BREP_render 0.1.0

//! [`History`] — the engine-owned, editable model recipe: the ordered feature
//! history plus the rollback index (the feature the model is currently built up
//! to). This is the SINGLE SOURCE OF TRUTH for the model — the UI keeps NO copy;
//! it mutates and reads the history only through [`crate::engine_state::EngineState`]
//! methods. That keeps one engine-owned history (UI-agnostic) and converges with
//! the in-flight "whole history in Rust" pipeline migration — later this sinks
//! into `brep-kernel-rs` proper without touching the UI.
//!
//! Rolling to a step re-runs `features[0..=rollback]` through the SAME kernel
//! pipeline: the full feature list stays in the request and `stopAtId` (the
//! editor's "stop at the expanded feature") halts execution AFTER the rolled-to
//! feature, so the kernel's incremental history cache is RETAINED across rolls
//! (no thrash) and the viewport shows the model as of that step.

use serde_json::Value;

/// The cap on the undo (and redo) stack depth — old entries fall off the bottom.
const MAX_UNDO: usize = 100;

/// A restorable model state: the whole document PLUS the rolled-to step, captured
/// together so an undo returns both the geometry and the view to the state they
/// were in right before the mutation.
#[derive(Debug, Clone)]
struct Snapshot {
    request: Value,
    rollback: usize,
}

/// The engine-owned mutable history.
#[derive(Debug, Clone)]
pub struct History {
    /// The whole `HistoryRequest` document
    /// (`{expressions, configurator, features: [...]}`).
    request: Value,
    /// Index into `features` the model is rolled to (clamped to the last).
    rollback: usize,
    /// Undo/redo over the MODEL document. A snapshot is pushed BEFORE each model
    /// mutation (edit / add / delete / reorder); roll-to-step is view state and
    /// is NOT snapshotted. Rapid same-target edits (a slider drag) coalesce into a
    /// single undo entry via `last_edit_key`. The stacks live here in the engine
    /// core — the model is engine-owned, so its undo history is too; the UI only
    /// triggers `undo()` / `redo()`.
    undo_stack: Vec<Snapshot>,
    redo_stack: Vec<Snapshot>,
    /// The coalescing token of the most recently recorded edit (see `checkpoint`).
    last_edit_key: Option<String>,
    /// The persistent GLOBAL feature counter: bumped by one on every new-feature
    /// mint ([`Self::next_feature_id`]), so a new id is `{shortName}{counter}`.
    /// MONOTONIC and NEVER reused — deleting a feature does not free its number,
    /// and (deliberately) undo does NOT rewind it, so re-doing an add can't collide
    /// with a number already handed out. It is kept OFF `self.request` in memory
    /// (so the undo snapshots that clone `request` never rewind it) and folded into
    /// the serialized document under `"featureCounter"` so it round-trips save/load
    /// (see [`Self::request_json`] / [`Self::from_request_json`]).
    feature_counter: u64,
}

impl Default for History {
    fn default() -> Self {
        Self {
            request: empty_request(),
            rollback: 0,
            undo_stack: Vec::new(),
            redo_stack: Vec::new(),
            last_edit_key: None,
            feature_counter: 0,
        }
    }
}

fn empty_request() -> Value {
    serde_json::json!({ "expressions": "", "configurator": {}, "features": [] })
}

/// The trailing run of ASCII digits of `id` read as a number (`"P.CU12"` → 12,
/// `"Box"` → 0). Trailing digits are ASCII (one byte each), so the slice boundary
/// is always a valid char boundary; a missing/overflowing run yields 0.
fn trailing_number(id: &str) -> u64 {
    let digit_bytes = id
        .bytes()
        .rev()
        .take_while(u8::is_ascii_digit)
        .count();
    id[id.len() - digit_bytes..].parse().unwrap_or(0)
}

