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-qxc7wri4/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: 69.113437\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-qxc7wri4/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 72.824248"
  },
  {
    "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 is clear - I can't define a class inside another class in TypeScript. I need to define the `Node` class outside of the `LRUCache` class, or use a different approach.\n\nLet me fix this by moving the `Node` class definition 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 wrong - can't define class inside another class\n}\n```\n\nI need to define `Node` at the module level, not inside `LRUCache`. Let me rewrite this properly.\n",
    "test_error": "TestContext.<anonymous> (/tmp/llmlab-ts-03-lru-cache-27ak5srt/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: 0.10745\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-27ak5srt/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-27ak5srt/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: 0.09571\n  type: 'test'\n  location: '/tmp/llmlab-ts-03-lru-cache-27ak5srt/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-27ak5srt/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: 0.18577\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 84.01918"
  },
  {
    "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 === this.head) {\n      return;\n    }\n\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": "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 `get` after the `put` should return the value, but it's returning `undefined`.\n\n2. \"capacity of 1 keeps only the newest\" - Expected `'one'` but got `undefined`. Similar issue - after putting a new key that exceeds capacity, the old key should be evicted, but the test expects the new key to be accessible.\n\nThe issue is likely in my `moveToFront` logic. When I move a node to the front, I need to be careful about the order of operations. Let me trace through what happens:\n\nFor `put` with an existing key:\n1. Get the node from map\n2. Update its value\n3. Call `moveToFront`\n\nFor `moveToFront`:\n- If node has prev, set prev.next to node.next\n- If node has next, set next.prev to node.prev\n- Set node.prev = null\n- Set node.next = this.head\n- If this.head exists, set head.prev = node\n- Set this.head = node\n\nWait, I think the issue is that when I update an existing key and call `moveToFront`, the node might already be at the head, or the logic might be breaking the doubly-linked list.\n\nLet me trace through a simple example:\n- put(\"a\", 1): Creates node A, adds to tail. head=A, tail=A\n- put(\"b\", 2): Creates node B, adds to tail. head=A, tail=B\n- get(\"a\"): Finds node A, calls moveToFront(A)\n  - A.prev is null, so head = A.next = B\n  - A.next is B, so B.prev = null\n  - A.prev = null\n  - A.next = head = B\n  - head.prev = A (B.prev = A)\n  - head = A\n  - Now head=A, tail=B, but A.next=B and B.prev=A - this is correct!\n  \nThe real problem emerges when I try to update an existing key. If node A is already at the head and I call moveToFront again, the logic breaks because I'm treating it as if it needs to be moved from somewhere else. The doubly-linked list connections get corrupted when the node is already at the front.\n\nLet me trace through what happens: when A is already head and I call moveToFront(A), the first condition `if (node.prev)` is false since A.prev is null, so I set head to A.next (which is B). Then I set A.next to the current head (B), creating a cycle where A.next points to B and B.prev points to A, but now head is also B. This breaks the list structure.\n\nThe real problem is that I'm not handling the case where the node is already at the front. I should check if the node is already head and skip the move operation entirely, or restructure the logic to avoid corrupting the list when the node is already in the correct position.\n\nActually, looking more carefully at the test failures - one expects `2` but gets `undefined`, another expects `'one'` but gets `undefined`. This suggests the `get` calls after `put` operations are returning `undefined` when they shouldn't. The issue might be in how I'm updating the node's position in the list when I modify its value.\n\nLet me trace through the capacity 1 test case: after putting \"one\" with value 1, then putting \"two\" with value 2, the cache should evict \"one\" and keep \"two\". When I call `get(\"two\")`, it should return 2, not undefined. This means either the node isn't being found in the map, or the value isn't being set correctly.\n\nLooking at my `put` method, I create a new node and add it to the tail, then check if the map size exceeds capacity. But I'm not properly handling the eviction logic - I need to make sure the old node is actually removed from the map and the linked list is updated correctly when evicting.\n\nThe real issue is in `moveToFront`. When I move a node to the front, I need to be careful about the order of operations. If the node is already at the head, I shouldn't try to move it again. Let me trace through what happens when I call `moveToFront` on a node that's already at the head - the logic might be breaking the doubly-linked list connections.\n\nActually, I think the problem is simpler. When I update an existing key's value, I call `moveToFront`, but if that node is already at the head, the logic still runs and might corrupt the list. I should add a check to skip the move if the node is already at the front.\n\nLet me trace through the capacity 1 case more carefully. After putting \"one\" with value 1, the map has one node at the head and tail. When I put \"two\" with value 2, I create a new node, add it to the tail, then evict the LRU (which is \"one\"). But I'm not properly updating the linked list pointers during eviction - I need to make sure the tail pointer is updated correctly and the evicted node is removed from the map.\n\nActually, looking at my eviction logic, I delete the node from the map and update the tail pointer, but I'm not handling the case where the tail becomes null correctly. When the map size exceeds capacity, I should evict the LRU node, but I need to ensure the linked list stays consistent.\n\nThe real issue is that when I update an existing key's value, I'm calling `moveToFront` which might be breaking the doubly-linked list. I should check if the node is already at the head before moving it, or restructure the logic to avoid corrupting the list during updates.\n\nLet me trace through what happens when I put a new key that exceeds capacity. I create the node, add it to the tail, then check if the map size exceeds capacity. If it does, I evict the LRU node. But I'm not properly updating the head and tail pointers during eviction - I need to make sure the linked list stays consistent.\n\nActually, I think the issue is simpler. When I call `moveToFront` on a node that's already at the head, the logic breaks. I should add a check to skip the move if the node is already at the front.\n\nLet me rewrite the `moveToFront` method to handle this case properly. I'll check if the node is already at the head, and if so, just return without doing anything. Otherwise, I'll move it to the front by updating the pointers correctly.\n\nActually, I realize the issue might be in how I'm handling the doubly-linked list. When I move a node to the front, I need to make sure I'm not breaking the connections. Let me trace through the logic more carefully.\n\nWhen I call `moveToFront` on a node that's already at the head, the current logic sets `this.head = node.next`, which would set head to null if the node is the only one. Then it sets `node.next = this.head`, which would set node.next to null. This breaks the list.\n\nI need to add a check at the beginning of `moveToFront` to return early if the node is already at the head.\n\n```typescript\nprivate moveToFront(node: Node<K, V>): void {\n  if (node === this.head) {\n    return;\n  }\n  \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\nThis should fix the issue where moving a node that's already at the head breaks the list.\n",
    "test_error": ""
  }
]