lex-vcs 0.4.0

//! The `Operation` enum + `OperationRecord` (operation plus its
//! causal parents and resulting `OpId`).
//!
//! See `lib.rs` for the design context and #129 for the issue.

use indexmap::IndexSet;
use serde::{Deserialize, Serialize};
use std::collections::{BTreeMap, BTreeSet};

use crate::canonical;

/// Signature identity of a function or type — the part that stays
/// stable across body edits. Wraps the same string identity
/// `lex-store` uses; we keep it as `String` here so this crate has
/// no dependency on `lex-store`'s internals.
pub type SigId = String;

/// Content hash of a single stage (function body, type def, ...).
/// Same string identity as the file under `<root>/stages/<SigId>/
/// implementations/<StageId>.ast.json`.
pub type StageId = String;

/// Identity of an operation. `(kind, payload, parents)` SHA-256 in
/// lowercase hex (64 chars). Two operations with identical payloads
/// and parent sets produce identical `OpId`s; the store dedupes on
/// this.
pub type OpId = String;

/// Sorted set of effect-kind strings (e.g. `["fs_write", "io"]`).
/// `BTreeSet` so the canonical form is order-independent for
/// hashing.
pub type EffectSet = BTreeSet<String>;

/// Reference to an imported module — either a stdlib name
/// (`std.io`) or a local path (`./helpers`). Kept as a string so
/// this crate doesn't pull in `lex-syntax`'s parser.
pub type ModuleRef = String;

/// Version tag for the operation canonical form (#244).
///
/// The pre-image bytes hashed to derive an `OpId` are not stable
/// across schema evolutions: adding a field to `OperationKind` or
/// changing its serde representation rotates every existing `OpId`.
/// This enum tags the encoding used so a long-lived store can detect
/// mismatches and migrate explicitly via [`crate::migrate`].
///
/// **Today only [`Self::V1`] is in production.** Adding a future
/// variant requires:
///
/// 1. A new arm in [`Operation::canonical_bytes_in`].
/// 2. An update to the canonical-form spec in [`crate::canonical`].
/// 3. A `CHANGELOG.md` entry under `### Internal` calling out the
///    `OpId` rotation.
/// 4. A migration recipe via [`crate::migrate::plan_migration`] —
///    the mechanism is encoder-agnostic, but each new variant needs
///    its own `canonical_bytes_in` arm.
#[derive(
    Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize, Default,
)]
#[serde(rename_all = "lowercase")]
pub enum OperationFormat {
    #[default]
    V1,
}

impl OperationFormat {
    /// The format every newly-emitted op uses today.
    pub const CURRENT: OperationFormat = OperationFormat::V1;

    /// `true` for the implicit format (V1). Used by the
    /// `skip_serializing_if` hook on [`OperationRecord::format_version`]
    /// so existing V1 stores keep byte-identical on-disk JSON —
    /// adding the version field doesn't itself rotate any `OpId`.
    pub fn is_implicit(&self) -> bool {
        matches!(self, OperationFormat::V1)
    }
}

/// Effect of applying an operation on a stage's content-addressed
/// identity. Used as the `produces` field of an [`OperationRecord`]
/// so consumers can answer "after this op, what's the head stage
/// for this SigId?" without rerunning the apply step.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(tag = "kind", rename_all = "snake_case")]
pub enum StageTransition {
    /// New SigId; produces a stage that didn't exist before.
    Create { sig_id: SigId, stage_id: StageId },
    /// Existing SigId; replaces its head stage.
    Replace { sig_id: SigId, from: StageId, to: StageId },
    /// SigId removed; no head stage afterwards.
    Remove { sig_id: SigId, last: StageId },
    /// SigId renamed; same body hash, different signature identity.
    Rename { from: SigId, to: SigId, body_stage_id: StageId },
    /// Import-only change; doesn't touch any stage.
    ImportOnly,
    /// Merge op result. `entries` lists only the sigs whose head
    /// changed relative to the merge op's first parent (`dst_head`):
    /// `Some(stage_id)` sets the head; `None` removes the sig.
    /// Sigs unaffected by the merge are not listed.
    ///
    /// **Canonical-form contract:** `BTreeMap` is load-bearing —
    /// iteration is sorted by `SigId`, so on-disk JSON for two
    /// callers that resolved the same conflicts in different
    /// orders produces byte-identical output. Switching to
    /// `HashMap` here would break canonical stability of the
    /// `OperationRecord` JSON file and is rejected by the
    /// canonical-form spec in `crate::canonical`.
    Merge {
        entries: BTreeMap<SigId, Option<StageId>>,
    },
}

