jak-project/goal_src/jak1/pc/util/knuth-rand.gc

;;-*-Lisp-*-
(in-package goal)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;; mimicking Java's util.Random https://docs.oracle.com/javase/8/docs/api/java/util/Random.html
;;;; This is a linear congruential pseudorandom number generator, as defined by D. H. Lehmer 
;;;;  and described by Donald E. Knuth in The Art of Computer Programming, Volume 3: Seminumerical Algorithms, section 3.2.1.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


(defun scf-time-to-int64 ()
  "pack system time into 64-bits for randomization purposes."
  (let ((date (new 'stack-no-clear 'scf-time)))
    (scf-get-time date)
    
    (logior (the int (-> date stat))
            (shl (-> date second) 8)
            (shl (-> date minute) 16)
            (shl (-> date hour) 24)
            (shl (-> date week) 32)
            (shl (-> date day) 40)
            (shl (-> date month) 48)
            (shl (-> date year) 56))
    )
  )


(deftype knuth-rand-state (structure)
  ((seed int64))
  )

(define *knuth-rand-state* (new 'static 'knuth-rand-state))


(defun knuth-rand-init ((seed int))
  (set! (-> *knuth-rand-state* seed) seed)
  )

(defun knuth-rand-advance-seed ()
  ;; seed =  (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)
  (set! (-> *knuth-rand-state* seed) (logand (+ (imul64 (-> *knuth-rand-state* seed) #x5deece66d) #xb) (- (shl 1 48) 1)))
  )

(defun knuth-rand-next ((bits int))
  (knuth-rand-advance-seed)
  ;; return (int)(seed >>> (48 - bits))
  (shr (-> *knuth-rand-state* seed) (- 48 bits))
  )

(defun knuth-rand-next-int () 
  (knuth-rand-advance-seed)
  (-> *knuth-rand-state* seed)
  )

;; returns a pseudorandom int in the range [min, max)
(defun knuth-rand-int-range ((min int) (max int))
  (let* ((bound (- max min))
      ;; int bits, val;
         (bits 0)
         (val 0))

    ;; if ((bound & -bound) == bound)  // i.e., bound is a power of 2
    (if (= (logand bound (- bound)) bound)
      ;; return (int)((bound * (long)next(31)) >> 31);
      (return (+ min (sar (imul64 bound (knuth-rand-next 31)) 31)))
      )

    ;;  do {
    ;;      bits = next(31);
    ;;      val = bits % bound;
    ;;  } while (bits - val + (bound-1) < 0);
    ;;  return val;
    (until (not (< (+ bits (- val) bound -1) 0))
      (set! bits (knuth-rand-next 31))
      (set! val (mod bits bound))
      )
    (+ min val))
  )

;; returns a pseudorandom float in the range [0, 1)
(defun knuth-rand-next-float ()
  ;; return next(24) / ((float)(1 << 24));
  (/ (the float (knuth-rand-next 24)) (shl 1 24))
  )

(knuth-rand-init (scf-time-to-int64))