combinations.rb

# Generates combinations of letters or numbers, or anything really. The combinations
# include repeating values, unlike Ruby's regular Array#permutation method.
# 
# Usage:
#   generate('a'..'c', 3) do |seq|
#     puts seq.inspect #=> [a,a,a], [a,a,b], [a,a,c], [a,b,a]...
#   end
def generate(range, comb_len)
  range = range.to_a
  range_len = range.length
  (range_len ** comb_len).times do |q|
    # Converts a number into digits of an arbitrary base,
    # eg. 13 in base 3 -> [1,1,1]
    indices = Array.new(comb_len) { q, r = q.divmod(range_len); r }.reverse
    # Uses base conversion indices to produce combinations.
    yield range.values_at(*indices)
  end
end