/// The kinds of operations that produce stage transitions. Mirrors
/// the initial set in #129; new kinds (`MoveBetweenFiles`,
/// `SplitFunction`, `ExtractType`) can be added later as long as
/// they're appended at the end of this enum or use explicit
/// `#[serde(rename = "...")]` tags so existing `OpId`s stay stable.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(tag = "op", rename_all = "snake_case")]
pub enum OperationKind {
    /// New function published. `effects` is the effect set declared
    /// in the signature; tracked here (not just inside the stage)
    /// so #130's write-time gate has a cheap path to check effect
    /// changes without rehydrating the AST.
    ///
    /// `budget_cost` (#247) records the function's declared
    /// `[budget(N)]` cost. Optional with `skip_serializing_if`, so
    /// pre-#247 ops without a declared budget continue to hash to
    /// their original `OpId` (additive serialization, same trick
    /// `intent_id` uses). `None` means the function declared no
    /// budget effect; `Some(n)` is the literal `n` from
    /// `[budget(n)]`.
    AddFunction {
        sig_id: SigId,
        stage_id: StageId,
        effects: EffectSet,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        budget_cost: Option<u64>,
    },
    /// Function removed; `last_stage_id` is the head before the
    /// remove (so blame can walk the predecessor without scanning).
    RemoveFunction {
        sig_id: SigId,
        last_stage_id: StageId,
    },
    /// Function body changed; signature unchanged.
    ///
    /// `from_budget` / `to_budget` (#247) record the declared
    /// `[budget(N)]` on each side. Same `Option` + `skip` discipline
    /// as `AddFunction.budget_cost` — pre-#247 ops keep their
    /// `OpId`s. The pair is what `lex op log --budget-drift` reads
    /// to surface "budget grew/shrank" diffs without rehydrating
    /// stages.
    ModifyBody {
        sig_id: SigId,
        from_stage_id: StageId,
        to_stage_id: StageId,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        from_budget: Option<u64>,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        to_budget: Option<u64>,
    },
    /// Symbol renamed. The body hash is preserved (`body_stage_id`)
    /// so two renames of the same body collapse to the same OpId
    /// and `lex blame` walks the rename as a single causal event
    /// rather than `delete + add`.
    RenameSymbol {
        from: SigId,
        to: SigId,
        body_stage_id: StageId,
    },
    /// Effect signature changed. Captures both old and new effect
    /// sets so the write-time gate (#130) can verify importers
    /// haven't silently broken.
    ///
    /// `from_budget` / `to_budget` (#247) capture the declared
    /// `[budget(N)]` on each side. ChangeEffectSig usually fires
    /// because the effect *list* changed; #247 makes budget drift
    /// visible without forcing a full effect-set diff.
    ChangeEffectSig {
        sig_id: SigId,
        from_stage_id: StageId,
        to_stage_id: StageId,
        from_effects: EffectSet,
        to_effects: EffectSet,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        from_budget: Option<u64>,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        to_budget: Option<u64>,
    },
    /// Import added to a file. `in_file` is the canonical path
    /// (relative to the repo root, forward-slashes) so two
    /// machines hashing the same edit get the same OpId.
    AddImport {
        in_file: String,
        module: ModuleRef,
    },
    RemoveImport {
        in_file: String,
        module: ModuleRef,
    },
    AddType {
        sig_id: SigId,
        stage_id: StageId,
    },
    RemoveType {
        sig_id: SigId,
        last_stage_id: StageId,
    },
    ModifyType {
        sig_id: SigId,
        from_stage_id: StageId,
        to_stage_id: StageId,
    },
    /// Merge of two branch heads. Carries only an informational count
    /// of resolved sigs so two structurally identical merges of
    /// different sizes don't collide on op_id; the per-sig deltas live
    /// in `OperationRecord::produces` (`StageTransition::Merge`).
    Merge {
        resolved: usize,
    },
}

