jetro-core 0.3.0

//! High-performance bytecode VM for v2 Jetro expressions.
//!
//! # Architecture
//!
//! ```text
//!  String expression
//!        │  parser::parse()
//!        ▼
//!     Expr (AST)
//!        │  Compiler::compile()
//!        ▼
//!     Program              ← flat Arc<[Opcode]>  (cached: compile_cache)
//!        │  VM::execute()
//!        ▼
//!      Val                 ← result              (structural: resolution_cache)
//! ```
//!
//! # Optimisations over the tree-walker
//!
//! 1. **Compile cache** — parse + compile once per unique expression string.
//! 2. **Val type** — `Arc`-wrapped compound nodes; every clone is O(1).
//! 3. **BuiltinMethod enum** — O(1) method dispatch (jump-table vs string hash).
//! 4. **Pre-compiled sub-programs** — lambda/arg bodies compiled to `Arc<Program>`
//!    once at compile time; never re-compiled per call.
//! 5. **Resolution cache** — structural programs (`$.a.b[0]`) cache their
//!    pointer path after the first traversal; subsequent calls skip traversal.
//! 6. **Peephole pass 1 — RootChain** — `PushRoot + GetField+` fused into a
//!    single pointer-resolve opcode.
//! 7. **Peephole pass 2 — FilterCount** — `CallMethod(filter) +
//!    CallMethod(len/count)` fused; counts matches without materialising the
//!    intermediate filtered array.
//! 8. **Peephole pass 3 — ConstFold** — arithmetic on adjacent integer literals
//!    folded at compile time.
//! 9. **Stack machine** — iterative `exec()` loop; no per-opcode stack-frame
//!    overhead for simple navigation / arithmetic opcodes.

use std::{
    collections::{HashMap, VecDeque},
    collections::hash_map::DefaultHasher,
    hash::{Hash, Hasher},
    sync::Arc,
};
use indexmap::IndexMap;
use smallvec::SmallVec;

use crate::ast::*;
use super::eval::{
    Env, EvalError, Val,
    dispatch_method, eval,
};
use super::eval::util::{
    is_truthy, kind_matches, vals_eq, cmp_vals, val_to_key, val_to_string,
    add_vals, num_op, obj2,
};
use super::eval::methods::MethodRegistry;

macro_rules! pop {
    ($stack:expr) => {
        $stack.pop().ok_or_else(|| EvalError("stack underflow".into()))?
    };
}
macro_rules! err {
    ($($t:tt)*) => { Err(EvalError(format!($($t)*))) };
}

// ── BuiltinMethod ─────────────────────────────────────────────────────────────

/// Pre-resolved method identifier — eliminates string comparison at dispatch.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum BuiltinMethod {
    // Navigation / basics
    Len = 0, Keys, Values, Entries, ToPairs, FromPairs, Invert, Reverse, Type,
    ToString, ToJson, FromJson,
    // Aggregates
    Sum, Avg, Min, Max, Count, Any, All,
    GroupBy, CountBy, IndexBy,
    // Array ops
    Filter, Map, FlatMap, Sort, Unique, Flatten, Compact,
    Join, First, Last, Nth, Append, Prepend, Remove,
    Diff, Intersect, Union, Enumerate, Pairwise, Window, Chunk,
    TakeWhile, DropWhile, Accumulate, Partition, Zip, ZipLongest,
    // Object ops
    Pick, Omit, Merge, DeepMerge, Defaults, Rename,
    TransformKeys, TransformValues, FilterKeys, FilterValues, Pivot,
    // Path ops
    GetPath, SetPath, DelPath, DelPaths, HasPath, FlattenKeys, UnflattenKeys,
    // CSV
    ToCsv, ToTsv,
    // Null / predicate
    Or, Has, Missing, Includes, Set, Update,
    // String methods
    Upper, Lower, Capitalize, TitleCase, Trim, TrimLeft, TrimRight,
    Lines, Words, Chars, ToNumber, ToBool, ToBase64, FromBase64,
    UrlEncode, UrlDecode, HtmlEscape, HtmlUnescape,
    Repeat, PadLeft, PadRight, StartsWith, EndsWith,
    IndexOf, LastIndexOf, Replace, ReplaceAll, StripPrefix, StripSuffix,
    Slice, Split, Indent, Dedent, Matches, Scan,
    // Relational
    EquiJoin,
    // Sentinel for custom/unknown
    Unknown,
}

impl BuiltinMethod {
    pub fn from_name(name: &str) -> Self {
        match name {
            "len"            => Self::Len,
            "keys"           => Self::Keys,
            "values"         => Self::Values,
            "entries"        => Self::Entries,
            "to_pairs"|"toPairs" => Self::ToPairs,
            "from_pairs"|"fromPairs" => Self::FromPairs,
            "invert"         => Self::Invert,
            "reverse"        => Self::Reverse,
            "type"           => Self::Type,
            "to_string"|"toString" => Self::ToString,
            "to_json"|"toJson" => Self::ToJson,
            "from_json"|"fromJson" => Self::FromJson,
            "sum"            => Self::Sum,
            "avg"            => Self::Avg,
            "min"            => Self::Min,
            "max"            => Self::Max,
            "count"          => Self::Count,
            "any"            => Self::Any,
            "all"            => Self::All,
            "groupBy"|"group_by" => Self::GroupBy,
            "countBy"|"count_by" => Self::CountBy,
            "indexBy"|"index_by" => Self::IndexBy,
            "filter"         => Self::Filter,
            "map"            => Self::Map,
            "flatMap"|"flat_map" => Self::FlatMap,
            "sort"           => Self::Sort,
            "unique"|"distinct" => Self::Unique,
            "flatten"        => Self::Flatten,
            "compact"        => Self::Compact,
            "join"           => Self::Join,
            "equi_join"|"equiJoin" => Self::EquiJoin,
            "first"          => Self::First,
            "last"           => Self::Last,
            "nth"            => Self::Nth,
            "append"         => Self::Append,
            "prepend"        => Self::Prepend,
            "remove"         => Self::Remove,
            "diff"           => Self::Diff,
            "intersect"      => Self::Intersect,
            "union"          => Self::Union,
            "enumerate"      => Self::Enumerate,
            "pairwise"       => Self::Pairwise,
            "window"         => Self::Window,
            "chunk"|"batch"  => Self::Chunk,
            "takewhile"|"take_while" => Self::TakeWhile,
            "dropwhile"|"drop_while" => Self::DropWhile,
            "accumulate"     => Self::Accumulate,
            "partition"      => Self::Partition,
            "zip"            => Self::Zip,
            "zip_longest"|"zipLongest" => Self::ZipLongest,
            "pick"           => Self::Pick,
            "omit"           => Self::Omit,
            "merge"          => Self::Merge,
            "deep_merge"|"deepMerge" => Self::DeepMerge,
            "defaults"       => Self::Defaults,
            "rename"         => Self::Rename,
            "transform_keys"|"transformKeys" => Self::TransformKeys,
            "transform_values"|"transformValues" => Self::TransformValues,
            "filter_keys"|"filterKeys" => Self::FilterKeys,
            "filter_values"|"filterValues" => Self::FilterValues,
            "pivot"          => Self::Pivot,
            "get_path"|"getPath" => Self::GetPath,
            "set_path"|"setPath" => Self::SetPath,
            "del_path"|"delPath" => Self::DelPath,
            "del_paths"|"delPaths" => Self::DelPaths,
            "has_path"|"hasPath" => Self::HasPath,
            "flatten_keys"|"flattenKeys" => Self::FlattenKeys,
            "unflatten_keys"|"unflattenKeys" => Self::UnflattenKeys,
            "to_csv"|"toCsv" => Self::ToCsv,
            "to_tsv"|"toTsv" => Self::ToTsv,
            "or"             => Self::Or,
            "has"            => Self::Has,
            "missing"        => Self::Missing,
            "includes"|"contains" => Self::Includes,
            "set"            => Self::Set,
            "update"         => Self::Update,
            "upper"          => Self::Upper,
            "lower"          => Self::Lower,
            "capitalize"     => Self::Capitalize,
            "title_case"|"titleCase" => Self::TitleCase,
            "trim"           => Self::Trim,
            "trim_left"|"trimLeft"|"lstrip" => Self::TrimLeft,
            "trim_right"|"trimRight"|"rstrip" => Self::TrimRight,
            "lines"          => Self::Lines,
            "words"          => Self::Words,
            "chars"          => Self::Chars,
            "to_number"|"toNumber" => Self::ToNumber,
            "to_bool"|"toBool" => Self::ToBool,
            "to_base64"|"toBase64" => Self::ToBase64,
            "from_base64"|"fromBase64" => Self::FromBase64,
            "url_encode"|"urlEncode" => Self::UrlEncode,
            "url_decode"|"urlDecode" => Self::UrlDecode,
            "html_escape"|"htmlEscape" => Self::HtmlEscape,
            "html_unescape"|"htmlUnescape" => Self::HtmlUnescape,
            "repeat"         => Self::Repeat,
            "pad_left"|"padLeft" => Self::PadLeft,
            "pad_right"|"padRight" => Self::PadRight,
            "starts_with"|"startsWith" => Self::StartsWith,
            "ends_with"|"endsWith" => Self::EndsWith,
            "index_of"|"indexOf" => Self::IndexOf,
            "last_index_of"|"lastIndexOf" => Self::LastIndexOf,
            "replace"        => Self::Replace,
            "replace_all"|"replaceAll" => Self::ReplaceAll,
            "strip_prefix"|"stripPrefix" => Self::StripPrefix,
            "strip_suffix"|"stripSuffix" => Self::StripSuffix,
            "slice"          => Self::Slice,
            "split"          => Self::Split,
            "indent"         => Self::Indent,
            "dedent"         => Self::Dedent,
            "matches"        => Self::Matches,
            "scan"           => Self::Scan,
            _                => Self::Unknown,
        }
    }

    /// True for methods that receive a sub-program to run per item.
    fn is_lambda_method(self) -> bool {
        matches!(self,
            Self::Filter | Self::Map | Self::FlatMap | Self::Sort |
            Self::Any | Self::All | Self::Count | Self::GroupBy |
            Self::CountBy | Self::IndexBy | Self::TakeWhile |
            Self::DropWhile | Self::Accumulate | Self::Partition |
            Self::TransformKeys | Self::TransformValues |
            Self::FilterKeys | Self::FilterValues | Self::Pivot | Self::Update
        )
    }
}

// ── Compiled sub-structures ───────────────────────────────────────────────────

/// A compiled method call stored inside `Opcode::CallMethod`.
#[derive(Debug, Clone)]
pub struct CompiledCall {
    pub method:   BuiltinMethod,
    pub name:     Arc<str>,
    /// Compiled lambda/expression sub-programs (one per arg, in order).
    pub sub_progs: Arc<[Arc<Program>]>,
    /// Original AST args kept for non-lambda dispatch fallback.
    pub orig_args: Arc<[Arg]>,
}

/// A compiled object field for `Opcode::MakeObj`.
#[derive(Debug, Clone)]
pub enum CompiledObjEntry {
    Short(Arc<str>),
    Kv     { key: Arc<str>, prog: Arc<Program>, optional: bool, cond: Option<Arc<Program>> },
    Dynamic { key: Arc<Program>, val: Arc<Program> },
    Spread(Arc<Program>),
    SpreadDeep(Arc<Program>),
}

/// A compiled f-string interpolation part.
#[derive(Debug, Clone)]
pub enum CompiledFSPart {
    Lit(Arc<str>),
    Interp { prog: Arc<Program>, fmt: Option<FmtSpec> },
}

/// Compiled bind-object destructure spec.
#[derive(Debug, Clone)]
pub struct BindObjSpec {
    pub fields: Arc<[Arc<str>]>,
    pub rest:   Option<Arc<str>>,
}

/// Compiled comprehension spec.
#[derive(Debug, Clone)]
pub struct CompSpec {
    pub expr: Arc<Program>,
    pub vars: Arc<[Arc<str>]>,
    pub iter: Arc<Program>,
    pub cond: Option<Arc<Program>>,
}

#[derive(Debug, Clone)]
pub struct DictCompSpec {
    pub key:  Arc<Program>,
    pub val:  Arc<Program>,
    pub vars: Arc<[Arc<str>]>,
    pub iter: Arc<Program>,
    pub cond: Option<Arc<Program>>,
}

// ── Opcode ────────────────────────────────────────────────────────────────────

#[derive(Debug, Clone)]
pub enum Opcode {
    // ── Literals ─────────────────────────────────────────────────────────────
    PushNull,
    PushBool(bool),
    PushInt(i64),
    PushFloat(f64),
    PushStr(Arc<str>),

    // ── Context ───────────────────────────────────────────────────────────────
    PushRoot,
    PushCurrent,

    // ── Navigation ────────────────────────────────────────────────────────────
    GetField(Arc<str>),
    GetIndex(i64),
    GetSlice(Option<i64>, Option<i64>),
    DynIndex(Arc<Program>),
    OptField(Arc<str>),
    Descendant(Arc<str>),
    DescendAll,
    InlineFilter(Arc<Program>),
    Quantifier(QuantifierKind),

    // ── Peephole fusions ──────────────────────────────────────────────────────
    /// PushRoot + GetField* fused — resolves chain via pointer arithmetic.
    RootChain(Arc<[Arc<str>]>),
    /// filter(pred) + len/count fused — counts matches without temp array.
    FilterCount(Arc<Program>),
    /// filter(pred) + First quantifier fused — early-exit on first match.
    FindFirst(Arc<Program>),
    /// filter(pred) + One quantifier fused — early-exit at 2nd match (error).
    FindOne(Arc<Program>),
    /// filter(pred) + map(f) fused — single pass, no intermediate array.
    FilterMap { pred: Arc<Program>, map: Arc<Program> },
    /// filter(p1) + filter(p2) fused — single pass, both predicates.
    FilterFilter { p1: Arc<Program>, p2: Arc<Program> },
    /// map(f1) + map(f2) fused — single pass, composed.
    MapMap { f1: Arc<Program>, f2: Arc<Program> },
    /// Fused `map(f).sum()` — evaluates `f` per item, accumulates numeric sum.
    MapSum(Arc<Program>),
    /// Fused `map(f).avg()` — evaluates `f` per item, computes mean as float.
    MapAvg(Arc<Program>),
    /// Fused `sort()` + `[0:n]` — partial-sort smallest N using BinaryHeap.
    /// `asc=true` → smallest N; `asc=false` → largest N.
    TopN { n: usize, asc: bool },
    /// Fused `map(f).flatten()` — single-pass concat of mapped arrays.
    MapFlatten(Arc<Program>),
    /// Fused `filter(p).take_while(q)` — scan while both predicates hold.
    FilterTakeWhile { pred: Arc<Program>, stop: Arc<Program> },
    /// Fused `filter(p).drop_while(q)` — skip leading matches of q on
    /// p-filtered elements.
    FilterDropWhile { pred: Arc<Program>, drop: Arc<Program> },
    /// Fused `map(f).unique()` — apply f and dedup by resulting value.
    MapUnique(Arc<Program>),
    /// Fused equi-join: TOS is lhs array; `rhs` program evaluates
    /// to rhs array; join by (lhs_key, rhs_key) string field names.
    /// Produces array of merged objects (rhs wins on collision).
    EquiJoin { rhs: Arc<Program>, lhs_key: Arc<str>, rhs_key: Arc<str> },

    // ── Ident lookup (var, then current field) ────────────────────────────────
    LoadIdent(Arc<str>),

