consistent_hash.rb

require 'digest'

class ConsistentHash
  Node = Data.define(:position, :updated_at)

  def initialize(size = 1)
    @hash_ring = {}
    (0...size).each { |i| add(i) }
  end

  def size
    @hash_ring.size
  end

  def add(pos)
    hash = hash_value(pos)
    @hash_ring[hash] = Node.new(pos, Time.now.utc)
  end

  def resolve(key)
    hash = hash_value(key)
    ring = @hash_ring.keys.sort
    k = ring.find { |h| h >= hash } || ring.first
    @hash_ring[k]
  end

  def checkin(pos)
    hash = hash_value(pos)
    add(pos) unless @hash_ring.key?(hash)

    ring = @hash_ring.keys.sort
    k = ring.find { |h| h >= hash } || ring.first
    node = @hash_ring[k]
    @hash_ring[k] = node.with(updated_at: Time.now.utc)
  end

  def remove(pos)
    hash = hash_value(pos)
    @hash_ring.delete(hash)
  end

  def rebalance
    now = Time.now.utc
    @hash_ring.each do |hash, node|
      @hash_ring.delete(hash) if (now - node.updated_at) > 10
    end
  end

  private

  def hash_value(value)
    Digest::SHA256.digest(value.to_s).sum % 360
  end
end