Ruby solution to New Scientist Enigma 1656

enigma1656.rb

#!/usr/bin/env ruby

def divisible a,b
  p = a.to_f/b
  (a - p.to_i*b) == 0
end

def indivisible a,b
  not divisible a,b
end

# Given a list, computes the "enigma test":  is the sum divisible by the 
# last array but not by any of the other entries.
#
def enigma_test list
  s = list.inject(:+)
  p = list.slice(0, list.length-1).map { |m| indivisible s, m } << divisible( s, list[-1] )
  p.inject( true ) { |s,i| s and i }
end

# Given an array of N arrays, returns an array of 9N arrays, each of the 
# original with the digits 1..9 tacked on the end.
#
def add_digit list
  list.map { |l| Array.new(9) { |i| [ *l, (i+1)] } }.flatten(1)
end

# Seed the original array...
#
a = (1..9).to_a.map { |i| [i] }

4.times { |j|
  a = add_digit a
  a.delete_if { |i| not enigma_test i }
  puts "For length #{j+2}"
  p a
}