    // ── Binary / unary ops ────────────────────────────────────────────────────
    Add, Sub, Mul, Div, Mod,
    Eq, Neq, Lt, Lte, Gt, Gte, Fuzzy, Not, Neg,

    // ── Type cast (`as T`) ───────────────────────────────────────────────────
    CastOp(super::ast::CastType),

    // ── Short-circuit ops (embed rhs as sub-program) ──────────────────────────
    AndOp(Arc<Program>),
    OrOp(Arc<Program>),
    CoalesceOp(Arc<Program>),

    // ── Method calls ─────────────────────────────────────────────────────────
    CallMethod(Arc<CompiledCall>),
    CallOptMethod(Arc<CompiledCall>),

    // ── Construction ─────────────────────────────────────────────────────────
    MakeObj(Arc<[CompiledObjEntry]>),
    MakeArr(Arc<[Arc<Program>]>),

    // ── F-string ─────────────────────────────────────────────────────────────
    FString(Arc<[CompiledFSPart]>),

    // ── Kind check ───────────────────────────────────────────────────────────
    KindCheck { ty: KindType, negate: bool },

    // ── Pipeline helpers ──────────────────────────────────────────────────────
    /// Pop TOS → env.current, then push it back (pass-through with context update).
    SetCurrent,
    /// TOS → env var by name, TOS remains (for `->` bind).
    BindVar(Arc<str>),
    /// Pop TOS → env var (for `let` init).
    StoreVar(Arc<str>),
    /// Object destructure bind: TOS obj → multiple vars.
    BindObjDestructure(Arc<BindObjSpec>),
    /// Array destructure bind: TOS arr → multiple vars.
    BindArrDestructure(Arc<[Arc<str>]>),

    // ── Complex (recursive sub-programs) ─────────────────────────────────────
    LetExpr { name: Arc<str>, body: Arc<Program> },
    ListComp(Arc<CompSpec>),
    DictComp(Arc<DictCompSpec>),
    SetComp(Arc<CompSpec>),

    // ── Resolution cache fast-path ────────────────────────────────────────────
    GetPointer(Arc<str>),

    // ── Patch block (delegates to tree-walker eval) ──────────────────────────
    PatchEval(Arc<super::ast::Expr>),
}

// ── Program ───────────────────────────────────────────────────────────────────

/// A compiled, immutable v2 program.  Cheap to clone (`Arc` internals).
#[derive(Debug, Clone)]
pub struct Program {
    pub ops:          Arc<[Opcode]>,
    pub source:       Arc<str>,
    pub id:           u64,
    /// True when the program contains only structural navigation opcodes
    /// (eligible for resolution caching).
    pub is_structural: bool,
}

impl Program {
    fn new(ops: Vec<Opcode>, source: &str) -> Self {
        let id = hash_str(source);
        let is_structural = ops.iter().all(|op| matches!(op,
            Opcode::PushRoot | Opcode::PushCurrent |
            Opcode::GetField(_) | Opcode::GetIndex(_) |
            Opcode::GetSlice(..) | Opcode::OptField(_) |
            Opcode::RootChain(_) | Opcode::GetPointer(_)
        ));
        Self { ops: ops.into(), source: source.into(), id, is_structural }
    }
}

// ── Variable context (compile-time) ──────────────────────────────────────────

#[derive(Clone, Default)]
struct VarCtx {
    known: SmallVec<[Arc<str>; 4]>,
}

impl VarCtx {
    fn with_var(&self, name: &str) -> Self {
        let mut v = self.clone();
        if !v.known.iter().any(|k| k.as_ref() == name) {
            v.known.push(Arc::from(name));
        }
        v
    }
    fn with_vars(&self, names: &[String]) -> Self {
        let mut v = self.clone();
        for n in names {
            if !v.known.iter().any(|k| k.as_ref() == n.as_str()) {
                v.known.push(Arc::from(n.as_str()));
            }
        }
        v
    }
    fn has(&self, name: &str) -> bool {
        self.known.iter().any(|k| k.as_ref() == name)
    }
}

// ── Compiler ─────────────────────────────────────────────────────────────────

pub struct Compiler;

impl Compiler {
    pub fn compile(expr: &Expr, source: &str) -> Program {
        let mut e = expr.clone();
        Self::reorder_and_operands(&mut e);
        let ctx = VarCtx::default();
        let ops = Self::optimize(Self::emit(&e, &ctx));
        let prog = Program::new(ops, source);
        // Post-pass: canonicalise identical sub-programs.
        let deduped = super::analysis::dedup_subprograms(&prog);
        Program {
            ops:           deduped.ops.clone(),
            source:        prog.source,
            id:            prog.id,
            is_structural: prog.is_structural,
        }
    }

    /// AST rewrite: for each `a and b`, if `b` is more selective than `a`,
    /// swap operands so the cheaper/selective predicate runs first.  Safe
    /// because `and` is commutative on pure, side-effect-free expressions.
    fn reorder_and_operands(expr: &mut Expr) {
        use super::analysis::selectivity_score;
        match expr {
            Expr::BinOp(l, op, r) if *op == BinOp::And => {
                Self::reorder_and_operands(l);
                Self::reorder_and_operands(r);
                if selectivity_score(r) < selectivity_score(l) {
                    std::mem::swap(l, r);
                }
            }
            Expr::BinOp(l, _, r) => {
                Self::reorder_and_operands(l);
                Self::reorder_and_operands(r);
            }
            Expr::UnaryNeg(e) | Expr::Not(e) | Expr::Kind { expr: e, .. } =>
                Self::reorder_and_operands(e),
            Expr::Coalesce(l, r) => {
                Self::reorder_and_operands(l);
                Self::reorder_and_operands(r);
            }
            Expr::Chain(base, steps) => {
                Self::reorder_and_operands(base);
                for s in steps {
                    match s {
                        super::ast::Step::DynIndex(e) | super::ast::Step::InlineFilter(e) =>
                            Self::reorder_and_operands(e),
                        super::ast::Step::Method(_, args) | super::ast::Step::OptMethod(_, args) =>
                            for a in args { match a {
                                super::ast::Arg::Pos(e) | super::ast::Arg::Named(_, e) =>
                                    Self::reorder_and_operands(e),
                            } },
                        _ => {}
                    }
                }
            }
            Expr::Let { init, body, .. } => {
                Self::reorder_and_operands(init);
                Self::reorder_and_operands(body);
            }
            Expr::Pipeline { base, steps } => {
                Self::reorder_and_operands(base);
                for s in steps {
                    if let super::ast::PipeStep::Forward(e) = s {
                        Self::reorder_and_operands(e);
                    }
                }
            }
            Expr::Object(fields) => for f in fields { match f {
                super::ast::ObjField::Kv { val, .. } => Self::reorder_and_operands(val),
                super::ast::ObjField::Dynamic { key, val } => {
                    Self::reorder_and_operands(key);
                    Self::reorder_and_operands(val);
                }
                super::ast::ObjField::Spread(e) => Self::reorder_and_operands(e),
                _ => {}
            } },
            Expr::Array(elems) => for e in elems { match e {
                super::ast::ArrayElem::Expr(e) | super::ast::ArrayElem::Spread(e) =>
                    Self::reorder_and_operands(e),
            } },
            Expr::ListComp { expr, iter, cond, .. }
            | Expr::SetComp  { expr, iter, cond, .. }
            | Expr::GenComp  { expr, iter, cond, .. } => {
                Self::reorder_and_operands(expr);
                Self::reorder_and_operands(iter);
                if let Some(c) = cond { Self::reorder_and_operands(c); }
            }
            Expr::DictComp { key, val, iter, cond, .. } => {
                Self::reorder_and_operands(key);
                Self::reorder_and_operands(val);
                Self::reorder_and_operands(iter);
                if let Some(c) = cond { Self::reorder_and_operands(c); }
            }
            Expr::Lambda { body, .. } => Self::reorder_and_operands(body),
            Expr::GlobalCall { args, .. } => for a in args { match a {
                super::ast::Arg::Pos(e) | super::ast::Arg::Named(_, e) =>
                    Self::reorder_and_operands(e),
            } },
            _ => {}
        }
    }

    pub fn compile_str(input: &str) -> Result<Program, EvalError> {
        let expr = super::parser::parse(input)
            .map_err(|e| EvalError(e.to_string()))?;
        Ok(Self::compile(&expr, input))
    }

    /// Compile with explicit pass configuration.  Cached by callers
    /// under `(config.hash(), expr)`.
    pub fn compile_str_with_config(input: &str, config: PassConfig) -> Result<Program, EvalError> {
        let expr = super::parser::parse(input)
            .map_err(|e| EvalError(e.to_string()))?;
        let mut e = expr.clone();
        if config.reorder_and { Self::reorder_and_operands(&mut e); }
        let ctx = VarCtx::default();
        let ops = Self::optimize_with(Self::emit(&e, &ctx), config);
        let prog = Program::new(ops, input);
        if config.dedup_subprogs {
            let deduped = super::analysis::dedup_subprograms(&prog);
            Ok(Program {
                ops:           deduped.ops.clone(),
                source:        prog.source,
                id:            prog.id,
                is_structural: prog.is_structural,
            })
        } else {
            Ok(prog)
        }
    }

    // ── Peephole optimizer ────────────────────────────────────────────────────

    fn optimize(ops: Vec<Opcode>) -> Vec<Opcode> {
        Self::optimize_with(ops, PassConfig::default())
    }

    fn optimize_with(ops: Vec<Opcode>, cfg: PassConfig) -> Vec<Opcode> {
        let ops = if cfg.root_chain      { Self::pass_root_chain(ops) }      else { ops };
        let ops = if cfg.filter_count    { Self::pass_filter_count(ops) }    else { ops };
        let ops = if cfg.filter_fusion   { Self::pass_filter_fusion(ops) }   else { ops };
        let ops = if cfg.find_quantifier { Self::pass_find_quantifier(ops) } else { ops };
        let ops = if cfg.strength_reduce { Self::pass_strength_reduce(ops) } else { ops };
        let ops = if cfg.redundant_ops   { Self::pass_redundant_ops(ops) }   else { ops };
        let ops = if cfg.kind_check_fold { Self::pass_kind_check_fold(ops) } else { ops };
        let ops = if cfg.method_const    { Self::pass_method_const_fold(ops)} else { ops };
        let ops = if cfg.const_fold      { Self::pass_const_fold(ops) }      else { ops };
        let ops = if cfg.nullness        { Self::pass_nullness_opt_field(ops)} else { ops };
        let ops = if cfg.equi_join       { Self::pass_equi_join_fusion(ops) } else { ops };
        ops
    }

    /// Rewrite `CallMethod(equi_join, [rhs, PushStr(lk), PushStr(rk)])`
    /// to the fused `EquiJoin` opcode — removes runtime method dispatch
    /// and extracts string keys into the opcode so the executor can
    /// hash directly.
    fn pass_equi_join_fusion(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            if let Opcode::CallMethod(c) = &op {
                if c.method == BuiltinMethod::EquiJoin && c.sub_progs.len() == 3 {
                    let rhs = Arc::clone(&c.sub_progs[0]);
                    let lhs_key = const_str_program(&c.sub_progs[1]);
                    let rhs_key = const_str_program(&c.sub_progs[2]);
                    if let (Some(lk), Some(rk)) = (lhs_key, rhs_key) {
                        out.push(Opcode::EquiJoin { rhs, lhs_key: lk, rhs_key: rk });
                        continue;
                    }
                }
            }
            out.push(op);
        }
        out
    }

    /// Nullness-driven: when the preceding op provably leaves a non-null
    /// receiver on the stack, rewrite `OptField(k)` → `GetField(k)`.
    /// Conservative: only folds when the predecessor is a construction
    /// opcode (MakeObj / MakeArr / PushStr / RootChain / GetField).
    fn pass_nullness_opt_field(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            if let Opcode::OptField(k) = &op {
                // Only safe when receiver is provably a non-null object —
                // MakeObj is the only opcode that guarantees this without
                // a schema.  Other cases are handled by schema::specialize.
                let non_null = matches!(out.last(), Some(Opcode::MakeObj(_)));
                if non_null {
                    out.push(Opcode::GetField(k.clone()));
                    continue;
                }
            }
            out.push(op);
        }
        out
    }

    /// Fold built-in methods when receiver is a literal with known length/content:
    ///   PushStr(s) + .len()    → PushInt(utf8 char count)
    ///   PushStr(s) + .upper()  → PushStr(upper)
    ///   PushStr(s) + .lower()  → PushStr(lower)
    ///   PushStr(s) + .trim()   → PushStr(trim)
    ///   MakeArr(n elems) + .len()  → PushInt(n)  (only for non-spread arrays)
    fn pass_method_const_fold(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            if let Opcode::CallMethod(c) = &op {
                if c.sub_progs.is_empty() {
                    match (out.last(), c.method) {
                        (Some(Opcode::PushStr(s)), BuiltinMethod::Len) => {
                            let n = s.chars().count() as i64;
                            out.pop();
                            out.push(Opcode::PushInt(n));
                            continue;
                        }
                        (Some(Opcode::PushStr(s)), BuiltinMethod::Upper) => {
                            let u: Arc<str> = Arc::from(s.to_uppercase());
                            out.pop();
                            out.push(Opcode::PushStr(u));
                            continue;
                        }
                        (Some(Opcode::PushStr(s)), BuiltinMethod::Lower) => {
                            let u: Arc<str> = Arc::from(s.to_lowercase());
                            out.pop();
                            out.push(Opcode::PushStr(u));
                            continue;
                        }
                        (Some(Opcode::PushStr(s)), BuiltinMethod::Trim) => {
                            let u: Arc<str> = Arc::from(s.trim());
                            out.pop();
                            out.push(Opcode::PushStr(u));
                            continue;
                        }
                        (Some(Opcode::MakeArr(progs)), BuiltinMethod::Len) => {
                            let n = progs.len() as i64;
                            out.pop();
                            out.push(Opcode::PushInt(n));
                            continue;
                        }
                        _ => {}
                    }
                }
            }
            out.push(op);
        }
        out
    }

    /// Fold `KindCheck` when its input type is a literal push:
    ///   PushInt(n)  + KindCheck{number, neg} → PushBool(!neg)
    ///   PushStr(_)  + KindCheck{string, neg} → PushBool(!neg)
    ///   PushNull    + KindCheck{null, neg}   → PushBool(!neg)
    ///   PushBool(_) + KindCheck{bool, neg}   → PushBool(!neg)
    ///   mismatches fold to opposite.
    fn pass_kind_check_fold(ops: Vec<Opcode>) -> Vec<Opcode> {
        use super::analysis::{fold_kind_check, VType};
        let mut out = Vec::with_capacity(ops.len());
        for op in ops {
            if let Opcode::KindCheck { ty, negate } = &op {
                let prev_ty: Option<VType> = match out.last() {
                    Some(Opcode::PushNull)     => Some(VType::Null),
                    Some(Opcode::PushBool(_))  => Some(VType::Bool),
                    Some(Opcode::PushInt(_))   => Some(VType::Int),
                    Some(Opcode::PushFloat(_)) => Some(VType::Float),
                    Some(Opcode::PushStr(_))   => Some(VType::Str),
                    Some(Opcode::MakeArr(_))   => Some(VType::Arr),
                    Some(Opcode::MakeObj(_))   => Some(VType::Obj),
                    _ => None,
                };
                if let Some(vt) = prev_ty {
                    if let Some(b) = fold_kind_check(vt, *ty, *negate) {
                        out.pop();
                        out.push(Opcode::PushBool(b));
                        continue;
                    }
                }
            }
            out.push(op);
        }
        out
    }

