ts-03-lru-cache

# Generic LRU cache

Implement **`solution.ts`** exporting a generic class:

```ts
export class LRUCache<K, V> {
  constructor(capacity: number);
  get(key: K): V | undefined;
  put(key: K, value: V): void;
  get size(): number;
}
```

A least-recently-used cache holding at most `capacity` entries.

- `get(key)` returns the stored value, or `undefined` if absent. A successful `get`
  counts as a **use** (it makes that key the most-recently used).
- `put(key, value)` inserts or updates a key. Updating an existing key also counts as a
  use. When inserting a **new** key would exceed `capacity`, evict the
  least-recently-used key first.
- `size` is the current number of entries (never exceeds `capacity`).
- Throw a `RangeError` if `capacity < 1`.

Example:

```ts
const c = new LRUCache<string, number>(2);
c.put("a", 1);
c.put("b", 2);
c.get("a");      // 1  -> "a" is now most-recently used
c.put("c", 3);   // evicts "b" (least-recently used)
c.get("b");      // undefined
c.get("a");      // 1
c.get("c");      // 3
c.size;          // 2
```

Keep it fully typed (must pass `tsc --noEmit` in strict mode). Do not use `any` in the
public API.

import { test } from "node:test";
import { strict as assert } from "node:assert";
import { LRUCache } from "./solution.ts";

test("stores and retrieves values", () => {
  const c = new LRUCache<string, number>(2);
  c.put("a", 1);
  c.put("b", 2);
  assert.equal(c.get("a"), 1);
  assert.equal(c.get("b"), 2);
  assert.equal(c.size, 2);
});

test("missing key returns undefined", () => {
  const c = new LRUCache<string, number>(2);
  assert.equal(c.get("nope"), undefined);
});

test("evicts least-recently-used on overflow", () => {
  const c = new LRUCache<string, number>(2);
  c.put("a", 1);
  c.put("b", 2);
  c.put("c", 3); // evicts "a"
  assert.equal(c.get("a"), undefined);
  assert.equal(c.get("b"), 2);
  assert.equal(c.get("c"), 3);
  assert.equal(c.size, 2);
});

test("get counts as a use and protects from eviction", () => {
  const c = new LRUCache<string, number>(2);
  c.put("a", 1);
  c.put("b", 2);
  assert.equal(c.get("a"), 1); // "a" now most-recently used
  c.put("c", 3); // evicts "b"
  assert.equal(c.get("b"), undefined);
  assert.equal(c.get("a"), 1);
  assert.equal(c.get("c"), 3);
});

test("put updates existing key and marks it used", () => {
  const c = new LRUCache<string, number>(2);
  c.put("a", 1);
  c.put("b", 2);
  c.put("a", 10); // update + use "a"
  c.put("c", 3); // evicts "b"
  assert.equal(c.get("a"), 10);
  assert.equal(c.get("b"), undefined);
  assert.equal(c.get("c"), 3);
  assert.equal(c.size, 2);
});

test("capacity of 1 keeps only the newest", () => {
  const c = new LRUCache<number, string>(1);
  c.put(1, "one");
  c.put(2, "two");
  assert.equal(c.get(1), undefined);
  assert.equal(c.get(2), "two");
  assert.equal(c.size, 1);
});

test("invalid capacity throws RangeError", () => {
  assert.throws(() => new LRUCache<string, number>(0), RangeError);
});

