ident("a".to_string())]);
        assert_eq!(
            lex("a # comment\nb"),
            vec![Token::Ident("a".to_string()), Token::Newline, Token::Ident("b".to_string())]
        );
    }

    #[test]
    fn comments_preserved_when_requested() {
        let tokens = tokenize_with_comments("a # comment")
            .expect("should succeed")
            .into_iter()
            .map(|s| s.token)
            .collect::<Vec<_>>();
        assert_eq!(tokens, vec![Token::Ident("a".to_string()), Token::Comment]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // String parsing
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn parse_simple_string() {
        assert_eq!(parse_string_literal(r#""hello""#).expect("ok"), "hello");
    }

    #[test]
    fn parse_string_with_escapes() {
        assert_eq!(
            parse_string_literal(r#""hello\nworld""#).expect("ok"),
            "hello\nworld"
        );
        assert_eq!(
            parse_string_literal(r#""tab\there""#).expect("ok"),
            "tab\there"
        );
        assert_eq!(
            parse_string_literal(r#""quote\"here""#).expect("ok"),
            "quote\"here"
        );
    }

    #[test]
    fn parse_string_with_unicode() {
        assert_eq!(
            parse_string_literal(r#""emoji \u2764""#).expect("ok"),
            "emoji ❤"
        );
    }

    #[test]
    fn parse_string_with_escaped_dollar() {
        // \$ produces a marker that parse_interpolated_string will convert to $
        // The marker __KAISH_ESCAPED_DOLLAR__ is used to prevent re-interpretation
        assert_eq!(
            parse_string_literal(r#""\$VAR""#).expect("ok"),
            "__KAISH_ESCAPED_DOLLAR__VAR"
        );
        assert_eq!(
            parse_string_literal(r#""cost: \$100""#).expect("ok"),
            "cost: __KAISH_ESCAPED_DOLLAR__100"
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Variable reference parsing
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn parse_simple_var() {
        assert_eq!(
            parse_var_ref("${X}").expect("ok"),
            vec!["X"]
        );
    }

    #[test]
    fn parse_var_with_field() {
        assert_eq!(
            parse_var_ref("${VAR.field}").expect("ok"),
            vec!["VAR", "field"]
        );
    }

    #[test]
    fn parse_var_with_index() {
        assert_eq!(
            parse_var_ref("${VAR[0]}").expect("ok"),
            vec!["VAR", "[0]"]
        );
    }

    #[test]
    fn parse_var_nested() {
        assert_eq!(
            parse_var_ref("${VAR.field[0].nested}").expect("ok"),
            vec!["VAR", "field", "[0]", "nested"]
        );
    }

    #[test]
    fn parse_last_result() {
        assert_eq!(
            parse_var_ref("${?}").expect("ok"),
            vec!["?"]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Number parsing
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn parse_integers() {
        assert_eq!(parse_int("0").expect("ok"), 0);
        assert_eq!(parse_int("42").expect("ok"), 42);
        assert_eq!(parse_int("-1").expect("ok"), -1);
    }

    #[test]
    fn parse_floats() {
        assert!((parse_float("3.14").expect("ok") - 3.14).abs() < f64::EPSILON);
        assert!((parse_float("-0.5").expect("ok") - (-0.5)).abs() < f64::EPSILON);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Edge cases and errors
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn empty_input() {
        assert_eq!(lex(""), vec![]);
    }

    #[test]
    fn only_whitespace() {
        assert_eq!(lex("   \t\t   "), vec![]);
    }

    #[test]
    fn json_array() {
        assert_eq!(
            lex(r#"[1, 2, 3]"#),
            vec![
                Token::LBracket,
                Token::Int(1),
                Token::Comma,
                Token::Int(2),
                Token::Comma,
                Token::Int(3),
                Token::RBracket
            ]
        );
    }

    #[test]
    fn json_object() {
        assert_eq!(
            lex(r#"{"key": "value"}"#),
            vec![
                Token::LBrace,
                Token::String("key".to_string()),
                Token::Colon,
                Token::String("value".to_string()),
                Token::RBrace
            ]
        );
    }

    #[test]
    fn redirect_operators() {
        assert_eq!(
            lex("cmd > file"),
            vec![Token::Ident("cmd".to_string()), Token::Gt, Token::Ident("file".to_string())]
        );
        assert_eq!(
            lex("cmd >> file"),
            vec![Token::Ident("cmd".to_string()), Token::GtGt, Token::Ident("file".to_string())]
        );
        assert_eq!(
            lex("cmd 2> err"),
            vec![Token::Ident("cmd".to_string()), Token::Stderr, Token::Ident("err".to_string())]
        );
        assert_eq!(
            lex("cmd &> all"),
            vec![Token::Ident("cmd".to_string()), Token::Both, Token::Ident("all".to_string())]
        );
    }

