mpl-macro 0.2.1

//! Static-codegen generator for the `FastParse` derive.
//!
//! Emits one `fn parse_<rule>(state, pos) -> Result<u32, ()>` per grammar
//! rule plus a small public surface (`var` ids, `recognize`, `parse`).
//! Layout matches `examples/paren_bench/src/mpl_fast_bub.rs`.

use crate::generator::{
    analyze_char_classes, analyze_first_byte_dispatch, classify_rules, compute_first_sets,
    is_left_recursive, is_tail_loop, DispatchCascade,
};
use crate::mplg::MplgOutput;
use mpl::symbols::{Metasymbol, TerminalSymbol, E};
use proc_macro2::TokenStream;
use quote::{format_ident, quote};
use std::collections::BTreeSet;
use syn::{Expr, ExprCall, ExprLit, ExprPath, Ident, Lit};

/// Convert PascalCase / CamelCase to snake_case so the generated fn names
/// follow Rust convention. `BubFn1` → `bub_fn1`, `OrOrExpr` → `or_or_expr`.
fn snake(s: &str) -> String {
    let mut out = String::with_capacity(s.len() + 4);
    for (i, c) in s.chars().enumerate() {
        if c.is_ascii_uppercase() && i > 0 {
            out.push('_');
        }
        out.push(c.to_ascii_lowercase());
    }
    out
}

fn fn_ident(name: &str) -> Ident {
    format_ident!("parse_{}", snake(name))
}

fn const_ident(name: &str) -> Ident {
    format_ident!("{}", snake(name).to_uppercase())
}

/// Emit code that evaluates `e` at the position bound to `pos_tok`. The
/// result is a `Result<u32, ()>` — `Ok(end_pos)` on success, `Err(())` on
/// failure. On success the appropriate token(s) have already been pushed
/// (only when `EMIT` is true; check-mode skips them).
fn eval_e(e: &E<&str, &str>, pos_tok: TokenStream) -> TokenStream {
    match e {
        E::V(v) => {
            let f = fn_ident(v);
            quote! { #f::<S, EMIT>(state, #pos_tok) }
        }
        E::T(t) => match t {
            TerminalSymbol::Metasymbol(m) => match m {
                Metasymbol::Empty => quote! {
                    {
                        let p = #pos_tok;
                        if EMIT {
                            state.push_leaf(::mpl::fast::kind::EMPTY, p, p);
                        }
                        Ok::<u32, ()>(p)
                    }
                },
                Metasymbol::Failure => quote! { Err::<u32, ()>(()) },
                Metasymbol::Any(n) => {
                    let n_lit = *n as u32;
                    quote! {
                        {
                            let p = #pos_tok;
                            let want = p as usize + #n_lit as usize;
                            if want <= state.input().len() {
                                if EMIT {
                                    state.push_leaf(::mpl::fast::kind::ANY, p, want as u32);
                                }
                                Ok::<u32, ()>(want as u32)
                            } else {
                                state.note_failure_at(p);
                                Err::<u32, ()>(())
                            }
                        }
                    }
                }
                Metasymbol::All => quote! {
                    {
                        let p = #pos_tok;
                        let end = state.input().len() as u32;
                        if EMIT {
                            state.push_leaf(::mpl::fast::kind::ALL, p, end);
                        }
                        Ok::<u32, ()>(end)
                    }
                },
                Metasymbol::Omit => unimplemented!("Omit metasymbol not supported in FastParse"),
            },
            TerminalSymbol::Original(raw) => emit_terminal(raw, pos_tok),
        },
    }
}

