Testing max's reasoning

kcompl.rb

# I'm assuming sum operations are cheap and I won't gain much, if at all, from
# memoizing sums.
def complementary_pairs(k, a)
  matches = 0
  a.each_index do |i|
    # sum the tail, the elements from i (excluded) to the end of a
    (i...a.length).each do |j|
      sum = a[i] + a[j]
      # we have a match but, ...
      if sum == k
        # if its the sum of itself, then there can only be one match
        if i == j
          matches += 1
        # if it is not, then account for the inverse of that pair
        else
          matches += 2
        end
      end
    end
  end
  matches
end


tests = {
  [6, [1,8,-3,0,1,3,-2,4,5]] => 7, # textbook case
  [6, [0]]                   => 0,
  [6, [0,0]]                 => 0, # better test too many cases than not enough...
  [6, [3,3]]                 => 4,
  [6, [3,3,3]]               => 9,
  [6, [1,5]]                 => 2
}

tests.each do |args, expected|
  got = complementary_pairs(*args)
  unless got == expected
    raise "expected #{expected}, not #{got} from complementary_pairs(#{args.inspect})"
  end
end


puts "All Green"