Issue #21564 has been updated by toy (Ivan Kuchin). @duerst When final number of permutations is small - there is no problem materialising all of them and joining in one array using `flat_map`, but when number of permutations/combinations is big and there is no need to get all results in memory, using flat_map would be wasteful. So for example following would be compact and efficient: ```ruby alphabet = [*'a'..'z'] alphabet.repeated_permutation(*3..6).find{ … } ``` Doing same with `flat_map` will take long time to materialise all permutations: ```ruby (3..6).flat_map{ |i| alphabet.repeated_permutation(i).to_a }.find{ … } ``` It is possible to resolve this using `lazy`: ```ruby (3..6).lazy.flat_map{ |i| alphabet.repeated_permutation(i).lazy }.find{ … } ``` Or using `chain`: ```ruby Enumerator::Chain.new(*(3..6).map{ |i| alphabet.repeated_permutation(i) }).find{ … } ``` But I think both are harder to write and read ---------------------------------------- Feature #21564: Extend permutation, repeated_permutation, combination and repeated_combination arguments https://bugs.ruby-lang.org/issues/21564#change-114556 * Author: toy (Ivan Kuchin) * Status: Open ---------------------------------------- When using functions `permutation`, `repeated_permutation`, `combination` and `repeated_combination`, often one needs not one, but multiple permutation/combination sizes. Currently all functions accept one Integer argument (for `permutation` it is optional and defaults to array size), and it would be more powerful (not require iteration of lengths and possibly flattening) if it would accept multiple lengths: ```ruby a = [1, 2, 3] a.permutation(*1..3).to_a # => [[1], [2], [3], # [1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2], # [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]] a.permutation(1, 3).to_a # => [[1], [2], [3], # [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]] a.permutation(3, 1).to_a # => [[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1], # [1], [2], [3]] a.repeated_permutation(2, 1).to_a # => [[1, 1], [1, 2], [1, 3], [2, 1], [2, 2], [2, 3], [3, 1], [3, 2], [3, 3], # [1], [2], [3]] a.combination(*1..3).to_a # => [[1], [2], [3], # [1, 2], [1, 3], [2, 3], # [1, 2, 3]] a.repeated_combination(*1..3).to_a # => [[1], [2], [3], # [1, 1], [1, 2], [1, 3], [2, 2], [2, 3], [3, 3], # [1, 1, 1], [1, 1, 2], [1, 1, 3], [1, 2, 2], [1, 2, 3], [1, 3, 3], [2, 2, 2], [2, 2, 3], [2, 3, 3], [3, 3, 3]] # find right combination of letters [*'a'..'z'].repeated_permutation(*3..6).find do |chars| correct_combination?(chars) end # naïve knapsack solution def max_knapsack_value(items, weight_max) items.combination(*1..items.length) .map{ |items| [items.sum(&:weight), items.sum(&:value)] } .reject{ |weight_sum, _| weight_sum > weight_max } .max_by{ |_, value_sum| value_sum } &.last || 0 end ``` Hack in code to enhance current methods would be: ```ruby class Array %i[ combination permutation repeated_combination repeated_permutation ].each do |method_name| alias_method "original_#{method_name}", method_name class_eval <<-RUBY, __FILE__, __LINE__ + 1 def #{method_name}(*counts, &block) return to_enum(__method__, *counts) unless block counts.each do |count| original_#{method_name}(count, &block) end end RUBY end end ``` -- https://bugs.ruby-lang.org/