    #[test]
    fn background_job() {
        assert_eq!(
            lex("cmd &"),
            vec![Token::Ident("cmd".to_string()), Token::Amp]
        );
    }

    #[test]
    fn command_substitution() {
        assert_eq!(
            lex("$(cmd)"),
            vec![Token::CmdSubstStart, Token::Ident("cmd".to_string()), Token::RParen]
        );
        assert_eq!(
            lex("$(cmd arg)"),
            vec![
                Token::CmdSubstStart,
                Token::Ident("cmd".to_string()),
                Token::Ident("arg".to_string()),
                Token::RParen
            ]
        );
        assert_eq!(
            lex("$(a | b)"),
            vec![
                Token::CmdSubstStart,
                Token::Ident("a".to_string()),
                Token::Pipe,
                Token::Ident("b".to_string()),
                Token::RParen
            ]
        );
    }

    #[test]
    fn complex_pipeline() {
        assert_eq!(
            lex(r#"cat file | grep pattern="foo" | head count=10"#),
            vec![
                Token::Ident("cat".to_string()),
                Token::Ident("file".to_string()),
                Token::Pipe,
                Token::Ident("grep".to_string()),
                Token::Ident("pattern".to_string()),
                Token::Eq,
                Token::String("foo".to_string()),
                Token::Pipe,
                Token::Ident("head".to_string()),
                Token::Ident("count".to_string()),
                Token::Eq,
                Token::Int(10),
            ]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Flag tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn short_flag() {
        assert_eq!(lex("-l"), vec![Token::ShortFlag("l".to_string())]);
        assert_eq!(lex("-a"), vec![Token::ShortFlag("a".to_string())]);
        assert_eq!(lex("-v"), vec![Token::ShortFlag("v".to_string())]);
    }

    #[test]
    fn short_flag_combined() {
        // Combined short flags like -la
        assert_eq!(lex("-la"), vec![Token::ShortFlag("la".to_string())]);
        assert_eq!(lex("-vvv"), vec![Token::ShortFlag("vvv".to_string())]);
    }

    #[test]
    fn job_spec_lexes_as_one_token() {
        // `%N` is the bash jobspec for wait/kill — used to be a lexer error.
        assert_eq!(lex("%1"), vec![Token::JobSpec("%1".to_string())]);
        assert_eq!(lex("%12"), vec![Token::JobSpec("%12".to_string())]);
        assert_eq!(
            lex("wait %1 %2"),
            vec![
                Token::Ident("wait".to_string()),
                Token::JobSpec("%1".to_string()),
                Token::JobSpec("%2".to_string()),
            ]
        );
    }

    #[test]
    fn short_flag_with_internal_hyphens_is_one_token() {
        // A dash-word with internal hyphens is ONE shell word, not three
        // flags — `-not-a-flag` must not fragment into `-not` `-a` `-flag`.
        // (Whether it's a flag or a literal is the binding layer's call.)
        assert_eq!(
            lex("-not-a-flag"),
            vec![Token::ShortFlag("not-a-flag".to_string())]
        );
        // The two-char terminator `--` is still DoubleDash, and a lone `-`
        // is still MinusAlone — the second char must be a letter to start a
        // short flag.
        assert_eq!(lex("--"), vec![Token::DoubleDash]);
        assert_eq!(lex("-"), vec![Token::MinusAlone]);
    }

    #[test]
    fn long_flag() {
        assert_eq!(lex("--force"), vec![Token::LongFlag("force".to_string())]);
        assert_eq!(lex("--verbose"), vec![Token::LongFlag("verbose".to_string())]);
        assert_eq!(lex("--foo-bar"), vec![Token::LongFlag("foo-bar".to_string())]);
    }

    #[test]
    fn double_dash() {
        // -- alone marks end of flags
        assert_eq!(lex("--"), vec![Token::DoubleDash]);
    }

    #[test]
    fn flags_vs_negative_numbers() {
        // -123 should be a negative integer, not a flag
        assert_eq!(lex("-123"), vec![Token::Int(-123)]);
        // -l should be a flag
        assert_eq!(lex("-l"), vec![Token::ShortFlag("l".to_string())]);
        // -1a is ambiguous - should be Int(-1) then Ident(a)
        // Actually the regex -[a-zA-Z] won't match -1a since 1 isn't a letter
        assert_eq!(
            lex("-1 a"),
            vec![Token::Int(-1), Token::Ident("a".to_string())]
        );
    }