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.

```typescript
export class LRUCache<K, V> {
  private capacity: number;
  private map: Map<K, Node<K, V>>;
  private head: Node<K, V> | null;
  private tail: Node<K, V> | null;

  private class Node<K, V> {
    key: K;
    value: V;
    prev: Node<K, V> | null;
    next: Node<K, V> | null;

    constructor(key: K, value: V) {
      this.key = key;
      this.value = value;
      this.prev = null;
      this.next = null;
    }
  }

  constructor(capacity: number) {
    if (capacity < 1) {
      throw new RangeError('capacity must be at least 1');
    }
    this.capacity = capacity;
    this.map = new Map();
    this.head = null;
    this.tail = null;
  }

  private moveToFront(node: Node<K, V>): void {
    if (node.prev) {
      node.prev.next = node.next;
    } else {
      this.head = node.next;
    }

    if (node.next) {
      node.next.prev = node.prev;
    } else {
      this.tail = node.prev;
    }

    node.prev = null;
    node.next = this.head;

    if (this.head) {
      this.head.prev = node;
    }

    this.head = node;
  }

  private addToTail(node: Node<K, V>): void {
    node.next = null;
    node.prev = this.tail;

    if (this.tail) {
      this.tail.next = node;
    } else {
      this.head = node;
    }

    this.tail = node;
  }

  private evictLRU(): void {
    if (this.tail) {
      const node = this.tail;
      this.map.delete(node.key);
      this.tail = node.prev;
      if (this.tail) {
        this.tail.next = null;
      } else {
        this.head = null;
      }
    }
  }

  get(key: K): V | undefined {
    const node = this.map.get(key);
    if (!node) {
      return undefined;
    }

    this.moveToFront(node);
    return node.value;
  }

  put(key: K, value: V): void {
    const node = this.map.get(key);

    if (node) {
      node.value = value;
      this.moveToFront(node);
    } else {
      const newNode = new Node(key, value);
      this.map.set(key, newNode);
      this.addToTail(newNode);

      if (this.map.size > this.capacity) {
        this.evictLRU();
      }
    }
  }

  get size(): number {
    return this.map.size;
  }
}
```

The user wants me to implement a generic LRU (Least Recently Used) cache in TypeScript. Let me break down the requirements:

1. Generic class `LRUCache<K, V>` with:
   - Constructor that takes `capacity: number`
   - `get(key: K): V | undefined` - returns value or undefined, updates LRU order on successful get
   - `put(key: K, value: V): void` - inserts/updates, evicts LRU if needed
   - `size: number` - current number of entries

2. Rules:
   - Successful `get` makes that key most-recently used
   - `put` with existing key updates it and makes it most-recently used
   - When inserting new key exceeds capacity, evict least-recently-used key
   - Throw `RangeError` if `capacity < 1`

3. Must be fully typed, no `any` in public API

For implementation, I'll use a combination of:
- A Map to store key-value pairs
- A doubly-linked list (or similar structure) to maintain LRU order

Actually, in TypeScript/JavaScript, I can use a Map with a custom approach. The most efficient way is to use a Map for O(1) lookups and maintain insertion order, but for true LRU we need to track which is least recently used.

A common approach is:
1. Use a Map for O(1) key-value access
2. Use a doubly-linked list to track LRU order
3. When accessing a key, move it to the front of the list
4. When capacity is exceeded, remove from the back of the list

Let me implement this:

```typescript
export class LRUCache<K, V> {
  private capacity: number;
  private map: Map<K, Node<K, V>>;
  private head: Node<K, V> | null;
  private tail: Node<K, V> | null;

  private class Node<K, V> {
    key: K;
    value: V;
    prev: Node<K, V> | null;
    next: Node<K, V> | null;
    
    constructor(key: K, value: V) {
      this.key = key;
      this.value = value;
      this.prev = null;
      this.next = null;
    }
  }

  constructor(capacity: number) {
    if (capacity < 1) {
      throw new RangeError('capacity must be at least 1');
    }
    this.capacity = capacity;
    this.map = new Map();
    this.head = null;
    this.tail = null;
  }

  private moveToFront(node: Node<K, V>): void {
    if (node.prev) {
      node.prev.next = node.next;
    } else {
      this.head = node.next;
    }
    
    if (node.next) {
      node.next.prev = node.prev;
    } else {
      this.tail = node.prev;
    }
    
    node.prev = null;
    node.next = this.head;
    
    if (this.head) {
      this.head.prev = node;
    }
    
    this.head = node;
  }

  private addToTail(node: Node<K, V>): void {
    node.next = null;
    node.prev = this.tail;
    
    if (this.tail) {
      this.tail.next = node;
    } else {
      this.head = node;
    }
    
    this.tail = node;
  }

  private evictLRU(): void {
    if (this.tail) {
      const node = this.tail;
      this.map.delete(node.key);
      this.tail = node.prev;
      if (this.tail) {
        this.tail.next = null;
      } else {
        this.head = null;
      }
    }
  }

  get(key: K): V | undefined {
    const node = this.map.get(key);
    if (!node) {
      return undefined;
    }
    
    this.moveToFront(node);
    return node.value;
  }

  put(key: K, value: V): void {
    const node = this.map.get(key);
    
    if (node) {
      node.value = value;
      this.moveToFront(node);
    } else {
      const newNode = new Node(key, value);
      this.map.set(key, newNode);
      this.addToTail(newNode);
      
      if (this.map.size > this.capacity) {
        this.evictLRU();
      }
    }
  }

  get size(): number {
    return this.map.size;
  }
}
```