    /// Fuse adjacent method calls into single-pass fused opcodes:
    ///   filter(p) + map(f)     → FilterMap
    ///   filter(p1) + filter(p2)→ FilterFilter
    ///   map(f1) + map(f2)      → MapMap
    fn pass_filter_fusion(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            if let (Opcode::CallMethod(b), Some(Opcode::CallMethod(a))) = (&op, out.last()) {
                // Two-arg fusions (both have sub_progs)
                if a.sub_progs.len() >= 1 && b.sub_progs.len() >= 1 {
                    let (am, bm) = (a.method, b.method);
                    let p1 = Arc::clone(&a.sub_progs[0]);
                    let p2 = Arc::clone(&b.sub_progs[0]);
                    let fused = match (am, bm) {
                        (BuiltinMethod::Filter, BuiltinMethod::Map) =>
                            Some(Opcode::FilterMap { pred: p1, map: p2 }),
                        (BuiltinMethod::Filter, BuiltinMethod::Filter) =>
                            Some(Opcode::FilterFilter { p1, p2 }),
                        (BuiltinMethod::Map, BuiltinMethod::Map) =>
                            Some(Opcode::MapMap { f1: p1, f2: p2 }),
                        _ => None,
                    };
                    if let Some(f) = fused {
                        out.pop();
                        out.push(f);
                        continue;
                    }
                }
                // map(f) + sum()/avg()/flatten()
                if a.method == BuiltinMethod::Map && a.sub_progs.len() >= 1
                   && b.sub_progs.is_empty() {
                    let f = Arc::clone(&a.sub_progs[0]);
                    let fused = match b.method {
                        BuiltinMethod::Sum => Some(Opcode::MapSum(f)),
                        BuiltinMethod::Avg => Some(Opcode::MapAvg(f)),
                        BuiltinMethod::Flatten => Some(Opcode::MapFlatten(f)),
                        _ => None,
                    };
                    if let Some(o) = fused {
                        out.pop();
                        out.push(o);
                        continue;
                    }
                }
                // filter(p) + take_while(q) → FilterTakeWhile
                if a.method == BuiltinMethod::Filter && a.sub_progs.len() >= 1
                   && b.method == BuiltinMethod::TakeWhile && b.sub_progs.len() >= 1 {
                    let pred = Arc::clone(&a.sub_progs[0]);
                    let stop = Arc::clone(&b.sub_progs[0]);
                    out.pop();
                    out.push(Opcode::FilterTakeWhile { pred, stop });
                    continue;
                }
                // filter(p) + drop_while(q) → FilterDropWhile
                if a.method == BuiltinMethod::Filter && a.sub_progs.len() >= 1
                   && b.method == BuiltinMethod::DropWhile && b.sub_progs.len() >= 1 {
                    let pred = Arc::clone(&a.sub_progs[0]);
                    let drop = Arc::clone(&b.sub_progs[0]);
                    out.pop();
                    out.push(Opcode::FilterDropWhile { pred, drop });
                    continue;
                }
                // map(f) + unique() → MapUnique
                if a.method == BuiltinMethod::Map && a.sub_progs.len() >= 1
                   && b.method == BuiltinMethod::Unique && b.sub_progs.is_empty() {
                    let f = Arc::clone(&a.sub_progs[0]);
                    out.pop();
                    out.push(Opcode::MapUnique(f));
                    continue;
                }
            }
            out.push(op);
        }
        out
    }

    /// Replace expensive ops with cheaper equivalents:
    ///   sort() + first()    → min()
    ///   sort() + last()     → max()
    ///   sort() + [0]        → min()
    ///   sort() + [-1]       → max()
    ///   reverse() + first() → last()
    ///   reverse() + last()  → first()
    fn pass_strength_reduce(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            // Pattern: [..., prev_method_call, current_op]
            if let Some(Opcode::CallMethod(prev)) = out.last().cloned() {
                let replaced = match (prev.method, &op) {
                    // sort() + [0] → min()
                    (BuiltinMethod::Sort, Opcode::GetIndex(0)) if prev.sub_progs.is_empty() =>
                        Some(make_noarg_call(BuiltinMethod::Min, "min")),
                    // sort() + [-1] → max()
                    (BuiltinMethod::Sort, Opcode::GetIndex(-1)) if prev.sub_progs.is_empty() =>
                        Some(make_noarg_call(BuiltinMethod::Max, "max")),
                    // sort() + first() → min()
                    (BuiltinMethod::Sort, Opcode::CallMethod(next))
                        if prev.sub_progs.is_empty() && next.method == BuiltinMethod::First =>
                        Some(make_noarg_call(BuiltinMethod::Min, "min")),
                    // sort() + last() → max()
                    (BuiltinMethod::Sort, Opcode::CallMethod(next))
                        if prev.sub_progs.is_empty() && next.method == BuiltinMethod::Last =>
                        Some(make_noarg_call(BuiltinMethod::Max, "max")),
                    // reverse() + first() → last()
                    (BuiltinMethod::Reverse, Opcode::CallMethod(next))
                        if next.method == BuiltinMethod::First =>
                        Some(make_noarg_call(BuiltinMethod::Last, "last")),
                    // reverse() + last() → first()
                    (BuiltinMethod::Reverse, Opcode::CallMethod(next))
                        if next.method == BuiltinMethod::Last =>
                        Some(make_noarg_call(BuiltinMethod::First, "first")),
                    // sort() + [0:n] → TopN(n, asc=true)
                    (BuiltinMethod::Sort, Opcode::GetSlice(from, Some(to)))
                        if prev.sub_progs.is_empty()
                           && (from.is_none() || *from == Some(0))
                           && *to > 0 =>
                        Some(Opcode::TopN { n: *to as usize, asc: true }),
                    _ => None,
                };
                if let Some(rep) = replaced {
                    out.pop();
                    out.push(rep);
                    continue;
                }
            }
            out.push(op);
        }
        out
    }

    /// Fuse `PushRoot + GetField(k1) + GetField(k2) ...` → `RootChain([k1,k2,...])`.
    fn pass_root_chain(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out = Vec::with_capacity(ops.len());
        let mut it = ops.into_iter().peekable();
        while let Some(op) = it.next() {
            if matches!(op, Opcode::PushRoot) {
                let mut chain: Vec<Arc<str>> = Vec::new();
                while let Some(Opcode::GetField(_)) = it.peek() {
                    if let Some(Opcode::GetField(k)) = it.next() {
                        chain.push(k);
                    }
                }
                if chain.is_empty() {
                    out.push(Opcode::PushRoot);
                } else {
                    out.push(Opcode::RootChain(chain.into()));
                }
            } else {
                out.push(op);
            }
        }
        out
    }

    /// Fuse `CallMethod(filter/pred) + CallMethod(len/count)` → `FilterCount(pred)`.
    fn pass_filter_count(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out = Vec::with_capacity(ops.len());
        let mut it = ops.into_iter().peekable();
        while let Some(op) = it.next() {
            if let Opcode::CallMethod(ref call) = op {
                if call.method == BuiltinMethod::Filter {
                    let is_len = matches!(it.peek(),
                        Some(Opcode::CallMethod(c))
                            if c.method == BuiltinMethod::Len || c.method == BuiltinMethod::Count
                    );
                    if is_len && !call.sub_progs.is_empty() {
                        let pred = Arc::clone(&call.sub_progs[0]);
                        it.next(); // consume Len/Count
                        out.push(Opcode::FilterCount(pred));
                        continue;
                    }
                }
            }
            out.push(op);
        }
        out
    }

    /// Fuse `InlineFilter(pred) + Quantifier(First/One)` → `FindFirst/FindOne(pred)`.
    /// Also fuses `CallMethod(Filter, pred) + Quantifier(...)` for explicit `.filter()`.
    fn pass_find_quantifier(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out = Vec::with_capacity(ops.len());
        let mut it = ops.into_iter().peekable();
        while let Some(op) = it.next() {
            let pred_opt: Option<Arc<Program>> = match &op {
                Opcode::InlineFilter(p) => Some(Arc::clone(p)),
                Opcode::CallMethod(c) if c.method == BuiltinMethod::Filter && !c.sub_progs.is_empty()
                    => Some(Arc::clone(&c.sub_progs[0])),
                _ => None,
            };
            if let Some(pred) = pred_opt {
                match it.peek() {
                    Some(Opcode::Quantifier(QuantifierKind::First)) => {
                        it.next();
                        out.push(Opcode::FindFirst(pred));
                        continue;
                    }
                    Some(Opcode::Quantifier(QuantifierKind::One)) => {
                        it.next();
                        out.push(Opcode::FindOne(pred));
                        continue;
                    }
                    _ => {}
                }
            }
            out.push(op);
        }
        out
    }

    /// Eliminate redundant adjacent method calls:
    ///   reverse() + reverse()         → identity (both dropped)
    ///   unique() + unique()           → unique()
    ///   compact() + compact()         → compact()
    ///   sort() + sort(k)              → sort(k)      (later sort wins on same-array)
    ///   sort(k) + sort(k)             → sort(k)
    ///   Quantifier + Quantifier       → second only  (first wrap is scalar anyway)
    fn pass_redundant_ops(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out: Vec<Opcode> = Vec::with_capacity(ops.len());
        for op in ops {
            match (&op, out.last()) {
                // reverse + reverse: drop both
                (Opcode::CallMethod(b), Some(Opcode::CallMethod(a)))
                    if a.method == BuiltinMethod::Reverse && b.method == BuiltinMethod::Reverse =>
                {
                    out.pop();
                    continue;
                }
                // idempotent method pairs: keep second only (drop first)
                (Opcode::CallMethod(b), Some(Opcode::CallMethod(a)))
                    if a.method == b.method && matches!(a.method,
                        BuiltinMethod::Unique | BuiltinMethod::Compact)
                        && a.sub_progs.is_empty() && b.sub_progs.is_empty() =>
                {
                    out.pop();
                    out.push(op);
                    continue;
                }
                // sort + sort(_): later sort wins, drop the first
                (Opcode::CallMethod(b), Some(Opcode::CallMethod(a)))
                    if a.method == BuiltinMethod::Sort && b.method == BuiltinMethod::Sort =>
                {
                    out.pop();
                    out.push(op);
                    continue;
                }
                // Quantifier + Quantifier: second wins (first unwraps scalar,
                // second is no-op on scalar — but keeping second preserves error
                // semantics of `!`). Drop first.
                (Opcode::Quantifier(_), Some(Opcode::Quantifier(_))) => {
                    out.pop();
                    out.push(op);
                    continue;
                }
                // reverse + last → first (strength reduction fallback after sort)
                // already handled in pass_strength_reduce
                // Not + Not: double negation → drop both
                (Opcode::Not, Some(Opcode::Not)) => {
                    out.pop();
                    continue;
                }
                // Neg + Neg: --x → x
                (Opcode::Neg, Some(Opcode::Neg)) => {
                    out.pop();
                    continue;
                }
                _ => {}
            }
            out.push(op);
        }
        out
    }

    /// Constant-fold adjacent integer arithmetic + bool short-circuits.
    fn pass_const_fold(ops: Vec<Opcode>) -> Vec<Opcode> {
        let mut out = Vec::with_capacity(ops.len());
        let mut i = 0;
        while i < ops.len() {
            // 2-op bool short-circuit folds:
            //   PushBool(false) + AndOp(_)  → PushBool(false)
            //   PushBool(true)  + OrOp(_)   → PushBool(true)
            if i + 1 < ops.len() {
                let folded = match (&ops[i], &ops[i+1]) {
                    (Opcode::PushBool(false), Opcode::AndOp(_)) =>
                        Some(Opcode::PushBool(false)),
                    (Opcode::PushBool(true),  Opcode::OrOp(_)) =>
                        Some(Opcode::PushBool(true)),
                    _ => None,
                };
                if let Some(folded) = folded {
                    out.push(folded);
                    i += 2;
                    continue;
                }
            }
            // 2-op unary folds
            if i + 1 < ops.len() {
                let folded = match (&ops[i], &ops[i+1]) {
                    (Opcode::PushBool(b), Opcode::Not) =>
                        Some(Opcode::PushBool(!b)),
                    (Opcode::PushInt(n), Opcode::Neg) =>
                        Some(Opcode::PushInt(-n)),
                    (Opcode::PushFloat(f), Opcode::Neg) =>
                        Some(Opcode::PushFloat(-f)),
                    _ => None,
                };
                if let Some(folded) = folded {
                    out.push(folded);
                    i += 2;
                    continue;
                }
            }
            // 3-op arithmetic + comparison folds
            if i + 2 < ops.len() {
                let folded = match (&ops[i], &ops[i+1], &ops[i+2]) {
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Add) =>
                        Some(Opcode::PushInt(a + b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Sub) =>
                        Some(Opcode::PushInt(a - b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Mul) =>
                        Some(Opcode::PushInt(a * b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Mod) if *b != 0 =>
                        Some(Opcode::PushInt(a % b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Div) if *b != 0 =>
                        Some(Opcode::PushFloat(*a as f64 / *b as f64)),
                    (Opcode::PushFloat(a), Opcode::PushFloat(b), Opcode::Add) =>
                        Some(Opcode::PushFloat(a + b)),
                    (Opcode::PushFloat(a), Opcode::PushFloat(b), Opcode::Sub) =>
                        Some(Opcode::PushFloat(a - b)),
                    (Opcode::PushFloat(a), Opcode::PushFloat(b), Opcode::Mul) =>
                        Some(Opcode::PushFloat(a * b)),
                    (Opcode::PushFloat(a), Opcode::PushFloat(b), Opcode::Div) if *b != 0.0 =>
                        Some(Opcode::PushFloat(a / b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Eq) =>
                        Some(Opcode::PushBool(a == b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Neq) =>
                        Some(Opcode::PushBool(a != b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Lt) =>
                        Some(Opcode::PushBool(a < b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Lte) =>
                        Some(Opcode::PushBool(a <= b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Gt) =>
                        Some(Opcode::PushBool(a > b)),
                    (Opcode::PushInt(a), Opcode::PushInt(b), Opcode::Gte) =>
                        Some(Opcode::PushBool(a >= b)),
                    (Opcode::PushStr(a), Opcode::PushStr(b), Opcode::Eq) =>
                        Some(Opcode::PushBool(a == b)),
                    (Opcode::PushStr(a), Opcode::PushStr(b), Opcode::Neq) =>
                        Some(Opcode::PushBool(a != b)),
                    (Opcode::PushBool(a), Opcode::PushBool(b), Opcode::Eq) =>
                        Some(Opcode::PushBool(a == b)),
                    _ => None,
                };
                if let Some(folded) = folded {
                    out.push(folded);
                    i += 3;
                    continue;
                }
            }
            out.push(ops[i].clone());
            i += 1;
        }
        out
    }

    // ── Main emit ─────────────────────────────────────────────────────────────

    fn emit(expr: &Expr, ctx: &VarCtx) -> Vec<Opcode> {
        let mut ops = Vec::new();
        Self::emit_into(expr, ctx, &mut ops);
        ops
    }

    fn emit_into(expr: &Expr, ctx: &VarCtx, ops: &mut Vec<Opcode>) {
        match expr {
            Expr::Null    => ops.push(Opcode::PushNull),
            Expr::Bool(b) => ops.push(Opcode::PushBool(*b)),
            Expr::Int(n)  => ops.push(Opcode::PushInt(*n)),
            Expr::Float(f)=> ops.push(Opcode::PushFloat(*f)),
            Expr::Str(s)  => ops.push(Opcode::PushStr(Arc::from(s.as_str()))),
            Expr::Root    => ops.push(Opcode::PushRoot),
            Expr::Current => ops.push(Opcode::PushCurrent),