    #[test]
    fn command_with_flags() {
        assert_eq!(
            lex("ls -l"),
            vec![
                Token::Ident("ls".to_string()),
                Token::ShortFlag("l".to_string()),
            ]
        );
        assert_eq!(
            lex("git commit -m"),
            vec![
                Token::Ident("git".to_string()),
                Token::Ident("commit".to_string()),
                Token::ShortFlag("m".to_string()),
            ]
        );
        assert_eq!(
            lex("git push --force"),
            vec![
                Token::Ident("git".to_string()),
                Token::Ident("push".to_string()),
                Token::LongFlag("force".to_string()),
            ]
        );
    }

    #[test]
    fn flag_with_value() {
        assert_eq!(
            lex(r#"git commit -m "message""#),
            vec![
                Token::Ident("git".to_string()),
                Token::Ident("commit".to_string()),
                Token::ShortFlag("m".to_string()),
                Token::String("message".to_string()),
            ]
        );
        assert_eq!(
            lex(r#"--message="hello""#),
            vec![
                Token::LongFlag("message".to_string()),
                Token::Eq,
                Token::String("hello".to_string()),
            ]
        );
    }

    #[test]
    fn end_of_flags_marker() {
        assert_eq!(
            lex("git checkout -- file"),
            vec![
                Token::Ident("git".to_string()),
                Token::Ident("checkout".to_string()),
                Token::DoubleDash,
                Token::Ident("file".to_string()),
            ]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Bash compatibility tokens
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn local_keyword() {
        assert_eq!(lex("local"), vec![Token::Local]);
        assert_eq!(
            lex("local X = 5"),
            vec![Token::Local, Token::Ident("X".to_string()), Token::Eq, Token::Int(5)]
        );
    }

    #[test]
    fn simple_var_ref() {
        assert_eq!(lex("$X"), vec![Token::SimpleVarRef("X".to_string())]);
        assert_eq!(lex("$foo"), vec![Token::SimpleVarRef("foo".to_string())]);
        assert_eq!(lex("$foo_bar"), vec![Token::SimpleVarRef("foo_bar".to_string())]);
        assert_eq!(lex("$_private"), vec![Token::SimpleVarRef("_private".to_string())]);
    }

    #[test]
    fn simple_var_ref_in_command() {
        assert_eq!(
            lex("echo $NAME"),
            vec![Token::Ident("echo".to_string()), Token::SimpleVarRef("NAME".to_string())]
        );
    }

    #[test]
    fn single_quoted_strings() {
        assert_eq!(lex("'hello'"), vec![Token::SingleString("hello".to_string())]);
        assert_eq!(lex("'hello world'"), vec![Token::SingleString("hello world".to_string())]);
        assert_eq!(lex("''"), vec![Token::SingleString("".to_string())]);
        // Single quotes don't process escapes or variables
        assert_eq!(lex(r"'no $VAR here'"), vec![Token::SingleString("no $VAR here".to_string())]);
        assert_eq!(lex(r"'backslash \n stays'"), vec![Token::SingleString(r"backslash \n stays".to_string())]);
    }

    #[test]
    fn test_brackets() {
        // [[ and ]] are now two separate bracket tokens to avoid conflicts with nested arrays
        assert_eq!(lex("[["), vec![Token::LBracket, Token::LBracket]);
        assert_eq!(lex("]]"), vec![Token::RBracket, Token::RBracket]);
        assert_eq!(
            lex("[[ -f file ]]"),
            vec![
                Token::LBracket,
                Token::LBracket,
                Token::ShortFlag("f".to_string()),
                Token::Ident("file".to_string()),
                Token::RBracket,
                Token::RBracket
            ]
        );
    }

    #[test]
    fn test_expression_syntax() {
        assert_eq!(
            lex(r#"[[ $X == "value" ]]"#),
            vec![
                Token::LBracket,
                Token::LBracket,
                Token::SimpleVarRef("X".to_string()),
                Token::EqEq,
                Token::String("value".to_string()),
                Token::RBracket,
                Token::RBracket
            ]
        );
    }

    #[test]
    fn bash_style_assignment() {
        // NAME="value" (no spaces) - lexer sees IDENT EQ STRING
        assert_eq!(
            lex(r#"NAME="value""#),
            vec![
                Token::Ident("NAME".to_string()),
                Token::Eq,
                Token::String("value".to_string())
            ]
        );
    }

    #[test]
    fn positional_params() {
        assert_eq!(lex("$0"), vec![Token::Positional(0)]);
        assert_eq!(lex("$1"), vec![Token::Positional(1)]);
        assert_eq!(lex("$9"), vec![Token::Positional(9)]);
        assert_eq!(lex("$@"), vec![Token::AllArgs]);
        assert_eq!(lex("$#"), vec![Token::ArgCount]);
    }

    #[test]
    fn positional_in_context() {
        assert_eq!(
            lex("echo $1 $2"),
            vec![
                Token::Ident("echo".to_string()),
                Token::Positional(1),
                Token::Positional(2),
            ]
        );
    }

