LRU cache

lru_cache.rb

# frozen_string_literal: true

class LRUCache
  attr_reader :capacity, :cache

  def initialize(capacity)
    @capacity = capacity
    @cache = {}
  end

  def get(key)
    cache[key] = cache.delete(key)
  end

  def put(key, value)
    cache.delete(key)

    cache[key] = value

    evict

    nil
  end

  private

  def evict
    return if cache.empty? || cache.count <= capacity

    cache.delete(cache.keys.first)
  end
end

cache = LRUCache.new(2)

cache.put(1, 1)   # cache is {1=1}
cache.put(2, 2)   # cache is {1=1, 2=2}
puts cache.get(1) # return 1
cache.put(3, 3)   # LRU key was 2, evicts key 2, cache is {1=1, 3=3}
puts cache.get(2) # returns -1 (not found)
cache.put(4, 4)   # LRU key was 1, evicts key 1, cache is {4=4, 3=3}
puts cache.get(1) # return -1 (not found)
puts cache.get(3) # return 3
puts cache.get(4) # return 4