double-factorial ( n -- n!! )

Vocabulary

math.factorials

Inputs

n

an object

Outputs

n!!

an object

Definition

USING: kernel math ranges sequences ;

IN: math.factorials

MEMO: double-factorial ( n -- n!! )
    dup [ even? ] [ 0 < ] bi [
        [ drop 1/0. ] [ 2 + -1 swap -2 <range> product recip ]
        if
    ] [ 2 3 ? swap 2 <range> product ] if ;