impl History {
    /// Load a whole history document (a saved part file parses as one). Rolls to
    /// the last feature. Ensures a `features` array exists.
    pub fn from_request_json(json: &str) -> Result<Self, String> {
        let mut request: Value =
            serde_json::from_str(json).map_err(|e| format!("history parse: {e}"))?;
        if !request.get("features").map(Value::is_array).unwrap_or(false) {
            if let Some(obj) = request.as_object_mut() {
                obj.insert("features".into(), Value::Array(Vec::new()));
            } else {
                request = empty_request();
            }
        }
        // Lift the persistent feature counter OUT of the document so it lives ONLY
        // in the struct field: kept off `self.request`, the undo snapshots (which
        // clone `request`) can never rewind it, and it can't be double-folded on
        // re-serialize. A document with no stored counter (fresh or saved before
        // this field existed) safe-inits below.
        let stored = request
            .as_object_mut()
            .and_then(|obj| obj.remove("featureCounter"))
            .and_then(|value| value.as_u64());
        let mut history = Self {
            request,
            rollback: 0,
            ..Self::default()
        };
        history.rollback = history.len().saturating_sub(1);
        // Safe init when unstored: start ABOVE the largest numeric suffix already
        // present among feature ids so the next mint (`{shortName}{counter+1}`)
        // cannot collide with an existing id. This holds because no shortName ends
        // in a digit (verified — even `IMPORT3D` ends in `D`), so an id's trailing
        // digits ARE its numeric suffix and `counter+1` strictly exceeds them all.
        history.feature_counter = stored.unwrap_or_else(|| history.max_id_suffix());
        Ok(history)
    }

    /// The largest trailing-integer suffix among all existing feature ids (0 when
    /// none carry one) — the floor for a safe counter init on a document with no
    /// stored `featureCounter` (see [`Self::from_request_json`]).
    fn max_id_suffix(&self) -> u64 {
        self.features()
            .iter()
            .filter_map(|f| {
                f.get("inputParams")
                    .and_then(|p| p.get("id"))
                    .and_then(Value::as_str)
            })
            .map(trailing_number)
            .max()
            .unwrap_or(0)
    }

    /// The features slice (empty if none).
    pub fn features(&self) -> &[Value] {
        self.request
            .get("features")
            .and_then(Value::as_array)
            .map(Vec::as_slice)
            .unwrap_or(&[])
    }

    fn features_mut(&mut self) -> &mut Vec<Value> {
        let obj = self
            .request
            .as_object_mut()
            .expect("history request is a JSON object");
        obj.entry("features")
            .or_insert_with(|| Value::Array(Vec::new()));
        obj.get_mut("features")
            .and_then(Value::as_array_mut)
            .expect("features is a JSON array")
    }

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

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// The rolled-to index, clamped to a valid feature (0 when empty).
    pub fn rollback(&self) -> usize {
        self.rollback.min(self.len().saturating_sub(1))
    }

    pub fn set_rollback(&mut self, index: usize) {
        self.rollback = if self.is_empty() {
            0
        } else {
            index.min(self.len() - 1)
        };
        // Rolling to a step is a view move, not a model edit: it records NO undo
        // snapshot, but it DOES break the edit-coalescing run so the next edit
        // starts a fresh undo entry rather than merging with a pre-roll edit.
        self.last_edit_key = None;
    }

    pub fn feature_type(&self, index: usize) -> Option<String> {
        self.features()
            .get(index)?
            .get("type")
            .and_then(Value::as_str)
            .map(String::from)
    }

    pub fn feature_id(&self, index: usize) -> Option<String> {
        self.features()
            .get(index)?
            .get("inputParams")
            .and_then(|p| p.get("id"))
            .and_then(Value::as_str)
            .map(String::from)
    }

    pub fn index_of(&self, id: &str) -> Option<usize> {
        self.features().iter().position(|f| {
            f.get("inputParams")
                .and_then(|p| p.get("id"))
                .and_then(Value::as_str)
                == Some(id)
        })
    }

    /// The `inputParams` document of the feature at `index` (for the dialog).
    pub fn feature_params(&self, index: usize) -> Option<Value> {
        self.features().get(index)?.get("inputParams").cloned()
    }

    pub fn set_feature_params(&mut self, index: usize, params: Value) {
        if index >= self.len() {
            return;
        }
        // Coalesce a slider drag (many consecutive edits of the SAME feature) into
        // one undo entry, keyed by the feature index.
        self.checkpoint(Some(&format!("param:{index}")));
        if let Some(feat) = self.features_mut().get_mut(index) {
            if let Some(obj) = feat.as_object_mut() {
                obj.insert("inputParams".into(), params);
            }
        }
    }

    pub fn push_feature(&mut self, feature: Value) {
        self.checkpoint(None);
        self.features_mut().push(feature);
    }

    pub fn remove_feature(&mut self, index: usize) {
        if index >= self.len() {
            return;
        }
        self.checkpoint(None);
        self.features_mut().remove(index);
    }

    pub fn swap(&mut self, a: usize, b: usize) {
        let len = self.len();
        if a < len && b < len && a != b {
            self.checkpoint(None);
            self.features_mut().swap(a, b);
        }
    }

