patchloom 0.20.0

//! Multi-strategy fallback chain for edit resolution.
//!
//! When a primary edit operation fails (e.g., target text not found), this
//! module provides progressively looser matching strategies before giving up.
//!
//! The fallback chain:
//! 1. **Exact match** (primary, handled by the caller)
//! 2. **Anchor-based matching** (structural landmarks around the edit target)
//! 3. **Similarity scoring** (Jaro-Winkler on surrounding lines)
//! 4. **Structured error** (diagnosis with suggestions)
//!
//! # Examples
//!
//! Suggest similar targets when a search misses:
//!
//! ```rust
//! use patchloom::fallback::{EditError, EditErrorKind, find_similar_targets};
//!
//! let content = "fn process_request() {}\nfn process_response() {}\n";
//! let similar = find_similar_targets(content, "process_requst", 3);
//! assert!(similar.iter().any(|s| s.contains("process_request")));
//! ```
//!
//! Run the full fallback chain (exact, anchor, similarity, structured error):
//!
//! ```rust
//! use patchloom::fallback::{resolve_with_fallback, MatchStrategy};
//!
//! let content = "fn setup() {}\nfn proccess_data(x: i32) {}\nfn cleanup() {}\n";
//! let result = resolve_with_fallback(
//!     content,
//!     "fn process_data(x: i32) {}",
//!     Some("fn setup() {}"),
//!     Some("fn cleanup() {}"),
//!).expect("anchor match should succeed");
//! assert_eq!(result.strategy, MatchStrategy::Anchor);
//! assert!(result.matched_text.contains("proccess_data"));
//! ```
//!
//! Validate an edit before applying it:
//!
//! ```rust
//! use patchloom::fallback::validate_edit;
//!
//! let json = r#"{"key": "value"}"#;
//! let result = validate_edit(json, "value", "new_value", Some("config.json"));
//! assert!(result.valid);
//! ```
//!
//! Validate with `--nth` semantics (only replace the Nth occurrence):
//!
//! ```rust
//! use patchloom::fallback::validate_edit_nth;
//!
//! let json = r#"{"a": "val", "b": "val"}"#;
//! let result = validate_edit_nth(json, "val", "new", Some("data.json"), Some(1));
//! assert!(result.valid);
//! ```
//!
//! size-waiver: accepted single-domain bulk (policy #1408). Multi-strategy edit recovery chain is one conceptual unit; do not split for LOC alone.

use serde::{Deserialize, Serialize};

/// Structured error type for edit operations with actionable diagnosis.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct EditError {
    /// What kind of error occurred.
    pub kind: EditErrorKind,
    /// Human-readable error message.
    pub message: String,
    /// A suggestion for how to fix the issue (if available).
    pub suggestion: Option<String>,
    /// Similar targets found in the file (for "did you mean?" hints).
    pub similar_targets: Vec<String>,
}

impl std::fmt::Display for EditError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}: {}", self.kind, self.message)?;
        if let Some(ref suggestion) = self.suggestion {
            write!(f, " (suggestion: {suggestion})")?;
        }
        if !self.similar_targets.is_empty() {
            write!(f, " [similar: {}]", self.similar_targets.join(", "))?;
        }
        Ok(())
    }
}

impl std::error::Error for EditError {}

/// Classification of edit errors.
///
/// Marked `non_exhaustive` so new honesty kinds (for example [`Self::TypeError`])
/// can land in minor releases without breaking external exhaustive matches.
///
/// **Append-only variants:** new kinds must be added **after** the last existing
/// variant. Inserting in the middle shifts discriminants of later variants and
/// fails `cargo-semver-checks` (`enum_no_repr_variant_discriminant_changed`),
/// which blocks patch releases (see #1950 order fix for #1951).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[non_exhaustive]
#[serde(rename_all = "snake_case")]
pub enum EditErrorKind {
    /// The edit target matched multiple locations in the file.
    AmbiguousTarget,
    /// The edit target was not found in the file.
    NoMatch,
    /// The edit would produce invalid syntax.
    SyntaxInvalid,
    /// Another edit already modified this region (for batch operations).
    ConflictingEdit,
    /// The file could not be parsed.
    ParseError,
    /// Path rejected by workspace PathGuard (#1492).
    GuardRejected,
    /// Invalid arguments or options for the edit.
    InvalidInput,
    /// Wrong value type for a doc selector (multi-document YAML bare key,
    /// navigate into array as object, etc.). CLI JSON uses `error_kind:
    /// "type_error"`. Distinct from [`Self::InvalidInput`] so hosts can
    /// recover with `0.key` / `[0].key` without scraping English (#1883).
    TypeError,
    /// I/O or other operational failure.
    OperationFailed,
    /// Post-write format/lint hook failed (`format_failed` / #1663).
    /// Distinct from generic [`Self::OperationFailed`] so hosts can branch
    /// without scraping English.
    FormatFailed,
    /// Create/rename destination already exists without force.
    /// CLI JSON uses `error_kind: "already_exists"`. Distinct from
    /// [`Self::InvalidInput`] (empty path, directory target) and content SoftSkip
    /// ([`Self::Binary`] / [`Self::InvalidEncoding`]) so hosts can hint
    /// `overwrite`/`force` without scraping English (#1947 / #1963).
    /// Appended after [`Self::FormatFailed`] so 0.18.0 discriminants stay stable.
    AlreadyExists,
    /// Path not found (`std::io::ErrorKind::NotFound`). CLI JSON uses
    /// `error_kind: "not_found"`. Distinct from generic [`Self::OperationFailed`]
    /// so hosts can treat missing paths separately from other I/O failures.
    NotFound,
    /// Patch/apply produced merge conflict markers. CLI JSON uses
    /// `error_kind: "conflicts"`. Distinct from [`Self::ConflictingEdit`]
    /// (batch region overlap) and generic [`Self::OperationFailed`].
    Conflicts,
    /// Check/preview reported pending changes, or soft assert-count mismatch
    /// (`error_kind: "changes_detected"`, exit 2). Distinct from
    /// [`Self::OperationFailed`] so hosts can mirror CLI exit-2 semantics
    /// without scraping English.
    ChangesDetected,
    /// Target contains NUL in the binary probe window (not agent-editable text).
    /// CLI JSON uses `error_kind: "binary"`. Distinct from [`Self::InvalidInput`]
    /// (empty path, directory target, empty pattern) so hosts can recover
    /// overwrite/delete/rename without treating all invalid input as binary (#1963).
    /// Appended after [`Self::ChangesDetected`] so 0.18/0.19 discriminants stay stable.
    Binary,
    /// Target is not valid UTF-8 (and not binary by NUL probe). CLI JSON uses
    /// `error_kind: "invalid_encoding"`. Distinct from [`Self::Binary`] and
    /// [`Self::InvalidInput`] (#1963).
    InvalidEncoding,
}

impl std::fmt::Display for EditErrorKind {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            EditErrorKind::AmbiguousTarget => write!(f, "ambiguous_target"),
            EditErrorKind::NoMatch => write!(f, "no_match"),
            EditErrorKind::SyntaxInvalid => write!(f, "syntax_invalid"),
            EditErrorKind::ConflictingEdit => write!(f, "conflicting_edit"),
            EditErrorKind::ParseError => write!(f, "parse_error"),
            EditErrorKind::GuardRejected => write!(f, "guard_rejected"),
            EditErrorKind::InvalidInput => write!(f, "invalid_input"),
            EditErrorKind::TypeError => write!(f, "type_error"),
            EditErrorKind::OperationFailed => write!(f, "operation_failed"),
            EditErrorKind::FormatFailed => write!(f, "format_failed"),
            EditErrorKind::AlreadyExists => write!(f, "already_exists"),
            EditErrorKind::NotFound => write!(f, "not_found"),
            EditErrorKind::Conflicts => write!(f, "conflicts"),
            EditErrorKind::ChangesDetected => write!(f, "changes_detected"),
            EditErrorKind::Binary => write!(f, "binary"),
            EditErrorKind::InvalidEncoding => write!(f, "invalid_encoding"),
        }
    }
}

impl EditError {
    /// Build a structured edit error.
    pub fn new(kind: EditErrorKind, message: impl Into<String>) -> Self {
        Self {
            kind,
            message: message.into(),
            suggestion: None,
            similar_targets: Vec::new(),
        }
    }

    /// PathGuard rejection as an `anyhow` error (`edit_error_kind` → [`EditErrorKind::GuardRejected`]).
    ///
    /// Prefer this over [`crate::exit::InvalidInputError`] for guard failures so
    /// library hosts and CLI JSON can branch on `guard_rejected` without scraping
    /// English (#1935 / engine PathGuard paths).
    pub fn guard_rejected(detail: impl std::fmt::Display) -> anyhow::Error {
        Self::new(
            EditErrorKind::GuardRejected,
            format!("path rejected by workspace guard: {detail}"),
        )
        .into()
    }

    /// Attach similar-target suggestions (did-you-mean).
    pub fn with_similar(mut self, similar: Vec<String>) -> Self {
        self.similar_targets = similar;
        self
    }

    /// Attach a single suggestion string.
    pub fn with_suggestion(mut self, suggestion: impl Into<String>) -> Self {
        self.suggestion = Some(suggestion.into());
        self
    }
}

/// Downcast an `anyhow` error chain to [`EditErrorKind`] when present.
///
/// Peels both [`EditError`] and the CLI/tx typed errors in [`crate::exit`]
/// (`InvalidInputError`, `NoMatchError`, …) so library hosts can branch on
/// kind without caring which construction path produced the failure.
///
/// Hosts that do **not** depend on `anyhow` should use [`classify_error`]
/// on a `dyn Error` instead (#1659).
pub fn edit_error_kind(err: &anyhow::Error) -> Option<EditErrorKind> {
    for cause in err.chain() {
        if let Some(kind) = classify_error(cause) {
            return Some(kind);
        }
    }
    None
}