/// Translate a `{ Char('c') }` / `{ Str("...") }` original-symbol into a
/// byte-level match. Other shapes fall back to a generic note-failure
/// stub (intentional: the FastParse path expects byte input only for now).
fn emit_terminal(raw: &str, pos_tok: TokenStream) -> TokenStream {
    let parsed: Expr = syn::parse_str(raw).unwrap_or_else(|_| {
        panic!("FastParse: cannot parse terminal expression `{raw}`");
    });

    if let Expr::Call(ExprCall { func, args, .. }) = &parsed {
        if let Expr::Path(ExprPath { path, .. }) = &**func {
            if path.is_ident("Char") {
                if let Some(Expr::Lit(ExprLit {
                    lit: Lit::Char(c), ..
                })) = args.first()
                {
                    let byte = c.value() as u32 as u8;
                    return quote! {
                        {
                            let p = #pos_tok;
                            let pi = p as usize;
                            if pi < state.input().len() && state.input()[pi] == #byte {
                                if EMIT {
                                    state.push_leaf(::mpl::fast::kind::TERMINAL, p, p + 1);
                                }
                                Ok::<u32, ()>(p + 1)
                            } else {
                                state.note_failure_at(p);
                                Err::<u32, ()>(())
                            }
                        }
                    };
                }
            } else if path.is_ident("Str") {
                if let Some(Expr::Lit(ExprLit {
                    lit: Lit::Str(s), ..
                })) = args.first()
                {
                    let bytes_lit = syn::LitByteStr::new(s.value().as_bytes(), s.span());
                    return quote! {
                        {
                            let p = #pos_tok;
                            let pi = p as usize;
                            let pat: &[u8] = #bytes_lit;
                            let end = pi + pat.len();
                            if end <= state.input().len() && &state.input()[pi..end] == pat {
                                if EMIT {
                                    state.push_leaf(::mpl::fast::kind::TERMINAL, p, end as u32);
                                }
                                Ok::<u32, ()>(end as u32)
                            } else {
                                state.note_failure_at(p);
                                Err::<u32, ()>(())
                            }
                        }
                    };
                }
            }
        }
    }

    panic!("FastParse: unsupported terminal expression `{raw}`")
}

/// True when this `E` is the `Failure` metasymbol — the second-choice
/// alternative is then a no-op rather than emitting a leaf.
fn is_failure(e: &E<&str, &str>) -> bool {
    matches!(e, E::T(TerminalSymbol::Metasymbol(Metasymbol::Failure)))
}

/// True when this `E` is a variable reference (so the second choice should
/// be wrapped in Start/End for the parent rule). Terminals as the second
/// choice are returned directly per current mpl semantics.
fn is_variable(e: &E<&str, &str>) -> bool {
    matches!(e, E::V(_))
}

/// Generate a `[bool; 256]` byte-table init expression that flags every
/// byte in `bytes` as accepted.
fn generate_byte_table(bytes: &BTreeSet<u8>) -> TokenStream {
    let entries = bytes.iter().map(|b| {
        let b = *b;
        quote! { __t[#b as usize] = true; }
    });
    quote! {
        {
            let mut __t = [false; 256];
            #(#entries)*
            __t
        }
    }
}

/// Emit an EMIT-mode-only fast path for char-class cascade rules. The
/// recognition path becomes a single byte-table lookup; the emit path
/// falls through to the original cascade body for AST-shape preservation.
fn generate_cascade_dispatcher(
    rule: &mpl::rules::Rule<&str, &str>,
    bytes: &BTreeSet<u8>,
    cascade_body: TokenStream,
) -> TokenStream {
    let name = rule.value;
    let f = fn_ident(name);
    let table_ident = format_ident!("__{}_BYTES", const_ident(name));
    let table_init = generate_byte_table(bytes);

    quote! {
        const #table_ident: [bool; 256] = #table_init;

        #[inline(always)]
        fn #f<S: ::mpl::fast::ParseState, const EMIT: bool>(
            state: &mut S,
            pos: u32,
        ) -> ::core::result::Result<u32, ()> {
            if !EMIT {
                let p = pos as usize;
                if p < state.input().len() && #table_ident[state.input()[p] as usize] {
                    return Ok(pos + 1);
                }
                state.note_failure_at(pos);
                return Err(());
            }
            // EMIT=true: original cascade preserves AST shape.
            #cascade_body
        }
    }
}

fn generate_rule_inner(rule: &mpl::rules::Rule<&str, &str>) -> TokenStream {
    let name = rule.value;
    let f = fn_ident(name);
    let kind_const = const_ident(name);

    let lhs = &rule.equal.first.lhs;
    let rhs = &rule.equal.first.rhs;
    let alt = &rule.equal.second.0;

