Generational hash table — useful for in-memory caches

lru_generational_hash.rb

#
# by Eric Lindvall <eric@5stops.com>
#

class LruGenerationalHash
  attr_reader :max_size

  def initialize(max_size, generations = 2)
    @generations = generations.times.collect { Hash.new }
    @max_size    = max_size
  end

  def clear
    @generations.each { |hash| hash.clear }
  end

  def max_size=(value)
    @max_size = value
    rotate
    @max_size
  end

  def size
    @generations.inject(0) { |memo, hash| memo + hash.size }
  end

  def []=(key, value)
    rotate

    @generations.each_with_index do |hash, idx|
      if idx == @generations.length - 1
        hash[key] = value
      else
        hash.delete(key)
      end
    end

    value
  end

  def [](key)
    @generations.each_with_index do |hash, idx|
      if idx == 0
        if value = hash[key]
          return value
        end
      else
        if value = hash.delete(key)
          @generations[idx - 1][key] = value
          return value
        end
      end
    end

    nil
  end

  def delete(key)
    @generations.each do |hash|
      hash.delete(key)
    end
  end

  def merge!(hash)
    hash.each { |k,v| self[k] = v }
  end

  private
  def rotate
    if size >= @max_size
      @generations.unshift(Hash.new)
      @generations.pop
    end
  end
end