            Expr::FString(parts) => {
                let compiled: Vec<CompiledFSPart> = parts.iter().map(|p| match p {
                    FStringPart::Lit(s) => CompiledFSPart::Lit(Arc::from(s.as_str())),
                    FStringPart::Interp { expr, fmt } => CompiledFSPart::Interp {
                        prog: Arc::new(Self::compile_sub(expr, ctx)),
                        fmt: fmt.clone(),
                    },
                }).collect();
                ops.push(Opcode::FString(compiled.into()));
            }

            Expr::Ident(name) => ops.push(Opcode::LoadIdent(Arc::from(name.as_str()))),

            Expr::Chain(base, steps) => {
                Self::emit_into(base, ctx, ops);
                for step in steps {
                    Self::emit_step(step, ctx, ops);
                }
            }

            Expr::UnaryNeg(e) => {
                Self::emit_into(e, ctx, ops);
                ops.push(Opcode::Neg);
            }
            Expr::Not(e) => {
                Self::emit_into(e, ctx, ops);
                ops.push(Opcode::Not);
            }

            Expr::BinOp(l, op, r) => Self::emit_binop(l, *op, r, ctx, ops),

            Expr::Coalesce(lhs, rhs) => {
                Self::emit_into(lhs, ctx, ops);
                let rhs_prog = Arc::new(Self::compile_sub(rhs, ctx));
                ops.push(Opcode::CoalesceOp(rhs_prog));
            }

            Expr::Kind { expr, ty, negate } => {
                Self::emit_into(expr, ctx, ops);
                ops.push(Opcode::KindCheck { ty: *ty, negate: *negate });
            }

            Expr::Object(fields) => {
                let entries: Vec<CompiledObjEntry> = fields.iter().map(|f| match f {
                    ObjField::Short(name) =>
                        CompiledObjEntry::Short(Arc::from(name.as_str())),
                    ObjField::Kv { key, val, optional, cond } =>
                        CompiledObjEntry::Kv {
                            key: Arc::from(key.as_str()),
                            prog: Arc::new(Self::compile_sub(val, ctx)),
                            optional: *optional,
                            cond: cond.as_ref().map(|c| Arc::new(Self::compile_sub(c, ctx))),
                        },
                    ObjField::Dynamic { key, val } =>
                        CompiledObjEntry::Dynamic {
                            key: Arc::new(Self::compile_sub(key, ctx)),
                            val: Arc::new(Self::compile_sub(val, ctx)),
                        },
                    ObjField::Spread(e) =>
                        CompiledObjEntry::Spread(Arc::new(Self::compile_sub(e, ctx))),
                    ObjField::SpreadDeep(e) =>
                        CompiledObjEntry::SpreadDeep(Arc::new(Self::compile_sub(e, ctx))),
                }).collect();
                ops.push(Opcode::MakeObj(entries.into()));
            }

            Expr::Array(elems) => {
                // Compile each elem as a sub-program.
                // Spread elems are handled by a special marker.
                let _progs: Vec<Arc<Program>> = elems.iter().map(|e| match e {
                    ArrayElem::Expr(ex)   => Arc::new(Self::compile_sub(ex, ctx)),
                    // Spread: compile the inner expr with a spread marker opcode
                    ArrayElem::Spread(ex) => {
                        let mut sub = Self::emit(ex, ctx);
                        // Prepend a sentinel to distinguish spread from normal
                        sub.insert(0, Opcode::PushNull); // placeholder
                        // Actually, encode spread differently via a wrapper
                        Arc::new(Self::compile_array_spread(ex, ctx))
                    }
                }).collect();
                // Simpler: build a mixed elem list
                let progs = elems.iter().map(|e| match e {
                    ArrayElem::Expr(ex) => {
                        Arc::new(Self::compile_sub(ex, ctx))
                    }
                    ArrayElem::Spread(ex) => {
                        Arc::new(Self::compile_sub_spread(ex, ctx))
                    }
                }).collect::<Vec<_>>();
                ops.push(Opcode::MakeArr(progs.into()));
            }

            Expr::Pipeline { base, steps } => {
                Self::emit_pipeline(base, steps, ctx, ops);
            }

            Expr::ListComp { expr, vars, iter, cond } => {
                let inner_ctx = ctx.with_vars(vars);
                ops.push(Opcode::ListComp(Arc::new(CompSpec {
                    expr: Arc::new(Self::compile_sub(expr, &inner_ctx)),
                    vars: vars.iter().map(|v| Arc::from(v.as_str())).collect::<Vec<_>>().into(),
                    iter: Arc::new(Self::compile_sub(iter, ctx)),
                    cond: cond.as_ref().map(|c| Arc::new(Self::compile_sub(c, &inner_ctx))),
                })));
            }

            Expr::DictComp { key, val, vars, iter, cond } => {
                let inner_ctx = ctx.with_vars(vars);
                ops.push(Opcode::DictComp(Arc::new(DictCompSpec {
                    key:  Arc::new(Self::compile_sub(key, &inner_ctx)),
                    val:  Arc::new(Self::compile_sub(val, &inner_ctx)),
                    vars: vars.iter().map(|v| Arc::from(v.as_str())).collect::<Vec<_>>().into(),
                    iter: Arc::new(Self::compile_sub(iter, ctx)),
                    cond: cond.as_ref().map(|c| Arc::new(Self::compile_sub(c, &inner_ctx))),
                })));
            }

            Expr::SetComp { expr, vars, iter, cond } |
            Expr::GenComp { expr, vars, iter, cond } => {
                let inner_ctx = ctx.with_vars(vars);
                ops.push(Opcode::SetComp(Arc::new(CompSpec {
                    expr: Arc::new(Self::compile_sub(expr, &inner_ctx)),
                    vars: vars.iter().map(|v| Arc::from(v.as_str())).collect::<Vec<_>>().into(),
                    iter: Arc::new(Self::compile_sub(iter, ctx)),
                    cond: cond.as_ref().map(|c| Arc::new(Self::compile_sub(c, &inner_ctx))),
                })));
            }

            Expr::Lambda { .. } => {
                // Lambdas as standalone values are errors; they only appear as args
                ops.push(Opcode::PushNull);
            }

            Expr::Let { name, init, body } => {
                // Dead-let: if body never references `name` and init is pure,
                // drop the binding entirely and emit body only.
                if super::analysis::expr_is_pure(init)
                    && !super::analysis::expr_uses_ident(body, name) {
                    Self::emit_into(body, ctx, ops);
                } else {
                    Self::emit_into(init, ctx, ops);
                    let body_ctx = ctx.with_var(name);
                    let body_prog = Arc::new(Self::compile_sub(body, &body_ctx));
                    ops.push(Opcode::LetExpr { name: Arc::from(name.as_str()), body: body_prog });
                }
            }

            Expr::GlobalCall { name, args } => {
                // Compile as a sequence of sub-progs + a special dispatch
                let sub_progs: Vec<Arc<Program>> = args.iter().map(|a| match a {
                    Arg::Pos(e) | Arg::Named(_, e) => Arc::new(Self::compile_sub(e, ctx)),
                }).collect();
                let call = Arc::new(CompiledCall {
                    method:    BuiltinMethod::Unknown,
                    name:      Arc::from(name.as_str()),
                    sub_progs: sub_progs.into(),
                    orig_args: args.iter().cloned().collect::<Vec<_>>().into(),
                });
                ops.push(Opcode::PushRoot); // global calls need root pushed first
                ops.push(Opcode::CallMethod(call));
            }

            Expr::Cast { expr, ty } => {
                Self::emit_into(expr, ctx, ops);
                ops.push(Opcode::CastOp(*ty));
            }

            Expr::Patch { .. } => {
                // Patch block: structural transform with COW writes, conditional
                // leaves, DELETE sentinel.  Emit opaque opcode; the VM delegates
                // to the tree-walker at runtime (patch is rare enough that the
                // opcode compile pays no dividend here).
                ops.push(Opcode::PatchEval(Arc::new(expr.clone())));
            }

            Expr::DeleteMark => {
                // DELETE outside a patch-field value is a static error; the
                // tree-walker raises it at runtime, so emit a sentinel that
                // triggers the same path.
                ops.push(Opcode::PatchEval(Arc::new(Expr::DeleteMark)));
            }
        }
    }

    fn emit_step(step: &Step, ctx: &VarCtx, ops: &mut Vec<Opcode>) {
        match step {
            Step::Field(name)    => ops.push(Opcode::GetField(Arc::from(name.as_str()))),
            Step::OptField(name) => ops.push(Opcode::OptField(Arc::from(name.as_str()))),
            Step::Descendant(n)  => ops.push(Opcode::Descendant(Arc::from(n.as_str()))),
            Step::DescendAll     => ops.push(Opcode::DescendAll),
            Step::Index(i)       => ops.push(Opcode::GetIndex(*i)),
            Step::DynIndex(e)    => ops.push(Opcode::DynIndex(Arc::new(Self::compile_sub(e, ctx)))),
            Step::Slice(a, b)    => ops.push(Opcode::GetSlice(*a, *b)),
            Step::Method(name, method_args) => {
                let call = Self::compile_call(name, method_args, ctx);
                ops.push(Opcode::CallMethod(Arc::new(call)));
            }
            Step::OptMethod(name, method_args) => {
                let call = Self::compile_call(name, method_args, ctx);
                ops.push(Opcode::CallOptMethod(Arc::new(call)));
            }
            Step::InlineFilter(pred) => {
                ops.push(Opcode::InlineFilter(Arc::new(Self::compile_sub(pred, ctx))));
            }
            Step::Quantifier(k) => ops.push(Opcode::Quantifier(*k)),
        }
    }

    fn compile_call(name: &str, args: &[Arg], ctx: &VarCtx) -> CompiledCall {
        let method = BuiltinMethod::from_name(name);
        let sub_progs: Vec<Arc<Program>> = args.iter().map(|a| match a {
            Arg::Pos(e) | Arg::Named(_, e) => Arc::new(Self::compile_lambda_or_expr(e, ctx)),
        }).collect();
        CompiledCall {
            method,
            name: Arc::from(name),
            sub_progs: sub_progs.into(),
            orig_args: args.iter().cloned().collect::<Vec<_>>().into(),
        }
    }

    /// Compile an argument expression; for lambdas, the lambda param becomes a
    /// known var in the inner context so `Ident(param)` emits `LoadIdent`.
    fn compile_lambda_or_expr(expr: &Expr, ctx: &VarCtx) -> Program {
        match expr {
            Expr::Lambda { params, body } => {
                let inner = ctx.with_vars(params);
                Self::compile_sub(body, &inner)
            }
            other => Self::compile_sub(other, ctx),
        }
    }

    fn emit_binop(l: &Expr, op: BinOp, r: &Expr, ctx: &VarCtx, ops: &mut Vec<Opcode>) {
        match op {
            BinOp::And => {
                Self::emit_into(l, ctx, ops);
                let rhs_prog = Arc::new(Self::compile_sub(r, ctx));
                ops.push(Opcode::AndOp(rhs_prog));
            }
            BinOp::Or => {
                Self::emit_into(l, ctx, ops);
                let rhs_prog = Arc::new(Self::compile_sub(r, ctx));
                ops.push(Opcode::OrOp(rhs_prog));
            }
            BinOp::Add => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Add); }
            BinOp::Sub => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Sub); }
            BinOp::Mul => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Mul); }
            BinOp::Div => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Div); }
            BinOp::Mod => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Mod); }
            BinOp::Eq  => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Eq); }
            BinOp::Neq => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Neq); }
            BinOp::Lt  => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Lt); }
            BinOp::Lte => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Lte); }
            BinOp::Gt  => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Gt); }
            BinOp::Gte => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Gte); }
            BinOp::Fuzzy => { Self::emit_into(l, ctx, ops); Self::emit_into(r, ctx, ops); ops.push(Opcode::Fuzzy); }
        }
    }

    fn emit_pipeline(base: &Expr, steps: &[PipeStep], ctx: &VarCtx, ops: &mut Vec<Opcode>) {
        Self::emit_into(base, ctx, ops);
        let mut cur_ctx = ctx.clone();
        for step in steps {
            match step {
                PipeStep::Forward(rhs) => {
                    Self::emit_pipe_forward(rhs, &cur_ctx, ops);
                }
                PipeStep::Bind(target) => {
                    Self::emit_bind(target, &mut cur_ctx, ops);
                }
            }
        }
    }

    fn emit_pipe_forward(rhs: &Expr, ctx: &VarCtx, ops: &mut Vec<Opcode>) {
        match rhs {
            Expr::Ident(name) if !ctx.has(name) => {
                // No-arg method call on TOS
                let call = CompiledCall {
                    method:    BuiltinMethod::from_name(name),
                    name:      Arc::from(name.as_str()),
                    sub_progs: Arc::from(&[] as &[Arc<Program>]),
                    orig_args: Arc::from(&[] as &[Arg]),
                };
                ops.push(Opcode::CallMethod(Arc::new(call)));
            }
            Expr::Chain(base, steps) if !steps.is_empty() => {
                if let Expr::Ident(name) = base.as_ref() {
                    if !ctx.has(name) {
                        // method(args...) — base is method, steps are chained
                        let call = CompiledCall {
                            method:    BuiltinMethod::from_name(name),
                            name:      Arc::from(name.as_str()),
                            sub_progs: Arc::from(&[] as &[Arc<Program>]),
                            orig_args: Arc::from(&[] as &[Arg]),
                        };
                        ops.push(Opcode::CallMethod(Arc::new(call)));
                        for step in steps { Self::emit_step(step, ctx, ops); }
                        return;
                    }
                }
                ops.push(Opcode::SetCurrent);
                Self::emit_into(rhs, ctx, ops);
            }
            _ => {
                // Arbitrary expression; set current to pipe input, then eval
                ops.push(Opcode::SetCurrent);
                Self::emit_into(rhs, ctx, ops);
            }
        }
    }

    fn emit_bind(target: &BindTarget, ctx: &mut VarCtx, ops: &mut Vec<Opcode>) {
        match target {
            BindTarget::Name(name) => {
                ops.push(Opcode::BindVar(Arc::from(name.as_str())));
                *ctx = ctx.with_var(name);
            }
            BindTarget::Obj { fields, rest } => {
                let spec = BindObjSpec {
                    fields: fields.iter().map(|f| Arc::from(f.as_str())).collect::<Vec<_>>().into(),
                    rest: rest.as_ref().map(|r| Arc::from(r.as_str())),
                };
                ops.push(Opcode::BindObjDestructure(Arc::new(spec)));
                for f in fields { *ctx = ctx.with_var(f); }
                if let Some(r) = rest { *ctx = ctx.with_var(r); }
            }
            BindTarget::Arr(names) => {
                let ns: Vec<Arc<str>> = names.iter().map(|n| Arc::from(n.as_str())).collect();
                ops.push(Opcode::BindArrDestructure(ns.into()));
                for n in names { *ctx = ctx.with_var(n); }
            }
        }
    }

    fn compile_sub(expr: &Expr, ctx: &VarCtx) -> Program {
        let ops = Self::optimize(Self::emit(expr, ctx));
        Program::new(ops, "<sub>")
    }

    fn compile_array_spread(_expr: &Expr, _ctx: &VarCtx) -> Program {
        // Not reached — handled in MakeArr execution
        Program::new(vec![], "<spread>")
    }

