require 'atomic'
require 'metriks/uniform_sample'
require 'metriks/exponentially_decaying_sample'

module Metriks
  class Histogram
    DEFAULT_SAMPLE_SIZE = 1028
    DEFAULT_ALPHA = 0.015

    def self.new_uniform
      new(Metriks::UniformSample.new(DEFAULT_SAMPLE_SIZE))
    end

    def self.new_exponentially_decaying
      new(Metriks::ExponentiallyDecayingSample.new(DEFAULT_SAMPLE_SIZE, DEFAULT_ALPHA))
    end

    def initialize(sample)
      @sample   = sample
      @count    = Atomic.new(0)
      @min      = Atomic.new(nil)
      @max      = Atomic.new(nil)
      @sum      = Atomic.new(0)
      @variance = Atomic.new([ -1, 0 ])
    end

    def clear
      @sample.clear
      @count.value = 0
      @min.value = nil
      @max.value = nil
      @sum.value = 0
      @variance.value = [ -1, 0 ]
    end

    def update(value)
      @count.update { |v| v + 1 }
      @sample.update(value)
      self.max = value
      self.min = value
      @sum.update { |v| v + value }
      update_variance(value)
    end

    def snapshot
      @sample.snapshot
    end

    def count
      @count.value
    end

    def max
      count > 0 ? @max.value : 0.0
    end

    def min
      count > 0 ? @min.value : 0.0
    end

    def mean
      count > 0 ? @sum.value / count : 0.0
    end

    def stddev
      count > 0 ? variance ** 0.5 : 0.0
    end

    def variance
      count <= 1 ? 0.0 : @variance.value[1] / (count - 1)
    end

    def max=(potential_max)
      done = false

      while !done
        current_max = @max.value
        done = (!current_max.nil? && current_max >= potential_max) || @max.compare_and_swap(current_max, potential_max)
      end
    end

    def min=(potential_min)
      done = false

      while !done
        current_min = @min.value
        done = (!current_min.nil? && current_min <= potential_min) || @min.compare_and_swap(current_min, potential_min)
      end
    end

    def update_variance(value)
      @variance.update do |old_values|
        new_values = Array.new(2)
        if old_values[0] == -1
          new_values[0] = value
          new_values[1] = 0
        else
          old_m = old_values[0]
          old_s = old_values[1]

          new_m = old_m + ((value - old_m) / count)
          new_s = old_s + ((value - old_m) * (value - new_m))

          new_values[0] = new_m
          new_values[1] = new_s
        end

        new_values
      end
    end
  end
end