    let eval_lhs = eval_e(lhs, quote! { pos });
    let eval_rhs = eval_e(rhs, quote! { end_b });

    let first_choice = quote! {
        {
            let cp = if EMIT { state.checkpoint() } else { 0 };
            let s = if EMIT { state.push_start(var::#kind_const, pos) } else { 0 };
            if let Ok(end_b) = #eval_lhs {
                if let Ok(end_c) = #eval_rhs {
                    if EMIT {
                        state.push_end(var::#kind_const, end_c, s);
                    }
                    return Ok(end_c);
                }
            }
            if EMIT {
                state.truncate(cp);
            }
        }
    };

    let second_choice = if is_failure(alt) {
        quote! { Err(()) }
    } else if is_variable(alt) {
        // Wrap variable result in Start/End for this rule.
        let eval_alt = eval_e(alt, quote! { pos });
        quote! {
            {
                let cp2 = if EMIT { state.checkpoint() } else { 0 };
                let s = if EMIT { state.push_start(var::#kind_const, pos) } else { 0 };
                if let Ok(end) = #eval_alt {
                    if EMIT {
                        state.push_end(var::#kind_const, end, s);
                    }
                    return Ok(end);
                }
                if EMIT {
                    state.truncate(cp2);
                }
                Err(())
            }
        }
    } else {
        // Terminal / metasymbol second choice: returned directly per mpl
        // semantics (no wrapping in this rule's Start/End).
        let eval_alt = eval_e(alt, quote! { pos });
        quote! {
            { #eval_alt }
        }
    };

    quote! {
        #[inline]
        fn #f<S: ::mpl::fast::ParseState, const EMIT: bool>(
            state: &mut S,
            pos: u32,
        ) -> ::core::result::Result<u32, ()> {
            #first_choice
            #second_choice
        }
    }
}

/// Like [`generate_rule_inner`] but returns just the body tokens (no
/// surrounding `fn` wrapper). Used by the cascade dispatcher to embed
/// the original cascade as the EMIT=true fallback.
fn generate_rule_body(rule: &mpl::rules::Rule<&str, &str>) -> TokenStream {
    let kind_const = const_ident(rule.value);
    let lhs = &rule.equal.first.lhs;
    let rhs = &rule.equal.first.rhs;
    let alt = &rule.equal.second.0;

    let eval_lhs = eval_e(lhs, quote! { pos });
    let eval_rhs = eval_e(rhs, quote! { end_b });

    let first_choice = quote! {
        {
            let cp = if EMIT { state.checkpoint() } else { 0 };
            let s = if EMIT { state.push_start(var::#kind_const, pos) } else { 0 };
            if let Ok(end_b) = #eval_lhs {
                if let Ok(end_c) = #eval_rhs {
                    if EMIT {
                        state.push_end(var::#kind_const, end_c, s);
                    }
                    return Ok(end_c);
                }
            }
            if EMIT {
                state.truncate(cp);
            }
        }
    };

    let second_choice = if is_failure(alt) {
        quote! { Err(()) }
    } else if is_variable(alt) {
        let eval_alt = eval_e(alt, quote! { pos });
        quote! {
            {
                let cp2 = if EMIT { state.checkpoint() } else { 0 };
                let s = if EMIT { state.push_start(var::#kind_const, pos) } else { 0 };
                if let Ok(end) = #eval_alt {
                    if EMIT {
                        state.push_end(var::#kind_const, end, s);
                    }
                    return Ok(end);
                }
                if EMIT {
                    state.truncate(cp2);
                }
                Err(())
            }
        }
    } else {
        let eval_alt = eval_e(alt, quote! { pos });
        quote! {
            { #eval_alt }
        }
    };

    quote! {
        #first_choice
        #second_choice
    }
}

/// Emit a first-byte-dispatch fast path for cascades with mostly disjoint
/// FIRST sets. Each alternative's FIRST bytes route to its parser; bytes
/// shared between alternatives are tried in cascade order via or_else.
fn generate_first_byte_dispatcher(
    rule: &mpl::rules::Rule<&str, &str>,
    cascade: &DispatchCascade<'_>,
    cascade_body: TokenStream,
) -> TokenStream {
    let name = rule.value;
    let f = fn_ident(name);

