harn_vm/value/

env.rs

use std::collections::BTreeMap;
use std::path::PathBuf;
use std::sync::{Arc, Weak};

use crate::chunk::CompiledFunctionRef;

use super::{VmError, VmMutex, VmValue};

/// A compiled closure value.
#[derive(Debug, Clone)]
pub struct VmClosure {
    pub func: CompiledFunctionRef,
    pub env: VmEnv,
    /// Source directory for this closure's originating module.
    /// When set, `render()` and other source-relative builtins resolve
    /// paths relative to this directory instead of the entry pipeline.
    pub source_dir: Option<PathBuf>,
    /// Module-local named functions that should resolve before builtin fallback.
    /// This lets selectively imported functions keep private sibling helpers
    /// without exporting them into the caller's environment.
    pub module_functions: Option<WeakModuleFunctionRegistry>,
    /// Shared, mutable module-level env: holds top-level `var` / `let`
    /// bindings declared at the module root (caches, counters, lazily
    /// initialized registries). All closures created from the same
    /// module import point at the same shared mutable env, so a
    /// mutation inside one function is visible to every other function
    /// in that module on subsequent calls. `closure.env` still holds
    /// the per-closure lexical snapshot (captured function args from
    /// enclosing scopes, etc.) and is unchanged by this — `module_state`
    /// is a separate lookup layer consulted after the local env and
    /// before globals. Created in `import_declarations` after the
    /// module's init chunk runs, so the initial values from `var x = ...`
    /// land in it.
    pub module_state: Option<WeakModuleState>,
}

pub type ModuleFunctionRegistry = Arc<VmMutex<BTreeMap<String, Arc<VmClosure>>>>;
pub type WeakModuleFunctionRegistry = Weak<VmMutex<BTreeMap<String, Arc<VmClosure>>>>;
pub type ModuleState = Arc<VmMutex<VmEnv>>;
pub type WeakModuleState = Weak<VmMutex<VmEnv>>;

impl VmClosure {
    pub(crate) fn module_functions(&self) -> Option<ModuleFunctionRegistry> {
        self.module_functions
            .as_ref()
            .and_then(WeakModuleFunctionRegistry::upgrade)
    }

    pub(crate) fn module_state(&self) -> Option<ModuleState> {
        self.module_state
            .as_ref()
            .and_then(WeakModuleState::upgrade)
    }
}

/// VM environment for variable storage.
///
/// `Scope::vars` is wrapped in `Arc` so that `VmEnv::clone()` is cheap
/// (Arc bump per scope) instead of a deep walk of every BTreeMap. The
/// VM saves and restores `env` snapshots on every function call, and
/// the call hot path dominates orchestration-heavy workloads. With
/// `Arc<BTreeMap<..>>`, the per-scope clone collapses to a refcount
/// bump, and `Arc::make_mut` only does a deep copy when the scope is
/// still shared with a saved snapshot — which is exactly the case where
/// the caller would have needed an isolated copy anyway. Reads still go
/// through the `BTreeMap` directly via `Deref`.
#[derive(Debug, Clone)]
pub struct VmEnv {
    pub(crate) scopes: Vec<Scope>,
}

#[derive(Debug, Clone)]
pub(crate) struct Scope {
    pub(crate) vars: Arc<BTreeMap<String, (VmValue, bool)>>, // (value, mutable)
}

/// Process-wide shared empty binding map.
///
/// Every block entry pushes a fresh [`Scope`], but inside a function body its
/// bindings compile to local slots (`DefLocalSlot`) rather than env writes, so
/// the pushed scope is overwhelmingly *empty* — a hot loop whose body is a
/// block would otherwise `Arc::new(BTreeMap::new())`-allocate (and free) one
/// map per iteration. Sharing a single immutable empty map makes
/// [`Scope::empty`] a refcount bump instead; the first real `define`/`assign`
/// copies-on-write away from this shared map via `Arc::make_mut` (the insert
/// paths already do), so a scope that never binds anything never allocates.
static EMPTY_SCOPE_VARS: std::sync::LazyLock<Arc<BTreeMap<String, (VmValue, bool)>>> =
    std::sync::LazyLock::new(|| Arc::new(BTreeMap::new()));