/// Downcast an `anyhow` error chain to [`EditError`] when present.
pub fn edit_error_ref(err: &anyhow::Error) -> Option<&EditError> {
    for cause in err.chain() {
        if let Some(e) = classify_error_ref(cause) {
            return Some(e);
        }
    }
    None
}

/// Classify a bare `dyn Error` (no `anyhow` required) into [`EditErrorKind`].
///
/// Walks `source()` and peels [`EditError`] plus the CLI/tx typed errors
/// (`NoMatchError`, `InvalidInputError`, `AmbiguousError`, …). Use this from
/// non-anyhow agent hosts that store `Box<dyn Error>` or `Arc<dyn Error>`.
///
/// ```rust
/// use patchloom::fallback::{classify_error, EditError, EditErrorKind};
///
/// let err: Box<dyn std::error::Error + Send + Sync> =
///     Box::new(EditError::new(EditErrorKind::NoMatch, "no matches for \"x\""));
/// assert_eq!(classify_error(err.as_ref()), Some(EditErrorKind::NoMatch));
/// ```
///
/// See #1659.
pub fn classify_error(err: &(dyn std::error::Error + 'static)) -> Option<EditErrorKind> {
    let mut current: Option<&(dyn std::error::Error + 'static)> = Some(err);
    while let Some(e) = current {
        if let Some(edit) = e.downcast_ref::<EditError>() {
            return Some(edit.kind);
        }
        if e.downcast_ref::<crate::exit::NoMatchError>().is_some() {
            return Some(EditErrorKind::NoMatch);
        }
        if e.downcast_ref::<crate::exit::AmbiguousError>().is_some() {
            return Some(EditErrorKind::AmbiguousTarget);
        }
        if e.downcast_ref::<crate::exit::InvalidInputError>().is_some() {
            return Some(EditErrorKind::InvalidInput);
        }
        if e.downcast_ref::<crate::exit::AlreadyExistsError>()
            .is_some()
        {
            return Some(EditErrorKind::AlreadyExists);
        }
        if e.downcast_ref::<crate::exit::TypeErrorError>().is_some() {
            return Some(EditErrorKind::TypeError);
        }
        if e.downcast_ref::<crate::exit::ParseErrorError>().is_some() {
            return Some(EditErrorKind::ParseError);
        }
        if e.downcast_ref::<crate::exit::FormatFailedError>().is_some() {
            return Some(EditErrorKind::FormatFailed);
        }
        if e.downcast_ref::<crate::exit::ConflictsError>().is_some() {
            return Some(EditErrorKind::Conflicts);
        }
        if e.downcast_ref::<crate::exit::ChangesDetectedError>()
            .is_some()
        {
            return Some(EditErrorKind::ChangesDetected);
        }
        if e.downcast_ref::<crate::exit::BinaryError>().is_some() {
            return Some(EditErrorKind::Binary);
        }
        if e.downcast_ref::<crate::exit::InvalidEncodingError>()
            .is_some()
        {
            return Some(EditErrorKind::InvalidEncoding);
        }
        if e.downcast_ref::<std::io::Error>()
            .is_some_and(|io| io.kind() == std::io::ErrorKind::NotFound)
        {
            return Some(EditErrorKind::NotFound);
        }
        current = e.source();
    }
    None
}

/// Same stable kind string CLI JSON uses (`already_exists`, `guard_rejected`, …).
///
/// Library hosts that mirror agent JSON envelopes should use this instead of
/// re-implementing [`crate::exit::classify_typed_error`] or scraping Display
/// (#1948). Returns `None` when the chain has no recognized typed kind.
///
/// Prefer this over matching only [`edit_error_kind`] when you need CLI-stable
/// strings (for example `no_matches` vs Display `no_match`).
#[must_use]
pub fn error_kind_str(err: &anyhow::Error) -> Option<&'static str> {
    crate::exit::classify_typed_error(err).map(|(kind, _)| kind)
}

/// Whether the error peels as dest-exists create/rename conflict.
///
/// True for both [`crate::exit::AlreadyExistsError`] and
/// [`EditError`] with [`EditErrorKind::AlreadyExists`], so the bool matches
/// what hosts get from [`edit_error_kind`] (#1947).
#[must_use]
pub fn is_already_exists(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::AlreadyExists)
}

/// Whether the error peels as missing path I/O ([`EditErrorKind::NotFound`]).
///
/// Matches IO `NotFound` (including through `anyhow::Context`) and
/// [`EditError`] with [`EditErrorKind::NotFound`]. Prefer this over scraping
/// Display for host recovery when a path is simply absent.
#[must_use]
pub fn is_not_found(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::NotFound)
}

/// Whether the error peels as patch/apply merge conflict markers
/// ([`EditErrorKind::Conflicts`]). Distinct from batch
/// [`EditErrorKind::ConflictingEdit`].
#[must_use]
pub fn is_conflicts(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::Conflicts)
}

/// Whether the error peels as check/preview pending changes or soft
/// assert-count mismatch ([`EditErrorKind::ChangesDetected`], CLI exit 2).
#[must_use]
pub fn is_changes_detected(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::ChangesDetected)
}

/// Whether the error peels as multi-doc / wrong-root type mismatch
/// ([`EditErrorKind::TypeError`]). Distinct from [`EditErrorKind::InvalidInput`].
#[must_use]
pub fn is_type_error(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::TypeError)
}

/// Whether the error peels as post-write format/lint failure
/// ([`EditErrorKind::FormatFailed`]). Files may already be written.
#[must_use]
pub fn is_format_failed(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::FormatFailed)
}

/// Whether the error peels as PathGuard / `--contain` rejection
/// ([`EditErrorKind::GuardRejected`]).
#[must_use]
pub fn is_guard_rejected(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::GuardRejected)
}

/// Whether the error peels as invalid arguments / sole binary / bad options
/// ([`EditErrorKind::InvalidInput`]).
#[must_use]
pub fn is_invalid_input(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::InvalidInput)
}

/// Whether the error peels as soft no-match ([`EditErrorKind::NoMatch`]).
/// CLI JSON kind is `no_matches` via [`error_kind_str`].
#[must_use]
pub fn is_no_match(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::NoMatch)
}

/// Whether the error peels as multi-match / unique-mode ambiguity
/// ([`EditErrorKind::AmbiguousTarget`]). Distinct from soft [`is_no_match`].
///
/// Hosts with `unique: true` / `require_change` multi-hit recovery should prefer
/// this over scraping Display.
#[must_use]
pub fn is_ambiguous(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::AmbiguousTarget)
}

/// Whether the error peels as binary/NUL content ([`EditErrorKind::Binary`]).
///
/// Distinct from [`is_invalid_input`] (argument mistakes) and
/// [`is_invalid_encoding`] (non-UTF-8 text without NUL). Hosts that recover
/// overwrite of non-text priors should branch on this (or [`is_invalid_encoding`])
/// rather than all of `InvalidInput` (#1963).
#[must_use]
pub fn is_binary(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::Binary)
}

/// Whether the error peels as invalid UTF-8 content
/// ([`EditErrorKind::InvalidEncoding`]). Distinct from [`is_binary`] and
/// [`is_invalid_input`] (#1963).
#[must_use]
pub fn is_invalid_encoding(err: &anyhow::Error) -> bool {
    edit_error_kind(err) == Some(EditErrorKind::InvalidEncoding)
}

/// One-shot peel of a library/CLI error for host tool envelopes (#1964).
///
/// Combines [`error_kind_str`], [`edit_error_ref`], and agent-facing message
/// so embedders do not reimplement `edit_error_kind` + `error_kind_str` +
/// Display formatting. Returns `None` when the chain has no recognized kind.
#[derive(Debug, Clone)]
pub struct PeeledError {
    /// CLI-stable kind string (`already_exists`, `binary`, …).
    pub kind_str: &'static str,
    /// Agent-facing message (same as CLI JSON `error` when possible).
    pub message: String,
    /// Optional suggestion from [`EditError`].
    pub suggestion: Option<String>,
    /// Similar targets from [`EditError`] (did-you-mean).
    pub similar_targets: Vec<String>,
}

/// Peel kind string + message + optional suggestion from an error chain (#1964).
#[must_use]
pub fn peel_error(err: &anyhow::Error) -> Option<PeeledError> {
    let kind_str = error_kind_str(err)?;
    let (suggestion, similar_targets) = match edit_error_ref(err) {
        Some(e) => (e.suggestion.clone(), e.similar_targets.clone()),
        None => (None, Vec::new()),
    };
    Some(PeeledError {
        kind_str,
        message: crate::exit::agent_error_message(err),
        suggestion,
        similar_targets,
    })
}

/// Downcast a bare `dyn Error` chain to [`EditError`] when present (#1659).
///
/// Prefer this when you need `similar_targets` / `suggestion` without
/// depending on `anyhow`.
pub fn classify_error_ref<'a>(err: &'a (dyn std::error::Error + 'static)) -> Option<&'a EditError> {
    let mut current: Option<&'a (dyn std::error::Error + 'static)> = Some(err);
    while let Some(e) = current {
        if let Some(edit) = e.downcast_ref::<EditError>() {
            return Some(edit);
        }
        current = e.source();
    }
    None
}

/// Result of `validate_edit()`: whether the edit would produce valid output.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ValidationResult {
    /// Whether the edit produces valid output.
    pub valid: bool,
    /// Syntax errors found (if invalid).
    pub errors: Vec<String>,
    /// Suspicious but technically valid issues.
    pub warnings: Vec<String>,
}