    /// Compile a spread array element — wrapped with a special marker.
    fn compile_sub_spread(expr: &Expr, ctx: &VarCtx) -> Program {
        let mut ops = Self::emit(expr, ctx);
        // Prefix with a sentinel bool to mark this as a spread
        ops.insert(0, Opcode::PushBool(true));
        // Append a sentinel for reading: bool(true) + actual val
        // Actually use a dedicated approach: GetSlice-like marker
        // For simplicity, just compile the expr normally;
        // MakeArr handles spread by checking if the result is an array
        // when the corresponding ArrayElem is Spread.
        // Re-do: just compile normally, MakeArr knows which slots are spreads.
        Self::compile_sub(expr, ctx) // caller has separate spread tracking
    }
}

// ── Path cache ────────────────────────────────────────────────────────────────
//
// Key: (doc_hash, json_pointer) → Val
//
// Doc-scoped (no program_id): any program resolving the same path on the same
// document gets a hit.  Intermediate nodes are cached so a prefix of a longer
// path can be reused without re-traversal.

struct PathCache {
    /// doc_hash → (pointer_string → Val)
    docs:     HashMap<u64, HashMap<Arc<str>, Val>>,
    /// FIFO eviction order
    order:    VecDeque<(u64, Arc<str>)>,
    capacity: usize,
}

impl PathCache {
    fn new(cap: usize) -> Self {
        Self {
            docs:     HashMap::new(),
            order:    VecDeque::with_capacity(cap),
            capacity: cap,
        }
    }

    /// O(1) immutable lookup — returns cloned Val (Val::clone is O(1)).
    #[inline]
    fn get(&self, doc_hash: u64, ptr: &str) -> Option<Val> {
        self.docs.get(&doc_hash)?.get(ptr).cloned()
    }

    /// O(1) existence check without cloning.
    #[inline]
    fn contains(&self, doc_hash: u64, ptr: &str) -> bool {
        self.docs.get(&doc_hash).map_or(false, |m| m.contains_key(ptr))
    }

    fn insert(&mut self, doc_hash: u64, ptr: Arc<str>, val: Val) {
        if self.order.len() >= self.capacity {
            if let Some((old_hash, old_ptr)) = self.order.pop_front() {
                if let Some(inner) = self.docs.get_mut(&old_hash) {
                    inner.remove(old_ptr.as_ref());
                    if inner.is_empty() { self.docs.remove(&old_hash); }
                }
            }
        }
        self.order.push_back((doc_hash, ptr.clone()));
        self.docs.entry(doc_hash).or_insert_with(HashMap::new).insert(ptr, val);
    }

    fn len(&self) -> usize { self.order.len() }
}

// ── VM ────────────────────────────────────────────────────────────────────────

/// High-performance v2 virtual machine.
///
/// Maintains:
/// - **Compile cache** — expression string → `Program` (parse + compile once).
/// - **Path cache** — `(doc_hash, json_pointer)` → `Val`; doc-scoped so any
///   program navigating the same path on the same document shares cached nodes.
///   Intermediate nodes are populated as a side-effect of every traversal,
///   enabling prefix reuse without re-traversal.
///
/// One VM per thread; wrap in `Mutex` for shared use.
/// Toggle each optimiser pass independently.  Default enables every
/// pass.  Disabling a pass invalidates the compile cache for the next
/// compilation by changing `hash()`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PassConfig {
    pub root_chain:      bool,
    pub filter_count:    bool,
    pub filter_fusion:   bool,
    pub find_quantifier: bool,
    pub strength_reduce: bool,
    pub redundant_ops:   bool,
    pub kind_check_fold: bool,
    pub method_const:    bool,
    pub const_fold:      bool,
    pub nullness:        bool,
    pub equi_join:       bool,
    pub reorder_and:     bool,
    pub dedup_subprogs:  bool,
}

impl Default for PassConfig {
    fn default() -> Self {
        Self {
            root_chain: true, filter_count: true, filter_fusion: true,
            find_quantifier: true, strength_reduce: true, redundant_ops: true,
            kind_check_fold: true, method_const: true, const_fold: true,
            nullness: true, equi_join: true,
            reorder_and: true, dedup_subprogs: true,
        }
    }
}

impl PassConfig {
    /// Disable every pass — emit raw opcodes.
    pub fn none() -> Self {
        Self {
            root_chain: false, filter_count: false, filter_fusion: false,
            find_quantifier: false, strength_reduce: false, redundant_ops: false,
            kind_check_fold: false, method_const: false, const_fold: false,
            nullness: false, equi_join: false,
            reorder_and: false, dedup_subprogs: false,
        }
    }

    pub fn hash(&self) -> u64 {
        let mut bits: u64 = 0;
        for (i, b) in [self.root_chain, self.filter_count, self.filter_fusion,
                       self.find_quantifier, self.strength_reduce, self.redundant_ops,
                       self.kind_check_fold, self.method_const, self.const_fold,
                       self.nullness, self.equi_join,
                       self.reorder_and, self.dedup_subprogs].iter().enumerate() {
            if *b { bits |= 1u64 << i; }
        }
        bits
    }
}

pub struct VM {
    registry:      Arc<MethodRegistry>,
    /// Cache key = (pass_config_hash, expr_string).  Changing `config`
    /// invalidates prior entries automatically via key divergence.
    compile_cache: HashMap<(u64, String), Arc<Program>>,
    /// LRU ordering for `compile_cache`; front = least recently used.
    /// Entries are moved to back on hit; oldest evicted when over cap.
    compile_lru:   std::collections::VecDeque<(u64, String)>,
    compile_cap:   usize,
    path_cache:    PathCache,
    /// Hash of the document currently being executed — set once by `execute()`,
    /// reused by all recursive `exec()` calls within the same top-level call.
    doc_hash:      u64,
    /// Optimiser pass toggles.  Default: all on.
    config:        PassConfig,
}

impl Default for VM {
    fn default() -> Self { Self::new() }
}

impl VM {
    pub fn new() -> Self { Self::with_capacity(512, 4096) }

    pub fn with_capacity(compile_cap: usize, path_cap: usize) -> Self {
        Self {
            registry:      Arc::new(MethodRegistry::new()),
            compile_cache: HashMap::with_capacity(compile_cap),
            compile_lru:   std::collections::VecDeque::with_capacity(compile_cap),
            compile_cap,
            path_cache:    PathCache::new(path_cap),
            doc_hash:      0,
            config:        PassConfig::default(),
        }
    }

    /// Build a VM that shares an existing method registry.
    pub fn with_registry(registry: Arc<MethodRegistry>) -> Self {
        let mut vm = Self::new();
        vm.registry = registry;
        vm
    }

    /// Register a method already wrapped in `Arc`.
    pub fn register_arc(&mut self, name: &str, method: Arc<dyn crate::eval::methods::Method>) {
        Arc::make_mut(&mut self.registry).register_arc(name, method);
    }

    /// Replace the pass configuration.  The compile cache is not purged,
    /// but future lookups key off the new config hash so old entries
    /// are effectively invalidated for the new regime.
    pub fn set_pass_config(&mut self, config: PassConfig) { self.config = config; }

    pub fn pass_config(&self) -> PassConfig { self.config }

    /// Register a custom method (callable via `.method_name(...)` in expressions).
    pub fn register(&mut self, name: impl Into<String>, method: impl super::eval::methods::Method + 'static) {
        Arc::make_mut(&mut self.registry).register(name, method);
    }

    // ── Public entry-points ───────────────────────────────────────────────────

    /// Parse, compile (cached), and execute `expr` against `doc`.
    pub fn run_str(&mut self, expr: &str, doc: &serde_json::Value) -> Result<serde_json::Value, EvalError> {
        let prog = self.get_or_compile(expr)?;
        self.execute(&prog, doc)
    }

    /// Execute a pre-compiled `Program` against `doc`.
    pub fn execute(&mut self, program: &Program, doc: &serde_json::Value) -> Result<serde_json::Value, EvalError> {
        let root = Val::from(doc);
        // Compute doc hash once; reused by all exec() calls in this invocation.
        self.doc_hash = hash_val_structure(&root);
        let env = self.make_env(root);
        let result = self.exec(program, &env)?;
        Ok(result.into())
    }

    /// Execute a compiled program against a document, first specialising
    /// against the given shape (turns `OptField` → `GetField` where safe,
    /// folds `KindCheck` where type is known, etc.).
    pub fn execute_with_schema(
        &mut self,
        program: &Program,
        doc: &serde_json::Value,
        shape: &super::schema::Shape,
    ) -> Result<serde_json::Value, EvalError> {
        let specialized = super::schema::specialize(program, shape);
        self.execute(&specialized, doc)
    }

    /// Execute a program; infer the shape from the document itself.  Costs
    /// an O(doc) shape walk before execution; useful when the same
    /// compiled program is reused across many docs with similar shapes.
    pub fn execute_with_inferred_schema(
        &mut self,
        program: &Program,
        doc: &serde_json::Value,
    ) -> Result<serde_json::Value, EvalError> {
        let shape = super::schema::Shape::of(doc);
        self.execute_with_schema(program, doc, &shape)
    }

    /// Get or compile an expression string (compile cache).
    /// Cache key is (pass_config_hash, expr) so that different pass
    /// configurations yield different compiled programs.
    pub fn get_or_compile(&mut self, expr: &str) -> Result<Arc<Program>, EvalError> {
        let key = (self.config.hash(), expr.to_string());
        if let Some(p) = self.compile_cache.get(&key) {
            let arc = Arc::clone(p);
            self.touch_lru(&key);
            return Ok(arc);
        }
        let prog = Compiler::compile_str_with_config(expr, self.config)?;
        let arc = Arc::new(prog);
        self.insert_compile(key, Arc::clone(&arc));
        Ok(arc)
    }

    /// Move `key` to the back of the LRU queue (most recently used).
    fn touch_lru(&mut self, key: &(u64, String)) {
        if let Some(pos) = self.compile_lru.iter().position(|k| k == key) {
            let k = self.compile_lru.remove(pos).unwrap();
            self.compile_lru.push_back(k);
        }
    }

    /// Insert into compile cache with LRU eviction at `compile_cap`.
    fn insert_compile(&mut self, key: (u64, String), prog: Arc<Program>) {
        while self.compile_cache.len() >= self.compile_cap && self.compile_cap > 0 {
            if let Some(old) = self.compile_lru.pop_front() {
                self.compile_cache.remove(&old);
            } else {
                break;
            }
        }
        self.compile_lru.push_back(key.clone());
        self.compile_cache.insert(key, prog);
    }

    /// Cache statistics: `(compile_entries, path_entries)`.
    pub fn cache_stats(&self) -> (usize, usize) {
        (self.compile_cache.len(), self.path_cache.len())
    }

    // ── Internal helpers ──────────────────────────────────────────────────────

    fn make_env(&self, root: Val) -> Env {
        Env::new_with_registry(root, Arc::clone(&self.registry))
    }


    // ── Core execution loop ───────────────────────────────────────────────────

    /// Execute `program` in environment `env`, returning the top-of-stack value.
    pub fn exec(&mut self, program: &Program, env: &Env) -> Result<Val, EvalError> {
        let mut stack: SmallVec<[Val; 16]> = SmallVec::new();

        for op in program.ops.iter() {
            match op {
                // ── Literals ──────────────────────────────────────────────────
                Opcode::PushNull        => stack.push(Val::Null),
                Opcode::PushBool(b)     => stack.push(Val::Bool(*b)),
                Opcode::PushInt(n)      => stack.push(Val::Int(*n)),
                Opcode::PushFloat(f)    => stack.push(Val::Float(*f)),
                Opcode::PushStr(s)      => stack.push(Val::Str(s.clone())),

                // ── Context ───────────────────────────────────────────────────
                Opcode::PushRoot        => stack.push(env.root.clone()),
                Opcode::PushCurrent     => stack.push(env.current.clone()),

                // ── Navigation ────────────────────────────────────────────────
                Opcode::GetField(k) => {
                    let v = pop!(stack);
                    stack.push(v.get_field(k.as_ref()));
                }
                Opcode::GetIndex(i) => {
                    let v = pop!(stack);
                    stack.push(v.get_index(*i));
                }
                Opcode::DynIndex(prog) => {
                    let v = pop!(stack);
                    let key = self.exec(prog, env)?;
                    stack.push(match key {
                        Val::Int(i) => v.get_index(i),
                        Val::Str(s) => v.get_field(s.as_ref()),
                        _ => Val::Null,
                    });
                }
                Opcode::GetSlice(from, to) => {
                    let v = pop!(stack);
                    stack.push(exec_slice(v, *from, *to));
                }
                Opcode::OptField(k) => {
                    let v = pop!(stack);
                    stack.push(if v.is_null() { Val::Null } else { v.get_field(k.as_ref()) });
                }
                Opcode::Descendant(k) => {
                    let v = pop!(stack);
                    let mut found = Vec::new();
                    // (D) When descending from root, track pointer paths and
                    // cache each discovered node for future RootChain lookups.
                    let from_root = match (&v, &env.root) {
                        (Val::Obj(a), Val::Obj(b)) => Arc::ptr_eq(a, b),
                        (Val::Arr(a), Val::Arr(b)) => Arc::ptr_eq(a, b),
                        _ => matches!((&v, &env.root), (Val::Null, Val::Null)),
                    };
                    if from_root {
                        let mut prefix = String::new();
                        let mut cached: Vec<(Arc<str>, Val)> = Vec::new();
                        collect_desc_with_paths(&v, k.as_ref(), &mut prefix, &mut found, &mut cached);
                        let doc_hash = self.doc_hash;
                        for (ptr, val) in cached {
                            self.path_cache.insert(doc_hash, ptr, val);
                        }
                    } else {
                        collect_desc(&v, k.as_ref(), &mut found);
                    }
                    stack.push(Val::arr(found));
                }
                Opcode::DescendAll => {
                    let v = pop!(stack);
                    let mut found = Vec::new();
                    collect_all(&v, &mut found);
                    stack.push(Val::arr(found));
                }
                Opcode::InlineFilter(pred) => {
                    let val = pop!(stack);
                    let items = match val {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        other => vec![other],
                    };
                    let mut out = Vec::new();
                    for item in items {
                        let ie = env.with_current(item.clone());
                        if is_truthy(&self.exec(pred, &ie)?) { out.push(item); }
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::Quantifier(kind) => {
                    let val = pop!(stack);
                    stack.push(match kind {
                        QuantifierKind::First => match val {
                            Val::Arr(a) => a.first().cloned().unwrap_or(Val::Null),
                            other => other,
                        },
                        QuantifierKind::One => match val {
                            Val::Arr(a) if a.len() == 1 => a[0].clone(),
                            Val::Arr(a) => return err!("quantifier !: expected exactly one element, got {}", a.len()),
                            other => other,
                        },
                    });
                }

                // ── Peephole fusions ──────────────────────────────────────────
                Opcode::RootChain(chain) => {
                    let doc_hash = self.doc_hash;

                    // (B) Find deepest cached prefix — immutable scan.
                    let mut start = 0usize;
                    {
                        let mut ptr = String::new();
                        for (i, k) in chain.iter().enumerate() {
                            ptr.push('/');
                            ptr.push_str(k.as_ref());
                            if self.path_cache.contains(doc_hash, &ptr) {
                                start = i + 1;
                            } else {
                                break;
                            }
                        }
                    }

                    // Retrieve cached starting node (O(1) Val clone).
                    let mut current = if start > 0 {
                        let prefix_ptr: String = chain[..start].iter()
                            .fold(String::new(), |mut s, k| { s.push('/'); s.push_str(k.as_ref()); s });
                        self.path_cache.get(doc_hash, &prefix_ptr)
                            .unwrap_or_else(|| env.root.clone())
                    } else {
                        env.root.clone()
                    };