    /// Mint the id for a NEW feature: `{base}{N}` where `base` is the feature's
    /// shortName and `N` is this history's persistent GLOBAL counter, bumped by one
    /// on every mint (`P.CU` → `P.CU7`, `S` → `S8`). GLOBAL across all feature
    /// types, MONOTONIC, and NEVER reused — a delete does not free a number and
    /// undo does not rewind the counter — and it persists across save/load, so two
    /// features can never receive the same id over the document's whole lifetime.
    pub fn next_feature_id(&mut self, base: &str) -> String {
        self.feature_counter += 1;
        format!("{base}{}", self.feature_counter)
    }

    /// The `stopAtId`-truncated request that stops AFTER the rolled-to feature —
    /// the roll-to-step request the pipeline runs. Empty history → empty request.
    pub fn prefix_request(&self) -> Value {
        let mut request = self.request.clone();
        if let Some(id) = self.feature_id(self.rollback()) {
            if let Some(obj) = request.as_object_mut() {
                obj.insert("stopAtId".into(), Value::String(id));
            }
        }
        request
    }

    /// The tree listing for the UI: `{ step, features: [{index, type, id}] }`.
    pub fn listing_json(&self) -> String {
        let features: Vec<Value> = self
            .features()
            .iter()
            .enumerate()
            .map(|(index, _)| {
                serde_json::json!({
                    "index": index,
                    "type": self.feature_type(index).unwrap_or_else(|| "?".into()),
                    "id": self.feature_id(index).unwrap_or_else(|| "(no id)".into()),
                })
            })
            .collect();
        serde_json::json!({ "step": self.rollback(), "features": features }).to_string()
    }

    /// The whole request document (for persistence / debugging), with the
    /// persistent global feature counter folded back in under `"featureCounter"`
    /// so it round-trips through save/load (the twin of [`Self::from_request_json`],
    /// which lifts it back out). Written only when non-zero, so a document that has
    /// never minted a feature persists byte-for-byte as before (mirrors the
    /// metadata field's "un-annotated model persists unchanged" convention).
    pub fn request_json(&self) -> String {
        if self.feature_counter == 0 {
            return self.request.to_string();
        }
        let mut document = self.request.clone();
        if let Some(obj) = document.as_object_mut() {
            obj.insert("featureCounter".into(), Value::from(self.feature_counter));
        }
        document.to_string()
    }

    // --- Undo / redo over the model document ------------------------------

    fn snapshot(&self) -> Snapshot {
        Snapshot {
            request: self.request.clone(),
            rollback: self.rollback,
        }
    }

    fn restore(&mut self, snap: Snapshot) {
        self.request = snap.request;
        let last = self.len().saturating_sub(1);
        self.rollback = snap.rollback.min(last);
    }

    /// Record a pre-mutation snapshot for undo. `coalesce_key` groups a run of
    /// rapid same-target edits (one slider drag) into a SINGLE undo entry: while
    /// the same non-empty key repeats, no new snapshot is pushed. A `None` key
    /// never coalesces, so every structural add/delete/reorder is its own entry.
    /// Any new snapshot clears the redo stack (a fresh edit forks the timeline)
    /// and the oldest entry falls off once the stack passes [`MAX_UNDO`].
    fn checkpoint(&mut self, coalesce_key: Option<&str>) {
        if coalesce_key.is_some() && coalesce_key == self.last_edit_key.as_deref() {
            return;
        }
        self.undo_stack.push(self.snapshot());
        if self.undo_stack.len() > MAX_UNDO {
            self.undo_stack.remove(0);
        }
        self.redo_stack.clear();
        self.last_edit_key = coalesce_key.map(str::to_string);
    }

    /// Whether an undo step is available (for enabling the toolbar button).
    pub fn can_undo(&self) -> bool {
        !self.undo_stack.is_empty()
    }

    /// Whether a redo step is available.
    pub fn can_redo(&self) -> bool {
        !self.redo_stack.is_empty()
    }

    /// Undo the last model mutation: push the current state onto the redo stack
    /// and restore the previous document + rolled-to step. Returns whether it
    /// changed anything (false when the undo stack is empty).
    pub fn undo(&mut self) -> bool {
        let Some(prev) = self.undo_stack.pop() else {
            return false;
        };
        self.redo_stack.push(self.snapshot());
        self.restore(prev);
        // A distinct undo breaks any coalescing run so the next edit is fresh.
        self.last_edit_key = None;
        true
    }

