<k-permutations> ( seq k -- permutations )

Vocabulary

math.combinatorics

Inputs

seq	a sequence
k	an integer

Outputs

permutations

a sequence

Word description

An efficient sequence containing the k lexicographical permutations of seq.

Definition

USING: combinators kernel math sequences ;

IN: math.combinatorics

:: <k-permutations> ( seq k -- permutations )
    seq length :> n n k nPk :> len {
        { [ len zero? ] [ { } ] }
        { [ n k = ] [ seq <permutations> ] }
        [ len n factorial over /i k seq k-permutations boa ]
    } cond ;