Measure the times it takes to access (hit and miss) elements in hashes with up to 2^21 elements.

hash_bench.rb

require 'benchmark'
require 'ascii_charts'

def measure
  GC.disable
  Benchmark.realtime do
    yield
  end
ensure
  GC.enable
end

def measure_hash_accesses(exp, count, miss = false)
  exp.times.map do |exponent|
    size = 2**exponent
    hash = {}

    (1..size).each do |n|
      index       = rand
      hash[index] = rand
    end

    keys    = hash.keys
    kick    = miss ? 1.0 : 0.0
    targets = count.times.map { keys.sample + kick }
    
    offset = measure do 
      targets.each do |k|
        val = k
      end
    end

    access_time = measure do
      targets.each do |k|
        val = hash[k]
      end
    end
    
    [size, access_time - offset]
  end
end

def draw(title, times)
  puts AsciiCharts::Cartesian.new(
    times,
    title:       title,
    y_step_size: 0.05,
    max_y_vals:  1.0,
    bar:         true,
    hide_zero:   true
  ).draw
end

count = 1_000_000

hit_times = measure_hash_accesses(21, count)
draw "#{count} successful hash reads", hit_times

miss_times = measure_hash_accesses(21, count, true)
draw "#{count} missing hash reads", miss_times