    /// Redo the last undone model mutation (symmetric with [`Self::undo`]).
    pub fn redo(&mut self) -> bool {
        let Some(next) = self.redo_stack.pop() else {
            return false;
        };
        self.undo_stack.push(self.snapshot());
        self.restore(next);
        self.last_edit_key = None;
        true
    }
}

// ============================================================================
// Expressions / configurator accessors (appended — the expressions/parameters
// panel slice). A SEPARATE `impl` block so concurrent edits to the primary block
// don't conflict; purely additive over the existing history API.
//
// The history document carries an `expressions` source string — the variable
// sheet feature params evaluate against (a numeric param may be the string
// `"boxW"`, evaluated by the pipeline's shared expression env). The panel edits
// this and re-runs; the `configurator` object (typed named inputs) is exposed
// read-only for display.
// ============================================================================
impl History {
    /// The history document's `expressions` source string (empty when absent or
    /// stored as `null`). The panel's editor binds to this.
    pub fn expressions(&self) -> String {
        self.request
            .get("expressions")
            .and_then(Value::as_str)
            .unwrap_or("")
            .to_string()
    }

    /// Replace the `expressions` source string. Snapshotted for undo, coalescing a
    /// run of keystroke edits into ONE undo entry (like a slider drag) via the
    /// shared `"expressions"` coalesce key, so a distinct add/edit/roll starts a
    /// fresh entry. A no-op re-set (same text) still records under the same key.
    pub fn set_expressions(&mut self, expressions: &str) {
        self.checkpoint(Some("expressions"));
        if let Some(obj) = self.request.as_object_mut() {
            obj.insert(
                "expressions".into(),
                Value::String(expressions.to_string()),
            );
        }
    }

    /// The `configurator` object (typed named inputs), or `{}` when absent —
    /// read-only for the panel's display (deeper configurator editing deferred).
    pub fn configurator(&self) -> Value {
        self.request
            .get("configurator")
            .cloned()
            .unwrap_or_else(|| Value::Object(serde_json::Map::new()))
    }
}

// ============================================================================
// Feature `persistentData` accessors (appended — the engine-native sketch mode
// slice). A SEPARATE `impl` block (like the expressions accessors above) so
// concurrent edits don't conflict; purely additive over the primary history API.
//
// A feature's `persistentData` is the kernel-persisted, non-input state (a
// SKETCH feature stores its solved `{points, geometries, constraints}` under the
// `sketch` key and the plane `basis` there). Sketch mode reads that state on
// enter and writes the edited doc back on commit — mirroring how the ref-select
// slice reads/writes `inputParams` through `feature_params` / `set_feature_params`.
// ============================================================================
impl History {
    /// The `persistentData` document of the feature at `index` (`None` if the
    /// feature or the field is absent) — the read twin of [`Self::feature_params`].
    pub fn feature_persistent_data(&self, index: usize) -> Option<Value> {
        self.features().get(index)?.get("persistentData").cloned()
    }

    /// Set a single `key` inside the feature-at-`index`'s `persistentData` object,
    /// creating (or replacing a non-object) `persistentData` as needed. Snapshotted
    /// for undo (a structural edit — never coalesced), like an add/delete.
    pub fn set_feature_persistent_field(&mut self, index: usize, key: &str, value: Value) {
        if index >= self.len() {
            return;
        }
        self.checkpoint(None);
        if let Some(feat) = self.features_mut().get_mut(index).and_then(Value::as_object_mut) {
            let entry = feat
                .entry("persistentData")
                .or_insert_with(|| Value::Object(serde_json::Map::new()));
            if !entry.is_object() {
                *entry = Value::Object(serde_json::Map::new());
            }
            if let Some(obj) = entry.as_object_mut() {
                obj.insert(key.to_string(), value);
            }
        }
    }
}

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

    fn seed() -> History {
        History::from_request_json(
            r#"{"features":[
                {"type":"P.CU","inputParams":{"id":"Box","sizeX":20}},
                {"type":"P.CY","inputParams":{"id":"Pin","radius":6}},
                {"type":"B","inputParams":{"id":"Cut","boolean":{"operation":"SUBTRACT","targets":["Pin"]}}}
            ]}"#,
        )
        .unwrap()
    }

    #[test]
    fn loads_and_rolls_to_last() {
        let h = seed();
        assert_eq!(h.len(), 3);
        assert_eq!(h.rollback(), 2);
        assert_eq!(h.feature_type(0).as_deref(), Some("P.CU"));
        assert_eq!(h.feature_id(2).as_deref(), Some("Cut"));
    }