    #[test]
    fn var_length() {
        assert_eq!(lex("${#X}"), vec![Token::VarLength("X".to_string())]);
        assert_eq!(lex("${#NAME}"), vec![Token::VarLength("NAME".to_string())]);
        assert_eq!(lex("${#foo_bar}"), vec![Token::VarLength("foo_bar".to_string())]);
    }

    #[test]
    fn var_length_with_subscript() {
        // The widened regex admits `[...]` subscripts so a length-of-path lexes
        // in expression position; the parser turns the inner into a VarPath.
        assert_eq!(lex("${#u[tags]}"), vec![Token::VarLength("u[tags]".to_string())]);
        assert_eq!(lex("${#a[0]}"), vec![Token::VarLength("a[0]".to_string())]);
        assert_eq!(lex("${#a[b][c]}"), vec![Token::VarLength("a[b][c]".to_string())]);
        assert_eq!(lex("${#r[$k]}"), vec![Token::VarLength("r[$k]".to_string())]);
    }

    #[test]
    fn var_length_in_context() {
        assert_eq!(
            lex("echo ${#NAME}"),
            vec![
                Token::Ident("echo".to_string()),
                Token::VarLength("NAME".to_string()),
            ]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Edge case tests: Flag ambiguities
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn plus_flag() {
        // Plus flags for set +e
        assert_eq!(lex("+e"), vec![Token::PlusFlag("e".to_string())]);
        assert_eq!(lex("+x"), vec![Token::PlusFlag("x".to_string())]);
        assert_eq!(lex("+ex"), vec![Token::PlusFlag("ex".to_string())]);
    }

    #[test]
    fn set_with_plus_flag() {
        assert_eq!(
            lex("set +e"),
            vec![
                Token::Set,
                Token::PlusFlag("e".to_string()),
            ]
        );
    }

    #[test]
    fn set_with_multiple_flags() {
        assert_eq!(
            lex("set -e -u"),
            vec![
                Token::Set,
                Token::ShortFlag("e".to_string()),
                Token::ShortFlag("u".to_string()),
            ]
        );
    }

    #[test]
    fn flags_vs_negative_numbers_edge_cases() {
        // -1a should be negative int followed by ident
        assert_eq!(
            lex("-1 a"),
            vec![Token::Int(-1), Token::Ident("a".to_string())]
        );
        // -l is a flag
        assert_eq!(lex("-l"), vec![Token::ShortFlag("l".to_string())]);
        // -123 is negative number
        assert_eq!(lex("-123"), vec![Token::Int(-123)]);
    }

    #[test]
    fn single_dash_is_minus_alone() {
        // Single dash alone - now handled as MinusAlone for `cat -` stdin indicator
        let result = tokenize("-").expect("should lex");
        assert_eq!(result.len(), 1);
        assert!(matches!(result[0].token, Token::MinusAlone));
    }

    #[test]
    fn plus_bare_for_date_format() {
        // `date +%s` - the +%s should be PlusBare
        let result = tokenize("+%s").expect("should lex");
        assert_eq!(result.len(), 1);
        assert!(matches!(result[0].token, Token::PlusBare(ref s) if s == "+%s"));

        // `date +%Y-%m-%d` - format string with dashes
        let result = tokenize("+%Y-%m-%d").expect("should lex");
        assert_eq!(result.len(), 1);
        assert!(matches!(result[0].token, Token::PlusBare(ref s) if s == "+%Y-%m-%d"));
    }

    #[test]
    fn plus_flag_still_works() {
        // `set +e` - should still be PlusFlag
        let result = tokenize("+e").expect("should lex");
        assert_eq!(result.len(), 1);
        assert!(matches!(result[0].token, Token::PlusFlag(ref s) if s == "e"));
    }

    #[test]
    fn while_keyword_vs_while_loop() {
        // 'while' as keyword in loop context
        assert_eq!(lex("while"), vec![Token::While]);
        // 'while' at start followed by condition
        assert_eq!(
            lex("while true"),
            vec![Token::While, Token::True]
        );
    }

    #[test]
    fn control_flow_keywords() {
        assert_eq!(lex("break"), vec![Token::Break]);
        assert_eq!(lex("continue"), vec![Token::Continue]);
        assert_eq!(lex("return"), vec![Token::Return]);
        assert_eq!(lex("exit"), vec![Token::Exit]);
    }