I'm reconsidering the Node class structure. The current implementation might have some complexity in managing the doubly-linked list. I'll need to carefully track the head and tail pointers to ensure efficient LRU cache operations.

The key challenge is maintaining the correct order of nodes while allowing quick access and removal. I'll need to ensure that when a node is accessed, it moves to the front of the list, and when the cache is full, the least recently used node is removed from the back.

My current approach uses a Map to store nodes and a doubly-linked list to track access order. This allows O(1) access and removal of nodes, which is crucial for an efficient LRU cache implementation.

The critical steps involve:
1. Checking if a node exists in the map
2. Moving accessed nodes to the front
3. Adding new nodes to the tail
4. Evicting the least recently used node when capacity is exceeded

I'll need to carefully manage the head and tail pointers to ensure the list remains correctly ordered.

[
  {
    "total": 1,
    "answer": "```typescript\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  private class Node<K, V> {\n    key: K;\n    value: V;\n    prev: Node<K, V> | null;\n    next: Node<K, V> | null;\n\n    constructor(key: K, value: V) {\n      this.key = key;\n      this.value = value;\n      this.prev = null;\n      this.next = null;\n    }\n  }\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private moveToFront(node: Node<K, V>): void {\n    if (node.prev) {\n      node.prev.next = node.next;\n    } else {\n      this.head = node.next;\n    }\n\n    if (node.next) {\n      node.next.prev = node.prev;\n    } else {\n      this.tail = node.prev;\n    }\n\n    node.prev = null;\n    node.next = this.head;\n\n    if (this.head) {\n      this.head.prev = node;\n    }\n\n    this.head = node;\n  }\n\n  private addToTail(node: Node<K, V>): void {\n    node.next = null;\n    node.prev = this.tail;\n\n    if (this.tail) {\n      this.tail.next = node;\n    } else {\n      this.head = node;\n    }\n\n    this.tail = node;\n  }\n\n  private evictLRU(): void {\n    if (this.tail) {\n      const node = this.tail;\n      this.map.delete(node.key);\n      this.tail = node.prev;\n      if (this.tail) {\n        this.tail.next = null;\n      } else {\n        this.head = null;\n      }\n    }\n  }\n\n  get(key: K): V | undefined {\n    const node = this.map.get(key);\n    if (!node) {\n      return undefined;\n    }\n\n    this.moveToFront(node);\n    return node.value;\n  }\n\n  put(key: K, value: V): void {\n    const node = this.map.get(key);\n\n    if (node) {\n      node.value = value;\n      this.moveToFront(node);\n    } else {\n      const newNode = new Node(key, value);\n      this.map.set(key, newNode);\n      this.addToTail(newNode);\n\n      if (this.map.size > this.capacity) {\n        this.evictLRU();\n      }\n    }\n  }\n\n  get size(): number {\n    return this.map.size;\n  }\n}\n```",
    "passed": 0,
    "reasoning": "The user wants me to implement a generic LRU (Least Recently Used) cache in TypeScript. Let me break down the requirements:\n\n1. Generic class `LRUCache<K, V>` with:\n   - Constructor that takes `capacity: number`\n   - `get(key: K): V | undefined` - returns value or undefined, updates LRU order on successful get\n   - `put(key: K, value: V): void` - inserts/updates, evicts LRU if needed\n   - `size: number` - current number of entries\n\n2. Rules:\n   - Successful `get` makes that key most-recently used\n   - `put` with existing key updates it and makes it most-recently used\n   - When inserting new key exceeds capacity, evict least-recently-used key\n   - Throw `RangeError` if `capacity < 1`\n\n3. Must be fully typed, no `any` in public API\n\nFor implementation, I'll use a combination of:\n- A Map to store key-value pairs\n- A doubly-linked list (or similar structure) to maintain LRU order\n\nActually, in TypeScript/JavaScript, I can use a Map with a custom approach. The most efficient way is to use a Map for O(1) lookups and maintain insertion order, but for true LRU we need to track which is least recently used.