    // Group alternatives by each byte they can start. The cascade stays
    // priority-ordered so earlier alternatives win on shared bytes.
    let mut byte_to_alts: [Vec<usize>; 256] = std::array::from_fn(|_| Vec::new());
    for (i, alt) in cascade.alternatives.iter().enumerate() {
        for b in alt.first.iter_bytes() {
            byte_to_alts[b as usize].push(i);
        }
    }

    // Bucket bytes by which alternative-list they map to so we can emit
    // a single match arm per group (e.g. all digits → Float-then-Integer).
    let mut groups: std::collections::BTreeMap<Vec<usize>, BTreeSet<u8>> =
        std::collections::BTreeMap::new();
    for (b, alts) in byte_to_alts.iter().enumerate() {
        if alts.is_empty() {
            continue;
        }
        groups.entry(alts.clone()).or_default().insert(b as u8);
    }

    let arms = groups.iter().map(|(alt_indices, bytes)| {
        let byte_pat: Vec<TokenStream> = bytes
            .iter()
            .map(|b| {
                let b = *b;
                quote! { #b }
            })
            .collect();
        let pat = if byte_pat.len() == 1 {
            let b = &byte_pat[0];
            quote! { #b }
        } else {
            quote! { #(#byte_pat)|* }
        };

        // Build chained `parse_<alt>::<S, EMIT>(state, pos).or_else(|_| ...)`
        // for shared-byte alternatives; single alternative → just call.
        let mut call_chain = TokenStream::new();
        for (idx, alt_idx) in alt_indices.iter().enumerate() {
            let alt_name = cascade.alternatives[*alt_idx].rule_name;
            let alt_fn = fn_ident(alt_name);
            if idx == 0 {
                call_chain = quote! { #alt_fn::<S, EMIT>(state, pos) };
            } else {
                call_chain = quote! {
                    #call_chain.or_else(|_| #alt_fn::<S, EMIT>(state, pos))
                };
            }
        }
        quote! { #pat => #call_chain, }
    });

    quote! {
        #[inline(always)]
        fn #f<S: ::mpl::fast::ParseState, const EMIT: bool>(
            state: &mut S,
            pos: u32,
        ) -> ::core::result::Result<u32, ()> {
            if EMIT {
                #cascade_body
            } else {
                let p = pos as usize;
                if p >= state.input().len() {
                    state.note_failure_at(pos);
                    return Err(());
                }
                match state.input()[p] {
                    #(#arms)*
                    _ => {
                        state.note_failure_at(pos);
                        Err(())
                    }
                }
            }
        }
    }
}

/// Emit a `loop {}`-based fast path for `R = X R / ()` rules. Rust does
/// not guarantee TCO; LLVM's `tailcallelim` typically fails for
/// `&mut State + Result<u32, ()>`-shaped recursive returns, leaving a
/// real call/ret pair per iteration. We sidestep it by generating an
/// explicit loop in the EMIT=false path; EMIT=true keeps the original
/// recursion so the AST shape (right-recursive Internal nodes) is preserved.
fn generate_tail_loop(
    rule: &mpl::rules::Rule<&str, &str>,
    cascade_body: TokenStream,
) -> TokenStream {
    let name = rule.value;
    let f = fn_ident(name);
    let lhs = &rule.equal.first.lhs;
    let eval_lhs_loop = eval_e(lhs, quote! { p });

    quote! {
        #[inline(always)]
        fn #f<S: ::mpl::fast::ParseState, const EMIT: bool>(
            state: &mut S,
            pos: u32,
        ) -> ::core::result::Result<u32, ()> {
            if EMIT {
                #cascade_body
            } else {
                let mut p = pos;
                loop {
                    match #eval_lhs_loop {
                        Ok(np) => p = np,
                        Err(_) => break,
                    }
                }
                Ok(p)
            }
        }
    }
}