/// Validate whether a replacement would produce valid output, without applying.
///
/// Performs the replacement in memory and checks basic structural validity
/// of the resulting content (JSON, YAML, TOML validation for structured files).
///
/// When `nth` is `Some(n)`, only the Nth (1-based) occurrence is replaced,
/// matching the behavior of the replace command's `--nth` flag. When `None`,
/// all occurrences are replaced.
pub fn validate_edit(
    content: &str,
    from: &str,
    to: &str,
    file_path: Option<&str>,
) -> ValidationResult {
    validate_edit_nth(content, from, to, file_path, None)
}

/// Like [`validate_edit`] but with an explicit `nth` occurrence parameter.
pub fn validate_edit_nth(
    content: &str,
    from: &str,
    to: &str,
    file_path: Option<&str>,
    nth: Option<usize>,
) -> ValidationResult {
    if from.is_empty() {
        return ValidationResult {
            valid: false,
            errors: vec!["empty search pattern".into()],
            warnings: vec![],
        };
    }

    if !content.contains(from) {
        return ValidationResult {
            valid: false,
            errors: vec![format!(
                "pattern '{}' not found in content",
                truncate_str(from, 60)
            )],
            warnings: vec![],
        };
    }

    let new_content = match nth {
        Some(0) => {
            return ValidationResult {
                valid: false,
                errors: vec!["nth must be >= 1 (1-based indexing)".into()],
                warnings: vec![],
            };
        }
        Some(n) => {
            // Replace only the Nth (1-based) occurrence.
            let mut count = 0usize;
            let mut result = String::with_capacity(content.len());
            let mut remaining = content;
            while let Some(pos) = remaining.find(from) {
                count += 1;
                if count == n {
                    result.push_str(&remaining[..pos]);
                    result.push_str(to);
                    result.push_str(&remaining[pos + from.len()..]);
                    break;
                }
                result.push_str(&remaining[..pos + from.len()]);
                remaining = &remaining[pos + from.len()..];
            }
            if count < n {
                return ValidationResult {
                    valid: false,
                    errors: vec![format!(
                        "occurrence {n} not found (only {count} occurrence{} exist{})",
                        if count == 1 { "" } else { "s" },
                        if count == 1 { "s" } else { "" },
                    )],
                    warnings: vec![],
                };
            }
            result
        }
        None => content.replace(from, to),
    };
    let mut errors = Vec::new();
    let mut warnings = Vec::new();

    // If we can detect the file format, validate the result.
    if let Some(path) = file_path
        && let Ok(fmt) = crate::ops::doc::detect_format(path)
    {
        let parse_err = match fmt {
            crate::ops::doc::FileFormat::Json => {
                serde_json::from_str::<serde_json::Value>(&new_content)
                    .err()
                    .map(|e| format!("result would be invalid JSON: {e}"))
            }
            crate::ops::doc::FileFormat::Yaml => {
                serde_yaml_ng::from_str::<serde_json::Value>(&new_content)
                    .err()
                    .map(|e| format!("result would be invalid YAML: {e}"))
            }
            crate::ops::doc::FileFormat::Toml => {
                toml_edit::de::from_str::<serde_json::Value>(&new_content)
                    .err()
                    .map(|e| format!("result would be invalid TOML: {e}"))
            }
        };
        if let Some(msg) = parse_err {
            errors.push(msg);
        }
    }

    // Warn if the replacement creates unbalanced brackets/braces.
    let open_parens =
        new_content.matches('(').count() as i64 - new_content.matches(')').count() as i64;
    let open_braces =
        new_content.matches('{').count() as i64 - new_content.matches('}').count() as i64;
    let open_brackets =
        new_content.matches('[').count() as i64 - new_content.matches(']').count() as i64;

    if open_parens != 0 {
        warnings.push(format!("unbalanced parentheses (delta: {open_parens})"));
    }
    if open_braces != 0 {
        warnings.push(format!("unbalanced braces (delta: {open_braces})"));
    }
    if open_brackets != 0 {
        warnings.push(format!("unbalanced brackets (delta: {open_brackets})"));
    }

    ValidationResult {
        valid: errors.is_empty(),
        errors,
        warnings,
    }
}

/// Minimum Jaro-Winkler score for a did-you-mean candidate.
///
/// Kept in line with anchor matching (0.85). The previous 0.7 floor
/// admitted noise like `nold` for long unrelated patterns when scanning
/// large trees (fixrealloop stress, 2026-07-23).
const SIMILAR_TARGET_MIN_SCORE: f64 = 0.85;

/// Whether `candidate` is a plausible typo of `target` by length.
///
/// Jaro-Winkler can score short tokens above the floor against long
/// strings (shared characters / prefixes). Reject extreme length skew
/// so agents do not chase nonsense suggestions.
fn similar_target_length_plausible(candidate: &str, target: &str) -> bool {
    let (ca, ta) = (candidate.len(), target.len());
    if ca == 0 || ta == 0 {
        return false;
    }
    let (shorter, longer) = if ca < ta { (ca, ta) } else { (ta, ca) };
    // Allow ~2x length difference, or a small absolute gap for short ids.
    shorter.saturating_mul(2) >= longer || longer - shorter <= 3
}

/// Find similar text targets in file content using Jaro-Winkler similarity.
///
/// Extracts identifiers and substrings from the content and returns the top
/// `max_results` matches sorted by similarity score (descending).
pub fn find_similar_targets(content: &str, target: &str, max_results: usize) -> Vec<String> {
    if target.is_empty() || content.is_empty() {
        return vec![];
    }

    let mut candidates: Vec<(String, f64)> = Vec::new();
    let mut seen = std::collections::HashSet::new();

    // Extract word-like tokens from the content.
    for line in content.lines() {
        for word in extract_identifiers(line) {
            if !seen.insert(word.clone()) {
                continue;
            }
            if word == target || !similar_target_length_plausible(&word, target) {
                continue;
            }
            let score = strsim::jaro_winkler(&word, target);
            if score > SIMILAR_TARGET_MIN_SCORE {
                candidates.push((word, score));
            }
        }
    }

    // Also try matching against whole lines for multi-word patterns.
    if target.contains(' ') || target.len() > 20 {
        for line in content.lines() {
            let trimmed = line.trim().to_string();
            if trimmed.is_empty() || seen.contains(&trimmed) {
                continue;
            }
            seen.insert(trimmed.clone());
            if trimmed == target || !similar_target_length_plausible(&trimmed, target) {
                continue;
            }
            let score = strsim::jaro_winkler(&trimmed, target);
            if score > SIMILAR_TARGET_MIN_SCORE {
                candidates.push((trimmed, score));
            }
        }
    }

    candidates.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap_or(std::cmp::Ordering::Equal));
    candidates.truncate(max_results);
    candidates.into_iter().map(|(s, _)| s).collect()
}

/// Try anchor-based matching: find the target text using surrounding context lines.
///
/// If `target` is not found exactly, looks for lines that share anchor text
/// (the lines immediately before and after the target in the original context)
/// and returns the matching region.
pub fn anchor_match(
    content: &str,
    target: &str,
    before_context: Option<&str>,
    after_context: Option<&str>,
) -> Option<AnchorMatchResult> {
    if target.is_empty() {
        return None;
    }

    // If exact match exists, return it directly.
    if let Some(pos) = content.find(target) {
        return Some(AnchorMatchResult {
            matched_text: target.to_string(),
            start_offset: pos,
            strategy: MatchStrategy::Exact,
            score: None,
        });
    }

    // Try anchor-based matching using before/after context.
    let lines: Vec<&str> = content.lines().collect();
    let target_lines: Vec<&str> = target.lines().collect();

    // Compute byte offset of each line start for accurate slicing,
    // avoiding CRLF vs LF mismatch (the old code used a global
    // heuristic that broke on mixed or CRLF line endings).
    let mut line_byte_starts: Vec<usize> = vec![0];
    for (i, b) in content.bytes().enumerate() {
        if b == b'\n' {
            line_byte_starts.push(i + 1);
        }
    }

    if target_lines.is_empty() {
        return None;
    }

    let first_target = target_lines[0].trim();
    let last_target = target_lines.last().map(|l| l.trim()).unwrap_or("");

    // Anchor matching requires at least one piece of structural context
    // (before_context or after_context). Without context, it would
    // degenerate into the same Jaro-Winkler line scan that the
    // similarity path already does, making the fallback chain redundant.
    if before_context.is_none() && after_context.is_none() {
        return None;
    }

    // Find candidate positions by matching the first line with anchors.
    for (i, line) in lines.iter().enumerate() {
        let trimmed = line.trim();

        // Check if this line is similar to the first target line.
        if strsim::jaro_winkler(trimmed, first_target) < 0.85 {
            continue;
        }

        // For multi-line targets, also verify the last line of the
        // candidate region is similar to the last target line.
        if target_lines.len() > 1 {
            let end_idx = i + target_lines.len();
            if end_idx > lines.len() {
                continue;
            }
            let candidate_last = lines[end_idx - 1].trim();
            if strsim::jaro_winkler(candidate_last, last_target) < 0.85 {
                continue;
            }
        }

        // If we have before_context, verify the preceding line matches.
        // For multi-line context, compare only the last line (the one
        // immediately before the target region).
        if let Some(before) = before_context {
            if i == 0 {
                continue;
            }
            let prev = lines[i - 1].trim();
            let before_line = before.lines().last().unwrap_or(before).trim();
            if strsim::jaro_winkler(prev, before_line) < 0.8 {
                continue;
            }
        }

        // If we have after_context, check the line after the candidate region.
        // For multi-line context, compare only the first line (the one
        // immediately after the target region).
        if let Some(after) = after_context {
            let end_idx = i + target_lines.len();
            if end_idx >= lines.len() {
                continue;
            }
            let next = lines[end_idx].trim();
            let after_line = after.lines().next().unwrap_or(after).trim();
            if strsim::jaro_winkler(next, after_line) < 0.8 {
                continue;
            }
        }