impl Scope {
    #[inline]
    fn empty() -> Self {
        Self {
            vars: Arc::clone(&EMPTY_SCOPE_VARS),
        }
    }
}

impl Drop for Scope {
    fn drop(&mut self) {
        // Deeply nested script values (e.g. `x = [x]` built in a loop, which
        // adds no VM call frames and so never trips `max_vm_frames`) live in
        // scope bindings. Their default recursive drop would overflow the
        // native stack and abort the whole process — an uncatchable failure.
        // When this scope holds the last reference to its bindings and any
        // value is a nested container, tear the bindings down iteratively
        // instead. `Arc::get_mut` succeeds only for a uniquely-owned scope, so
        // shared snapshots fall through to the cheap default drop and the real
        // teardown happens later at the last owner (also a `Scope`).
        if let Some(map) = Arc::get_mut(&mut self.vars) {
            if map
                .values()
                .any(|(value, _)| super::recursion::is_recursive_container(value))
            {
                let bindings = std::mem::take(map);
                super::recursion::dismantle_values(bindings.into_values().map(|(value, _)| value));
            }
        }
    }
}

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

impl VmEnv {
    pub fn new() -> Self {
        Self {
            scopes: vec![Scope::empty()],
        }
    }

    pub fn push_scope(&mut self) {
        self.scopes.push(Scope::empty());
    }

    /// Clone the scope stack for a fresh call frame, reserving room for the
    /// one empty scope every invocation pushes for the callee's body.
    ///
    /// `Vec::clone` allocates at exactly `len` capacity, so the `push_scope`
    /// that immediately follows on the call hot path would otherwise force a
    /// reallocation and copy of the whole scope stack. Reserving the extra
    /// slot up front folds those two allocations into one. When a caller does
    /// not end up pushing (no path currently does, but it stays correct if one
    /// is added), the only cost is a single unused `Scope` slot of capacity.
    pub(crate) fn cloned_for_call(&self) -> VmEnv {
        let mut scopes = Vec::with_capacity(self.scopes.len() + 1);
        scopes.extend(self.scopes.iter().cloned());
        VmEnv { scopes }
    }

    pub fn pop_scope(&mut self) {
        if self.scopes.len() > 1 {
            self.scopes.pop();
        }
    }

    pub fn scope_depth(&self) -> usize {
        self.scopes.len()
    }

    pub fn truncate_scopes(&mut self, target_depth: usize) {
        let min_depth = target_depth.max(1);
        while self.scopes.len() > min_depth {
            self.scopes.pop();
        }
    }

    pub fn get(&self, name: &str) -> Option<VmValue> {
        for scope in self.scopes.iter().rev() {
            if let Some((val, _)) = scope.vars.get(name) {
                return Some(val.clone());
            }
        }
        None
    }

    pub(crate) fn contains(&self, name: &str) -> bool {
        self.scopes
            .iter()
            .rev()
            .any(|scope| scope.vars.contains_key(name))
    }

    pub fn define(&mut self, name: &str, value: VmValue, mutable: bool) -> Result<(), VmError> {
        if let Some(scope) = self.scopes.last_mut() {
            if let Some((_, existing_mutable)) = scope.vars.get(name) {
                if !existing_mutable && !mutable {
                    return Err(VmError::Runtime(format!(
                        "Cannot redeclare immutable variable '{name}' in the same scope (use 'var' for mutable bindings)"
                    )));
                }
            }
            if let Some((previous, _)) =
                Arc::make_mut(&mut scope.vars).insert(name.to_string(), (value, mutable))
            {
                super::recursion::dismantle(previous);
            }
        }
        Ok(())
    }

    pub fn all_variables(&self) -> crate::value::DictMap {
        let mut vars = crate::value::DictMap::new();
        for scope in &self.scopes {
            for (name, (value, _)) in scope.vars.iter() {
                vars.insert(crate::value::intern_key(name), value.clone());
            }
        }
        vars
    }

    pub fn assign(&mut self, name: &str, value: VmValue) -> Result<(), VmError> {
        for scope in self.scopes.iter_mut().rev() {
            if let Some((_, mutable)) = scope.vars.get(name) {
                if !mutable {
                    return Err(VmError::ImmutableAssignment(name.to_string()));
                }
                if let Some((previous, _)) =
                    Arc::make_mut(&mut scope.vars).insert(name.to_string(), (value, true))
                {
                    // Iterative teardown so overwriting a deeply nested binding
                    // cannot overflow the stack on drop (scalars are a no-op).
                    super::recursion::dismantle(previous);
                }
                return Ok(());
            }
        }
        Err(VmError::UndefinedVariable(name.to_string()))
    }