/// Squirrel-style seed-growing wrapper for directly left-recursive rules.
/// EMIT=false uses position-only seed (matches [`crate::parser::squirrel_recognize`]
/// — same algorithmic family as peg's `#[cache_left_rec]`). EMIT=true
/// panics: AST construction with seed-growing is O(n²) in this codegen
/// path, so the user is steered to [`crate::parser::squirrel_parse`] for
/// the parse tree.
fn generate_left_recursive(
    rule: &mpl::rules::Rule<&str, &str>,
    cascade_body: TokenStream,
) -> TokenStream {
    let name = rule.value;
    let f = fn_ident(name);
    let kind_const = const_ident(name);

    quote! {
        #[inline(always)]
        fn #f<S: ::mpl::fast::ParseState, const EMIT: bool>(
            state: &mut S,
            pos: u32,
        ) -> ::core::result::Result<u32, ()> {
            if EMIT {
                // The token-emitting AST path is not seed-growing-aware
                // in this codegen; use `Parser::squirrel_parse` instead.
                // We still emit the cascade body so a non-LR call site
                // (e.g. when the LR alternative cannot fire) parses correctly.
                #cascade_body
            } else {
                // Position-only seed-growing — peg-compatible LR family.
                let key_kind = var::#kind_const;
                if let Some(entry) = state.lr_memo_get(key_kind, pos) {
                    return match entry {
                        ::mpl::fast::LrMemoEntry::Done { result } |
                        ::mpl::fast::LrMemoEntry::InProgress { seed: result } => {
                            result.map(Ok).unwrap_or(Err(()))
                        }
                    };
                }

                state.lr_memo_set(
                    key_kind,
                    pos,
                    ::mpl::fast::LrMemoEntry::InProgress { seed: None },
                );

                let mut best: Option<u32> = None;
                loop {
                    let result: ::core::result::Result<u32, ()> = (|| -> ::core::result::Result<u32, ()> {
                        #cascade_body
                    })();
                    let new_end = result.ok();
                    let grew = match (new_end, best) {
                        (Some(_), None) => true,
                        (Some(n), Some(b)) => n > b,
                        _ => false,
                    };
                    if !grew { break; }
                    best = new_end;
                    state.lr_memo_set(
                        key_kind,
                        pos,
                        ::mpl::fast::LrMemoEntry::InProgress { seed: best },
                    );
                }

                state.lr_memo_set(
                    key_kind,
                    pos,
                    ::mpl::fast::LrMemoEntry::Done { result: best },
                );
                best.map(Ok).unwrap_or(Err(()))
            }
        }
    }
}

/// Decide which fast path (if any) a rule gets:
/// 1. Squirrel seed-growing for directly left-recursive rules.
/// 2. Byte-table when the cascade is a true char class (≥ 4 bytes).
/// 3. First-byte dispatch when it's a multi-prefix cascade with a useful
///    char class somewhere in the alternatives.
/// 4. `loop {}` for `R = X R / ()` tail recursion.
/// 5. Otherwise the original cascade with `#[inline]`.
fn generate_rule(
    rule: &mpl::rules::Rule<&str, &str>,
    char_classes: &std::collections::HashMap<&str, BTreeSet<u8>>,
    dispatches: &std::collections::HashMap<&str, DispatchCascade<'_>>,
) -> TokenStream {
    if is_left_recursive(rule) {
        let body = generate_rule_body(rule);
        return generate_left_recursive(rule, body);
    }
    if let Some(bytes) = char_classes.get(rule.value) {
        if bytes.len() >= 4 {
            let body = generate_rule_body(rule);
            return generate_cascade_dispatcher(rule, bytes, body);
        }
    }
    if let Some(cascade) = dispatches.get(rule.value) {
        let body = generate_rule_body(rule);
        return generate_first_byte_dispatcher(rule, cascade, body);
    }
    if is_tail_loop(rule) {
        let body = generate_rule_body(rule);
        return generate_tail_loop(rule, body);
    }
    generate_rule_inner(rule)
}

pub fn generate_fast(parser_ident: &Ident, lines: &[MplgOutput]) -> TokenStream {
    let rules: Vec<&mpl::rules::Rule<&str, &str>> = lines
        .iter()
        .filter_map(|line| match line {
            MplgOutput::Rule(rule) => Some(rule),
            _ => None,
        })
        .collect();

    if rules.is_empty() {
        return quote! {};
    }

    // Each variable becomes a u32 id in declaration order. The first rule
    // is the start variable.
    let var_consts = rules.iter().enumerate().map(|(i, rule)| {
        let cid = const_ident(rule.value);
        let i = i as u32;
        quote! { pub const #cid: u32 = #i; }
    });

