binary-reduce ( seq start quot: ( elt1 elt2 -- newelt ) -- value )

Vocabulary

sequences

Inputs

seq	a sequence
start	an integer
quot	a quotation with stack effect ( elt1 elt2 -- newelt )

Outputs

value

an object

Word description

Like reduce, but splits the sequence in half recursively until each sequence is small enough, and calls the quotation on these smaller sequences. If the quotation computes values that depend on the size of their input, such as bignum arithmetic, then this algorithm can be more efficient than using reduce.

Examples

Computing factorial:

USING: prettyprint sequences math ; 40 <iota> rest-slice 1 [ * ] binary-reduce .
20397882081197443358640281739902897356800000000

Definition

USING: accessors kernel math math.order sequences.private ;

IN: sequences

: binary-reduce
    ( seq start quot: ( elt1 elt2 -- newelt ) -- value )
    pick dup slice?
    [ [ seq>> ] 3dip [ from>> 0 max ] [ to>> 0 max over - ] bi ]
    [ length 0 max 0 swap ] if (binary-reduce) ; inline