        // Found a match. Extract the matched region directly from the
        // original content so line endings are preserved exactly.
        let end_idx = (i + target_lines.len()).min(lines.len());
        let start_offset = line_byte_starts[i];
        let end_offset = if end_idx < line_byte_starts.len() {
            line_byte_starts[end_idx]
        } else {
            content.len()
        };
        // Strip exactly one trailing line ending to match the format
        // of exact-match results (which carry no trailing newline).
        let slice = &content[start_offset..end_offset];
        let matched_text = slice
            .strip_suffix("\r\n")
            .or_else(|| slice.strip_suffix('\n'))
            .unwrap_or(slice)
            .to_string();

        return Some(AnchorMatchResult {
            matched_text,
            start_offset,
            strategy: MatchStrategy::Anchor,
            score: None,
        });
    }

    None
}

/// Result of anchor-based matching.
#[derive(Debug, Clone)]
pub struct AnchorMatchResult {
    /// The text that was matched.
    pub matched_text: String,
    /// Byte offset of the match start in the content.
    pub start_offset: usize,
    /// Which matching strategy succeeded.
    pub strategy: MatchStrategy,
    /// Similarity score when [`MatchStrategy::Similarity`] succeeded (#1662).
    pub score: Option<f64>,
}

/// Which matching strategy found the result.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum MatchStrategy {
    /// Exact literal match.
    Exact,
    /// Anchor-based matching using surrounding context.
    Anchor,
    /// Similarity-based fuzzy matching.
    Similarity,
}

impl std::fmt::Display for MatchStrategy {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            MatchStrategy::Exact => write!(f, "exact"),
            MatchStrategy::Anchor => write!(f, "anchor"),
            MatchStrategy::Similarity => write!(f, "similarity"),
        }
    }
}

/// Run the full fallback chain: exact -> anchor -> similarity -> structured error.
///
/// Returns the first successful match or a structured error with diagnosis.
///
/// Stable 4-arg surface. Callers that already enforced a stricter exact path
/// (for example `--word-boundary`) should use [`resolve_with_fallback_skip_exact`]
/// so bare `find` does not re-accept a substring the primary path rejected.
pub fn resolve_with_fallback(
    content: &str,
    target: &str,
    before_context: Option<&str>,
    after_context: Option<&str>,
) -> Result<AnchorMatchResult, EditError> {
    resolve_with_fallback_skip_exact(content, target, before_context, after_context, false)
}

/// Like [`resolve_with_fallback`], with control over the bare Exact tier.
///
/// When `skip_exact` is true, bare `find` Exact matching is skipped. Callers that
/// already ran a stricter exact path (for example `--word-boundary`) must set
/// this so fallback does not re-accept a substring that word boundaries rejected
/// (#1755). Similarity and anchor strategies still run (whole-token typo recovery).
pub fn resolve_with_fallback_skip_exact(
    content: &str,
    target: &str,
    before_context: Option<&str>,
    after_context: Option<&str>,
    skip_exact: bool,
) -> Result<AnchorMatchResult, EditError> {
    // Step 1: Exact match (unless caller already enforced a stricter exact path).
    if !skip_exact && let Some(pos) = content.find(target) {
        return Ok(AnchorMatchResult {
            matched_text: target.to_string(),
            start_offset: pos,
            strategy: MatchStrategy::Exact,
            score: None,
        });
    }

    // Step 2: Anchor-based matching.
    if let Some(result) = anchor_match(content, target, before_context, after_context) {
        // anchor_match may still surface Exact via its own find; reject that when
        // the caller asked to skip bare exact (word_boundary).
        if !(skip_exact && result.strategy == MatchStrategy::Exact) {
            return Ok(result);
        }
    }

    // Step 3: Similarity-based matching (#1694).
    //
    // Single-token targets (identifiers / typos) must match **tokens**, not whole
    // lines. Jaro-Winkler on a full source line vs a short identifier scores high
    // because of shared prefixes, then the apply path replaces the entire line and
    // deletes surrounding syntax (`const`, types, etc.).
    let target_lines: Vec<&str> = target.lines().collect();
    if target_lines.len() == 1 {
        let target_trim = target.trim();
        if is_token_like_target(target_trim) {
            if let Some(hit) = best_token_similarity(content, target_trim) {
                return Ok(hit);
            }
            // Do not fall through to whole-line similarity for token-like targets.
        } else if let Some(hit) = best_line_similarity(content, target_trim) {
            return Ok(hit);
        }
    }

    // Step 4: Structured error with diagnosis.
    let similar = find_similar_targets(content, target.lines().next().unwrap_or(target), 5);
    let suggestion = if !similar.is_empty() {
        Some(format!("did you mean: {}?", similar[0]))
    } else {
        None
    };

    Err(EditError {
        kind: EditErrorKind::NoMatch,
        message: format!("target not found: '{}'", truncate_str(target, 80)),
        suggestion,
        similar_targets: similar,
    })
}

/// Whether a fuzzy match should be rejected by `min_fuzzy_score` (#1687).
///
/// Fail closed when score is missing: hosts that set a floor must not apply
/// unscored fuzzy matches (anchor/legacy paths without a score cannot prove
/// they clear the floor).
pub(crate) fn fuzzy_fails_min_floor(score: Option<f64>, min: f64) -> bool {
    match score {
        Some(s) => s < min,
        None => true,
    }
}

/// #1758: refuse Similarity/fuzzy apply when exact `old` was absent unless opt-in.
///
/// Anchored (explicit context) matches are not refused here — the host supplied
/// landmarks. Only when the host opted into `fuzzy` and the resolve path returns
/// Similarity/Fuzzy mode do we fail closed by default. Context-only fallback
/// (before/after without `fuzzy`) may still land Similarity without this gate
/// so structural recovery stays usable.
pub(crate) fn should_refuse_fuzzy_absent_old(
    fuzzy_requested: bool,
    is_fuzzy_mode: bool,
    allow_absent_old: bool,
) -> bool {
    fuzzy_requested && is_fuzzy_mode && !allow_absent_old
}

/// Diagnostic when refusing fuzzy apply because exact `old` is not in the file.
pub(crate) fn fuzzy_absent_old_refuse_message(
    old: &str,
    matched_text: &str,
    score: Option<f64>,
) -> String {
    let score_s = score
        .map(|s| format!("{s:.3}"))
        .unwrap_or_else(|| "none".into());
    format!(
        "exact old absent for {:?}; best fuzzy candidate {:?} score {} \
         (set allow_absent_old / --allow-absent-old to apply)",
        truncate_str(old, 60),
        truncate_str(matched_text, 60),
        score_s
    )
}

/// True when `target` is a single identifier-like token (no whitespace).
///
/// Multi-word / snippet targets keep whole-line similarity. Token-like targets
/// use identifier-level matching so fuzzy typo recovery does not expand the
/// replace span to an entire source line (#1694).
fn is_token_like_target(target: &str) -> bool {
    if target.is_empty() || target.chars().any(|c| c.is_whitespace()) {
        return false;
    }
    // Match extract_identifiers: alphanumeric + underscore only. Hyphens would
    // mark the target as "token-like" while extraction splits on `-`, so a
    // kebab-case typo never matched a full token and skipped line fallback.
    target.chars().any(|c| c.is_alphanumeric())
        && target.chars().all(|c| c.is_alphanumeric() || c == '_')
}

/// Best identifier similarity hit, if score > 0.85.
fn best_token_similarity(content: &str, target: &str) -> Option<AnchorMatchResult> {
    let mut best_score = 0.0f64;
    let mut best_match = String::new();
    let mut best_offset = 0usize;
    let mut offset = 0usize;
    for line in content.lines() {
        for (ident, col) in extract_identifiers_with_offsets(line) {
            if ident == target {
                continue;
            }
            let score = strsim::jaro_winkler(&ident, target);
            if score > best_score {
                best_score = score;
                best_match = ident;
                best_offset = offset + col;
            }
        }
        offset = advance_line_offset(content, offset, line);
    }
    if best_score > 0.85 {
        Some(AnchorMatchResult {
            matched_text: best_match,
            start_offset: best_offset,
            strategy: MatchStrategy::Similarity,
            score: Some(best_score),
        })
    } else {
        None
    }
}

/// Whole-line similarity for multi-word / snippet targets.
///
/// Refuses a match when the line is much longer than the target (ratio > 2)
/// so accidental line expansion stays rare even for long snippets (#1694).
fn best_line_similarity(content: &str, target: &str) -> Option<AnchorMatchResult> {
    let mut best_score = 0.0f64;
    let mut best_match = String::new();
    let mut best_offset = 0usize;
    let mut offset = 0usize;
    for line in content.lines() {
        let score = strsim::jaro_winkler(line.trim(), target);
        if score > best_score {
            best_score = score;
            best_match = line.to_string();
            best_offset = offset;
        }
        offset = advance_line_offset(content, offset, line);
    }
    if best_score <= 0.85 {
        return None;
    }
    let line_len = best_match.trim().len();
    let target_len = target.len().max(1);
    if line_len > target_len.saturating_mul(2) && line_len > target_len + 16 {
        // Too expansive: treat as no similarity match (suggestions still help).
        return None;
    }
    Some(AnchorMatchResult {
        matched_text: best_match,
        start_offset: best_offset,
        strategy: MatchStrategy::Similarity,
        score: Some(best_score),
    })
}

fn advance_line_offset(content: &str, mut offset: usize, line: &str) -> usize {
    // Advance past the line content and the actual line ending (\r\n or \n).
    offset += line.len();
    if content.as_bytes().get(offset) == Some(&b'\r') {
        offset += 1;
    }
    if content.as_bytes().get(offset) == Some(&b'\n') {
        offset += 1;
    }
    offset
}

