Choosing random array members deterministically based on contents of array and a name string

0_deterministic_sample.rb

require 'backports/1.9.1' if RUBY_VERSION < '1.9.1'
require 'backports/1.9.2' if RUBY_VERSION < '1.9.2'
require 'digest'

class Array
  def deterministic_sample(n, name='')
    seed = Digest::SHA1.hexdigest(self.sort.map(&:to_s).join << name).to_i(16)
    prng = Random.new(seed)
    self.sort.sample(n, :random => prng)
  end
end

# output for ruby 1.8.7

p ['a', 'b', 'c'].deterministic_sample(2)
# => ['c', 'a']
p ['c', 'b', 'a'].deterministic_sample(2)
# => ['c', 'a']

p
p ['a', 'b', 'c'].deterministic_sample(2, "foo")
# => ['b', 'c']
p ['c', 'b', 'a'].deterministic_sample(2, "foo")
# => ['b', 'c']

p
p ['a', 'b', 'c', 'd'].deterministic_sample(2)
# => ['c', 'd']
p ['d', 'a', 'c', 'b'].deterministic_sample(2)
# => ['c', 'd']

p
p ['a', 'b', 'c', 'd', 'e'].deterministic_sample(2)
# => ['d', 'b']
p ['d', 'a', 'c', 'e', 'b'].deterministic_sample(2)
# => ['d', 'b']

Gemfile

source 'https://rubygems.org'

gem 'backports' if RUBY_VERSION < '1.9.3'