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Adds Haskell style list comprehensions to the Ruby Array
class Array
#
# Iterates over each permutation of the enumerable values within the
# {Array}.
#
# @yield [list]
# The given block will be passed each permutation of the enumerable
# values in the {Array}.
#
# @yieldparam [Array] list
# A permutation of the enumerable values within the {Array}.
#
# @return [Enumerator]
# If no block is given, an enumerator object will be returned.
#
# @example
# [(1..5),(1..4),(1..3)].comprehension.to_a
# # => [
# [1, 1, 1], [1, 1, 2], [1, 1, 3],
# [1, 2, 1], [1, 2, 2], [1, 2, 3],
# [1, 3, 1], [1, 3, 2], [1, 3, 3],
# [1, 4, 1], [1, 4, 2], [1, 4, 3],
# [2, 1, 1], [2, 1, 2], [2, 1, 3],
# [2, 2, 1], [2, 2, 2], [2, 2, 3],
# [2, 3, 1], [2, 3, 2], [2, 3, 3],
# [2, 4, 1], [2, 4, 2], [2, 4, 3],
# [3, 1, 1], [3, 1, 2], [3, 1, 3],
# [3, 2, 1], [3, 2, 2], [3, 2, 3],
# [3, 3, 1], [3, 3, 2], [3, 3, 3],
# [3, 4, 1], [3, 4, 2], [3, 4, 3],
# [4, 1, 1], [4, 1, 2], [4, 1, 3],
# [4, 2, 1], [4, 2, 2], [4, 2, 3],
# [4, 3, 1], [4, 3, 2], [4, 3, 3],
# [4, 4, 1], [4, 4, 2], [4, 4, 3],
# [5, 1, 1], [5, 1, 2], [5, 1, 3],
# [5, 2, 1], [5, 2, 2], [5, 2, 3],
# [5, 3, 1], [5, 3, 2], [5, 3, 3],
# [5, 4, 1], [5, 4, 2], [5, 4, 3]
# ]
#
# @note
# This currently will not run under Ruby 1.8, due to the fact that the
# Ruby 1.8 `Enumerable` class not provide the `#peek` method. Additionally,
# the `Enumerator` implementation in Ruby 1.8 is much slower than that of
# Ruby 1.9. Please upgrade to 1.9.2 for the best Ruby experience.
#
def comprehension
return enum_for(:comprehension) unless block_given?
ranges = self.map do |range|
if range.kind_of?(Enumerable)
range
elsif range.kind_of?(Proc)
Enumerator.new(&range)
else
(range..range)
end
end
cycles = ranges.map { |range| range.cycle }
loop do
yield cycles.map { |cycle| cycle.peek }
(cycles.length - 1).downto(0) do |index|
cycles[index].next
break unless cycles[index].peek == ranges[index].first
return nil if index == 0
end
end
end
end
Owner

postmodern commented Oct 1, 2010

What has Science doooone?!

dkubb commented Jan 17, 2012

I wonder if there are nice ways to include predicates in this, or is it already there and my eyes glossed over it?

Owner

postmodern commented Jan 17, 2012

Current version of this is in combinatorics.

I suppose I could add support for lambdas by passing them the half-built Array.

[(0..255), ->(a) { a[0].chr }]
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