impl OperationKind {
    /// The `(SigId, Option<StageId>)` an op kind targets, as used by
    /// `StageTransition::Merge::entries`. Used by the merge-commit
    /// path (#134) to translate a `Resolution::Custom { op }` into
    /// the head-map delta the merge op records:
    ///
    /// * Adds → `(sig, Some(stage_id))`
    /// * Modifies → `(sig, Some(to_stage_id))`
    /// * Removes → `(sig, None)`
    /// * Renames → `(to_sig, Some(body_stage_id))`
    /// * `AddImport` / `RemoveImport` / nested `Merge` → `None`
    ///   (no single sig→stage delta)
    pub fn merge_target(&self) -> Option<(SigId, Option<StageId>)> {
        use OperationKind::*;
        match self {
            AddFunction { sig_id, stage_id, .. }
            | AddType { sig_id, stage_id }
                => Some((sig_id.clone(), Some(stage_id.clone()))),
            ModifyBody { sig_id, to_stage_id, .. }
            | ChangeEffectSig { sig_id, to_stage_id, .. }
            | ModifyType { sig_id, to_stage_id, .. }
                => Some((sig_id.clone(), Some(to_stage_id.clone()))),
            RemoveFunction { sig_id, .. }
            | RemoveType { sig_id, .. }
                => Some((sig_id.clone(), None)),
            RenameSymbol { to, body_stage_id, .. }
                => Some((to.clone(), Some(body_stage_id.clone()))),
            AddImport { .. } | RemoveImport { .. } | Merge { .. } => None,
        }
    }

    /// `(from_budget, to_budget)` for ops that carry a budget delta
    /// (#247). `(None, None)` for ops where the budget isn't part
    /// of the canonical payload — `RemoveFunction`, `RenameSymbol`,
    /// imports, and merges. `AddFunction` reports `(None,
    /// Some(cost))` for "this is the initial cost." Used by `lex op
    /// show`, `lex op log --budget-drift`, and `lex audit --budget`.
    pub fn budget_delta(&self) -> (Option<u64>, Option<u64>) {
        use OperationKind::*;
        match self {
            AddFunction { budget_cost, .. } => (None, *budget_cost),
            ModifyBody { from_budget, to_budget, .. }
            | ChangeEffectSig { from_budget, to_budget, .. } => (*from_budget, *to_budget),
            _ => (None, None),
        }
    }

    /// The `SigId` an op touches if it carries a budget — used for
    /// per-sig audit rollups in `lex audit --budget`. Returns `None`
    /// for ops without a relevant budget (the same set as the
    /// `_ => (None, None)` arm of [`Self::budget_delta`]).
    pub fn budget_sig(&self) -> Option<&SigId> {
        use OperationKind::*;
        match self {
            AddFunction { sig_id, .. }
            | ModifyBody { sig_id, .. }
            | ChangeEffectSig { sig_id, .. } => Some(sig_id),
            _ => None,
        }
    }
}

/// Extract the declared `[budget(N)]` integer from an [`EffectSet`],
/// if any (#247).
///
/// Effect labels in [`EffectSet`] are produced by
/// [`crate::compute_diff::effect_label`]: a `[budget(50)]`
/// declaration becomes the literal string `"budget(50)"`. This
/// helper parses that literal back to the integer; bare `"budget"`
/// (no arg) returns `None` because the magnitude is unknown. A
/// stage with multiple budget declarations — which the type-
/// checker should reject anyway — picks the smallest, conservative
/// answer for `lex audit --budget`.
pub fn budget_from_effects(effects: &EffectSet) -> Option<u64> {
    let mut min_cost: Option<u64> = None;
    for label in effects {
        let Some(rest) = label.strip_prefix("budget(") else { continue };
        let Some(inner) = rest.strip_suffix(')') else { continue };
        let Ok(n) = inner.parse::<u64>() else { continue };
        min_cost = Some(min_cost.map(|c| c.min(n)).unwrap_or(n));
    }
    min_cost
}