    #[test]
    fn control_flow_with_numbers() {
        assert_eq!(
            lex("break 2"),
            vec![Token::Break, Token::Int(2)]
        );
        assert_eq!(
            lex("continue 3"),
            vec![Token::Continue, Token::Int(3)]
        );
        assert_eq!(
            lex("exit 1"),
            vec![Token::Exit, Token::Int(1)]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Here-doc tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn heredoc_simple() {
        let source = "cat <<EOF\nhello\nworld\nEOF";
        let tokens = lex(source);
        // body_start_offset = byte offset of 'h' in "hello", i.e. just after "cat <<EOF\n"
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "hello\nworld\n".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 10,
            }),
            Token::Newline,
        ]);
    }

    #[test]
    fn heredoc_empty() {
        let source = "cat <<EOF\nEOF";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 10,
            }),
            Token::Newline,
        ]);
    }

    #[test]
    fn heredoc_with_special_chars() {
        let source = "cat <<EOF\n$VAR and \"quoted\" 'single'\nEOF";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "$VAR and \"quoted\" 'single'\n".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 10,
            }),
            Token::Newline,
        ]);
    }

    #[test]
    fn heredoc_multiline() {
        let source = "cat <<END\nline1\nline2\nline3\nEND";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "line1\nline2\nline3\n".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 10,
            }),
            Token::Newline,
        ]);
    }

    #[test]
    fn heredoc_in_command() {
        let source = "cat <<EOF\nhello\nEOF\necho goodbye";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "hello\n".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 10,
            }),
            Token::Newline,
            Token::Ident("echo".to_string()),
            Token::Ident("goodbye".to_string()),
        ]);
    }

    #[test]
    fn heredoc_strip_tabs() {
        let source = "cat <<-EOF\n\thello\n\tworld\n\tEOF";
        let tokens = lex(source);
        // Content keeps tabs verbatim — strip_tabs is recorded on the token so
        // the interpreter can apply POSIX leading-tab stripping at materialization
        // without disturbing source byte offsets used for span tracking.
        assert_eq!(tokens, vec![
            Token::Ident("cat".to_string()),
            Token::HereDocStart,
            Token::HereDoc(HereDocData {
                content: "\thello\n\tworld\n".to_string(),
                literal: false,
                strip_tabs: true,
                body_start_offset: 11,
            }),
            Token::Newline,
        ]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Arithmetic expression tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn arithmetic_simple() {
        let source = "$((1 + 2))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![Token::Arithmetic("1 + 2".to_string())]);
    }

    #[test]
    fn arithmetic_in_assignment() {
        let source = "X=$((5 * 3))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("X".to_string()),
            Token::Eq,
            Token::Arithmetic("5 * 3".to_string()),
        ]);
    }

    #[test]
    fn arithmetic_with_nested_parens() {
        let source = "$((2 * (3 + 4)))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![Token::Arithmetic("2 * (3 + 4)".to_string())]);
    }

    #[test]
    fn arithmetic_with_variable() {
        let source = "$((X + 1))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![Token::Arithmetic("X + 1".to_string())]);
    }

    #[test]
    fn arithmetic_command_subst_not_confused() {
        // $( should not be treated as arithmetic
        let source = "$(echo hello)";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::CmdSubstStart,
            Token::Ident("echo".to_string()),
            Token::Ident("hello".to_string()),
            Token::RParen,
        ]);
    }

    #[test]
    fn arithmetic_nesting_limit() {
        // Create deeply nested parens that exceed MAX_PAREN_DEPTH (256)
        let open_parens = "(".repeat(300);
        let close_parens = ")".repeat(300);
        let source = format!("$(({}1{}))", open_parens, close_parens);
        let result = tokenize(&source);
        assert!(result.is_err());
        let errors = result.unwrap_err();
        assert_eq!(errors.len(), 1);
        assert_eq!(errors[0].token, LexerError::NestingTooDeep);
    }

    #[test]
    fn arithmetic_nesting_within_limit() {
        // Nesting within limit should work
        let source = "$((((1 + 2) * 3)))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![Token::Arithmetic("((1 + 2) * 3)".to_string())]);
    }

    // ═══════════════════════════════════════════════════════════════════
    // Arithmetic preprocessor + comment interaction
    //
    // The preprocessor used to walk raw characters tracking only quote
    // state. An apostrophe inside a `#` comment would open single-quote
    // mode and swallow real `$((..))` later in the file; `$((..))` *inside*
    // a comment would itself be preprocessed into a marker, misplacing
    // tokens. Surfaced from kaijutsu's seed scripts (see gotcha memory
    // `gotcha-kaish-comment-arithmetic`).
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn arithmetic_after_apostrophe_in_comment() {
        // The bare apostrophe in "doesn't" used to open single-quote mode
        // in the preprocessor and swallow the $((..)) below.
        let source = "# this doesn't work\necho $((1+2))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Newline,
            Token::Ident("echo".to_string()),
            Token::Arithmetic("1+2".to_string()),
        ]);
    }