    #[test]
    fn prefix_request_stops_at_rolled_step() {
        let mut h = seed();
        h.set_rollback(0);
        let req = h.prefix_request();
        assert_eq!(req["stopAtId"].as_str(), Some("Box"));
        // The full feature list is retained (cache retention), only stopAtId moves.
        assert_eq!(req["features"].as_array().unwrap().len(), 3);
    }

    #[test]
    fn edit_add_delete_reorder() {
        let mut h = seed();
        // edit params of "Pin"
        let idx = h.index_of("Pin").unwrap();
        h.set_feature_params(idx, serde_json::json!({"id":"Pin","radius":9}));
        assert_eq!(h.feature_params(idx).unwrap()["radius"], 9);
        // add
        h.push_feature(serde_json::json!({"type":"P.CU","inputParams":{"id":"Box"}}));
        // the persistent counter mints the next id (seed ids carry no numeric
        // suffix → the counter safe-inits to 0, so the first mint is 1).
        assert_eq!(h.next_feature_id("Box"), "Box1");
        // reorder swap 0<->1
        h.swap(0, 1);
        assert_eq!(h.feature_id(0).as_deref(), Some("Pin"));
        // delete "Cut"
        let cut = h.index_of("Cut").unwrap();
        h.remove_feature(cut);
        assert!(h.index_of("Cut").is_none());
    }

    #[test]
    fn undo_redo_over_edit_and_add() {
        let mut h = seed();
        // A freshly loaded history has nothing to undo/redo.
        assert!(!h.can_undo());
        assert!(!h.can_redo());
        // A roll is view state: it records NO undo entry.
        h.set_rollback(0);
        assert!(!h.can_undo());

        // Edit a param → undo reverts it, redo re-applies it.
        let box_idx = h.index_of("Box").unwrap();
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 20);
        h.set_feature_params(box_idx, serde_json::json!({ "id": "Box", "sizeX": 50 }));
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 50);
        assert!(h.can_undo());
        assert!(h.undo());
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 20);
        assert!(h.can_redo());
        assert!(h.redo());
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 50);

        // Add a feature → undo removes it, redo brings it back.
        let n = h.len();
        h.push_feature(serde_json::json!({ "type": "P.CU", "inputParams": { "id": "Extra" } }));
        assert_eq!(h.len(), n + 1);
        assert!(h.undo());
        assert_eq!(h.len(), n);
        assert!(h.index_of("Extra").is_none());
        assert!(h.redo());
        assert_eq!(h.len(), n + 1);
        assert!(h.index_of("Extra").is_some());

        // A NEW mutation forks the timeline (clears redo).
        h.undo();
        assert!(h.can_redo());
        h.push_feature(serde_json::json!({ "type": "P.CU", "inputParams": { "id": "Fork" } }));
        assert!(!h.can_redo());
    }

    #[test]
    fn drag_edits_coalesce_into_one_undo() {
        let mut h = seed();
        h.set_rollback(0);
        let box_idx = h.index_of("Box").unwrap();
        // Simulate a slider DRAG: many consecutive edits of the same feature.
        for v in [21, 22, 23, 24, 25] {
            h.set_feature_params(box_idx, serde_json::json!({ "id": "Box", "sizeX": v }));
        }
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 25);
        // ONE undo reverts the WHOLE drag to the pre-drag value — and that is the
        // only undo entry the drag produced.
        assert!(h.undo());
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 20);
        assert!(!h.can_undo());

        // A roll between two edits of the same feature breaks coalescing, so the
        // two edits become two distinct undo entries.
        h.set_feature_params(box_idx, serde_json::json!({ "id": "Box", "sizeX": 30 }));
        h.set_rollback(0);
        h.set_feature_params(box_idx, serde_json::json!({ "id": "Box", "sizeX": 40 }));
        assert!(h.undo());
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 30);
        assert!(h.undo());
        assert_eq!(h.feature_params(box_idx).unwrap()["sizeX"], 20);
    }

    #[test]
    fn expressions_get_set_and_undo_coalesces() {
        let mut h = seed();
        assert_eq!(h.expressions(), ""); // seed has no expressions field
        h.set_expressions("boxW = 30;");
        assert_eq!(h.expressions(), "boxW = 30;");
        // Consecutive expression edits coalesce into a single undo entry.
        h.set_expressions("boxW = 40;");
        h.set_expressions("boxW = 50;");
        assert_eq!(h.expressions(), "boxW = 50;");
        assert!(h.undo());
        // ONE undo reverts the whole coalesced run to the pre-edit (empty) value.
        assert_eq!(h.expressions(), "");
        // The configurator accessor returns an object (empty here).
        assert!(h.configurator().is_object());
    }

