fix(#7): use auto-incrementing counter for item IDs

app.py

@@ -1,3 +1,5 @@
+import itertools
+
 from flask import Flask, jsonify, request
 from datetime import datetime, timezone
 
@@ -27,7 +29,7 @@ def create_item():
         return jsonify(error="name is required"), 400
 
     item = {
-        "id": len(_get_items()) + 1,
+        "id": _new_id(),
         "name": data["name"],
         "done": False,
     }
@@ -63,6 +65,11 @@ def delete_item(item_id):
 
 
 _items_store = []
+_id_counter = itertools.count(1)
+
+
+def _new_id():
+    return next(_id_counter)
 
 
 def _get_items():
@@ -71,7 +78,9 @@ def _get_items():
 
 def reset_items():
     """Reset the in-memory store (used by tests)."""
+    global _id_counter
     _items_store.clear()
+    _id_counter = itertools.count(1)
 
 
 if __name__ == "__main__":

test_app.py

@@ -63,3 +63,48 @@ def test_delete_item(client):
 def test_delete_item_not_found(client):
     resp = client.delete("/items/999")
     assert resp.status_code == 404
+
+
+def test_no_id_collision_after_delete(client):
+    """IDs must never be reused after deletion."""
+    resp1 = client.post("/items", json={"name": "A"})
+    resp2 = client.post("/items", json={"name": "B"})
+    id_a = resp1.get_json()["id"]
+    id_b = resp2.get_json()["id"]
+
+    # Delete first item
+    client.delete(f"/items/{id_a}")
+
+    # Create a new item — should NOT collide with B's id
+    resp3 = client.post("/items", json={"name": "C"})
+    id_c = resp3.get_json()["id"]
+    assert id_c != id_b, "New item ID collides with existing item"
+
+    # Verify both remaining items exist in the list
+    all_items = client.get("/items").get_json()["items"]
+    all_ids = [item["id"] for item in all_items]
+    assert id_b in all_ids
+    assert id_c in all_ids
+    assert len(all_items) == 2
+
+
+def test_no_id_reuse_after_multiple_deletes(client):
+    """IDs stay unique after several deletions followed by several creates."""
+    # Create three items
+    ids = []
+    for name in ("X", "Y", "Z"):
+        resp = client.post("/items", json={"name": name})
+        ids.append(resp.get_json()["id"])
+
+    # Delete all three
+    for item_id in ids:
+        client.delete(f"/items/{item_id}")
+
+    # Create two more items — their IDs must not collide with any prior ID
+    new_ids = []
+    for name in ("P", "Q"):
+        resp = client.post("/items", json={"name": name})
+        new_ids.append(resp.get_json()["id"])
+
+    all_assigned = ids + new_ids
+    assert len(all_assigned) == len(set(all_assigned)), "ID reuse detected"