A bounded hash that expires keys using LRU (roughly)

bounded_hash.rb

# frozen_string_literal: true

require "delegate"
require "set"

# Hash that can only hold a certain number of keys, keys expire on a LRU basis
class BoundedHash < DelegateClass(Hash)

  NO_OP = proc {}

  def initialize(size = 100, &block)
    @size = size
    @keys = []
    @keyset = Set.new
    @init_block = block_given? ? block : NO_OP
    super({})
  end

  def []=(key, value)
    __getobj__[key] = value

    if @keyset.include?(key)
      lru_swap!(key)
    else
      @keys.insert(0, key)
      @keyset << key
    end

    return value unless @keys.length > @size

    # housekeeping
    oldest = @keys.pop
    @keyset.delete(oldest)
    __getobj__.delete(oldest)

    value
  end

  def [](key)
    lru_swap!(key)

    __getobj__.fetch(key) { @init_block.call(self, key) }
  end

  def clear
    @keys.clear
    @keyset.clear
    __getobj__.clear
  end

  private

  def lru_swap!(key)
    return unless @keyset.include?(key)
    return if @keys[0] == key

    index = @keys.index(key)
    @keys[0], @keys[index] = @keys[index], @keys[0]
  end

end