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From: minforth@gmx.net (minforth)
Newsgroups: comp.lang.forth
Subject: Re: exercise in double number arithmetic
Date: Mon, 15 Jul 2024 07:38:50 +0000
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Yet another fast variant without looping/recursion:

\ LOGBINOM

100 FLOATS BUFFER: LCS{ \ log sum array
: }  ( a i -- f: n ) floats + f@ ;
: }! ( a i f: n -- ) floats + f! ;
: PREP-LCS ( -- ) \ prepare array
  0e lcs{ 0 }! 100 1 DO
    lcs{ i 1- } i s>f flog f+ lcs{ i }!
  LOOP ;

: LBINOM ( n k -- binom ) \ calc binom coeff by array lookup
  2dup = over 0= or
  IF 0e
  ELSE 2dup lcs{ swap } lcs{ swap } - lcs{ swap } f- fswap f-
  THEN falog fround f>d ;

PREP-LCS
42 21 LBINOM D.