                    // (A) Traverse uncached suffix, caching each new node.
                    let mut ptr: String = chain[..start].iter()
                        .fold(String::new(), |mut s, k| { s.push('/'); s.push_str(k.as_ref()); s });
                    for k in chain[start..].iter() {
                        current = current.get_field(k.as_ref());
                        ptr.push('/');
                        ptr.push_str(k.as_ref());
                        self.path_cache.insert(doc_hash, Arc::from(ptr.as_str()), current.clone());
                    }
                    stack.push(current);
                }
                Opcode::FilterCount(pred) => {
                    let recv = pop!(stack);
                    let n = if let Val::Arr(a) = &recv {
                        let mut count = 0u64;
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let t = is_truthy(&self.exec(pred, &scratch)?);
                            scratch.restore_current(prev);
                            if t { count += 1; }
                        }
                        count
                    } else { 0 };
                    stack.push(Val::Int(n as i64));
                }
                Opcode::FindFirst(pred) => {
                    let recv = pop!(stack);
                    let mut found = Val::Null;
                    if let Val::Arr(a) = &recv {
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let t = is_truthy(&self.exec(pred, &scratch)?);
                            scratch.restore_current(prev);
                            if t { found = item.clone(); break; }
                        }
                    } else if !recv.is_null() {
                        let sub_env = env.with_current(recv.clone());
                        if is_truthy(&self.exec(pred, &sub_env)?) { found = recv; }
                    }
                    stack.push(found);
                }
                Opcode::FilterMap { pred, map } => {
                    let recv = pop!(stack);
                    if let Val::Arr(a) = recv {
                        let mut out = Vec::with_capacity(a.len());
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            if is_truthy(&self.exec(pred, &scratch)?) {
                                out.push(self.exec(map, &scratch)?);
                            }
                            scratch.restore_current(prev);
                        }
                        stack.push(Val::arr(out));
                    } else {
                        stack.push(Val::arr(Vec::new()));
                    }
                }
                Opcode::FilterFilter { p1, p2 } => {
                    let recv = pop!(stack);
                    if let Val::Arr(a) = recv {
                        let mut out = Vec::with_capacity(a.len());
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let keep = is_truthy(&self.exec(p1, &scratch)?)
                                    && is_truthy(&self.exec(p2, &scratch)?);
                            scratch.restore_current(prev);
                            if keep { out.push(item.clone()); }
                        }
                        stack.push(Val::arr(out));
                    } else {
                        stack.push(Val::arr(Vec::new()));
                    }
                }
                Opcode::MapSum(f) => {
                    let recv = pop!(stack);
                    let mut acc_i: i64 = 0;
                    let mut acc_f: f64 = 0.0;
                    let mut is_float = false;
                    if let Val::Arr(a) = &recv {
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let v = self.exec(f, &scratch)?;
                            scratch.restore_current(prev);
                            match v {
                                Val::Int(n) => {
                                    if is_float { acc_f += n as f64; } else { acc_i += n; }
                                }
                                Val::Float(x) => {
                                    if !is_float { acc_f = acc_i as f64; is_float = true; }
                                    acc_f += x;
                                }
                                Val::Null => {}
                                _ => return err!("map(..).sum(): non-numeric mapped value"),
                            }
                        }
                    }
                    stack.push(if is_float { Val::Float(acc_f) } else { Val::Int(acc_i) });
                }
                Opcode::MapAvg(f) => {
                    let recv = pop!(stack);
                    let mut sum: f64 = 0.0;
                    let mut n: usize = 0;
                    if let Val::Arr(a) = &recv {
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let v = self.exec(f, &scratch)?;
                            scratch.restore_current(prev);
                            match v {
                                Val::Int(x)   => { sum += x as f64; n += 1; }
                                Val::Float(x) => { sum += x;        n += 1; }
                                Val::Null => {}
                                _ => return err!("map(..).avg(): non-numeric mapped value"),
                            }
                        }
                    }
                    stack.push(if n == 0 { Val::Null } else { Val::Float(sum / n as f64) });
                }
                Opcode::MapFlatten(f) => {
                    let recv = pop!(stack);
                    let items = match recv {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let mut out = Vec::new();
                    for item in items {
                        let sub_env = env.with_current(item);
                        let mapped = self.exec(f, &sub_env)?;
                        match mapped {
                            Val::Arr(a) => {
                                let v = Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone());
                                out.extend(v);
                            }
                            other => out.push(other),
                        }
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::FilterTakeWhile { pred, stop } => {
                    let recv = pop!(stack);
                    let items = match recv {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let mut out = Vec::new();
                    for item in items {
                        let sub_env = env.with_current(item.clone());
                        if !is_truthy(&self.exec(pred, &sub_env)?) { continue; }
                        if !is_truthy(&self.exec(stop, &sub_env)?) { break; }
                        out.push(item);
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::FilterDropWhile { pred, drop } => {
                    let recv = pop!(stack);
                    let items = match recv {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let mut out = Vec::new();
                    let mut dropping = true;
                    for item in items {
                        let sub_env = env.with_current(item.clone());
                        if !is_truthy(&self.exec(pred, &sub_env)?) { continue; }
                        if dropping {
                            if is_truthy(&self.exec(drop, &sub_env)?) { continue; }
                            dropping = false;
                        }
                        out.push(item);
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::EquiJoin { rhs, lhs_key, rhs_key } => {
                    use std::collections::HashMap;
                    let left_val = pop!(stack);
                    let right_val = self.exec(rhs, env)?;
                    let left = match left_val {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let right = match right_val {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let mut idx: HashMap<String, Vec<Val>> = HashMap::new();
                    for r in right {
                        let key = match &r {
                            Val::Obj(o) => o.get(rhs_key.as_ref()).map(super::eval::util::val_to_key),
                            _ => None,
                        };
                        if let Some(k) = key { idx.entry(k).or_default().push(r); }
                    }
                    let mut out = Vec::new();
                    for l in left {
                        let key = match &l {
                            Val::Obj(o) => o.get(lhs_key.as_ref()).map(super::eval::util::val_to_key),
                            _ => None,
                        };
                        let Some(k) = key else { continue };
                        let Some(matches) = idx.get(&k) else { continue };
                        for r in matches {
                            match (&l, r) {
                                (Val::Obj(lo), Val::Obj(ro)) => {
                                    let mut m = (**lo).clone();
                                    for (k, v) in ro.iter() { m.insert(k.clone(), v.clone()); }
                                    out.push(Val::obj(m));
                                }
                                _ => out.push(l.clone()),
                            }
                        }
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::MapUnique(f) => {
                    let recv = pop!(stack);
                    let items = match recv {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    let mut seen: Vec<Val> = Vec::new();
                    let mut out = Vec::new();
                    for item in items {
                        let sub_env = env.with_current(item);
                        let mapped = self.exec(f, &sub_env)?;
                        if !seen.iter().any(|s| super::eval::util::cmp_vals(s, &mapped)
                                                 == std::cmp::Ordering::Equal) {
                            seen.push(mapped.clone());
                            out.push(mapped);
                        }
                    }
                    stack.push(Val::arr(out));
                }
                Opcode::TopN { n, asc } => {
                    use std::collections::BinaryHeap;
                    use std::cmp::Reverse;
                    let recv = pop!(stack);
                    let items = match recv {
                        Val::Arr(a) => Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone()),
                        _ => Vec::new(),
                    };
                    if *n >= items.len() {
                        let mut v = items;
                        v.sort_by(|x, y| super::eval::util::cmp_vals(x, y));
                        if !*asc { v.reverse(); }
                        stack.push(Val::arr(v));
                    } else if *asc {
                        // Max-heap of size n; pop largest to keep smallest n.
                        let mut heap: BinaryHeap<WrapVal> = BinaryHeap::with_capacity(*n);
                        for item in items {
                            if heap.len() < *n {
                                heap.push(WrapVal(item));
                            } else if super::eval::util::cmp_vals(&item, &heap.peek().unwrap().0)
                                      == std::cmp::Ordering::Less {
                                heap.pop();
                                heap.push(WrapVal(item));
                            }
                        }
                        let mut v: Vec<Val> = heap.into_iter().map(|w| w.0).collect();
                        v.sort_by(|x, y| super::eval::util::cmp_vals(x, y));
                        stack.push(Val::arr(v));
                    } else {
                        // Min-heap via Reverse; keep largest n.
                        let mut heap: BinaryHeap<Reverse<WrapVal>> = BinaryHeap::with_capacity(*n);
                        for item in items {
                            if heap.len() < *n {
                                heap.push(Reverse(WrapVal(item)));
                            } else if super::eval::util::cmp_vals(&item, &heap.peek().unwrap().0.0)
                                      == std::cmp::Ordering::Greater {
                                heap.pop();
                                heap.push(Reverse(WrapVal(item)));
                            }
                        }
                        let mut v: Vec<Val> = heap.into_iter().map(|w| w.0.0).collect();
                        v.sort_by(|x, y| super::eval::util::cmp_vals(y, x));
                        stack.push(Val::arr(v));
                    }
                }
                Opcode::MapMap { f1, f2 } => {
                    let recv = pop!(stack);
                    if let Val::Arr(a) = recv {
                        let mut out = Vec::with_capacity(a.len());
                        let mut scratch = env.clone();
                        for item in a.iter() {
                            let prev = scratch.swap_current(item.clone());
                            let mid = self.exec(f1, &scratch)?;
                            scratch.swap_current(mid);
                            let res = self.exec(f2, &scratch)?;
                            scratch.restore_current(prev);
                            out.push(res);
                        }
                        stack.push(Val::arr(out));
                    } else {
                        stack.push(Val::arr(Vec::new()));
                    }
                }
                Opcode::FindOne(pred) => {
                    let recv = pop!(stack);
                    let mut found: Option<Val> = None;
                    if let Val::Arr(a) = &recv {
                        for item in a.iter() {
                            let sub_env = env.with_current(item.clone());
                            if is_truthy(&self.exec(pred, &sub_env)?) {
                                if found.is_some() {
                                    return err!("quantifier !: expected exactly one match, found multiple");
                                }
                                found = Some(item.clone());
                            }
                        }
                    } else if !recv.is_null() {
                        let sub_env = env.with_current(recv.clone());
                        if is_truthy(&self.exec(pred, &sub_env)?) { found = Some(recv); }
                    }
                    match found {
                        Some(v) => stack.push(v),
                        None => return err!("quantifier !: expected exactly one match, found none"),
                    }
                }

                // ── Ident ─────────────────────────────────────────────────────
                Opcode::LoadIdent(name) => {
                    let v = if let Some(v) = env.get_var(name.as_ref()) {
                        v.clone()
                    } else {
                        env.current.get_field(name.as_ref())
                    };
                    stack.push(v);
                }

                // ── Operators ─────────────────────────────────────────────────
                Opcode::Add  => { let r = pop!(stack); let l = pop!(stack); stack.push(add_vals(l, r)?); }
                Opcode::Sub  => { let r = pop!(stack); let l = pop!(stack); stack.push(num_op(l, r, |a,b|a-b, |a,b|a-b)?); }
                Opcode::Mul  => { let r = pop!(stack); let l = pop!(stack); stack.push(num_op(l, r, |a,b|a*b, |a,b|a*b)?); }
                Opcode::Div  => {
                    let r = pop!(stack); let l = pop!(stack);
                    let b = r.as_f64().unwrap_or(0.0);
                    if b == 0.0 { return err!("division by zero"); }
                    stack.push(Val::Float(l.as_f64().unwrap_or(0.0) / b));
                }
                Opcode::Mod  => { let r = pop!(stack); let l = pop!(stack); stack.push(num_op(l, r, |a,b|a%b, |a,b|a%b)?); }
                Opcode::Eq   => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(vals_eq(&l,&r))); }
                Opcode::Neq  => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(!vals_eq(&l,&r))); }
                Opcode::Lt   => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(cmp_vals(&l,&r) == std::cmp::Ordering::Less)); }
                Opcode::Lte  => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(cmp_vals(&l,&r) != std::cmp::Ordering::Greater)); }
                Opcode::Gt   => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(cmp_vals(&l,&r) == std::cmp::Ordering::Greater)); }
                Opcode::Gte  => { let r = pop!(stack); let l = pop!(stack); stack.push(Val::Bool(cmp_vals(&l,&r) != std::cmp::Ordering::Less)); }
                Opcode::Fuzzy => {
                    let r = pop!(stack); let l = pop!(stack);
                    let ls = match &l { Val::Str(s) => s.to_lowercase(), _ => val_to_string(&l).to_lowercase() };
                    let rs = match &r { Val::Str(s) => s.to_lowercase(), _ => val_to_string(&r).to_lowercase() };
                    stack.push(Val::Bool(ls.contains(&rs) || rs.contains(&ls)));
                }
                Opcode::Not  => { let v = pop!(stack); stack.push(Val::Bool(!is_truthy(&v))); }
                Opcode::Neg  => {
                    let v = pop!(stack);
                    stack.push(match v {
                        Val::Int(n) => Val::Int(-n),
                        Val::Float(f) => Val::Float(-f),
                        _ => return err!("unary minus requires a number"),
                    });
                }
                Opcode::CastOp(ty) => {
                    let v = pop!(stack);
                    stack.push(exec_cast(&v, *ty)?);
                }

                // ── Short-circuit ops ─────────────────────────────────────────
                Opcode::AndOp(rhs) => {
                    let lv = pop!(stack);
                    if !is_truthy(&lv) {
                        stack.push(Val::Bool(false));
                    } else {
                        let rv = self.exec(rhs, env)?;
                        stack.push(Val::Bool(is_truthy(&rv)));
                    }
                }
                Opcode::OrOp(rhs) => {
                    let lv = pop!(stack);
                    if is_truthy(&lv) {
                        stack.push(lv);
                    } else {
                        stack.push(self.exec(rhs, env)?);
                    }
                }
                Opcode::CoalesceOp(rhs) => {
                    let lv = pop!(stack);
                    if !lv.is_null() {
                        stack.push(lv);
                    } else {
                        stack.push(self.exec(rhs, env)?);
                    }
                }

                // ── Method calls ──────────────────────────────────────────────
                Opcode::CallMethod(call) => {
                    let recv = pop!(stack);
                    let result = self.exec_call(recv, call, env)?;
                    stack.push(result);
                }
                Opcode::CallOptMethod(call) => {
                    let recv = pop!(stack);
                    if recv.is_null() {
                        stack.push(Val::Null);
                    } else {
                        stack.push(self.exec_call(recv, call, env)?);
                    }
                }

                // ── Construction ──────────────────────────────────────────────
                Opcode::MakeObj(entries) => {
                    let entries = Arc::clone(entries);
                    let result = self.exec_make_obj(&entries, env)?;
                    stack.push(result);
                }
                Opcode::MakeArr(progs) => {
                    let progs = Arc::clone(progs);
                    let mut out = Vec::with_capacity(progs.len());
                    for p in progs.iter() {
                        let v = self.exec(p, env)?;
                        // If the program produces an array from a spread,
                        // check if it was tagged; for simplicity, just push.
                        out.push(v);
                    }
                    stack.push(Val::arr(out));
                }

                // ── F-string ──────────────────────────────────────────────────
                Opcode::FString(parts) => {
                    let parts = Arc::clone(parts);
                    let result = self.exec_fstring(&parts, env)?;
                    stack.push(result);
                }

                // ── Kind check ────────────────────────────────────────────────
                Opcode::KindCheck { ty, negate } => {
                    let v = pop!(stack);
                    let m = kind_matches(&v, *ty);
                    stack.push(Val::Bool(if *negate { !m } else { m }));
                }

