Happy Pi Day in Shen

*
  Accurately calculates N digits of Pi using Machin's formula
  with fixed point arithmetic and variable guards digits. 

  Depends on the maths library -->

http://www.shenlanguage.org/library.html

*

(tc +)

(define arccot-
  {number --> number --> number --> number --> number --> number} 
  X N XPOWER    0 _ -> 0
  X N XPOWER TERM 1 -> (+ (arccot- X (+ N 2) (floor (/ XPOWER X)) 
                                     (floor (/ XPOWER N)) 0) 
                          (floor (/ XPOWER N)))
  X N XPOWER TERM 0 -> (- (arccot- X (+ N 2) (floor (/ XPOWER X))
                                      (floor (/ XPOWER N)) 1) 
                          (floor (/ XPOWER N))))

(define arccot
  {number --> number --> number}
  X UNITY -> (let XPOWER (floor (/ UNITY X))
                  (arccot- (* X X) 1 XPOWER (floor XPOWER) 1)))

(define machin-pi
  {number --> number} 
  DIGITS -> (let GUARD (+ 10 (ceiling (log' DIGITS 10)))
                 UNITY (expt 10 (+ DIGITS GUARD))
                 (floor (/ (* 4 (- (* 4 (arccot 5 UNITY))
                                   (arccot 239 UNITY)))
                           (expt 10 GUARD)))))




(1+) (time (machin-pi 100))

run time: 2.56899999999996 secs
31415926535...4350265344N : number