\n\nA common approach is:\n1. Use a Map for O(1) key-value access\n2. Use a doubly-linked list to track LRU order\n3. When accessing a key, move it to the front of the list\n4. When capacity is exceeded, remove from the back of the list\n\nLet me implement this:\n\n```typescript\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  private class Node<K, V> {\n    key: K;\n    value: V;\n    prev: Node<K, V> | null;\n    next: Node<K, V> | null;\n    \n    constructor(key: K, value: V) {\n      this.key = key;\n      this.value = value;\n      this.prev = null;\n      this.next = null;\n    }\n  }\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private moveToFront(node: Node<K, V>): void {\n    if (node.prev) {\n      node.prev.next = node.next;\n    } else {\n      this.head = node.next;\n    }\n    \n    if (node.next) {\n      node.next.prev = node.prev;\n    } else {\n      this.tail = node.prev;\n    }\n    \n    node.prev = null;\n    node.next = this.head;\n    \n    if (this.head) {\n      this.head.prev = node;\n    }\n    \n    this.head = node;\n  }\n\n  private addToTail(node: Node<K, V>): void {\n    node.next = null;\n    node.prev = this.tail;\n    \n    if (this.tail) {\n      this.tail.next = node;\n    } else {\n      this.head = node;\n    }\n    \n    this.tail = node;\n  }\n\n  private evictLRU(): void {\n    if (this.tail) {\n      const node = this.tail;\n      this.map.delete(node.key);\n      this.tail = node.prev;\n      if (this.tail) {\n        this.tail.next = null;\n      } else {\n        this.head = null;\n      }\n    }\n  }\n\n  get(key: K): V | undefined {\n    const node = this.map.get(key);\n    if (!node) {\n      return undefined;\n    }\n    \n    this.moveToFront(node);\n    return node.value;\n  }\n\n  put(key: K, value: V): void {\n    const node = this.map.get(key);\n    \n    if (node) {\n      node.value = value;\n      this.moveToFront(node);\n    } else {\n      const newNode = new Node(key, value);\n      this.map.set(key, newNode);\n      this.addToTail(newNode);\n      \n      if (this.map.size > this.capacity) {\n        this.evictLRU();\n      }\n    }\n  }\n\n  get size(): number {\n    return this.map.size;\n  }\n}\n```\n\nI'm reconsidering the Node class structure. The current implementation might have some complexity in managing the doubly-linked list. I'll need to carefully track the head and tail pointers to ensure efficient LRU cache operations.\n\nThe key challenge is maintaining the correct order of nodes while allowing quick access and removal. I'll need to ensure that when a node is accessed, it moves to the front of the list, and when the cache is full, the least recently used node is removed from the back.\n\nMy current approach uses a Map to store nodes and a doubly-linked list to track access order. This allows O(1) access and removal of nodes, which is crucial for an efficient LRU cache implementation.\n\nThe critical steps involve:\n1. Checking if a node exists in the map\n2. Moving accessed nodes to the front\n3. Adding new nodes to the tail\n4. Evicting the least recently used node when capacity is exceeded\n\nI'll need to carefully manage the head and tail pointers to ensure the list remains correctly ordered.\n",
    "test_error": "TAP version 13\n# node:internal/modules/run_main:107\n#     triggerUncaughtException(\n#     ^\n# Error: Transform failed with 1 error:\n# /tmp/llmlab-ts-03-lru-cache-ux35zu76/solution.ts:7:16: ERROR: Expected \";\" but found \"Node\"\n#     at failureErrorWithLog (/home/psc/opt/node/lib/node_modules/tsx/node_modules/esbuild/lib/main.js:1748:15)\n#     at /home/psc/opt/node/lib/node_modules/tsx/node_modules/esbuild/lib/main.js:1017:50\n#     at responseCallbacks.