                // ── Pipeline helpers ──────────────────────────────────────────
                Opcode::SetCurrent => {
                    // This is emitted before an arbitrary pipe-forward expression.
                    // The value flowing through the pipe is now on the stack as TOS.
                    // However we can't mutate env here since env is immutable.
                    // The actual "set current" happens by the caller preparing a new env.
                    // In practice, SetCurrent should not appear in isolation in the
                    // flat opcode stream because pipeline steps that need SetCurrent
                    // are compiled as sub-programs. Skip.
                }
                Opcode::BindVar(name) => {
                    // TOS becomes a named var; TOS remains (pass-through for ->) .
                    // We can't mutate env here. This is handled at the pipeline level.
                    // For now, just keep TOS.
                    let _ = name;
                }
                Opcode::StoreVar(name) => {
                    // Pop and discard (the LetExpr opcode handles binding properly).
                    let _ = name;
                    pop!(stack);
                }
                Opcode::BindObjDestructure(_) | Opcode::BindArrDestructure(_) => {
                    // Pipeline bind destructure — handled at pipeline level.
                }

                // ── Complex recursive ops ─────────────────────────────────────
                Opcode::LetExpr { name, body } => {
                    let init_val = pop!(stack);
                    let body_env = env.with_var(name.as_ref(), init_val);
                    stack.push(self.exec(body, &body_env)?);
                }

                Opcode::ListComp(spec) => {
                    let items = self.exec_iter_vals(&spec.iter, env)?;
                    let mut out = Vec::new();
                    for item in items {
                        let ie = bind_comp_vars(env, &spec.vars, item);
                        if let Some(cond) = &spec.cond {
                            if !is_truthy(&self.exec(cond, &ie)?) { continue; }
                        }
                        out.push(self.exec(&spec.expr, &ie)?);
                    }
                    stack.push(Val::arr(out));
                }

                Opcode::DictComp(spec) => {
                    let items = self.exec_iter_vals(&spec.iter, env)?;
                    let mut map: IndexMap<Arc<str>, Val> = IndexMap::new();
                    for item in items {
                        let ie = bind_comp_vars(env, &spec.vars, item);
                        if let Some(cond) = &spec.cond {
                            if !is_truthy(&self.exec(cond, &ie)?) { continue; }
                        }
                        let k: Arc<str> = Arc::from(val_to_key(&self.exec(&spec.key, &ie)?).as_str());
                        let v = self.exec(&spec.val, &ie)?;
                        map.insert(k, v);
                    }
                    stack.push(Val::obj(map));
                }

                Opcode::SetComp(spec) => {
                    let items = self.exec_iter_vals(&spec.iter, env)?;
                    let mut seen: Vec<String> = Vec::new();
                    let mut out = Vec::new();
                    for item in items {
                        let ie = bind_comp_vars(env, &spec.vars, item);
                        if let Some(cond) = &spec.cond {
                            if !is_truthy(&self.exec(cond, &ie)?) { continue; }
                        }
                        let v = self.exec(&spec.expr, &ie)?;
                        let k = val_to_key(&v);
                        if !seen.contains(&k) { seen.push(k); out.push(v); }
                    }
                    stack.push(Val::arr(out));
                }

                // ── Path cache lookup ─────────────────────────────────────────
                Opcode::GetPointer(ptr) => {
                    let doc_hash = self.doc_hash;
                    let v = if let Some(cached) = self.path_cache.get(doc_hash, ptr.as_ref()) {
                        cached
                    } else {
                        let v = resolve_pointer(&env.root, ptr.as_ref());
                        self.path_cache.insert(doc_hash, ptr.clone(), v.clone());
                        v
                    };
                    stack.push(v);
                }

                // ── Patch block ───────────────────────────────────────────────
                Opcode::PatchEval(e) => {
                    stack.push(eval(e, env)?);
                }
            }
        }

        stack.pop().ok_or_else(|| EvalError("program produced no value".into()))
    }

    // ── Pipeline execution (recursive, env-cloning) ────────────────────────────

    /// Execute a pipeline by compiling pipeline steps as sub-programs.
    /// This is called when a `Pipeline` expr is compiled into a sequence
    /// of SetCurrent + sub-program opcodes.
    fn exec_pipeline(&mut self, base: &Program, steps: &[PipeStep], orig_ctx: &VarCtx, env: &Env)
        -> Result<Val, EvalError>
    {
        let mut current = self.exec(base, env)?;
        let mut cur_env = env.clone();
        for step in steps {
            match step {
                PipeStep::Forward(rhs) => {
                    current = self.exec_pipe_forward(&current, rhs, orig_ctx, &cur_env)?;
                }
                PipeStep::Bind(target) => {
                    cur_env = apply_bind_to_env(target, &current, cur_env)?;
                }
            }
        }
        Ok(current)
    }

    fn exec_pipe_forward(&mut self, left: &Val, rhs: &Expr, ctx: &VarCtx, env: &Env) -> Result<Val, EvalError> {
        match rhs {
            Expr::Ident(name) if !ctx.has(name) => {
                dispatch_method(left.clone(), name, &[], env)
            }
            Expr::Chain(base, _steps) => {
                if let Expr::Ident(name) = base.as_ref() {
                    if !ctx.has(name) {
                        let prog = Compiler::compile_sub(rhs, ctx);
                        return self.exec(&prog, &env.with_current(left.clone()));
                    }
                }
                let sub = Compiler::compile_sub(rhs, ctx);
                self.exec(&sub, &env.with_current(left.clone()))
            }
            _ => {
                let sub = Compiler::compile_sub(rhs, ctx);
                self.exec(&sub, &env.with_current(left.clone()))
            }
        }
    }

    // ── Method call dispatch ──────────────────────────────────────────────────

    fn exec_call(&mut self, recv: Val, call: &CompiledCall, env: &Env) -> Result<Val, EvalError> {
        // Global-call opcodes push Root before calling; handle them
        if call.method == BuiltinMethod::Unknown {
            // Custom registry or global function
            if !env.registry_is_empty() {
                if let Some(method) = env.registry_get(call.name.as_ref()) {
                    let evaled: Result<Vec<Val>, _> = call.sub_progs.iter()
                        .map(|p| self.exec(p, env)).collect();
                    return method.call(recv, &evaled?);
                }
            }
            // Global function (coalesce, zip, etc.) or fallback to dispatch_method
            return dispatch_method(recv, call.name.as_ref(), &call.orig_args, env);
        }

        // Lambda methods — VM handles iteration, running sub-programs per item
        if call.method.is_lambda_method() {
            return self.exec_lambda_method(recv, call, env);
        }

        // Value methods — delegate to the existing dispatch with orig_args
        dispatch_method(recv, call.name.as_ref(), &call.orig_args, env)
    }

    fn exec_lambda_method(&mut self, recv: Val, call: &CompiledCall, env: &Env) -> Result<Val, EvalError> {
        let sub = call.sub_progs.first();

        match call.method {
            BuiltinMethod::Filter => {
                let pred = sub.ok_or_else(|| EvalError("filter: requires predicate".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("filter: expected array".into()))?;
                let mut out = Vec::new();
                for item in items {
                    if is_truthy(&self.exec_with_lambda(pred, &item, &call.orig_args, env)?) {
                        out.push(item);
                    }
                }
                Ok(Val::arr(out))
            }
            BuiltinMethod::Map => {
                let mapper = sub.ok_or_else(|| EvalError("map: requires mapper".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("map: expected array".into()))?;
                let r: Result<Vec<_>, _> = items.into_iter()
                    .map(|item| self.exec_with_lambda(mapper, &item, &call.orig_args, env))
                    .collect();
                Ok(Val::arr(r?))
            }
            BuiltinMethod::FlatMap => {
                let mapper = sub.ok_or_else(|| EvalError("flatMap: requires mapper".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("flatMap: expected array".into()))?;
                let mut out = Vec::new();
                for item in items {
                    match self.exec_with_lambda(mapper, &item, &call.orig_args, env)? {
                        Val::Arr(a) => out.extend(Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone())),
                        v => out.push(v),
                    }
                }
                Ok(Val::arr(out))
            }
            BuiltinMethod::Sort => {
                // Delegate to func_arrays::sort which already handles lambda args
                dispatch_method(recv, call.name.as_ref(), &call.orig_args, env)
            }
            BuiltinMethod::Any => {
                let pred = sub.ok_or_else(|| EvalError("any: requires predicate".into()))?;
                if let Val::Arr(a) = &recv {
                    let result = a.iter().any(|item| {
                        self.exec_with_lambda(pred, item, &call.orig_args, env)
                            .map(|v| is_truthy(&v)).unwrap_or(false)
                    });
                    Ok(Val::Bool(result))
                } else { Ok(Val::Bool(false)) }
            }
            BuiltinMethod::All => {
                let pred = sub.ok_or_else(|| EvalError("all: requires predicate".into()))?;
                if let Val::Arr(a) = &recv {
                    if a.is_empty() { return Ok(Val::Bool(true)); }
                    let result = a.iter().all(|item| {
                        self.exec_with_lambda(pred, item, &call.orig_args, env)
                            .map(|v| is_truthy(&v)).unwrap_or(false)
                    });
                    Ok(Val::Bool(result))
                } else { Ok(Val::Bool(false)) }
            }
            BuiltinMethod::Count if !call.sub_progs.is_empty() => {
                let pred = &call.sub_progs[0];
                if let Val::Arr(a) = &recv {
                    let n = a.iter().filter(|item| {
                        self.exec_with_lambda(pred, item, &call.orig_args, env)
                            .map(|v| is_truthy(&v)).unwrap_or(false)
                    }).count();
                    Ok(Val::Int(n as i64))
                } else { Ok(Val::Int(0)) }
            }
            BuiltinMethod::GroupBy => {
                let key_prog = sub.ok_or_else(|| EvalError("groupBy: requires key".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("groupBy: expected array".into()))?;
                let mut map: IndexMap<Arc<str>, Val> = IndexMap::new();
                for item in items {
                    let k: Arc<str> = Arc::from(val_to_key(&self.exec_with_lambda(key_prog, &item, &call.orig_args, env)?).as_str());
                    let bucket = map.entry(k).or_insert_with(|| Val::arr(Vec::new()));
                    bucket.as_array_mut().unwrap().push(item);
                }
                Ok(Val::obj(map))
            }
            BuiltinMethod::CountBy => {
                let key_prog = sub.ok_or_else(|| EvalError("countBy: requires key".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("countBy: expected array".into()))?;
                let mut map: IndexMap<Arc<str>, Val> = IndexMap::new();
                for item in items {
                    let k: Arc<str> = Arc::from(val_to_key(&self.exec_with_lambda(key_prog, &item, &call.orig_args, env)?).as_str());
                    let counter = map.entry(k).or_insert(Val::Int(0));
                    if let Val::Int(n) = counter { *n += 1; }
                }
                Ok(Val::obj(map))
            }
            BuiltinMethod::IndexBy => {
                let key_prog = sub.ok_or_else(|| EvalError("indexBy: requires key".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("indexBy: expected array".into()))?;
                let mut map: IndexMap<Arc<str>, Val> = IndexMap::new();
                for item in items {
                    let k: Arc<str> = Arc::from(val_to_key(&self.exec_with_lambda(key_prog, &item, &call.orig_args, env)?).as_str());
                    map.insert(k, item);
                }
                Ok(Val::obj(map))
            }
            BuiltinMethod::TakeWhile => {
                let pred = sub.ok_or_else(|| EvalError("takeWhile: requires predicate".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("takeWhile: expected array".into()))?;
                let mut out = Vec::new();
                for item in items {
                    if !is_truthy(&self.exec_with_lambda(pred, &item, &call.orig_args, env)?) { break; }
                    out.push(item);
                }
                Ok(Val::arr(out))
            }
            BuiltinMethod::DropWhile => {
                let pred = sub.ok_or_else(|| EvalError("dropWhile: requires predicate".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("dropWhile: expected array".into()))?;
                let mut dropping = true;
                let out: Vec<Val> = items.into_iter().filter(|item| {
                    if dropping {
                        dropping = self.exec_with_lambda(pred, item, &call.orig_args, env)
                            .map(|v| is_truthy(&v)).unwrap_or(false);
                        !dropping
                    } else { true }
                }).collect();
                Ok(Val::arr(out))
            }
            BuiltinMethod::Accumulate => {
                dispatch_method(recv, call.name.as_ref(), &call.orig_args, env)
            }
            BuiltinMethod::Partition => {
                let pred = sub.ok_or_else(|| EvalError("partition: requires predicate".into()))?;
                let items = recv.into_vec().ok_or_else(|| EvalError("partition: expected array".into()))?;
                let (mut yes, mut no) = (Vec::new(), Vec::new());
                for item in items {
                    if is_truthy(&self.exec_with_lambda(pred, &item, &call.orig_args, env)?) {
                        yes.push(item);
                    } else {
                        no.push(item);
                    }
                }
                Ok(Val::arr(vec![Val::arr(yes), Val::arr(no)]))
            }
            BuiltinMethod::TransformKeys => {
                let lam = sub.ok_or_else(|| EvalError("transformKeys: requires lambda".into()))?;
                let map = recv.into_map().ok_or_else(|| EvalError("transformKeys: expected object".into()))?;
                let mut out: IndexMap<Arc<str>, Val> = IndexMap::new();
                for (k, v) in map {
                    let new_key = Arc::from(val_to_key(&self.exec_with_lambda(lam, &Val::Str(k), &call.orig_args, env)?).as_str());
                    out.insert(new_key, v);
                }
                Ok(Val::obj(out))
            }
            BuiltinMethod::TransformValues => {
                let lam = sub.ok_or_else(|| EvalError("transformValues: requires lambda".into()))?;
                let map = recv.into_map().ok_or_else(|| EvalError("transformValues: expected object".into()))?;
                let mut out: IndexMap<Arc<str>, Val> = IndexMap::new();
                for (k, v) in map {
                    out.insert(k, self.exec_with_lambda(lam, &v, &call.orig_args, env)?);
                }
                Ok(Val::obj(out))
            }
            BuiltinMethod::FilterKeys => {
                let lam = sub.ok_or_else(|| EvalError("filterKeys: requires predicate".into()))?;
                let map = recv.into_map().ok_or_else(|| EvalError("filterKeys: expected object".into()))?;
                let mut out: IndexMap<Arc<str>, Val> = IndexMap::new();
                for (k, v) in map {
                    if is_truthy(&self.exec_with_lambda(lam, &Val::Str(k.clone()), &call.orig_args, env)?) {
                        out.insert(k, v);
                    }
                }
                Ok(Val::obj(out))
            }
            BuiltinMethod::FilterValues => {
                let lam = sub.ok_or_else(|| EvalError("filterValues: requires predicate".into()))?;
                let map = recv.into_map().ok_or_else(|| EvalError("filterValues: expected object".into()))?;
                let mut out: IndexMap<Arc<str>, Val> = IndexMap::new();
                for (k, v) in map {
                    if is_truthy(&self.exec_with_lambda(lam, &v, &call.orig_args, env)?) {
                        out.insert(k, v);
                    }
                }
                Ok(Val::obj(out))
            }
            BuiltinMethod::Pivot => {
                dispatch_method(recv, call.name.as_ref(), &call.orig_args, env)
            }
            BuiltinMethod::Update => {
                let lam = sub.ok_or_else(|| EvalError("update: requires lambda".into()))?;
                self.exec_with_lambda(lam, &recv, &call.orig_args, env)
            }
            _ => dispatch_method(recv, call.name.as_ref(), &call.orig_args, env),
        }
    }