/// The operation as a whole — its kind and the causal predecessors
/// it assumes. The `OpId` is computed from this plus a sorted view
/// of `parents`.
///
/// Operations without parents are valid and represent "applies to
/// the empty repository" or "applies to the synthetic genesis
/// state." `lex store migrate v1→v2` will produce parentless ops
/// for stages it can't trace back to a clear predecessor.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Operation {
    #[serde(flatten)]
    pub kind: OperationKind,
    /// Operations whose `produces` this op assumes. Sorted before
    /// hashing for canonical form. Empty for ops against the empty
    /// repo.
    #[serde(default, skip_serializing_if = "Vec::is_empty")]
    pub parents: Vec<OpId>,
    /// The intent that caused this op, if known. Optional because
    /// operations produced outside an agent harness (e.g. a human
    /// running `lex publish` directly) don't have one.
    ///
    /// Including the intent in the canonical hash means the same
    /// logical change made under different intents produces
    /// different `OpId`s — causally distinct events should hash
    /// distinctly. Ops with `intent_id: None` keep their existing
    /// hashes (the field is omitted from the canonical JSON via
    /// `skip_serializing_if`), so this is backwards-compatible
    /// for stores written before #131.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub intent_id: Option<crate::intent::IntentId>,
}

impl Operation {
    /// Construct an operation against zero or more parents. Caller
    /// supplies parents in any order; canonicalization sorts them
    /// before hashing.
    pub fn new(kind: OperationKind, parents: impl IntoIterator<Item = OpId>) -> Self {
        let mut parents: Vec<OpId> = parents.into_iter().collect();
        parents.sort();
        parents.dedup();
        Self { kind, parents, intent_id: None }
    }

    /// Tag this operation with the intent that produced it. The
    /// builder shape keeps existing call sites untouched; agent
    /// harnesses that record intent call this once before
    /// applying the op.
    pub fn with_intent(mut self, intent_id: impl Into<crate::intent::IntentId>) -> Self {
        self.intent_id = Some(intent_id.into());
        self
    }

    /// Compute this operation's content-addressed identity under the
    /// current production canonical form ([`OperationFormat::CURRENT`]).
    ///
    /// Stable across runs and machines: same `(kind, payload,
    /// sorted parents, intent_id)` produces the same `OpId`. The
    /// invariant #129's automatic-dedup behavior relies on.
    pub fn op_id(&self) -> OpId {
        self.op_id_in(OperationFormat::CURRENT)
    }

    /// Compute the `OpId` under a specific canonical-form version.
    ///
    /// Used by [`crate::migrate`] to derive new `OpId`s when porting
    /// a store across format versions. Production code should call
    /// [`Self::op_id`].
    pub fn op_id_in(&self, format: OperationFormat) -> OpId {
        canonical::hash_bytes(&self.canonical_bytes_in(format))
    }

    /// The byte sequence that gets hashed to produce [`Self::op_id`]
    /// under the current canonical form. Equivalent to
    /// `self.canonical_bytes_in(OperationFormat::CURRENT)`.
    ///
    /// Exposed (not just consumed by `op_id`) so golden tests can pin
    /// the exact pre-image. **Not** equal to `serde_json::to_vec(&op)`
    /// in general — the on-disk JSON skips empty `parents` and
    /// `None` `intent_id`, while the canonical form always emits a
    /// (sorted, deduped) `parents` array. See `canonical.rs` for the
    /// full V1 canonical-form spec.
    pub fn canonical_bytes(&self) -> Vec<u8> {
        self.canonical_bytes_in(OperationFormat::CURRENT)
    }

    /// The pre-image hashed under a specific canonical-form version.
    ///
    /// Today every `OperationFormat` variant routes to V1's encoder
    /// (only V1 exists in production). When V2 lands, this match
    /// gains an arm and the migration tool's encoder closure routes
    /// here.
    pub fn canonical_bytes_in(&self, format: OperationFormat) -> Vec<u8> {
        match format {
            OperationFormat::V1 => self.canonical_bytes_v1(),
        }
    }

    fn canonical_bytes_v1(&self) -> Vec<u8> {
        // Build a transient hashable view rather than hashing
        // `self` directly so the parent ordering is canonical
        // even if a caller hand-constructs an `Operation` with
        // unsorted parents.
        let canonical = CanonicalView {
            kind: &self.kind,
            parents: self.parents.iter().collect::<IndexSet<_>>().into_iter().collect::<BTreeSet<_>>(),
            intent_id: self.intent_id.as_deref(),
        };
        serde_json::to_vec(&canonical).expect("canonical serialization")
    }
}

