kcompl.rb

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


def complementary_pairs(k, a)
  a.sort!
  puts "A: #{a.inspect}"
  matches = 0
  left = 0
  right = a.length - 1
  # lets hand over obvious fails quickly
  if k < (a.first * 2) or k > (a.last * 2)
    return 0
  end
  # also if k = 2Umax or 2Umin, then return 2
  begin
    puts "Doing: #{a[left]} + #{a[right]} = #{a[left] + a[right]}"
    puts "Left: #{left} Right: #{right}"
    o = matches
    case a[left] + a[right] <=> k
    when -1
      left += 1
    when 0
      matches += 2
      matches += 1 if left < right && (a[left] == a[right-1] || a[left+1] == a[right])
      left += 1
    when 1
      right -= 1
    end
    puts "Took: #{matches - o}"
    puts "~~~~~~~~~~~~~~~~~~~~~"
  end until left == right
  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]]                   => 2,
  [6, [3,3]]                 => 6,
  [6, [3,3,3]]               => 16,
  [6, [1,5]]                 => 2,
  [6, [6]]                   => 0
}


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"