    #[test]
    fn arithmetic_inside_comment_is_not_expanded() {
        // `$((y))` inside a `#` comment must stay comment text.
        let source = "# the $((y)) syntax explained\necho hello";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Newline,
            Token::Ident("echo".to_string()),
            Token::Ident("hello".to_string()),
        ]);
    }

    #[test]
    fn backticked_arithmetic_in_comment_is_not_expanded() {
        // The original kaijutsu repro: `$((x))` inside a comment.
        // Backticks-in-comments used to leak the inner $((..)) to the
        // preprocessor; with comment-skip they stay inert.
        let source = "# the `$((x))` syntax explained\necho $((3+4))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Newline,
            Token::Ident("echo".to_string()),
            Token::Arithmetic("3+4".to_string()),
        ]);
    }

    #[test]
    fn arithmetic_still_works_outside_comments() {
        // Regression guard: comment-skip must not shrink the arithmetic
        // preprocessor's scope on normal `$((..))` usages.
        let source = "X=$((1+2)); Y=$((3*4))";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("X".to_string()),
            Token::Eq,
            Token::Arithmetic("1+2".to_string()),
            Token::Semi,
            Token::Ident("Y".to_string()),
            Token::Eq,
            Token::Arithmetic("3*4".to_string()),
        ]);
    }

    #[test]
    fn arithmetic_inside_double_quotes_still_expands() {
        // `#` inside a double-quoted string is a literal character, not a
        // comment introducer — arithmetic must still expand around it.
        let source = "echo \"# $((1+2))\"";
        let tokens = lex(source);
        // The string token contains the `#` and the arithmetic marker;
        // the exact post-processing happens at interpret time. What we
        // assert here is that lexing succeeds and produces a String token
        // (i.e. the comment skip didn't trigger inside the string).
        assert_eq!(tokens.len(), 2);
        assert!(matches!(tokens[0], Token::Ident(_)));
        assert!(matches!(tokens[1], Token::String(_)));
    }

    // ═══════════════════════════════════════════════════════════════════
    // Backtick rejection
    //
    // Backticks are an explicitly dropped feature (see CLAUDE.md,
    // docs/LANGUAGE.md, help/limits.md, help/overview.md). We surface a
    // dedicated error rather than the generic `UnexpectedCharacter` so
    // users get a hint to use `$(cmd)`. Comments, single-quoted strings,
    // double-quoted strings, and heredoc bodies are all matched as single
    // tokens (or extracted before logos runs), so the rejection only
    // fires on bare backticks in source code.
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn backtick_in_source_is_rejected() {
        let result = tokenize("echo `date`");
        assert!(result.is_err());
        let errors = result.unwrap_err();
        assert!(errors.iter().any(|e| e.token == LexerError::BackticksNotSupported));
    }

    #[test]
    fn backtick_in_comment_is_just_comment_text() {
        // Backticks are only rejected when they reach the top-level
        // lexer. Inside a comment they're part of the comment body.
        let source = "# use `date` here\necho hi";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Newline,
            Token::Ident("echo".to_string()),
            Token::Ident("hi".to_string()),
        ]);
    }

    #[test]
    fn backtick_in_single_quoted_string_is_literal() {
        // Single-quoted strings are matched as one token by logos; the
        // backticks inside never reach the rejecting matcher.
        let source = "echo '`date`'";
        let tokens = lex(source);
        assert_eq!(tokens, vec![
            Token::Ident("echo".to_string()),
            Token::SingleString("`date`".to_string()),
        ]);
    }

    #[test]
    fn backtick_in_double_quoted_string_is_literal() {
        // Kaish does not activate command substitution from backticks
        // inside double-quoted strings either — clear divergence from
        // POSIX but matches the "backticks don't exist" stance. The
        // double-quoted string token absorbs them as literal characters.
        let source = "echo \"`date`\"";
        let tokens = lex(source);
        assert_eq!(tokens.len(), 2);
        assert!(matches!(tokens[0], Token::Ident(_)));
        match &tokens[1] {
            Token::String(s) => assert!(s.contains('`')),
            other => panic!("expected Token::String, got {:?}", other),
        }
    }

    #[test]
    fn backtick_in_heredoc_body_is_preserved() {
        // Heredoc bodies are extracted by preprocess_heredocs before
        // logos runs, so backticks inside them survive as content.
        let source = "cat <<EOF\n`date`\nEOF\n";
        let tokens = lex(source);
        let heredoc = tokens.iter().find(|t| matches!(t, Token::HereDoc(_)));
        assert!(heredoc.is_some(), "expected a HereDoc token");
        if let Some(Token::HereDoc(d)) = heredoc {
            assert!(d.content.contains('`'));
        }
    }