/// Hashable shadow of [`Operation`] with parents in a `BTreeSet` so
/// the serialization is order-independent regardless of how the
/// caller constructed the live operation. Never persisted; lives
/// only as a transient for hashing.
#[derive(Serialize)]
struct CanonicalView<'a> {
    #[serde(flatten)]
    kind: &'a OperationKind,
    parents: BTreeSet<&'a OpId>,
    /// `skip_serializing_if = "Option::is_none"` keeps existing
    /// `OpId`s stable for ops without an intent — the field is
    /// omitted from the canonical JSON entirely.
    #[serde(skip_serializing_if = "Option::is_none")]
    intent_id: Option<&'a str>,
}

/// An operation paired with its computed `OpId` and the resulting
/// stage transition. This is what gets persisted under
/// `<root>/ops/<OpId>.json`.
///
/// `format_version` records the canonical form the `op_id` was
/// computed under. Pre-#244 stores didn't emit this field; reading
/// such records deserializes to [`OperationFormat::V1`] (the
/// implicit pre-versioning format), and writing V1 records continues
/// to omit it (`skip_serializing_if = is_implicit`) so adding the
/// field doesn't rotate any existing `OpId` or change any on-disk
/// byte. Records written under a future format will explicitly
/// carry their version tag.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct OperationRecord {
    pub op_id: OpId,
    #[serde(default, skip_serializing_if = "OperationFormat::is_implicit")]
    pub format_version: OperationFormat,
    #[serde(flatten)]
    pub op: Operation,
    pub produces: StageTransition,
}

impl OperationRecord {
    pub fn new(op: Operation, produces: StageTransition) -> Self {
        let op_id = op.op_id();
        Self { op_id, format_version: OperationFormat::CURRENT, op, produces }
    }
}

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

    fn add_factorial() -> OperationKind {
        OperationKind::AddFunction {
            sig_id: "fac::Int->Int".into(),
            stage_id: "abc123".into(),
            effects: BTreeSet::new(),
            budget_cost: None,
        }
    }

    #[test]
    fn identical_operations_have_identical_op_ids() {
        let a = Operation::new(add_factorial(), []);
        let b = Operation::new(add_factorial(), []);
        assert_eq!(a.op_id(), b.op_id());
    }

    #[test]
    fn different_operations_have_different_op_ids() {
        let a = Operation::new(add_factorial(), []);
        let b = Operation::new(
            OperationKind::AddFunction {
                sig_id: "double::Int->Int".into(),
                stage_id: "abc123".into(),
                effects: BTreeSet::new(),
                budget_cost: None,
            },
            [],
        );
        assert_ne!(a.op_id(), b.op_id());
    }

    #[test]
    fn parent_set_changes_op_id() {
        let no_parent = Operation::new(add_factorial(), []);
        let with_parent = Operation::new(add_factorial(), ["op-parent-1".into()]);
        assert_ne!(no_parent.op_id(), with_parent.op_id());
    }

    #[test]
    fn parent_order_does_not_affect_op_id() {
        let a = Operation::new(add_factorial(), ["b".into(), "a".into(), "c".into()]);
        let b = Operation::new(add_factorial(), ["c".into(), "a".into(), "b".into()]);
        assert_eq!(a.op_id(), b.op_id());
        // and the stored form is sorted.
        assert_eq!(a.parents, vec!["a".to_string(), "b".to_string(), "c".to_string()]);
    }

    #[test]
    fn duplicate_parents_are_deduped() {
        let with_dups = Operation::new(
            add_factorial(),
            ["a".into(), "a".into(), "b".into()],
        );
        let no_dups = Operation::new(
            add_factorial(),
            ["a".into(), "b".into()],
        );
        assert_eq!(with_dups.op_id(), no_dups.op_id());
        assert_eq!(with_dups.parents, vec!["a".to_string(), "b".to_string()]);
    }

