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

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.

let pg = PermutationGenerator8::new(14);
assert_eq!(PermutationGeneratorError::TooManyElements, pg.unwrap_err());

If only a single permutation is needed for a known idx

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

let last_perm_iter = PermutationGenerator8::nth_absolute(4, 4*3*2*1 - 1).unwrap();
assert_eq!(&[3, 2, 1, 0], last_perm_iter.collect::<Vec<_>>().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.

let pg = PermutationGenerator32::new(30);
pg.count() // -> panics!