    // FIRST set fixed point: shared with Mizushima cut analysis below
    // and the first-byte dispatch detector above.
    let firsts = compute_first_sets(&rules);

    // Char-class cascade analysis: detect rules of the form
    // `A = X () / Y` where every X resolves to a single byte, and emit
    // a byte-table fast path for those. AST shape is preserved by
    // routing the EMIT=true path through the original cascade body.
    let char_classes = analyze_char_classes(&rules);

    // First-byte dispatch analysis: for cascades whose alternatives
    // have mostly disjoint FIRST sets but aren't pure single-byte (e.g.
    // `Atom = ExprInParens / FloatLiteral / IntegerLiteral / ...`).
    let dispatches = analyze_first_byte_dispatch(&rules, &firsts, &char_classes);

    let rule_fns = rules
        .iter()
        .map(|rule| generate_rule(rule, &char_classes, &dispatches));

    let start_fn = fn_ident(rules[0].value);

    // Mizushima cut analysis: classify each rule as LL(1)-equivalent
    // (FIRST sets disjoint, no memo needed) or backtracking (memo helpful).
    let classification = classify_rules(&rules, &firsts);

    let needs_memo_entries = classification.iter().map(|(_, needs)| {
        let v = *needs;
        quote! { #v }
    });
    let needs_memo_count = classification.len();

    let backtracking_count = classification.iter().filter(|(_, b)| *b).count();
    let analysis_summary = format!(
        "Mizushima cut analysis: {}/{} rules require memoisation",
        backtracking_count, needs_memo_count
    );

    let memo_rule_names: Vec<String> = classification
        .iter()
        .filter(|(_, b)| *b)
        .map(|(name, _)| name.clone())
        .collect();
    let memo_rule_list = if memo_rule_names.is_empty() {
        "(none)".to_string()
    } else {
        memo_rule_names.join(", ")
    };
    let needs_memo_doc = format!(
        "{}\n\nRules needing memoisation: {}",
        analysis_summary, memo_rule_list
    );

    let mod_ident = format_ident!("__{}_fast", snake(&parser_ident.to_string()));

    quote! {
        #[allow(non_snake_case, unused, clippy::all)]
        mod #mod_ident {
            use ::mpl::fast::ParserState;

            pub mod var {
                #(#var_consts)*
            }

            /// Mizushima cut analysis result, indexed by variable id.
            ///
            /// `false` ⇒ LL(1)-equivalent at the choice point; the rule
            /// will never re-evaluate at the same input position, so a
            /// future memoising backend can skip its cache slot.
            ///
            /// `true` ⇒ FIRST sets overlap (or both nullable); the rule
            /// is a candidate for packrat / Squirrel memoisation.
            #[doc = #needs_memo_doc]
            pub const NEEDS_MEMO: [bool; #needs_memo_count] = [#(#needs_memo_entries),*];

            #(#rule_fns)*

            /// Recognise + emit tokens, return only `bool`.
            pub fn recognize(input: &[u8]) -> bool {
                let mut state = ParserState::new(input);
                matches!(
                    #start_fn::<ParserState<'_>, true>(&mut state, 0),
                    Ok(end) if end as usize == input.len()
                )
            }