    #[test]
    fn rename_with_same_body_hashes_equal_across_runs() {
        // Two independent runs producing the same rename against the
        // same parent should produce the same OpId — this is the
        // automatic-dedup property #129 relies on for distributed
        // agents.
        let kind = OperationKind::RenameSymbol {
            from: "parse::Str->Int".into(),
            to: "parse_int::Str->Int".into(),
            body_stage_id: "abc123".into(),
        };
        let a = Operation::new(kind.clone(), ["op-parent".into()]);
        let b = Operation::new(kind, ["op-parent".into()]);
        assert_eq!(a.op_id(), b.op_id());
    }

    #[test]
    fn rename_does_not_collide_with_delete_plus_add() {
        // The whole point of `RenameSymbol` is that it's a different
        // OpId from the (semantically-equivalent) `RemoveFunction +
        // AddFunction` pair. Causal history sees one event, not two.
        let rename = Operation::new(
            OperationKind::RenameSymbol {
                from: "parse::Str->Int".into(),
                to: "parse_int::Str->Int".into(),
                body_stage_id: "abc123".into(),
            },
            ["op-parent".into()],
        );
        let remove = Operation::new(
            OperationKind::RemoveFunction {
                sig_id: "parse::Str->Int".into(),
                last_stage_id: "abc123".into(),
            },
            ["op-parent".into()],
        );
        let add = Operation::new(
            OperationKind::AddFunction {
                sig_id: "parse_int::Str->Int".into(),
                stage_id: "abc123".into(),
                effects: BTreeSet::new(),
                budget_cost: None,
            },
            ["op-parent".into()],
        );
        assert_ne!(rename.op_id(), remove.op_id());
        assert_ne!(rename.op_id(), add.op_id());
    }

    #[test]
    fn effect_set_order_does_not_affect_op_id() {
        // Effects are a BTreeSet so iteration is sorted. Build two
        // ops via different insertion orders and confirm the
        // canonical form is identical.
        let a_effects: EffectSet = ["io".into(), "fs_write".into()].into_iter().collect();
        let b_effects: EffectSet = ["fs_write".into(), "io".into()].into_iter().collect();
        let a = Operation::new(
            OperationKind::AddFunction {
                sig_id: "x".into(), stage_id: "s".into(), effects: a_effects,
                budget_cost: None,
            },
            [],
        );
        let b = Operation::new(
            OperationKind::AddFunction {
                sig_id: "x".into(), stage_id: "s".into(), effects: b_effects,
                budget_cost: None,
            },
            [],
        );
        assert_eq!(a.op_id(), b.op_id());
    }

    #[test]
    fn op_id_is_64_char_lowercase_hex() {
        let id = Operation::new(add_factorial(), []).op_id();
        assert_eq!(id.len(), 64);
        assert!(id.chars().all(|c| c.is_ascii_digit() || ('a'..='f').contains(&c)));
    }

    #[test]
    fn round_trip_through_serde_json() {
        let op = Operation::new(
            OperationKind::ChangeEffectSig {
                sig_id: "f".into(),
                from_stage_id: "old".into(),
                to_stage_id: "new".into(),
                from_effects: BTreeSet::new(),
                to_effects: ["io".into()].into_iter().collect(),
                from_budget: None,
                to_budget: None,
            },
            ["op-parent".into()],
        );
        let json = serde_json::to_string(&op).expect("serialize");
        let back: Operation = serde_json::from_str(&json).expect("deserialize");
        assert_eq!(op, back);
        assert_eq!(op.op_id(), back.op_id());
    }

    #[test]
    fn operation_record_carries_op_id() {
        let op = Operation::new(add_factorial(), []);
        let expected = op.op_id();
        let rec = OperationRecord::new(
            op,
            StageTransition::Create {
                sig_id: "fac::Int->Int".into(),
                stage_id: "abc123".into(),
            },
        );
        assert_eq!(rec.op_id, expected);
    }

