recursive multiplication without (-)

recursive_multiplication.coffee

# Finds product of two positive int's
# using only if, (+), and (===).
# Usage: mult(x, y)
mult = (n1, n2, top=null, orig=null) ->
  # sanity check (still assuming positive int's)
  if (n1 is 0) or (n2 is 0)
    return 0

  # on first call
  if top is null
    throw new Error() if not (orig is null)
    orig = n1
    top = n2
    n2 = 1

  # recursion over
  if n2 is top
    return n1

  # recurse!
  return mult(orig + n1, n2 + 1, top=top, orig=orig)

console.log mult 5, 0
console.log mult 0, 5
console.log mult 1, 30
console.log mult 1, 1
console.log mult 2, 10
console.log mult 10, 5