feat: binary-heap event queue with stable tie-breaking

src/sim/events.js

@@ -0,0 +1,54 @@
+// Binary min-heap keyed by (time, insertion sequence).
+export class EventQueue {
+  constructor() {
+    this.heap = [];
+    this.seq = 0;
+  }
+
+  get size() {
+    return this.heap.length;
+  }
+
+  push(time, type, data) {
+    const e = { time, seq: this.seq++, type, data };
+    const h = this.heap;
+    h.push(e);
+    let i = h.length - 1;
+    while (i > 0) {
+      const p = (i - 1) >> 1;
+      if (!before(h[i], h[p])) break;
+      [h[i], h[p]] = [h[p], h[i]];
+      i = p;
+    }
+    return e;
+  }
+
+  peek() {
+    return this.heap[0] ?? null;
+  }
+
+  pop() {
+    const h = this.heap;
+    if (h.length === 0) return null;
+    const top = h[0];
+    const last = h.pop();
+    if (h.length > 0) {
+      h[0] = last;
+      let i = 0;
+      for (;;) {
+        const l = 2 * i + 1, r = 2 * i + 2;
+        let m = i;
+        if (l < h.length && before(h[l], h[m])) m = l;
+        if (r < h.length && before(h[r], h[m])) m = r;
+        if (m === i) break;
+        [h[i], h[m]] = [h[m], h[i]];
+        i = m;
+      }
+    }
+    return top;
+  }
+}
+
+function before(a, b) {
+  return a.time < b.time || (a.time === b.time && a.seq < b.seq);
+}

tests/events.test.js

@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest';
+import { EventQueue } from '../src/sim/events.js';
+
+describe('EventQueue', () => {
+  it('pops events in time order regardless of insertion order', () => {
+    const q = new EventQueue();
+    q.push(5, 'c', {});
+    q.push(1, 'a', {});
+    q.push(3, 'b', {});
+    expect(q.pop().type).toBe('a');
+    expect(q.pop().type).toBe('b');
+    expect(q.pop().type).toBe('c');
+    expect(q.pop()).toBeNull();
+  });
+
+  it('breaks time ties by insertion order', () => {
+    const q = new EventQueue();
+    q.push(2, 'first', {});
+    q.push(2, 'second', {});
+    expect(q.pop().type).toBe('first');
+    expect(q.pop().type).toBe('second');
+  });
+
+  it('peek does not remove; size tracks contents', () => {
+    const q = new EventQueue();
+    q.push(1, 'a', { n: 7 });
+    expect(q.size).toBe(1);
+    expect(q.peek().time).toBe(1);
+    expect(q.size).toBe(1);
+    expect(q.pop().data.n).toBe(7);
+    expect(q.size).toBe(0);
+    expect(q.peek()).toBeNull();
+  });
+
+  it('survives a large randomized load in order', () => {
+    const q = new EventQueue();
+    const times = Array.from({ length: 500 }, (_, i) => (i * 7919) % 1000);
+    for (const t of times) q.push(t, 'e', {});
+    let last = -1;
+    while (q.size > 0) {
+      const e = q.pop();
+      expect(e.time).toBeGreaterThanOrEqual(last);
+      last = e.time;
+    }
+  });
+});