hextriclosan opened a new pull request, #721:
URL: https://github.com/apache/commons-collections/pull/721
Add an Iterator<List<E>> that generates the permutations of a collection in
lexicographical order, complementing PermutationIterator, which uses the
Steinhaus-Johnson-Trotter ordering.
Elements are ordered by their natural ordering, or by a Comparator supplied
to the two-argument constructor, which also allows permuting elements that do
not implement Comparable. Each call to next() advances by the standard
next-permutation step: locate the pivot, swap it with its successor, then
reverse the descending tail. Equal elements are not distinguished, so an input
with duplicates yields fewer than n! permutations. An empty collection yields
exactly one empty list, as 0! = 1. remove() is unsupported.
Comparator dispatch follows the java.util.TreeMap pattern of testing the
comparator field for null on each comparison; benchmarking showed no measurable
difference against normalizing null to Comparator.naturalOrder() in the
constructor.
Tests extend AbstractIteratorTest to cover the Iterator contract, and add
cases for lexicographical exhaustivity, duplicate handling, custom and reverse
comparators, non-Comparable elements, stream traversal, exhaustion, and
equals/hashCode.
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