/// Extract identifier-like tokens from a line of code.
fn extract_identifiers(line: &str) -> Vec<String> {
    extract_identifiers_with_offsets(line)
        .into_iter()
        .map(|(s, _)| s)
        .collect()
}

/// Identifier tokens with byte offsets into `line` (#1694).
fn extract_identifiers_with_offsets(line: &str) -> Vec<(String, usize)> {
    let mut identifiers = Vec::new();
    let mut current = String::new();
    let mut start = 0usize;
    let mut i = 0usize;

    for ch in line.chars() {
        let clen = ch.len_utf8();
        if ch.is_alphanumeric() || ch == '_' {
            if current.is_empty() {
                start = i;
            }
            current.push(ch);
        } else {
            if current.len() >= 3 {
                identifiers.push((std::mem::take(&mut current), start));
            } else {
                current.clear();
            }
        }
        i += clen;
    }
    if current.len() >= 3 {
        identifiers.push((current, start));
    }

    identifiers
}

/// Truncate a string for display in error messages.
///
/// Truncates at the last char boundary at or before `max_len` bytes,
/// so this is safe for multi-byte UTF-8 input.
pub(crate) fn truncate_str(s: &str, max_len: usize) -> &str {
    if s.len() <= max_len {
        s
    } else {
        // Find the last char boundary at or before max_len.
        let mut end = max_len;
        while end > 0 && !s.is_char_boundary(end) {
            end -= 1;
        }
        &s[..end]
    }
}

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

    #[test]
    fn edit_error_display() {
        let err = EditError {
            kind: EditErrorKind::NoMatch,
            message: "target not found".into(),
            suggestion: Some("try 'process_request'".into()),
            similar_targets: vec!["process_request".into()],
        };
        let display = err.to_string();
        assert!(display.contains("no_match"));
        assert!(display.contains("target not found"));
        assert!(display.contains("process_request"));
    }

    #[test]
    fn edit_error_kind_display() {
        assert_eq!(
            EditErrorKind::AmbiguousTarget.to_string(),
            "ambiguous_target"
        );
        assert_eq!(EditErrorKind::NoMatch.to_string(), "no_match");
        assert_eq!(EditErrorKind::SyntaxInvalid.to_string(), "syntax_invalid");
        assert_eq!(
            EditErrorKind::ConflictingEdit.to_string(),
            "conflicting_edit"
        );
        assert_eq!(EditErrorKind::ParseError.to_string(), "parse_error");
        assert_eq!(EditErrorKind::TypeError.to_string(), "type_error");
        assert_eq!(EditErrorKind::InvalidInput.to_string(), "invalid_input");
        assert_eq!(EditErrorKind::AlreadyExists.to_string(), "already_exists");
        assert_eq!(EditErrorKind::NotFound.to_string(), "not_found");
        assert_eq!(EditErrorKind::Conflicts.to_string(), "conflicts");
        assert_eq!(
            EditErrorKind::ChangesDetected.to_string(),
            "changes_detected"
        );
        assert_eq!(EditErrorKind::FormatFailed.to_string(), "format_failed");
        assert_eq!(EditErrorKind::Binary.to_string(), "binary");
        assert_eq!(
            EditErrorKind::InvalidEncoding.to_string(),
            "invalid_encoding"
        );
    }

    /// Locks discriminants published in 0.18.0 so new kinds stay append-only
    /// (`enum_no_repr_variant_discriminant_changed` / release PR #1951).
    #[test]
    fn edit_error_kind_stable_discriminants_since_0_18_0() {
        assert_eq!(EditErrorKind::AmbiguousTarget as u8, 0);
        assert_eq!(EditErrorKind::NoMatch as u8, 1);
        assert_eq!(EditErrorKind::SyntaxInvalid as u8, 2);
        assert_eq!(EditErrorKind::ConflictingEdit as u8, 3);
        assert_eq!(EditErrorKind::ParseError as u8, 4);
        assert_eq!(EditErrorKind::GuardRejected as u8, 5);
        assert_eq!(EditErrorKind::InvalidInput as u8, 6);
        assert_eq!(EditErrorKind::TypeError as u8, 7);
        assert_eq!(EditErrorKind::OperationFailed as u8, 8);
        assert_eq!(EditErrorKind::FormatFailed as u8, 9);
        // #1947/#1950 kinds must follow FormatFailed (never insert above).
        assert_eq!(EditErrorKind::AlreadyExists as u8, 10);
        assert_eq!(EditErrorKind::NotFound as u8, 11);
        assert_eq!(EditErrorKind::Conflicts as u8, 12);
        assert_eq!(EditErrorKind::ChangesDetected as u8, 13);
        // #1963 kinds append after ChangesDetected (never insert above).
        assert_eq!(EditErrorKind::Binary as u8, 14);
        assert_eq!(EditErrorKind::InvalidEncoding as u8, 15);
    }

    #[test]
    fn classify_error_peels_edit_error_and_typed() {
        use std::error::Error;
        let e = EditError::new(EditErrorKind::AmbiguousTarget, "many");
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::AmbiguousTarget)
        );
        let e = crate::exit::NoMatchError { msg: "none".into() };
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::NoMatch)
        );
        let e = crate::exit::FormatFailedError::new("fmt");
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::FormatFailed)
        );
        let e = crate::exit::TypeErrorError {
            msg: "parent is an array".into(),
        };
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::TypeError)
        );
        let e = crate::exit::AlreadyExistsError {
            msg: "exists".into(),
        };
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::AlreadyExists)
        );
        let e = crate::exit::ConflictsError {
            msg: "conflict".into(),
        };
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::Conflicts)
        );
        let e = crate::exit::ChangesDetectedError {
            msg: "would change".into(),
        };
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::ChangesDetected)
        );
        let e = std::io::Error::new(std::io::ErrorKind::NotFound, "gone");
        assert_eq!(
            classify_error(&e as &(dyn Error + 'static)),
            Some(EditErrorKind::NotFound)
        );
    }

    #[test]
    fn edit_error_kind_maps_exit_typed_errors() {
        let invalid: anyhow::Error = crate::exit::InvalidInputError {
            msg: "empty search pattern".into(),
        }
        .into();
        assert_eq!(edit_error_kind(&invalid), Some(EditErrorKind::InvalidInput));
        assert!(is_invalid_input(&invalid));
        assert!(!is_guard_rejected(&invalid));

        let no_match: anyhow::Error = crate::exit::NoMatchError {
            msg: "no matches".into(),
        }
        .into();
        assert_eq!(edit_error_kind(&no_match), Some(EditErrorKind::NoMatch));
        assert!(is_no_match(&no_match));
        assert_eq!(error_kind_str(&no_match), Some("no_matches"));
        assert!(!is_not_found(&no_match));

        let ambiguous: anyhow::Error = crate::exit::AmbiguousError {
            msg: "multiple matches".into(),
        }
        .into();
        assert_eq!(
            edit_error_kind(&ambiguous),
            Some(EditErrorKind::AmbiguousTarget)
        );
        assert!(is_ambiguous(&ambiguous));
        assert_eq!(error_kind_str(&ambiguous), Some("ambiguous"));
        assert!(!is_no_match(&ambiguous));

        let parse: anyhow::Error = crate::exit::ParseErrorError {
            msg: "bad yaml".into(),
        }
        .into();
        assert_eq!(edit_error_kind(&parse), Some(EditErrorKind::ParseError));

        let exists: anyhow::Error = crate::exit::AlreadyExistsError {
            msg: "file already exists".into(),
        }
        .into();
        assert_eq!(
            edit_error_kind(&exists),
            Some(EditErrorKind::AlreadyExists),
            "AlreadyExistsError must not collapse to InvalidInput (#1947)"
        );
        assert!(is_already_exists(&exists));
        assert_eq!(error_kind_str(&exists), Some("already_exists"));

        let exists_edit: anyhow::Error =
            EditError::new(EditErrorKind::AlreadyExists, "dest taken").into();
        assert!(
            is_already_exists(&exists_edit),
            "is_already_exists must match EditErrorKind::AlreadyExists too"
        );
        assert_eq!(
            edit_error_kind(&exists_edit),
            Some(EditErrorKind::AlreadyExists)
        );

        let type_err: anyhow::Error = crate::exit::TypeErrorError {
            msg: "parent is an array, not an object".into(),
        }
        .into();
        assert_eq!(
            edit_error_kind(&type_err),
            Some(EditErrorKind::TypeError),
            "TypeErrorError must not collapse to InvalidInput (#1883)"
        );
        assert!(is_type_error(&type_err));
        assert!(!is_invalid_input(&type_err));

        let format_failed: anyhow::Error =
            crate::exit::FormatFailedError::new("format command failed").into();
        assert_eq!(
            edit_error_kind(&format_failed),
            Some(EditErrorKind::FormatFailed)
        );
        assert!(is_format_failed(&format_failed));
        assert!(!is_type_error(&format_failed));

        let guard: anyhow::Error =
            EditError::new(EditErrorKind::GuardRejected, "path escapes").into();
        assert!(is_guard_rejected(&guard));
        assert_eq!(error_kind_str(&guard), Some("guard_rejected"));
        assert!(!is_invalid_input(&guard));

        let not_found: anyhow::Error =
            std::io::Error::new(std::io::ErrorKind::NotFound, "missing").into();
        let not_found = not_found.context("failed to read path");
        assert_eq!(
            edit_error_kind(&not_found),
            Some(EditErrorKind::NotFound),
            "IO NotFound must peel as NotFound not OperationFailed"
        );
        assert_eq!(error_kind_str(&not_found), Some("not_found"));
        assert!(
            is_not_found(&not_found),
            "is_not_found must match IO NotFound peel"
        );
        assert!(!is_already_exists(&not_found));

        let conflicts: anyhow::Error = crate::exit::ConflictsError {
            msg: "merge conflict".into(),
        }
        .into();
        assert_eq!(edit_error_kind(&conflicts), Some(EditErrorKind::Conflicts));
        assert_eq!(error_kind_str(&conflicts), Some("conflicts"));
        assert!(is_conflicts(&conflicts));
        assert!(!is_not_found(&conflicts));

        let changes: anyhow::Error = crate::exit::ChangesDetectedError {
            msg: "would change".into(),
        }
        .into();
        assert_eq!(
            edit_error_kind(&changes),
            Some(EditErrorKind::ChangesDetected)
        );
        assert_eq!(error_kind_str(&changes), Some("changes_detected"));
        assert!(is_changes_detected(&changes));
        assert!(!is_conflicts(&changes));

