円周率をマチンの公式を使って10000桁計算

calc-pi.lisp

;; 出力結果は3の後ろに小数点が無いのと最後の4桁は未収束の為除いたものが正しい値

(defpackage #:pi
  (:use #:cl))
(in-package #:pi)

(declaim (optimize (speed 3) (debug 0) (safety 0)))

(defun mpiarccot (x)
  (loop with base = (expt 10 (+ 10000 4))
        for a = x then (* a x x)
        for b from 1 by 2
        for pos = (truncate base (* a b))
        while (> pos 1)
        sum (if (logbitp 1 b) (- pos) pos)))

(defun calc ()
  (* 4 (- (* 4 (mpiarccot 5))
          (mpiarccot 239))))

(defparameter mypi 0)
(time (setq mypi (calc)))
(print mypi)