1 files changed, 55 insertions, 0 deletions
diff --git a/utils/equal/deep.ts b/utils/equal/deep.ts
new file mode 100644
index 0000000..d2c6ce2
--- /dev/null
+++ b/utils/equal/deep.ts
@@ -0,0 +1,55 @@
+/**
+ * Structural deep equality, used by determinism tests to compare
+ * simulation snapshots. Handles plain objects, arrays, Maps, Sets, Dates,
+ * and primitives. Does not handle class instances with custom equality or
+ * cyclic object graphs — simulation state is a tree, not a graph.
+ */
+export function deepEqual(a: unknown, b: unknown): boolean {
+  if (Object.is(a, b)) return true
+
+  if (typeof a !== typeof b) return false
+  if (a === null || b === null) return false
+  if (typeof a !== 'object') return false
+
+  if (a instanceof Date || b instanceof Date) {
+    return a instanceof Date && b instanceof Date && a.getTime() === b.getTime()
+  }
+
+  if (Array.isArray(a) || Array.isArray(b)) {
+    if (!Array.isArray(a) || !Array.isArray(b)) return false
+    if (a.length !== b.length) return false
+    for (let i = 0; i < a.length; i++) {
+      if (!deepEqual(a[i], b[i])) return false
+    }
+    return true
+  }
+
+  if (a instanceof Map || b instanceof Map) {
+    if (!(a instanceof Map) || !(b instanceof Map)) return false
+    if (a.size !== b.size) return false
+    for (const [key, value] of a) {
+      if (!b.has(key) || !deepEqual(value, b.get(key))) return false
+    }
+    return true
+  }
+
+  if (a instanceof Set || b instanceof Set) {
+    if (!(a instanceof Set) || !(b instanceof Set)) return false
+    if (a.size !== b.size) return false
+    for (const value of a) {
+      if (!b.has(value)) return false
+    }
+    return true
+  }
+
+  const aObj = a as Record<string, unknown>
+  const bObj = b as Record<string, unknown>
+  const aKeys = Object.keys(aObj)
+  const bKeys = Object.keys(bObj)
+  if (aKeys.length !== bKeys.length) return false
+  for (const key of aKeys) {
+    if (!Object.prototype.hasOwnProperty.call(bObj, key)) return false
+    if (!deepEqual(aObj[key], bObj[key])) return false
+  }
+  return true
+}