        // Intermediate .context() must not hide the typed kind.
        let wrapped = invalid.context("operation 1 (replace) failed");
        assert_eq!(edit_error_kind(&wrapped), Some(EditErrorKind::InvalidInput));

        let type_wrapped = type_err.context("operation 1 (doc.set) failed");
        assert_eq!(
            edit_error_kind(&type_wrapped),
            Some(EditErrorKind::TypeError)
        );

        let plain = anyhow::anyhow!("plain error");
        assert_eq!(edit_error_kind(&plain), None);
    }

    #[test]
    fn validate_edit_empty_pattern() {
        let result = validate_edit("content", "", "replacement", None);
        assert!(!result.valid);
        assert!(result.errors[0].contains("empty search pattern"));
    }

    #[test]
    fn validate_edit_pattern_not_found() {
        let result = validate_edit("hello world", "missing", "replacement", None);
        assert!(!result.valid);
        assert!(result.errors[0].contains("not found"));
    }

    #[test]
    fn validate_edit_valid_replacement() {
        let result = validate_edit("hello world", "hello", "goodbye", None);
        assert!(result.valid);
        assert!(result.errors.is_empty());
    }

    #[test]
    fn validate_edit_json_syntax_check() {
        let json = r#"{"key": "value"}"#;
        // Valid replacement.
        let result = validate_edit(json, "value", "new_value", Some("config.json"));
        assert!(result.valid);

        // Invalid replacement (breaks JSON).
        let result = validate_edit(json, "\"key\":", "broken", Some("config.json"));
        assert!(!result.valid);
        assert!(result.errors[0].contains("invalid JSON"));
    }

    #[test]
    fn validate_edit_yaml_syntax_check() {
        let yaml = "key: value\n";
        let result = validate_edit(yaml, "value", "new_value", Some("config.yaml"));
        assert!(result.valid);
    }

    #[test]
    fn validate_edit_warns_unbalanced_braces() {
        let content = "fn main() { hello }";
        let result = validate_edit(content, "{ hello }", "{ hello", None);
        assert!(result.valid); // Still valid (we can't know the language syntax).
        assert!(
            result
                .warnings
                .iter()
                .any(|w| w.contains("unbalanced braces"))
        );
    }

    #[test]
    fn find_similar_targets_finds_typos() {
        let content = "fn process_request() {}\nfn process_response() {}\nfn handle_error() {}\n";
        let similar = find_similar_targets(content, "process_requst", 3);
        assert!(!similar.is_empty());
        assert!(similar.iter().any(|s| s.contains("process_request")));
    }

    #[test]
    fn find_similar_targets_rejects_length_skew_noise() {
        // Real multi-file no-match noise (fixrealloop stress): long invented
        // pattern vs short tokens like "nold" can clear a loose JW floor.
        let content = "fn nold() {}\nlet compute = 1;\nfn process_request() {}\n";
        let similar = find_similar_targets(content, "this_string_should_not_exist_xyzzy", 5);
        assert!(
            similar.is_empty(),
            "unrelated long pattern must not suggest short tokens: {similar:?}"
        );
        let similar = find_similar_targets(content, "completely_bogus_token_zzz", 5);
        assert!(
            !similar
                .iter()
                .any(|s| s == "compute" || s == "let" || s == "nold"),
            "bogus pattern must not surface weak short-token hints: {similar:?}"
        );
    }

    #[test]
    fn find_similar_targets_empty_content() {
        let similar = find_similar_targets("", "target", 3);
        assert!(similar.is_empty());
    }

    #[test]
    fn find_similar_targets_empty_target() {
        let similar = find_similar_targets("content", "", 3);
        assert!(similar.is_empty());
    }

    #[test]
    fn anchor_match_exact() {
        let content = "line1\nline2\nline3\n";
        let result = anchor_match(content, "line2", None, None).unwrap();
        assert_eq!(result.matched_text, "line2");
        assert_eq!(result.strategy, MatchStrategy::Exact);
    }

    #[test]
    fn anchor_match_with_context() {
        // Simulate a case where the target line changed slightly.
        let content = "fn setup() {}\nfn proccess_data(x: i32) {}\nfn cleanup() {}\n";
        let result = anchor_match(
            content,
            "fn process_data(x: i32) {}",
            Some("fn setup() {}"),
            Some("fn cleanup() {}"),
        );
        let r = result.expect("anchor match should find a fuzzy match");
        assert_eq!(r.strategy, MatchStrategy::Anchor);
        assert!(r.matched_text.contains("proccess_data"));
    }

    #[test]
    fn anchor_match_no_match() {
        let content = "completely different content\n";
        let result = anchor_match(content, "not here at all", None, None);
        assert!(result.is_none());
    }

    #[test]
    fn anchor_match_requires_context_for_fuzzy() {
        // Without context, anchor matching skips the fuzzy path and returns
        // None even if a similar line exists. This prevents anchor from
        // duplicating the similarity path in the fallback chain.
        let content = "fn process_request(x: i32) {}\n";
        let result = anchor_match(content, "fn process_requst(x: i32) {}", None, None);
        assert!(result.is_none());
    }

    #[test]
    fn anchor_match_multi_line_verifies_last_line() {
        // Multi-line anchor matching must check both the first and last
        // lines of the candidate region, not just the first.
        let content = "fn setup() {}\nfn process(x: i32) {}\nfn teardown() {}\nfn other() {}\n";
        // Target has a matching first line but wrong last line.
        let result = anchor_match(
            content,
            "fn process(x: i32) {}\nfn completely_wrong() {}",
            Some("fn setup() {}"),
            None,
        );
        assert!(result.is_none());
    }

    #[test]
    fn anchor_match_crlf_offset() {
        let content = "line1\r\nline2\r\nline3\r\n";
        let result = anchor_match(content, "line2", Some("line1"), None).unwrap();
        // "line1\r\n" is 7 bytes, so line2 starts at offset 7.
        assert_eq!(result.start_offset, 7);
    }

    /// Regression: anchor match on CRLF content must produce a matched_text
    /// that is an exact substring of the original content. The old code
    /// joined lines with "\n" which produced LF-only text, wrong for CRLF.
    #[test]
    fn anchor_match_crlf_matched_text_preserves_endings() {
        let content =
            "fn setup() {}\r\nfn proccess_data(x: i32) {}\r\nfn more() {}\r\nfn cleanup() {}\r\n";
        let result = anchor_match(
            content,
            "fn process_data(x: i32) {}\nfn more() {}",
            Some("fn setup() {}"),
            Some("fn cleanup() {}"),
        )
        .unwrap();
        // matched_text must be verifiable against the original content.
        let end = result.start_offset + result.matched_text.len();
        assert_eq!(
            &content[result.start_offset..end],
            result.matched_text,
            "matched_text must be an exact slice of the original content"
        );
        // And it should contain the CRLF between lines.
        assert!(
            result.matched_text.contains("\r\n"),
            "matched text should preserve CRLF line endings"
        );
    }

    /// Regression: anchor match on mixed line endings (some CRLF, some LF)
    /// must produce correct offsets for each line, not a global decision.
    #[test]
    fn anchor_match_mixed_endings_correct_offset() {
        let content = "header\r\nfn proccess(x: i32) {}\nfooter\n";
        let result = anchor_match(
            content,
            "fn process(x: i32) {}",
            Some("header"),
            Some("footer"),
        )
        .unwrap();
        // "header\r\n" is 8 bytes, so line 2 starts at offset 8.
        assert_eq!(result.start_offset, 8);
        let end = result.start_offset + result.matched_text.len();
        assert_eq!(&content[result.start_offset..end], result.matched_text);
    }

    #[test]
    fn resolve_with_fallback_exact_match() {
        let content = "fn hello() {}\n";
        let result = resolve_with_fallback(content, "fn hello()", None, None).unwrap();
        assert_eq!(result.strategy, MatchStrategy::Exact);
    }

    /// #1755: skip_exact must not re-accept a bare substring after a stricter
    /// primary path (e.g. word_boundary) already rejected it.
    #[test]
    fn resolve_with_fallback_skip_exact_rejects_substring() {
        let content = "process_data process_data_extra\n";
        // Without skip_exact, bare find would match "process_dat" inside process_data.
        let bare = resolve_with_fallback(content, "process_dat", None, None).unwrap();
        assert_eq!(bare.strategy, MatchStrategy::Exact);
        assert_eq!(bare.matched_text, "process_dat");

        // With skip_exact, Exact is skipped; token similarity may recover the
        // full identifier (process_data) or miss — but never the bare substring.
        match resolve_with_fallback_skip_exact(content, "process_dat", None, None, true) {
            Ok(hit) => {
                assert_ne!(
                    hit.strategy,
                    MatchStrategy::Exact,
                    "skip_exact must not return Exact"
                );
                assert_ne!(
                    hit.matched_text, "process_dat",
                    "must not re-accept word_boundary-rejected substring"
                );
            }
            Err(e) => assert_eq!(e.kind, EditErrorKind::NoMatch),
        }
    }

