falling-factorial ( x n -- (x)n )

Vocabulary

math.factorials

Inputs

x	an object
n	an object

Outputs

(x)n

an object

Definition

USING: combinators kernel math ranges sequences ;

IN: math.factorials

: falling-factorial ( x n -- (x)n )
    {
        { 1 [ ] }
        { 0 [ drop 0 ] }
        [
            dup 0 < [ neg [ + ] keep t ] [ f ] if
            [ dupd - swap (a..b] product ] dip [ recip ] when
        ]
    } case ;