2622 - Cache with time limit

The problem statement

Write a class that allows getting and setting key-value pairs, however a time until expiration is associated with each key.

The class has three public methods:

set(key, value, duration): accepts an integer key, an integer value, and a duration in milliseconds. Once the duration has elapsed, the key should be inaccessible. The method should return true if the same un-expired key already exists and false otherwise. Both the value and duration should be overwritten if the key already exists.

get(key): if an un-expired key exists, it should return the associated value. Otherwise it should return -1.

count(): returns the count of un-expired keys.

Understanding the problem

What they are asking for is pretty simple.

Create a map which holds our key value pairs. Each value should consist of the numerical value and a timeoutid that is a reference to the setTimeout we create.

Examples

Example 1:

Input:

actions = ["TimeLimitedCache", "set", "get", "count", "get"]

values = [[], [1, 42, 100], [1], [], [1]]

timeDelays = [0, 0, 50, 50, 150]

Output: [null, false, 42, 1, -1]

Explanation:

• At t=0, the cache is constructed.
• At t=0, a key-value pair (1: 42) is added with a time limit of 100ms. The value doesn't exist so false is returned.
• At t=50, key=1 is requested and the value of 42 is returned.
• At t=50, count() is called and there is one active key in the cache.
• At t=100, key=1 expires.
• At t=150, get(1) is called but -1 is returned because the cache is empty.

Example 2:

Input:

actions = ["TimeLimitedCache", "set", "set", "get", "get", "get", "count"]

values = [[], [1, 42, 50], [1, 50, 100], [1], [1], [1], []]

timeDelays = [0, 0, 40, 50, 120, 200, 250]

Output: [null, false, true, 50, 50, -1, 0]

Explanation:

• At t=0, the cache is constructed.
• At t=0, a key-value pair (1: 42) is added with a time limit of 50ms. The value doesn't exist so false is returned.
• At t=40, a key-value pair (1: 50) is added with a time limit of 100ms. A non-expired value already existed so true is returned and the old value was overwritten.
• At t=50, get(1) is called which returned 50.
• At t=120, get(1) is called which returned 50.
• At t=140, key=1 expires.
• At t=200, get(1) is called but the cache is empty so -1 is returned.
• At t=250, count() returns 0 because the cache is empty.

Code

class TimeLimitedCache {
  private cache: Map<
    number,
    { value: number; timeoutId: ReturnType<typeof setTimeout> }
  >;
  constructor() {
    this.cache = new Map<
      number,
      { value: number; timeoutId: ReturnType<typeof setTimeout> }
    >();
  }

  set(key: number, value: number, duration: number): boolean {
    const hasEntry = this.cache.has(key);
    if (hasEntry) {
      clearTimeout(this.cache.get(key).timeoutId);
    }
    const timeoutId = setTimeout(() => this.cache.delete(key), duration);
    this.cache.set(key, { value, timeoutId });
    return hasEntry;
  }

  get(key: number): number {
    return this.cache.has(key) ? this.cache.get(key).value : -1;
  }

  count(): number {
    return this.cache.size;
  }
}

/**
 * const timeLimitedCache = new TimeLimitedCache()
 * timeLimitedCache.set(1, 42, 1000); // false
 * timeLimitedCache.get(1) // 42
 * timeLimitedCache.count() // 1
 */

Key Takeaways

1. A Map has a property on it called .size which is like array.length.