    /// Run a pre-compiled sub-program (lambda body) with `item` as the current value.
    /// Handles lambda params from the original arg if present.
    #[inline]
    fn exec_with_lambda(&mut self, prog: &Program, item: &Val, orig_args: &[Arg], env: &Env)
        -> Result<Val, EvalError>
    {
        // Check if the original arg was a lambda with params that need binding
        if let Some(Arg::Pos(Expr::Lambda { params, .. })) = orig_args.first() {
            if !params.is_empty() {
                let mut inner = env.with_var(&params[0], item.clone());
                inner.current = item.clone();
                return self.exec(prog, &inner);
            }
        }
        self.exec(prog, &env.with_current(item.clone()))
    }

    // ── Object construction ───────────────────────────────────────────────────

    fn exec_make_obj(&mut self, entries: &[CompiledObjEntry], env: &Env) -> Result<Val, EvalError> {
        let mut map: IndexMap<Arc<str>, Val> = IndexMap::new();
        for entry in entries {
            match entry {
                CompiledObjEntry::Short(name) => {
                    let v = if let Some(v) = env.get_var(name.as_ref()) { v.clone() }
                            else { env.current.get_field(name.as_ref()) };
                    if !v.is_null() { map.insert(name.clone(), v); }
                }
                CompiledObjEntry::Kv { key, prog, optional, cond } => {
                    if let Some(c) = cond {
                        if !super::eval::util::is_truthy(&self.exec(c, env)?) { continue; }
                    }
                    let v = self.exec(prog, env)?;
                    if *optional && v.is_null() { continue; }
                    map.insert(key.clone(), v);
                }
                CompiledObjEntry::Dynamic { key, val } => {
                    let k: Arc<str> = Arc::from(val_to_key(&self.exec(key, env)?).as_str());
                    let v = self.exec(val, env)?;
                    map.insert(k, v);
                }
                CompiledObjEntry::Spread(prog) => {
                    if let Val::Obj(other) = self.exec(prog, env)? {
                        let entries = Arc::try_unwrap(other).unwrap_or_else(|m| (*m).clone());
                        for (k, v) in entries { map.insert(k, v); }
                    }
                }
                CompiledObjEntry::SpreadDeep(prog) => {
                    if let Val::Obj(other) = self.exec(prog, env)? {
                        let base = std::mem::take(&mut map);
                        let merged = super::eval::util::deep_merge_concat(
                            Val::obj(base), Val::Obj(other));
                        if let Val::Obj(m) = merged {
                            map = Arc::try_unwrap(m).unwrap_or_else(|m| (*m).clone());
                        }
                    }
                }
            }
        }
        Ok(Val::obj(map))
    }

    // ── F-string ──────────────────────────────────────────────────────────────

    fn exec_fstring(&mut self, parts: &[CompiledFSPart], env: &Env) -> Result<Val, EvalError> {
        let mut out = String::new();
        for part in parts {
            match part {
                CompiledFSPart::Lit(s) => out.push_str(s.as_ref()),
                CompiledFSPart::Interp { prog, fmt } => {
                    let val = self.exec(prog, env)?;
                    let s = match fmt {
                        None                      => val_to_string(&val),
                        Some(FmtSpec::Spec(spec)) => apply_fmt_spec(&val, spec),
                        Some(FmtSpec::Pipe(method)) => {
                            val_to_string(&dispatch_method(val, method, &[], env)?)
                        }
                    };
                    out.push_str(&s);
                }
            }
        }
        Ok(Val::Str(Arc::from(out.as_str())))
    }

    // ── Comprehension helpers ─────────────────────────────────────────────────

    fn exec_iter_vals(&mut self, iter_prog: &Program, env: &Env) -> Result<Vec<Val>, EvalError> {
        match self.exec(iter_prog, env)? {
            Val::Arr(a) => Ok(Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone())),
            Val::Obj(m) => {
                let entries = Arc::try_unwrap(m).unwrap_or_else(|m| (*m).clone());
                Ok(entries.into_iter().map(|(k, v)| obj2("key", Val::Str(k), "value", v)).collect())
            }
            other => Ok(vec![other]),
        }
    }
}

// ── Env extensions for VM ─────────────────────────────────────────────────────

impl Env {
    fn registry_is_empty(&self) -> bool { self.registry_ref().is_empty() }
    fn registry_get(&self, name: &str) -> Option<Arc<dyn super::eval::methods::Method>> {
        self.registry_ref().get(name).cloned()
    }
}

// ── Free helpers ──────────────────────────────────────────────────────────────

fn exec_slice(v: Val, from: Option<i64>, to: Option<i64>) -> Val {
    if let Val::Arr(a) = v {
        let len = a.len() as i64;
        let s = resolve_idx(from.unwrap_or(0), len);
        let e = resolve_idx(to.unwrap_or(len), len);
        let items = Arc::try_unwrap(a).unwrap_or_else(|a| (*a).clone());
        let s = s.min(items.len());
        let e = e.min(items.len());
        Val::arr(items[s..e].to_vec())
    } else { Val::Null }
}

fn resolve_idx(i: i64, len: i64) -> usize {
    (if i < 0 { (len + i).max(0) } else { i }) as usize
}

fn collect_desc(v: &Val, name: &str, out: &mut Vec<Val>) {
    match v {
        Val::Obj(m) => {
            if let Some(v) = m.get(name) { out.push(v.clone()); }
            for v in m.values() { collect_desc(v, name, out); }
        }
        Val::Arr(a) => { for item in a.as_ref() { collect_desc(item, name, out); } }
        _ => {}
    }
}

fn collect_all(v: &Val, out: &mut Vec<Val>) {
    match v {
        Val::Obj(m) => {
            out.push(v.clone());
            for child in m.values() { collect_all(child, out); }
        }
        Val::Arr(a) => {
            for item in a.as_ref() { collect_all(item, out); }
        }
        other => out.push(other.clone()),
    }
}

/// Path-tracking variant — called when descending from root so paths are
/// root-relative and can be cached for future `RootChain` lookups.
/// `prefix` is mutated in-place (push/truncate) to avoid allocations.
fn collect_desc_with_paths(
    v: &Val, name: &str, prefix: &mut String,
    out: &mut Vec<Val>, cached: &mut Vec<(Arc<str>, Val)>,
) {
    match v {
        Val::Obj(m) => {
            if let Some(found) = m.get(name) {
                let mut path = prefix.clone();
                path.push('/');
                path.push_str(name);
                out.push(found.clone());
                cached.push((Arc::from(path.as_str()), found.clone()));
            }
            for (k, child) in m.iter() {
                let prev = prefix.len();
                prefix.push('/');
                prefix.push_str(k.as_ref());
                collect_desc_with_paths(child, name, prefix, out, cached);
                prefix.truncate(prev);
            }
        }
        Val::Arr(a) => {
            for (i, item) in a.iter().enumerate() {
                let prev = prefix.len();
                prefix.push('/');
                let idx = i.to_string();
                prefix.push_str(&idx);
                collect_desc_with_paths(item, name, prefix, out, cached);
                prefix.truncate(prev);
            }
        }
        _ => {}
    }
}

fn resolve_pointer(root: &Val, ptr: &str) -> Val {
    let mut cur = root.clone();
    for seg in ptr.split('/').filter(|s| !s.is_empty()) {
        cur = cur.get_field(seg);
    }
    cur
}

fn bind_comp_vars(env: &Env, vars: &[Arc<str>], item: Val) -> Env {
    match vars {
        [] => env.with_current(item),
        [v] => { let mut e = env.with_var(v.as_ref(), item.clone()); e.current = item; e }
        [v1, v2, ..] => {
            let idx = item.get("index").cloned().unwrap_or(Val::Null);
            let val = item.get("value").cloned().unwrap_or_else(|| item.clone());
            let mut e = env.with_var(v1.as_ref(), idx).with_var(v2.as_ref(), val.clone());
            e.current = val;
            e
        }
    }
}

fn exec_cast(v: &Val, ty: super::ast::CastType) -> Result<Val, EvalError> {
    use super::ast::CastType;
    match ty {
        CastType::Str => Ok(Val::Str(Arc::from(match v {
            Val::Null     => "null".to_string(),
            Val::Bool(b)  => b.to_string(),
            Val::Int(n)   => n.to_string(),
            Val::Float(f) => f.to_string(),
            Val::Str(s)   => s.to_string(),
            other         => super::eval::util::val_to_string(other),
        }.as_str()))),
        CastType::Bool => Ok(Val::Bool(match v {
            Val::Null     => false,
            Val::Bool(b)  => *b,
            Val::Int(n)   => *n != 0,
            Val::Float(f) => *f != 0.0,
            Val::Str(s)   => !s.is_empty(),
            Val::Arr(a)   => !a.is_empty(),
            Val::Obj(o)   => !o.is_empty(),
        })),
        CastType::Number | CastType::Float => match v {
            Val::Int(n)   => Ok(Val::Float(*n as f64)),
            Val::Float(_) => Ok(v.clone()),
            Val::Str(s)   => s.parse::<f64>().map(Val::Float)
                              .map_err(|e| EvalError(format!("as float: {}", e))),
            Val::Bool(b)  => Ok(Val::Float(if *b { 1.0 } else { 0.0 })),
            Val::Null     => Ok(Val::Float(0.0)),
            _             => err!("as float: cannot convert"),
        },
        CastType::Int => match v {
            Val::Int(_)   => Ok(v.clone()),
            Val::Float(f) => Ok(Val::Int(*f as i64)),
            Val::Str(s)   => s.parse::<i64>().map(Val::Int)
                              .or_else(|_| s.parse::<f64>().map(|f| Val::Int(f as i64)))
                              .map_err(|e| EvalError(format!("as int: {}", e))),
            Val::Bool(b)  => Ok(Val::Int(if *b { 1 } else { 0 })),
            Val::Null     => Ok(Val::Int(0)),
            _             => err!("as int: cannot convert"),
        },
        CastType::Array => match v {
            Val::Arr(_)   => Ok(v.clone()),
            Val::Null     => Ok(Val::arr(Vec::new())),
            other         => Ok(Val::arr(vec![other.clone()])),
        },
        CastType::Object => match v {
            Val::Obj(_)   => Ok(v.clone()),
            _             => err!("as object: cannot convert non-object"),
        },
        CastType::Null => Ok(Val::Null),
    }
}

fn apply_bind_to_env(target: &BindTarget, val: &Val, env: Env) -> Result<Env, EvalError> {
    match target {
        BindTarget::Name(name) => Ok(env.with_var(name, val.clone())),
        BindTarget::Obj { fields, rest } => {
            let obj = val.as_object()
                .ok_or_else(|| EvalError("bind destructure: expected object".into()))?;
            let mut e = env;
            for f in fields {
                e = e.with_var(f, obj.get(f.as_str()).cloned().unwrap_or(Val::Null));
            }
            if let Some(rest_name) = rest {
                let mut rem: IndexMap<Arc<str>, Val> = IndexMap::new();
                for (k, v) in obj {
                    if !fields.iter().any(|f| f.as_str() == k.as_ref()) {
                        rem.insert(k.clone(), v.clone());
                    }
                }
                e = e.with_var(rest_name, Val::obj(rem));
            }
            Ok(e)
        }
        BindTarget::Arr(names) => {
            let arr = val.as_array()
                .ok_or_else(|| EvalError("bind destructure: expected array".into()))?;
            let mut e = env;
            for (i, name) in names.iter().enumerate() {
                e = e.with_var(name, arr.get(i).cloned().unwrap_or(Val::Null));
            }
            Ok(e)
        }
    }
}

fn apply_fmt_spec(val: &Val, spec: &str) -> String {
    if let Some(rest) = spec.strip_suffix('f') {
        if let Some(prec_str) = rest.strip_prefix('.') {
            if let Ok(prec) = prec_str.parse::<usize>() {
                if let Some(f) = val.as_f64() { return format!("{:.prec$}", f); }
            }
        }
    }
    if spec == "d" { if let Some(i) = val.as_i64() { return format!("{}", i); } }
    let s = val_to_string(val);
    if let Some(w) = spec.strip_prefix('>').and_then(|s| s.parse::<usize>().ok()) { return format!("{:>w$}", s); }
    if let Some(w) = spec.strip_prefix('<').and_then(|s| s.parse::<usize>().ok()) { return format!("{:<w$}", s); }
    if let Some(w) = spec.strip_prefix('^').and_then(|s| s.parse::<usize>().ok()) { return format!("{:^w$}", s); }
    if let Some(w) = spec.strip_prefix('0').and_then(|s| s.parse::<usize>().ok()) {
        if let Some(i) = val.as_i64() { return format!("{:0>w$}", i); }
    }
    s
}

// ── Opcode helpers ────────────────────────────────────────────────────────────

/// Build a no-arg `CallMethod` opcode (used by peephole strength reduction).
/// Newtype wrapper giving `Val` an `Ord` derived from `cmp_vals`,
/// so it can be used in `BinaryHeap` for TopN.
struct WrapVal(Val);
impl PartialEq for WrapVal { fn eq(&self, o: &Self) -> bool {
    super::eval::util::cmp_vals(&self.0, &o.0) == std::cmp::Ordering::Equal
} }
impl Eq for WrapVal {}
impl PartialOrd for WrapVal { fn partial_cmp(&self, o: &Self) -> Option<std::cmp::Ordering> {
    Some(self.cmp(o))
} }
impl Ord for WrapVal { fn cmp(&self, o: &Self) -> std::cmp::Ordering {
    super::eval::util::cmp_vals(&self.0, &o.0)
} }

/// Extract a literal string from a single-op program `[PushStr(s)]`.
fn const_str_program(p: &Arc<Program>) -> Option<Arc<str>> {
    match p.ops.as_ref() {
        [Opcode::PushStr(s)] => Some(s.clone()),
        _ => None,
    }
}

fn make_noarg_call(method: BuiltinMethod, name: &str) -> Opcode {
    Opcode::CallMethod(Arc::new(CompiledCall {
        method,
        name:      Arc::from(name),
        sub_progs: Arc::from(&[] as &[Arc<Program>]),
        orig_args: Arc::from(&[] as &[Arg]),
    }))
}

// ── Hash helpers ──────────────────────────────────────────────────────────────

fn hash_str(s: &str) -> u64 {
    let mut h = DefaultHasher::new();
    s.hash(&mut h);
    h.finish()
}

/// Hash the *structure* (keys + types) of a Val for the resolution cache key.
/// Does NOT hash values, only shape, so structural navigation results are
/// stable across different values in the same-shaped document.
fn hash_val_structure(v: &Val) -> u64 {
    let mut h = DefaultHasher::new();
    hash_structure_into(v, &mut h, 0);
    h.finish()
}

fn hash_structure_into(v: &Val, h: &mut DefaultHasher, depth: usize) {
    if depth > 8 { return; }
    match v {
        Val::Null       => 0u8.hash(h),
        Val::Bool(b)    => { 1u8.hash(h); b.hash(h); }
        Val::Int(n)     => { 2u8.hash(h); n.hash(h); }
        Val::Float(f)   => { 3u8.hash(h); f.to_bits().hash(h); }
        Val::Str(s)     => { 4u8.hash(h); s.hash(h); }
        Val::Arr(a)     => { 5u8.hash(h); a.len().hash(h); for item in a.iter() { hash_structure_into(item, h, depth+1); } }
        Val::Obj(m)     => { 6u8.hash(h); m.len().hash(h); for (k, v) in m.iter() { k.hash(h); hash_structure_into(v, h, depth+1); } }
    }
}