<computed> (/home/psc/opt/node/lib/node_modules/tsx/node_modules/esbuild/lib/main.js:884:9)\n#     at handleIncomingPacket (/home/psc/opt/node/lib/node_modules/tsx/node_modules/esbuild/lib/main.js:939:12)\n#     at Socket.readFromStdout (/home/psc/opt/node/lib/node_modules/tsx/node_modules/esbuild/lib/main.js:862:7)\n#     at Socket.emit (node:events:509:28)\n#     at addChunk (node:internal/streams/readable:563:12)\n#     at readableAddChunkPushByteMode (node:internal/streams/readable:514:3)\n#     at Readable.push (node:internal/streams/readable:394:5)\n#     at Pipe.onStreamRead (node:internal/stream_base_commons:189:23) {\n#   name: 'TransformError'\n# }\n# Node.js v24.16.0\n# Subtest: solution.test.ts\nnot ok 1 - solution.test.ts\n  ---\n  duration_ms: 65.403819\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-ux35zu76/solution.test.ts:1:1'\n  failureType: 'testCodeFailure'\n  exitCode: 1\n  signal: ~\n  error: 'test failed'\n  code: 'ERR_TEST_FAILURE'\n  ...\n1..1\n# tests 1\n# suites 0\n# pass 0\n# fail 1\n# cancelled 0\n# skipped 0\n# todo 0\n# duration_ms 69.245001"
  },
  {
    "total": 7,
    "answer": "```typescript\nclass Node<K, V> {\n  key: K;\n  value: V;\n  prev: Node<K, V> | null;\n  next: Node<K, V> | null;\n\n  constructor(key: K, value: V) {\n    this.key = key;\n    this.value = value;\n    this.prev = null;\n    this.next = null;\n  }\n}\n\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private moveToFront(node: Node<K, V>): void {\n    if (node.prev) {\n      node.prev.next = node.next;\n    } else {\n      this.head = node.next;\n    }\n\n    if (node.next) {\n      node.next.prev = node.prev;\n    } else {\n      this.tail = node.prev;\n    }\n\n    node.prev = null;\n    node.next = this.head;\n\n    if (this.head) {\n      this.head.prev = node;\n    }\n\n    this.head = node;\n  }\n\n  private addToTail(node: Node<K, V>): void {\n    node.next = null;\n    node.prev = this.tail;\n\n    if (this.tail) {\n      this.tail.next = node;\n    } else {\n      this.head = node;\n    }\n\n    this.tail = node;\n  }\n\n  private evictLRU(): void {\n    if (this.tail) {\n      const node = this.tail;\n      this.map.delete(node.key);\n      this.tail = node.prev;\n      if (this.tail) {\n        this.tail.next = null;\n      } else {\n        this.head = null;\n      }\n    }\n  }\n\n  get(key: K): V | undefined {\n    const node = this.map.get(key);\n    if (!node) {\n      return undefined;\n    }\n\n    this.moveToFront(node);\n    return node.value;\n  }\n\n  put(key: K, value: V): void {\n    const node = this.map.get(key);\n\n    if (node) {\n      node.value = value;\n      this.moveToFront(node);\n    } else {\n      const newNode = new Node(key, value);\n      this.map.set(key, newNode);\n      this.addToTail(newNode);\n\n      if (this.map.size > this.capacity) {\n        this.evictLRU();\n      }\n    }\n  }\n\n  get size(): number {\n    return this.map.size;\n  }\n}\n```",
    "passed": 3,
    "reasoning": "The error message shows that there's a syntax error at line 7: \"Expected ';' but found 'Node'\". This is because I tried to define a class inside another class, which is not valid TypeScript syntax. I need to define the `Node` class at the top level instead of inside `LRUCache`.\n\nLet me fix this by moving the `Node` class outside of `LRUCache`:\n\n```typescript\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private class Node<K, V> {\n    // ...\n  }\n  // This is invalid - can't have class inside class\n}\n```\n\nI need to move the Node class to the top level of the file. Let me rewrite the solution:\n\n```typescript\nclass Node<K, V> {\n  key: K;\n  value: V;\n  prev: Node<K, V> | null;\n  next: Node<K, V> | null;\n\n  constructor(key: K, value: V) {\n    this.key = key;\n    this.value = value;\n    this.prev = null;\n    this.next = null;\n  }\n}\n\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private moveToFront(node: Node<K, V>): void {\n    if (node.prev) {\n      node.prev.next = node.next;\n    } else {\n      this.head = node.next;\n    }\n\n    if (node.next) {\n      