    /// Debugger-only variant of `assign` that rebinds the name even if
    /// the existing binding was declared with `let`. Pipeline authors
    /// overwhelmingly use `let`, so a strict mutability check would
    /// make the DAP `setVariable` request useless for "what-if"
    /// iteration — which is the whole point of the feature. Preserves
    /// the original mutability flag so the VM's runtime behavior is
    /// unchanged after the debugger overrides.
    pub fn assign_debug(&mut self, name: &str, value: VmValue) -> Result<(), VmError> {
        for scope in self.scopes.iter_mut().rev() {
            if let Some((_, mutable)) = scope.vars.get(name) {
                let mutable = *mutable;
                Arc::make_mut(&mut scope.vars).insert(name.to_string(), (value, mutable));
                return Ok(());
            }
        }
        Err(VmError::UndefinedVariable(name.to_string()))
    }
}

/// Compute Levenshtein edit distance between two strings.
fn levenshtein(a: &str, b: &str) -> usize {
    let a: Vec<char> = a.chars().collect();
    let b: Vec<char> = b.chars().collect();
    let (m, n) = (a.len(), b.len());
    let mut prev = (0..=n).collect::<Vec<_>>();
    let mut curr = vec![0; n + 1];
    for i in 1..=m {
        curr[0] = i;
        for j in 1..=n {
            let cost = usize::from(a[i - 1] != b[j - 1]);
            curr[j] = (prev[j] + 1).min(curr[j - 1] + 1).min(prev[j - 1] + cost);
        }
        std::mem::swap(&mut prev, &mut curr);
    }
    prev[n]
}

/// Find the closest match from a list of candidates using Levenshtein distance.
/// Returns `Some(suggestion)` if a candidate is within `max_dist` edits.
pub fn closest_match<'a>(name: &str, candidates: impl Iterator<Item = &'a str>) -> Option<String> {
    let max_dist = match name.len() {
        0..=2 => 1,
        3..=5 => 2,
        _ => 3,
    };
    candidates
        .filter(|c| *c != name && !c.starts_with("__"))
        .map(|c| (c, levenshtein(name, c)))
        .filter(|(_, d)| *d <= max_dist)
        // Prefer smallest distance, then closest length to original, then alphabetical
        .min_by(|(a, da), (b, db)| {
            da.cmp(db)
                .then_with(|| {
                    let a_diff = (a.len() as isize - name.len() as isize).unsigned_abs();
                    let b_diff = (b.len() as isize - name.len() as isize).unsigned_abs();
                    a_diff.cmp(&b_diff)
                })
                .then_with(|| a.cmp(b))
        })
        .map(|(c, _)| c.to_string())
}

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

    #[test]
    fn empty_scopes_share_one_backing_map() {
        // Pushing block scopes (the per-iteration cost in a loop body) must not
        // allocate: every empty scope shares the process-wide empty map.
        let mut env = VmEnv::new();
        env.push_scope();
        env.push_scope();
        for scope in &env.scopes {
            assert!(Arc::ptr_eq(&scope.vars, &EMPTY_SCOPE_VARS));
        }
    }

    #[test]
    fn define_copies_on_write_without_disturbing_siblings() {
        let mut env = VmEnv::new();
        env.push_scope(); // shares EMPTY
        env.define("x", VmValue::Int(1), true).unwrap();
        // The bound scope copied on write away from the shared empty map...
        let top = env.scopes.last().unwrap();
        assert!(!Arc::ptr_eq(&top.vars, &EMPTY_SCOPE_VARS));
        // ...while the root scope (untouched) still shares it.
        assert!(Arc::ptr_eq(&env.scopes[0].vars, &EMPTY_SCOPE_VARS));
        assert!(matches!(env.get("x"), Some(VmValue::Int(1))));
        // Popping the scope drops the binding entirely.
        env.pop_scope();
        assert!(env.get("x").is_none());
    }
}