            /// Pure recognition over [`::mpl::fast::CheckState`], the thin
            /// state (input + furthest only). The recursive call chain
            /// holds the parser state in registers — matches `peg`'s
            /// `(__input, __pos) -> end_pos` calling convention.
            pub fn recognize_only(input: &[u8]) -> bool {
                let mut state = ::mpl::fast::CheckState::new(input);
                matches!(
                    #start_fn::<::mpl::fast::CheckState<'_>, false>(&mut state, 0),
                    Ok(end) if end as usize == input.len()
                )
            }

            pub fn parse(
                input: &[u8],
            ) -> ::core::result::Result<Vec<::mpl::fast::Token>, ::mpl::fast::ParseError> {
                let mut state = ParserState::new(input);
                use ::mpl::fast::ParseState as _;
                match #start_fn::<ParserState<'_>, true>(&mut state, 0) {
                    Ok(end) if end as usize == input.len() => Ok(state.tokens),
                    Ok(end) => Err(::mpl::fast::ParseError::at(end)),
                    Err(_) => Err(::mpl::fast::ParseError::at(state.furthest())),
                }
            }

            /// Recognition path that uses a caller-supplied bump arena —
            /// useful for memoising backends that store cache tables there.
            /// Token storage still uses `Vec` (faster on push-heavy paths).
            pub fn recognize_in(input: &[u8], arena: &::mpl::fast::bumpalo::Bump) -> bool {
                let mut state = ParserState::new_in(input, arena);
                matches!(
                    #start_fn::<ParserState<'_>, true>(&mut state, 0),
                    Ok(end) if end as usize == input.len()
                )
            }

            /// Variant of [`parse`] that returns a [`ParseError`](::mpl::fast::ParseError)
            /// pointing at the furthest input position any rule reached
            /// (the most informative location for diagnostics by PEG/TDPL
            /// convention) when recognition fails. Same fast path as
            /// [`recognize_only`] for the success determination.
            pub fn check(
                input: &[u8],
            ) -> ::core::result::Result<(), ::mpl::fast::ParseError> {
                let mut state = ::mpl::fast::CheckState::new(input);
                use ::mpl::fast::ParseState as _;
                match #start_fn::<::mpl::fast::CheckState<'_>, false>(&mut state, 0) {
                    Ok(end) if end as usize == input.len() => Ok(()),
                    Ok(end) => Err(::mpl::fast::ParseError::at(end)),
                    Err(_) => Err(::mpl::fast::ParseError::at(state.furthest())),
                }
            }
        }

        impl #parser_ident {
            /// Recognise the input via the static-codegen fast path.
            /// Builds the flat token buffer; equivalent to `pest`'s shape.
            pub fn fast_recognize(&self, input: &[u8]) -> bool {
                #mod_ident::recognize(input)
            }

            /// Pure recognition: no token buffer is allocated and no
            /// token writes are emitted (the EMIT=false instantiation
            /// of the per-rule fns elides them at compile time).
            /// Apples-to-apples with `peg`'s `Result<(), _>` return.
            pub fn fast_recognize_only(&self, input: &[u8]) -> bool {
                #mod_ident::recognize_only(input)
            }

            /// Parse the input, returning the flat token buffer on success
            /// or a [`ParseError`](::mpl::fast::ParseError) pointing at the
            /// furthest input position the parser reached.
            pub fn fast_parse(&self, input: &[u8])
                -> ::core::result::Result<Vec<::mpl::fast::Token>, ::mpl::fast::ParseError>
            {
                #mod_ident::parse(input)
            }

            /// Recognition with diagnostic. Same fast path as
            /// [`fast_recognize_only`] but returns a [`ParseError`](::mpl::fast::ParseError)
            /// pinpointing where matching got stuck.
            pub fn fast_check(&self, input: &[u8])
                -> ::core::result::Result<(), ::mpl::fast::ParseError>
            {
                #mod_ident::check(input)
            }

            /// Mizushima cut analysis: which rules require memoisation
            /// for correctness against backtracking re-evaluation.
            /// Indexed by variable id.
            pub const fn fast_needs_memo() -> &'static [bool] {
                &#mod_ident::NEEDS_MEMO
            }

            /// Recognition path that reuses an externally-supplied bumpalo
            /// arena. Call `arena.reset()` between parses to avoid
            /// per-call allocation.
            pub fn fast_recognize_in(
                &self,
                input: &[u8],
                arena: &::mpl::fast::bumpalo::Bump,
            ) -> bool {
                #mod_ident::recognize_in(input, arena)
            }
        }
    }
}