A bunch of improvements

crates/compiler/checkmate/www/src/engine/engine.tsx

@@ -44,7 +44,7 @@ function getFlatEvents(events: Event[]): {
 export class Engine {
   #eventIndexMap: Map<Event, EventIndex>;
   #events: Event[];
-  #subs: Subs = new Subs();
+  #subs: Subs = Subs.new();
   #reverseEvents: Map<EventIndex, RollbackChange> = new Map();
 
   #nextIndexForward: EventIndex = 0 as EventIndex;
@@ -82,6 +82,10 @@ export class Engine {
     return this.#subs;
   }
 
+  lastEventIndex(): EventIndex {
+    return (this.#events.length - 1) as EventIndex;
+  }
+
   private stepForward(eventIndex: EventIndex): void {
     const event = this.#events[eventIndex];
     if (!isApplicable(event)) {

crates/compiler/checkmate/www/src/engine/subs.tsx

@@ -59,7 +59,15 @@ export type ChangeEvent =
 export type Change = ChangeEvent | RollbackChange;
 
 export class Subs {
-  #map: Map<Variable, VarType> = new Map();
+  #map: Map<Variable, VarType>;
+
+  private constructor(map: Map<Variable, VarType>) {
+    this.#map = map;
+  }
+
+  static new(): Subs {
+    return new Subs(new Map());
+  }
 
   get(variable: Variable): VarType | undefined {
     return this.#map.get(variable);
@@ -80,6 +88,41 @@ export class Subs {
     }
   }
 
+  get_root_key(variable: Variable): Variable {
+    const type = this.get(variable);
+    if (type === undefined) {
+      return variable;
+    }
+    switch (type.type) {
+      case "descriptor":
+        return variable;
+      case "link":
+        return this.get_root_key(type.to);
+      default:
+        assertExhaustive(type);
+    }
+  }
+
+  snapshot(): SubsSnapshot {
+    const snapshotMap = new Map<Variable, VarType>();
+    for (const [key, value] of this.#map) {
+      snapshotMap.set(key, { ...value });
+    }
+    const snapshot = new Subs(snapshotMap);
+    return {
+      get(variable: Variable): VarType | undefined {
+        return snapshot.get(variable);
+      },
+      get_root(variable: Variable): TypeDescriptor | undefined {
+        return snapshot.get_root(variable);
+      },
+      get_root_key(variable: Variable): Variable {
+        return snapshot.get_root_key(variable);
+      },
+      __snapshot__: SnapshotSymbol,
+    };
+  }
+
   apply(change: Change): void {
     switch (change.type) {
       case "VariableUnified": {
@@ -119,3 +162,12 @@ export class Subs {
     }
   }
 }
+
+const SnapshotSymbol = Symbol("Snapshot");
+
+export interface SubsSnapshot {
+  get(variable: Variable): VarType | undefined;
+  get_root(variable: Variable): TypeDescriptor | undefined;
+  get_root_key(variable: Variable): Variable;
+  __snapshot__: typeof SnapshotSymbol;
+}