node.next.prev = node.prev;\n    } else {\n      this.tail = node.prev;\n    }\n\n    node.prev = null;\n    node.next = this.head;\n\n    if (this.head) {\n      this.head.prev = node;\n    }\n\n    this.head = node;\n  }\n\n  private addToTail(node: Node<K, V>): void {\n    node.next = null;\n    node.prev = this.tail;\n\n    if (this.tail) {\n      this.tail.next = node;\n    } else {\n      this.head = node;\n    }\n\n    this.tail = node;\n  }\n\n  private evictLRU(): void {\n    if (this.tail) {\n      const node = this.tail;\n      this.map.delete(node.key);\n      this.tail = node.prev;\n      if (this.tail) {\n        this.tail.next = null;\n      } else {\n        this.head = null;\n      }\n    }\n  }\n\n  get(key: K): V | undefined {\n    const node = this.map.get(key);\n    if (!node) {\n      return undefined;\n    }\n\n    this.moveToFront(node);\n    return node.value;\n  }\n\n  put(key: K, value: V): void {\n    const node = this.map.get(key);\n\n    if (node) {\n      node.value = value;\n      this.moveToFront(node);\n    } else {\n      const newNode = new Node(key, value);\n      this.map.set(key, newNode);\n      this.addToTail(newNode);\n\n      if (this.map.size > this.capacity) {\n        this.evictLRU();\n      }\n    }\n  }\n\n  get size(): number {\n    return this.map.size;\n  }\n}\n```\n\nI'll complete the implementation by ensuring the Node class is properly defined at the top level and all methods are correctly implemented for the LRU cache functionality.\n",
    "test_error": "Context.<anonymous> (/tmp/llmlab-ts-03-lru-cache-xpiuyxus/solution.test.ts:36:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n# Subtest: put updates existing key and marks it used\nnot ok 5 - put updates existing key and marks it used\n  ---\n  duration_ms: 0.111441\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-xpiuyxus/solution.test.ts:1:841'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    2 !== undefined\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  actual: 2\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (/tmp/llmlab-ts-03-lru-cache-xpiuyxus/solution.test.ts:48:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n# Subtest: capacity of 1 keeps only the newest\nnot ok 6 - capacity of 1 keeps only the newest\n  ---\n  duration_ms: 0.095251\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-xpiuyxus/solution.test.ts:1:1085'\n  failureType: 'testCodeFailure'\n  error: |-\n    Expected values to be strictly equal:\n    \n    'one' !== undefined\n    \n  code: 'ERR_ASSERTION'\n  name: 'AssertionError'\n  actual: 'one'\n  operator: 'strictEqual'\n  stack: |-\n    TestContext.<anonymous> (/tmp/llmlab-ts-03-lru-cache-xpiuyxus/solution.test.ts:57:10)\n    Test.runInAsyncScope (node:async_hooks:227:14)\n    Test.run (node:internal/test_runner/test:1306:25)\n    Test.processPendingSubtests (node:internal/test_runner/test:897:18)\n    Test.postRun (node:internal/test_runner/test:1447:19)\n    Test.run (node:internal/test_runner/test:1372:12)\n    async Test.processPendingSubtests (node:internal/test_runner/test:897:7)\n  ...\n# Subtest: invalid capacity throws RangeError\nok 7 - invalid capacity throws RangeError\n  ---\n  duration_ms: 0.181743\n  type: 'test'\n  ...\n1..7\n# tests 7\n# suites 0\n# pass 3\n# fail 4\n# cancelled 0\n# skipped 0\n# todo 0\n# duration_ms 75.695937"
  },
  {
    "total": 7,
