Gemfile

source 'https://rubygems.org'

gem 'rspec'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    diff-lcs (1.4.4)
    rspec (3.10.0)
      rspec-core (~> 3.10.0)
      rspec-expectations (~> 3.10.0)
      rspec-mocks (~> 3.10.0)
    rspec-core (3.10.1)
      rspec-support (~> 3.10.0)
    rspec-expectations (3.10.1)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.10.0)
    rspec-mocks (3.10.2)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.10.0)
    rspec-support (3.10.2)

PLATFORMS
  x86_64-darwin-20

DEPENDENCIES
  rspec

BUNDLED WITH
   2.2.16

unique_pair.rb

# frozen_string_literal: true

# Write a function which, given an array of integers of value greater or equal to 0, returns all unique pairs which
# sum to 100.
#
# Example data:
#
# input = [0, 1, 100, 99, 0, 10, 90, 30, 55, 33, 55, 75, 50, 51, 49, 50, 51, 49, 51]
# output = [[1,99], [0,100], [10,90], [51,49], [50,50]]
class UniquePair
  def self.unique_pairs(numbers, sum=100)
    Array(numbers)
      .combination(2)
      .map do |x, y|
        x, y = y, x if x > y
        [x, y] if x + y == sum
      end.compact.sort.uniq || []
  end
end

unique_pairs_spec.rb

require 'rspec'
require './unique_pair'

RSpec.describe UniquePair do
  describe '.unique_pairs' do
    subject { described_class.unique_pairs(input) }

    context 'when an array of numbers is provided' do
      let(:input) { [0, 1, 100, 99, 0, 10, 90, 30, 55, 33, 55, 75, 50, 51, 49, 50, 51, 49, 51] }
      let(:output) { [[1,99], [0,100], [10,90], [49,51], [50,50]] }

      it 'returns all unique pairs which sum up to 100' do
        expect(subject).to match_array output
      end
    end

    context 'when no input is provided' do
      let(:input) { nil }
      let(:output) { [] }

      it 'returns an empty array' do
        expect(subject).to match_array output
      end
    end
  end
end