    #[test]
    fn intent_id_is_part_of_op_id_canonical_hash() {
        // The dedup property: same `(kind, parents, intent_id)`
        // produces the same OpId. Different intent_ids on
        // otherwise-identical ops produce different OpIds, so
        // causally distinct events (different prompts) hash
        // distinctly.
        let no_intent = Operation::new(add_factorial(), []);
        let with_intent_a = Operation::new(add_factorial(), [])
            .with_intent("intent-a");
        let with_intent_b = Operation::new(add_factorial(), [])
            .with_intent("intent-b");
        let with_intent_a_again = Operation::new(add_factorial(), [])
            .with_intent("intent-a");

        // No-intent op is distinct from any intent-tagged variant.
        assert_ne!(no_intent.op_id(), with_intent_a.op_id());
        // Different intents → different OpIds.
        assert_ne!(with_intent_a.op_id(), with_intent_b.op_id());
        // Same intent → same OpId (the load-bearing dedup invariant).
        assert_eq!(with_intent_a.op_id(), with_intent_a_again.op_id());
    }

    #[test]
    fn op_without_intent_keeps_pre_intent_op_id() {
        // Backwards-compat invariant: an op constructed without an
        // intent must hash to the same value as it would have
        // before #131 added the field. The golden test below pins
        // the exact hash; this one asserts that adding then
        // resetting to None doesn't drift.
        let mut op = Operation::new(add_factorial(), []);
        let baseline = op.op_id();
        op.intent_id = Some("transient".into());
        let with_intent = op.op_id();
        assert_ne!(baseline, with_intent);
        op.intent_id = None;
        let back = op.op_id();
        assert_eq!(baseline, back);
    }

    /// Golden hash. If this changes, the canonical form has shifted
    /// and *every* op_id in every existing store has changed too —
    /// that's a major-version event for the data model and should
    /// be a deliberate decision, not an accident from reordering
    /// fields. Update with care.
    #[test]
    fn canonical_form_is_stable_for_a_known_input() {
        let op = Operation::new(
            OperationKind::AddFunction {
                sig_id: "fac::Int->Int".into(),
                stage_id: "abc123".into(),
                effects: BTreeSet::new(),
                budget_cost: None,
            },
            [],
        );
        assert_eq!(
            op.op_id(),
            "f112990d31ef2a63f3e5ca5680637ed36a54bc7e8230510ae0c0e93fcb39d104"
        );
    }

    #[test]
    fn merge_kind_round_trips() {
        let op = Operation::new(
            OperationKind::Merge { resolved: 3 },
            ["op-a".into(), "op-b".into()],
        );
        let json = serde_json::to_string(&op).expect("ser");
        let back: Operation = serde_json::from_str(&json).expect("de");
        assert_eq!(op, back);
        assert_eq!(op.op_id(), back.op_id());
    }

    #[test]
    fn merge_stage_transition_round_trips() {
        let mut entries = BTreeMap::new();
        entries.insert("sig-a".to_string(), Some("stage-a".to_string()));
        entries.insert("sig-b".to_string(), None); // removed by merge
        let t = StageTransition::Merge { entries };
        let json = serde_json::to_string(&t).expect("ser");
        let back: StageTransition = serde_json::from_str(&json).expect("de");
        assert_eq!(t, back);
    }

    #[test]
    fn merge_resolved_count_changes_op_id() {
        // Two merges with the same parents but different resolved counts
        // must hash differently — keeps structurally distinct merges from
        // colliding on op_id.
        let parents: Vec<OpId> = vec!["op-a".into(), "op-b".into()];
        let one = Operation::new(OperationKind::Merge { resolved: 1 }, parents.clone());
        let two = Operation::new(OperationKind::Merge { resolved: 2 }, parents);
        assert_ne!(one.op_id(), two.op_id());
    }

    #[test]
    fn existing_add_function_op_id_is_unchanged_after_merge_added() {
        // Constructing the new Merge variant in the same enum must not
        // perturb the canonical bytes of existing variants. The golden
        // hash test below checks the literal value; this one verifies
        // the property holds even after a Merge op has been built.
        let _merge = Operation::new(
            OperationKind::Merge { resolved: 0 },
            ["op-x".into(), "op-y".into()],
        );
        let op = Operation::new(add_factorial(), []);
        assert_eq!(
            op.op_id(),
            "f112990d31ef2a63f3e5ca5680637ed36a54bc7e8230510ae0c0e93fcb39d104"
        );
    }
}