    "answer": "Looking at the test failures, the issue is in the `moveToFront` method. When a node is already at the head, we shouldn't try to move it (which causes issues with the doubly-linked list). Also, there's a bug where when updating an existing key, the node might not be properly maintained in the list.\n\nLet me fix the `moveToFront` logic:\n\n```typescript\nclass Node<K, V> {\n  key: K;\n  value: V;\n  prev: Node<K, V> | null;\n  next: Node<K, V> | null;\n\n  constructor(key: K, value: V) {\n    this.key = key;\n    this.value = value;\n    this.prev = null;\n    this.next = null;\n  }\n}\n\nexport class LRUCache<K, V> {\n  private capacity: number;\n  private map: Map<K, Node<K, V>>;\n  private head: Node<K, V> | null;\n  private tail: Node<K, V> | null;\n\n  constructor(capacity: number) {\n    if (capacity < 1) {\n      throw new RangeError('capacity must be at least 1');\n    }\n    this.capacity = capacity;\n    this.map = new Map();\n    this.head = null;\n    this.tail = null;\n  }\n\n  private moveToFront(node: Node<K, V>): void {\n    if (node === this.head) {\n      return;\n    }\n\n    // Remove from current position\n    if (node.prev) {\n      node.prev.next = node.next;\n    } else {\n      this.head = node.next;\n    }\n\n    if (node.next) {\n      node.next.prev = node.prev;\n    } else {\n      this.tail = node.prev;\n    }\n\n    // Add to front\n    node.prev = null;\n    node.next = this.head;\n\n    if (this.head) {\n      this.head.prev = node;\n    }\n\n    this.head = node;\n  }\n\n  private addToTail(node: Node<K, V>): void {\n    node.next = null;\n    node.prev = this.tail;\n\n    if (this.tail) {\n      this.tail.next = node;\n    } else {\n      this.head = node;\n    }\n\n    this.tail = node;\n  }\n\n  private evictLRU(): void {\n    if (this.tail) {\n      const node = this.tail;\n      this.map.delete(node.key);\n      this.tail = node.prev;\n      if (this.tail) {\n        this.tail.next = null;\n      } else {\n        this.head = null;\n      }\n    }\n  }\n\n  get(key: K): V | undefined {\n    const node = this.map.get(key);\n    if (!node) {\n      return undefined;\n    }\n\n    this.moveToFront(node);\n    return node.value;\n  }\n\n  put(key: K, value: V): void {\n    const node = this.map.get(key);\n\n    if (node) {\n      node.value = value;\n      this.moveToFront(node);\n    } else {\n      const newNode = new Node(key, value);\n      this.map.set(key, newNode);\n      this.addToTail(newNode);\n\n      if (this.map.size > this.capacity) {\n        this.evictLRU();\n      }\n    }\n  }\n\n  get size(): number {\n    return this.map.size;\n  }\n}\n```",
    "passed": 3,
    "reasoning": "Looking at the test failures:\n\n1. \"put updates existing key and marks it used\" - Expected `2` but got `undefined`. This suggests that when we update an existing key, the test is checking if the key is still in the cache after the update, but it's returning undefined.\n\n2. \"capacity of 1 keeps only the newest\" - Expected `'one'` but got `undefined`. Similar issue - after updating/putting, the old key should be evicted but the new key should be accessible.\n\nThe issue is likely in my `moveToFront` logic. When I move a node to the front, I need to make sure I'm not breaking the doubly-linked list connections.\n\nLet me trace through what happens:\n\n1. When we `put(\"a\", 1)` with capacity 2:\n   - Create node for \"a\"\n   - Add to tail (head = tail = node \"a\")\n   \n2. When we `put(\"b\", 2)`:\n   - Create node for \"b\"\n   - Add to tail (head = \"a\", tail = \"b\")\n   \n3. When we `get(\"a\")`:\n   - Find node \"a\"\n   - Call `moveToFront(node \"a\")`\n   - This should move \"a\" to the front (head)\n   - After move: head = \"a\", tail = \"b\"\n   - Return 1\n\n4. When we `put(\"c\", 3)`:\n   - Create node for \"c\"\n   - Add to tail (head = \"a\", tail = \"c\")\n   - Now size = 3 > capacity = 2\n   - Evict LRU (tail = \"c\")\n   - Wait, that's wrong! The tail is the LRU, but we just added \"c\" to the tail!\n\nThe issue is that when we add a new node, we add it to the tail (which is the LRU position). But then when we check if we need to evict, we evict the tail. This is correct for a new insertion.\n\n\nBut the real problem emerges when updating an existing key. If \"a\" was already in the cache and we update it, we need to move it to the front (most recently used). However, my current `moveToFront` implementation has a flaw - when the node is already at the head, I'm still trying to rewire the list, which breaks the connections. I need to check if the node is already at the head before attempting to move it, or handle that case specially.