    /// Regression: similarity matching on CRLF content must compute
    /// correct byte offsets. The old code used `offset += line.len() + 1`
    /// which hardcoded LF (1 byte), wrong for CRLF (2 bytes).
    #[test]
    fn resolve_with_fallback_similarity_crlf_offset() {
        let content = "fn alpha() {}\r\nfn process_requets(data: &str) {}\r\nfn gamma() {}\r\n";
        let result =
            resolve_with_fallback(content, "fn process_requests(data: &str) {}", None, None)
                .unwrap();
        assert_eq!(result.strategy, MatchStrategy::Similarity);
        // "fn alpha() {}\r\n" is 15 bytes, so the match starts at offset 15.
        assert_eq!(result.start_offset, 15);
        // Verify the offset points to the correct position in content.
        assert!(
            content[result.start_offset..].starts_with(&result.matched_text),
            "start_offset must point to matched_text in content"
        );
    }

    #[test]
    fn resolve_with_fallback_similarity_match() {
        // Without context, anchor matching is skipped (it requires at least
        // before_context or after_context to avoid degenerating into the same
        // Jaro-Winkler scan that similarity already does). The similarity
        // path catches the misspelled target instead.
        let content = "fn process_request(data: &str) -> Result<()> {\n    Ok(())\n}\n";
        let result = resolve_with_fallback(
            content,
            "fn process_requets(data: &str) -> Result<()> {",
            None,
            None,
        );
        let r = result.expect("similarity fallback should succeed");
        assert_eq!(r.strategy, MatchStrategy::Similarity);
        // Multi-word snippet → whole-line match (not a bare identifier).
        assert!(
            r.matched_text.contains("process_request"),
            "snippet match: {:?}",
            r.matched_text
        );
    }

    /// #1694: single-token fuzzy must not expand to the whole source line.
    #[test]
    fn resolve_token_typo_does_not_match_whole_line() {
        let content = "const CONFIGURATION_VALUE_PRIMARY: i32 = 1;\nfn use_it() -> i32 { CONFIGURATION_VALUE_PRIMARY }\n";
        let r = resolve_with_fallback(content, "CONFIGURATION_VALUE_PRIMRY", None, None)
            .expect("token typo should fuzzy-match the identifier");
        assert_eq!(r.strategy, MatchStrategy::Similarity);
        assert_eq!(
            r.matched_text, "CONFIGURATION_VALUE_PRIMARY",
            "must match the identifier token only, not the whole line"
        );
        assert!(
            content[r.start_offset..].starts_with("CONFIGURATION_VALUE_PRIMARY"),
            "offset must point at the token"
        );
        // Surrounding syntax must still be outside the match span.
        assert!(!r.matched_text.contains("const"));
        assert!(!r.matched_text.contains("i32"));
    }

    /// #1694: intentional full-line target still uses line similarity.
    #[test]
    fn resolve_full_line_snippet_still_matches_line() {
        let content = "const FOO: i32 = 1;\n";
        let r = resolve_with_fallback(content, "const FO: i32 = 1;", None, None)
            .expect("near-full-line snippet should match the line");
        assert_eq!(r.strategy, MatchStrategy::Similarity);
        assert!(
            r.matched_text.contains("const") && r.matched_text.contains("FOO"),
            "line match: {:?}",
            r.matched_text
        );
    }

    /// Embedder-style identifier typos across real source shapes (#1694 contract).
    #[test]
    fn resolve_token_typo_matrix_preserves_non_identifier_syntax() {
        // (content, typo_target, expected_matched_token, must_not_be_in_match)
        let cases: &[(&str, &str, &str, &[&str])] = &[
            // Rust const + use site (Bline #1694 repro shape).
            (
                "const CONFIGURATION_VALUE_PRIMARY: i32 = 1;\nfn use_it() -> i32 { CONFIGURATION_VALUE_PRIMARY }\n",
                "CONFIGURATION_VALUE_PRIMRY",
                "CONFIGURATION_VALUE_PRIMARY",
                &["const", "i32", "fn"],
            ),
            // Rust function name typo (token, not full signature).
            (
                "fn process_request(data: &str) -> Result<()> {\n    Ok(())\n}\n",
                "process_requets",
                "process_request",
                &["fn", "Result", "data"],
            ),
            // Python def.
            (
                "def load_configuration_value():\n    return 1\n",
                "load_configration_value",
                "load_configuration_value",
                &["def", "return"],
            ),
            // JS/TS const + call.
            (
                "const getUserProfile = () => null;\nexport { getUserProfile };\n",
                "getUserProfle",
                "getUserProfile",
                &["const", "export"],
            ),
            // YAML-ish key in a line with punctuation (identifier extraction).
            (
                "server_port_primary: 8080\n",
                "server_port_primry",
                "server_port_primary",
                &[":", "8080"],
            ),
            // camelCase method.
            (
                "    obj.fetchUserDetails(id);\n",
                "fetchUserDetials",
                "fetchUserDetails",
                &["obj", "id"],
            ),
        ];

        for (content, typo, expected, forbidden) in cases {
            let r = resolve_with_fallback(content, typo, None, None)
                .unwrap_or_else(|e| panic!("token typo {typo:?} should match in {content:?}: {e}"));
            assert_eq!(r.strategy, MatchStrategy::Similarity, "typo={typo:?}");
            assert_eq!(
                r.matched_text, *expected,
                "typo={typo:?} content={content:?}"
            );
            assert!(
                content[r.start_offset..].starts_with(expected),
                "offset wrong for {typo:?}"
            );
            for bad in *forbidden {
                assert!(
                    !r.matched_text.contains(bad),
                    "span leaked {bad:?} for typo={typo:?} match={:?}",
                    r.matched_text
                );
            }
            // Token-like targets must never expand to multi-word spans.
            assert!(
                !r.matched_text.contains(' '),
                "token match must not contain spaces: {:?}",
                r.matched_text
            );
        }
    }

    #[test]
    fn resolve_token_typo_picks_first_best_occurrence() {
        let content = "FOO_PRIMARY=1\nFOO_PRIMARY=2\n";
        let r = resolve_with_fallback(content, "FOO_PRIMRY", None, None).unwrap();
        assert_eq!(r.matched_text, "FOO_PRIMARY");
        // First line occurrence (offset 0).
        assert_eq!(r.start_offset, 0);
    }

    #[test]
    fn resolve_token_unrelated_is_no_match() {
        let content = "const ALPHA: i32 = 1;\nconst BETA: i32 = 2;\n";
        let err =
            resolve_with_fallback(content, "ZZZZ_COMPLETELY_UNRELATED", None, None).unwrap_err();
        assert_eq!(err.kind, EditErrorKind::NoMatch);
    }

    #[test]
    fn is_token_like_target_classification() {
        assert!(is_token_like_target("CONFIGURATION_VALUE_PRIMRY"));
        assert!(is_token_like_target("getUserProfile"));
        // Hyphenated names are not identifier tokens (extraction splits on `-`).
        assert!(!is_token_like_target("kebab-case-name"));
        assert!(!is_token_like_target("fn process_data() {}"));
        assert!(!is_token_like_target("const FOO: i32 = 1;"));
        assert!(!is_token_like_target("server.port"));
        assert!(!is_token_like_target(""));
        assert!(!is_token_like_target("   "));
    }

    #[test]
    fn fuzzy_fails_min_floor_fail_closed_without_score() {
        assert!(fuzzy_fails_min_floor(None, 0.8));
        assert!(fuzzy_fails_min_floor(Some(0.5), 0.8));
        assert!(!fuzzy_fails_min_floor(Some(0.9), 0.8));
        assert!(!fuzzy_fails_min_floor(Some(0.8), 0.8));
    }

    /// Kebab-case targets use line similarity (not broken token path).
    #[test]
    fn resolve_kebab_case_uses_line_similarity_not_broken_token_path() {
        let content = "font-weight-primary: bold;\n";
        // Near-full-line typo; must still resolve (line path), not dead-end as token.
        let r = resolve_with_fallback(content, "font-weight-primry: bold;", None, None)
            .expect("kebab line snippet should match via line similarity");
        assert_eq!(r.strategy, MatchStrategy::Similarity);
        assert!(
            r.matched_text.contains("font-weight-primary"),
            "{:?}",
            r.matched_text
        );
    }

    #[test]
    fn resolve_with_fallback_structured_error() {
        let content = "fn alpha() {}\nfn beta() {}\n";
        let result = resolve_with_fallback(content, "fn completely_unrelated_xyz()", None, None);
        let err = result.unwrap_err();
        assert_eq!(err.kind, EditErrorKind::NoMatch);
        assert!(err.message.contains("target not found"));
    }

    #[test]
    fn resolve_with_fallback_anchor_over_similarity() {
        // When context is provided, anchor matching fires before similarity
        // and should win.
        let content = "fn setup() {}\nfn proces_data(x: i32) {}\nfn cleanup() {}\n";
        let result = resolve_with_fallback(
            content,
            "fn process_data(x: i32) {}",
            Some("fn setup() {}"),
            Some("fn cleanup() {}"),
        );
        let r = result.expect("anchor fallback should succeed");
        assert_eq!(r.strategy, MatchStrategy::Anchor);
    }

    #[test]
    fn edit_error_serializes_to_json() {
        let err = EditError {
            kind: EditErrorKind::AmbiguousTarget,
            message: "found 3 matches".into(),
            suggestion: Some("use --nth to select one".into()),
            similar_targets: vec!["match1".into(), "match2".into()],
        };
        let json = serde_json::to_string(&err).unwrap();
        let deserialized: EditError = serde_json::from_str(&json).unwrap();
        assert_eq!(deserialized.kind, EditErrorKind::AmbiguousTarget);
        assert_eq!(deserialized.similar_targets.len(), 2);
    }

