An algorithm for eating M&Ms in pairs

mm-sorter.rb

class MMSorter
  def initialize(preference)
    @preference = preference
    @reverse = preference.each_with_index.to_h
  end

  def pairs(candy)
    # Find the candies which aren't part of a pair and sort them in preference
    # order.
    leftovers = candy.select { |_c, cnt| cnt.odd? }.map { |p| p[0] }
      .sort_by { |c| @reverse[c] }
    # Collect the pairs.
    pairs = candy.map { |color, count| [color, color] * (count / 2) }
    # Form the extras into pairs.
    pairs += leftovers.each_slice(2).to_a
    # Order pairs by their most preferred color, preferring identical pairs over
    # odd pairs.
    pairs.sort_by { |p| @reverse[p[0]] * @reverse.length + @reverse[p[1]] }
  end
end

mm-sorter_spec.rb

require_relative 'mm-sorter'

describe MMSorter do
  before(:each) do
    @prefs = %i(blue green red yellow orange brown)
    @sorter = MMSorter.new(@prefs)
  end

  it 'should sort paired items in order' do
    candy = {
      blue: 2,
      brown: 2,
      green: 2,
      orange: 2,
      red: 2,
      yellow: 2
    }
    result = [
      %i(blue blue),
      %i(green green),
      %i(red red),
      %i(yellow yellow),
      %i(orange orange),
      %i(brown brown),
    ]
    expect(@sorter.pairs(candy)).to eq result
  end

  it 'should sort extra items by first color then by second' do
    candy = {
      blue: 3,
      brown: 3,
      green: 3,
      orange: 3,
      red: 3,
      yellow: 3
    }
    result = [
      %i(blue blue),
      %i(blue green),
      %i(green green),
      %i(red red),
      %i(red yellow),
      %i(yellow yellow),
      %i(orange orange),
      %i(orange brown),
      %i(brown brown),
    ]
    expect(@sorter.pairs(candy)).to eq result
  end

  it 'should sort extra items by first color' do
    candy = {
      blue: 3,
      green: 2,
      orange: 3,
      red: 2,
    }
    result = [
      %i(blue blue),
      %i(blue orange),
      %i(green green),
      %i(red red),
      %i(orange orange),
    ]
    expect(@sorter.pairs(candy)).to eq result
  end
end