    // ═══════════════════════════════════════════════════════════════════
    // Token category tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn token_categories() {
        // Keywords
        assert_eq!(Token::If.category(), TokenCategory::Keyword);
        assert_eq!(Token::Then.category(), TokenCategory::Keyword);
        assert_eq!(Token::For.category(), TokenCategory::Keyword);
        assert_eq!(Token::Function.category(), TokenCategory::Keyword);
        assert_eq!(Token::True.category(), TokenCategory::Keyword);
        assert_eq!(Token::TypeString.category(), TokenCategory::Keyword);

        // Operators
        assert_eq!(Token::Pipe.category(), TokenCategory::Operator);
        assert_eq!(Token::And.category(), TokenCategory::Operator);
        assert_eq!(Token::Or.category(), TokenCategory::Operator);
        assert_eq!(Token::StderrToStdout.category(), TokenCategory::Operator);
        assert_eq!(Token::GtGt.category(), TokenCategory::Operator);

        // Strings
        assert_eq!(Token::String("test".to_string()).category(), TokenCategory::String);
        assert_eq!(Token::SingleString("test".to_string()).category(), TokenCategory::String);
        assert_eq!(
            Token::HereDoc(HereDocData {
                content: "test".to_string(),
                literal: false,
                strip_tabs: false,
                body_start_offset: 0,
            }).category(),
            TokenCategory::String,
        );

        // Numbers
        assert_eq!(Token::Int(42).category(), TokenCategory::Number);
        assert_eq!(Token::Float(3.14).category(), TokenCategory::Number);
        assert_eq!(Token::Arithmetic("1+2".to_string()).category(), TokenCategory::Number);

        // Variables
        assert_eq!(Token::SimpleVarRef("X".to_string()).category(), TokenCategory::Variable);
        assert_eq!(Token::VarRef("${X}".to_string()).category(), TokenCategory::Variable);
        assert_eq!(Token::Positional(1).category(), TokenCategory::Variable);
        assert_eq!(Token::AllArgs.category(), TokenCategory::Variable);
        assert_eq!(Token::ArgCount.category(), TokenCategory::Variable);
        assert_eq!(Token::LastExitCode.category(), TokenCategory::Variable);
        assert_eq!(Token::CurrentPid.category(), TokenCategory::Variable);

        // Flags
        assert_eq!(Token::ShortFlag("l".to_string()).category(), TokenCategory::Flag);
        assert_eq!(Token::LongFlag("verbose".to_string()).category(), TokenCategory::Flag);
        assert_eq!(Token::PlusFlag("e".to_string()).category(), TokenCategory::Flag);
        assert_eq!(Token::DoubleDash.category(), TokenCategory::Flag);

        // Punctuation
        assert_eq!(Token::Semi.category(), TokenCategory::Punctuation);
        assert_eq!(Token::LParen.category(), TokenCategory::Punctuation);
        assert_eq!(Token::LBracket.category(), TokenCategory::Punctuation);
        assert_eq!(Token::Newline.category(), TokenCategory::Punctuation);

        // Comments
        assert_eq!(Token::Comment.category(), TokenCategory::Comment);

        // Paths
        assert_eq!(Token::Path("/tmp/file".to_string()).category(), TokenCategory::Path);

        // Commands
        assert_eq!(Token::Ident("echo".to_string()).category(), TokenCategory::Command);
        assert_eq!(Token::NumberIdent("019dda1c".to_string()).category(), TokenCategory::Command);
        assert_eq!(Token::DottedIdent(".gitignore".to_string()).category(), TokenCategory::Command);

        // Errors
        assert_eq!(Token::InvalidFloatNoLeading.category(), TokenCategory::Error);
        assert_eq!(Token::InvalidFloatNoTrailing.category(), TokenCategory::Error);
    }

    #[test]
    fn test_heredoc_piped_to_command() {
        // Bug 4: "cat <<EOF | jq" should produce: cat <<heredoc | jq
        // Not: cat | jq <<heredoc
        let tokens = tokenize("cat <<EOF | jq\n{\"key\": \"val\"}\nEOF").unwrap();
        let heredoc_pos = tokens.iter().position(|t| matches!(t.token, Token::HereDoc(_)));
        let pipe_pos = tokens.iter().position(|t| matches!(t.token, Token::Pipe));
        assert!(heredoc_pos.is_some(), "should have a heredoc token");
        assert!(pipe_pos.is_some(), "should have a pipe token");
        assert!(
            pipe_pos.unwrap() > heredoc_pos.unwrap(),
            "Pipe must come after heredoc, got heredoc at {}, pipe at {}. Tokens: {:?}",
            heredoc_pos.unwrap(), pipe_pos.unwrap(), tokens,
        );
    }