\n\nActually, looking more carefully at the logic: when `node.prev` is null, the node is already at the head, so I shouldn't try to remove it from its current position. The same applies when `node.next` is null - the node is already at the tail. I need to add guards to prevent unnecessary list manipulation when the node is already in its target position.\n\nLet me trace through the `moveToFront` logic more carefully. When moving a node to the front, I need to:\n1. Remove it from its current position (if not already at head)\n2. Add it to the front (if not already at head)\n\nThe current implementation tries to handle both cases, but there's a bug in how it reconnects the list. When I set `node.prev = null` and `node.next = this.head`, I'm not properly updating the old head's `prev` pointer before moving the node.\n\nActually, I see the issue now. When I do `this.head.prev = node` after setting `node.next = this.head`, I'm updating the old head's prev pointer to point to the node I'm moving. But this happens after I've already set `node.prev = null`, which is correct.\n\nLet me trace through a specific example:\n- Initial state: head = \"a\", tail = \"b\", \"a\".next = \"b\", \"b\".prev = \"a\"\n- Move \"a\" to front:\n  - node.prev is null, so skip the first if\n  - node.next is \"b\", so set \"b\".prev = null\n  - node.prev = null, node.next = head (\"a\")\n  - head is \"a\", so set \"a\".prev = \"a\" (this is wrong!)\n  - head = \"a\"\n\nThe problem is that when the node is already at the head, I'm setting `this.head.prev = node`, which makes the head point to itself. I need to check if the node is already at the head before doing any reconnection.\n\nActually, I think the real issue is simpler. When I move a node to the front, I should:\n1. If the node is already at the head, do nothing\n2. Otherwise, remove it from its current position and add it to the front\n\nLet me rewrite `moveToFront` to handle this properly:\n\n```typescript\nprivate moveToFront(node: Node<K, V>): void {\n  if (node === this.head) {\n    return;\n  }\n\n  // Remove from current position\n  if (node.prev) {\n    node.prev.next = node.next;\n  } else {\n    this.head = node.next;\n  }\n\n  if (node.next) {\n    node.next.prev = node.prev;\n  } else {\n    this.tail = node.prev;\n  }\n\n  // Add to front\n  node.prev = null;\n  node.next = this.head;\n\n  if (this.head) {\n    this.head.prev = node;\n  }\n\n  this.head = node;\n}\n```\n\nThis should work correctly. The key insight is to check if the node is already at the head before attempting to move it.\n\nWait, I need to reconsider the test failures. The first failure shows `2 !== undefined` when checking `c.get(\"a\")` after `c.put(\"a\", 2)`. This suggests the value isn't being retrieved correctly. The second failure shows `'one' !== undefined` when checking `c.get(\"one\")` after `c.put(\"one\", \"one\")`. Both indicate that after updating an existing key, the subsequent `get` call returns undefined instead of the updated value.\n\nThis points to a problem with how I'm handling existing keys in the `put` method. When I update a key that already exists, I need to ensure the node is properly maintained in the cache structure.\n\nLooking at my `put` implementation, I'm calling `moveToFront` on the existing node, which should update its position. But the test failures suggest the node might not be in the map correctly, or there's an issue with how I'm managing the doubly-linked list connections.\n\nI should verify that when I update an existing key, the node remains in the map and its linked list pointers are correctly maintained. The `moveToFront` function needs to properly handle the case where the node is already at the head of the list.\n\nLet me trace through the logic more carefully to identify where the disconnect occurs between the map and the linked list structure.\n",
    "test_error": ""
  }
]