    #[test]
    fn next_feature_id_is_shortname_plus_global_counter() {
        let mut h = History::default();
        // A NEW feature's id is `{shortName}{N}` with N the global counter, bumped
        // once per mint — the exact ids for a couple of types (incl. dotted ones).
        assert_eq!(h.next_feature_id("S"), "S1");
        assert_eq!(h.next_feature_id("P.CU"), "P.CU2");
        assert_eq!(h.next_feature_id("E"), "E3");
    }

    #[test]
    fn counter_is_global_across_types_and_never_reused_on_delete() {
        let mut h = History::default();
        // Sketch, then cube, then extrude → S1, P.CU2, E3 (ONE global counter,
        // not per-type).
        let s = h.next_feature_id("S");
        h.push_feature(serde_json::json!({ "type": "S", "inputParams": { "id": s } }));
        let cu = h.next_feature_id("P.CU");
        h.push_feature(serde_json::json!({ "type": "P.CU", "inputParams": { "id": cu } }));
        let e = h.next_feature_id("E");
        h.push_feature(serde_json::json!({ "type": "E", "inputParams": { "id": e } }));
        assert_eq!(h.feature_id(0).as_deref(), Some("S1"));
        assert_eq!(h.feature_id(1).as_deref(), Some("P.CU2"));
        assert_eq!(h.feature_id(2).as_deref(), Some("E3"));

        // Delete the FIRST feature (S1): its number 1 must NOT be reused — the next
        // mint is strictly greater than every number handed out so far.
        let idx = h.index_of("S1").unwrap();
        h.remove_feature(idx);
        assert_eq!(h.next_feature_id("P.CY"), "P.CY4");
    }

    #[test]
    fn counter_persists_across_serialize_reload() {
        let mut h = History::default();
        let first = h.next_feature_id("S"); // S1
        h.push_feature(serde_json::json!({ "type": "S", "inputParams": { "id": first } }));

        // Serialize → reload through the REAL persistence path (`request_json` →
        // `from_request_json`), not a hand-built document.
        let json = h.request_json();
        assert!(json.contains("\"featureCounter\":1"), "counter is folded in: {json}");
        let mut reloaded = History::from_request_json(&json).unwrap();

        // The counter CONTINUES from where it left off — no restart, and the new id
        // cannot collide with the persisted S1.
        assert_eq!(reloaded.next_feature_id("P.CU"), "P.CU2");
    }

    #[test]
    fn unmutated_document_omits_the_counter_key() {
        // The seed's ids carry no numeric suffix → counter safe-inits to 0 → the
        // document persists byte-for-byte with NO `featureCounter` key.
        let h = seed();
        assert!(!h.request_json().contains("featureCounter"));
    }

    #[test]
    fn undo_does_not_rewind_the_counter() {
        // Snapshots capture only `request` + `rollback`; the counter lives OFF the
        // request, so undoing an add (a delete) must NOT free the number — otherwise
        // a redo/new-add could reuse it.
        let mut h = History::default();
        let a = h.next_feature_id("P.CU"); // P.CU1
        h.push_feature(serde_json::json!({ "type": "P.CU", "inputParams": { "id": a } }));
        assert!(h.undo());
        assert!(h.index_of("P.CU1").is_none());
        assert_eq!(h.next_feature_id("P.CU"), "P.CU2");
    }

    #[test]
    fn counter_safe_inits_above_existing_id_suffixes() {
        // A legacy document with NO stored counter: safe-init lifts the counter above
        // the largest existing numeric suffix so a new id can't collide.
        let mut h = History::from_request_json(
            r#"{"features":[
                {"type":"P.CU","inputParams":{"id":"P.CU5"}},
                {"type":"S","inputParams":{"id":"S2"}}
            ]}"#,
        )
        .unwrap();
        // Max suffix is 5 → the next mint is 6.
        assert_eq!(h.next_feature_id("E"), "E6");
    }

    #[test]
    fn trailing_number_reads_the_suffix() {
        assert_eq!(trailing_number("P.CU12"), 12);
        assert_eq!(trailing_number("S1"), 1);
        assert_eq!(trailing_number("Box"), 0);
        assert_eq!(trailing_number("IMPORT3D"), 0); // ends in a letter
    }
}