Permutation Generator

Generates the basic permutations of n elements and length n in a direct fashion. It works index-based, not by iterating over previous permutations.

Currently only available on nightly, because it relies on #![feature(min_type_alias_impl_trait)].

Optimized versions: - PermutationGenerator8: for basic permutations upto 8 elements - PermutationGenerator16: for basic permutations upto 16 elements - PermutationGenerator32: for basic permutations upto 32 elements

Permutations of more than 32 elements are not provided, since the index of permutation cannot be represented by a single u128.

PermutationGenerators implement Iterator<Item = impl Iterator<Item = u8>>.

Usage

Iterate over the permutations of 4 elements rust let mut pg = PermutationGenerator8::new(4).unwrap(); assert_eq!(&[0, 1, 2, 3], pg.next().unwrap().collect::<Vec<_>>().as_slice());

If the specified size of the permutations exceeds the capacity of the used PermutationGenerator, and Err(PermutationError) is returned. rust let pg = PermutationGenerator8::new(14); assert_eq!(PermutationGeneratorError::TooManyElements, pg.unwrap_err());

If only a single permutation is needed for a known idx ````rust let mut pg = PermutationGenerator8::new(4).unwrap(); asserteq!(&[0, 1, 2, 3], pg.next().unwrap().collect::>().asslice());

let lastpermiter = PermutationGenerator8::nthabsolute(4, 4*3*2*1 - 1).unwrap(); asserteq!(&[3, 2, 1, 0], lastpermiter.collect::>().as_slice())); ````

Panics

The number of permutations of size larger than 20, cannot be represented by u128. Collecting all, or querying the count, will panic. rust let pg = PermutationGenerator32::new(30); pg.count() // -> panics!