    #[test]
    fn match_strategy_display() {
        assert_eq!(MatchStrategy::Exact.to_string(), "exact");
        assert_eq!(MatchStrategy::Anchor.to_string(), "anchor");
        assert_eq!(MatchStrategy::Similarity.to_string(), "similarity");
    }

    #[test]
    fn extract_identifiers_from_code() {
        let line = "fn process_request(data: &str) -> Result<()> {";
        let ids = extract_identifiers(line);
        assert!(ids.contains(&"process_request".to_string()));
        assert!(ids.contains(&"data".to_string()));
        assert!(ids.contains(&"str".to_string()));
        assert!(ids.contains(&"Result".to_string()));
    }

    #[test]
    fn validation_result_serializes() {
        let result = ValidationResult {
            valid: true,
            errors: vec![],
            warnings: vec!["some warning".into()],
        };
        let json = serde_json::to_string(&result).unwrap();
        let deserialized: ValidationResult = serde_json::from_str(&json).unwrap();
        assert!(deserialized.valid);
        assert_eq!(deserialized.warnings.len(), 1);
    }

    #[test]
    fn truncate_safe_on_multibyte_utf8() {
        // "café" has bytes: c(1) a(1) f(1) é(2) = 5 bytes, 4 chars.
        let s = "café";
        assert_eq!(s.len(), 5);
        // Truncate at 4 would land in the middle of 'é' (bytes 3-4).
        // Must not panic; should truncate before the multi-byte char.
        let t = truncate_str(s, 4);
        assert_eq!(t, "caf");
        // Truncate at 5 returns the whole string.
        assert_eq!(truncate_str(s, 5), "café");
        // Truncate at 3 is a clean boundary.
        assert_eq!(truncate_str(s, 3), "caf");
        // Truncate at 0.
        assert_eq!(truncate_str(s, 0), "");
    }

    #[test]
    fn validate_edit_toml_syntax_check() {
        let toml = "[database]\nhost = \"localhost\"\n";
        // Valid replacement.
        let result = validate_edit(toml, "localhost", "remotehost", Some("config.toml"));
        assert!(result.valid);

        // Invalid replacement (breaks TOML).
        let result = validate_edit(
            toml,
            "[database]",
            "not valid toml {{{{",
            Some("config.toml"),
        );
        assert!(!result.valid);
        assert!(result.errors[0].contains("invalid TOML"));
    }

    #[test]
    fn find_similar_targets_multi_word_pattern() {
        let content = "fn process_all_requests(data: &str) -> bool {\n    true\n}\n";
        // Multi-word pattern triggers whole-line matching path.
        let similar = find_similar_targets(content, "fn process_all_reqests(data: &str)", 3);
        assert!(
            !similar.is_empty(),
            "should find similar multi-word targets"
        );
    }

    #[test]
    fn resolve_fallback_multi_line_no_similarity() {
        // Multi-line targets skip the similarity path and go to error.
        let content = "fn alpha() {}\nfn beta() {}\nfn gamma() {}\n";
        let result = resolve_with_fallback(content, "fn alphax() {}\nfn betax() {}", None, None);
        assert!(
            result.is_err(),
            "multi-line targets without context should not match via similarity"
        );
        let err = result.unwrap_err();
        assert_eq!(err.kind, EditErrorKind::NoMatch);
    }

    #[test]
    fn resolve_with_fallback_multi_line_anchor_match() {
        // Multi-line target with context should resolve via anchor matching
        // through the full fallback chain (not just the anchor_match helper).
        let content = "fn header() {}\nfn proccess_data(x: i32) {\n    x + 1\n}\nfn footer() {}\n";
        let result = resolve_with_fallback(
            content,
            "fn process_data(x: i32) {\n    x + 1\n}",
            Some("fn header() {}"),
            Some("fn footer() {}"),
        );
        let r = result.expect("multi-line anchor should succeed");
        assert_eq!(r.strategy, MatchStrategy::Anchor);
        assert!(r.matched_text.contains("proccess_data"));
        assert!(r.matched_text.contains("x + 1"));
    }

    #[test]
    fn validate_edit_yml_extension() {
        // The .yml extension (not just .yaml) should trigger YAML validation.
        let yaml = "key: value\nlist:\n  - item1\n";
        // Valid replacement.
        let result = validate_edit(yaml, "value", "new_value", Some("config.yml"));
        assert!(result.valid);

        // Invalid replacement that breaks YAML structure.
        let result = validate_edit(yaml, "key: value", ":\n  :\n  - :", Some("config.yml"));
        assert!(
            !result.valid,
            ".yml extension should trigger YAML validation"
        );
        assert!(
            result.errors[0].contains("invalid YAML"),
            "expected YAML error, got: {}",
            result.errors[0]
        );
    }

    // -- validate_edit_nth (#1061) -------------------------------------------

    #[test]
    fn validate_edit_nth_replaces_only_nth_occurrence() {
        // Two occurrences of "val"; replacing only the 1st is valid JSON.
        let json = r#"{"a": "val", "b": "val"}"#;
        let result = validate_edit_nth(json, "val", "new", Some("data.json"), Some(1));
        assert!(result.valid, "nth=1 should produce valid JSON");
    }

    #[test]
    fn validate_edit_nth_none_replaces_all() {
        // nth=None replaces all occurrences (same as validate_edit).
        let content = "aXbXc";
        let result = validate_edit_nth(content, "X", "Y", None, None);
        assert!(result.valid);
    }

    #[test]
    fn validate_edit_nth_out_of_range() {
        // nth=5 but only 2 occurrences: should return invalid with descriptive error.
        let content = "aXbXc";
        let result = validate_edit_nth(content, "X", "Y", None, Some(5));
        assert!(
            !result.valid,
            "nth beyond occurrence count should be invalid"
        );
        assert!(
            result.errors[0].contains("occurrence 5 not found"),
            "error should mention the missing occurrence: {:?}",
            result.errors
        );
    }

    #[test]
    fn validate_edit_nth_zero_rejected() {
        // nth=0 is invalid (1-based indexing); must not truncate content.
        let content = r#"{"a": "val"}"#;
        let result = validate_edit_nth(content, "val", "X", Some("f.json"), Some(0));
        assert!(!result.valid);
        assert!(result.errors[0].contains("nth must be >= 1"));
    }

    #[test]
    fn validate_edit_nth_detects_invalid_json_for_single_occurrence() {
        // Replacing only the 1st "value" with a broken fragment is invalid.
        let json = r#"{"a": "value", "b": "value"}"#;
        let result = validate_edit_nth(json, "\"value\"", "broken}", Some("config.json"), Some(1));
        assert!(!result.valid, "nth=1 should detect broken JSON");
    }

    // -- anchor_match and resolve_with_fallback edge cases (#978) -----------

    #[test]
    fn anchor_match_after_only_context() {
        // after_context without before_context hits the code path where
        // before_context is None but after_context is checked.
        let content = "fn setup() {}\nfn proccess_data(x: i32) {}\nfn cleanup() {}\n";
        let result = anchor_match(
            content,
            "fn process_data(x: i32) {}",
            None,
            Some("fn cleanup() {}"),
        );
        let r = result.expect("after-only context should find anchor match");
        assert_eq!(r.strategy, MatchStrategy::Anchor);
        assert!(r.matched_text.contains("proccess_data"));
    }

    #[test]
    fn anchor_match_multiple_candidates_returns_first() {
        // Target appears (fuzzily) twice; anchor_match returns the first match.
        let content = "fn header() {}\nfn proccess(x: i32) {}\nfn middle() {}\nfn proccess(y: bool) {}\nfn footer() {}\n";
        let result = anchor_match(
            content,
            "fn process(x: i32) {}",
            Some("fn header() {}"),
            Some("fn middle() {}"),
        );
        let r = result.expect("should match first candidate");
        assert_eq!(r.strategy, MatchStrategy::Anchor);
        // The first candidate contains "x: i32", not "y: bool".
        assert!(
            r.matched_text.contains("x: i32"),
            "should match first occurrence, got: {}",
            r.matched_text
        );
    }

    #[test]
    fn resolve_with_fallback_empty_content() {
        let result = resolve_with_fallback("", "fn hello()", None, None);
        let err = result.unwrap_err();
        assert_eq!(err.kind, EditErrorKind::NoMatch);
    }

    #[test]
    fn anchor_match_multiline_before_context() {
        // Multi-line before_context should use the last line for matching,
        // not the entire multi-line string.
        let content = "fn setup() {}\nfn target() {}\nfn cleanup() {}\n";
        let result = anchor_match(
            content,
            "fn target() {}",
            Some("fn other() {}\nfn setup() {}"),
            None,
        );
        assert!(
            result.is_some(),
            "anchor_match should match using the last line of multi-line before_context"
        );
        assert_eq!(result.unwrap().matched_text, "fn target() {}");
    }

    #[test]
    fn anchor_match_multiline_after_context() {
        // Multi-line after_context should use the first line for matching.
        let content = "fn setup() {}\nfn target() {}\nfn cleanup() {}\n";
        let result = anchor_match(
            content,
            "fn target() {}",
            None,
            Some("fn cleanup() {}\nfn extra() {}"),
        );
        assert!(
            result.is_some(),
            "anchor_match should match using the first line of multi-line after_context"
        );
        assert_eq!(result.unwrap().matched_text, "fn target() {}");
    }

    // Static assertions: types must be Send + Sync.
    const _: () = {
        fn _assert<T: Send + Sync>() {}
        let _ = _assert::<EditError>;
        let _ = _assert::<EditErrorKind>;
        let _ = _assert::<ValidationResult>;
        let _ = _assert::<AnchorMatchResult>;
        let _ = _assert::<MatchStrategy>;
    };
}