    #[test]
    fn test_heredoc_standalone_still_works() {
        // Regression: standalone heredoc (no pipe) must still work
        let tokens = tokenize("cat <<EOF\nhello\nEOF").unwrap();
        assert!(tokens.iter().any(|t| matches!(t.token, Token::HereDoc(_))));
        assert!(!tokens.iter().any(|t| matches!(t.token, Token::Pipe)));
    }

    #[test]
    fn test_heredoc_preserves_leading_empty_lines() {
        // Bug B: heredoc starting with a blank line must preserve it
        let tokens = tokenize("cat <<EOF\n\nhello\nEOF").unwrap();
        let heredoc = tokens.iter().find_map(|t| {
            if let Token::HereDoc(data) = &t.token {
                Some(data.clone())
            } else {
                None
            }
        });
        assert!(heredoc.is_some(), "should have a heredoc token");
        let data = heredoc.unwrap();
        assert!(data.content.starts_with('\n'), "leading empty line must be preserved, got: {:?}", data.content);
        assert_eq!(data.content, "\nhello\n");
    }

    #[test]
    fn test_heredoc_quoted_delimiter_sets_literal() {
        // Bug N: quoted delimiter (<<'EOF') should set literal=true
        let tokens = tokenize("cat <<'EOF'\nhello $HOME\nEOF").unwrap();
        let heredoc = tokens.iter().find_map(|t| {
            if let Token::HereDoc(data) = &t.token {
                Some(data.clone())
            } else {
                None
            }
        });
        assert!(heredoc.is_some(), "should have a heredoc token");
        let data = heredoc.unwrap();
        assert!(data.literal, "quoted delimiter should set literal=true");
        assert_eq!(data.content, "hello $HOME\n");
    }

    #[test]
    fn test_heredoc_unquoted_delimiter_not_literal() {
        // Bug N: unquoted delimiter (<<EOF) should have literal=false
        let tokens = tokenize("cat <<EOF\nhello $HOME\nEOF").unwrap();
        let heredoc = tokens.iter().find_map(|t| {
            if let Token::HereDoc(data) = &t.token {
                Some(data.clone())
            } else {
                None
            }
        });
        assert!(heredoc.is_some(), "should have a heredoc token");
        let data = heredoc.unwrap();
        assert!(!data.literal, "unquoted delimiter should have literal=false");
    }

    // ═══════════════════════════════════════════════════════════════════
    // Colon merge tests
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn colon_double_in_word() {
        assert_eq!(lex("foo::bar"), vec![Token::Ident("foo::bar".into())]);
    }

    #[test]
    fn colon_single_in_word() {
        assert_eq!(lex("a:b:c"), vec![Token::Ident("a:b:c".into())]);
    }

    #[test]
    fn colon_with_port() {
        assert_eq!(lex("host:8080"), vec![Token::Ident("host:8080".into())]);
    }

    #[test]
    fn colon_standalone() {
        assert_eq!(lex(":"), vec![Token::Colon]);
    }

    #[test]
    fn colon_spaced_no_merge() {
        assert_eq!(
            lex("foo : bar"),
            vec![
                Token::Ident("foo".into()),
                Token::Colon,
                Token::Ident("bar".into()),
            ]
        );
    }

    #[test]
    fn colon_in_command_arg() {
        assert_eq!(
            lex("echo foo::bar"),
            vec![
                Token::Ident("echo".into()),
                Token::Ident("foo::bar".into()),
            ]
        );
    }

    #[test]
    fn colon_trailing() {
        // Trailing colon merges with preceding ident
        assert_eq!(lex("foo:"), vec![Token::Ident("foo:".into())]);
    }

    #[test]
    fn colon_leading() {
        // Leading colon merges with following ident
        assert_eq!(lex(":foo"), vec![Token::Ident(":foo".into())]);
    }

    #[test]
    fn colon_with_path() {
        // Path token + colon + int
        assert_eq!(
            lex("/usr/bin:8080"),
            vec![Token::Ident("/usr/bin:8080".into())]
        );
    }

    // ═══════════════════════════════════════════════════════════════════
    // Token predicate coverage (is_keyword / starts_statement)
    // ═══════════════════════════════════════════════════════════════════

    #[test]
    fn is_keyword_covers_control_flow() {
        for t in [
            Token::While,
            Token::Return,
            Token::Break,
            Token::Continue,
            Token::Exit,
        ] {
            assert!(t.is_keyword(), "{t:?} should be a keyword");
        }
    }

    #[test]
    fn starts_statement_covers_while() {
        assert!(Token::While.starts_statement());
    }

    #[test]
    fn is_keyword_rejects_operators() {
        for t in [Token::Pipe, Token::Amp, Token::Eq, Token::LBrace] {
            assert!(!t.is_keyword(), "{t:?} should not be a keyword");
        }
    }
}