;; In Emacs, open this file in -*- Scheme -*- mode.

;; A PCG random number generator. (Specifically, this is the variant
;; called XSH RR in the paper, with a 16-bit state and 8-bit output).
;; See http://www.pcg-random.org/

(def %rng-to-uint16
     (fun (i) (int-bitwise-and #xFFFF i)))

(def %rng-to-uint8
     (fun (i) (int-bitwise-and #xFF i)))

(def rng-make
     (fun (seed)
          (let ((rng (@block-alloc-50 1)))
            (@block-set! rng 0 (%rng-to-uint16 seed))
            rng)))

(def rng?
     (fun (o)
          (and (@block? o) (= (@block-tag o) 50))))

(def %rng-get-state
     (fun (rng)
          (@block-get rng 0)))

(def %rng-set-state!
     (fun (rng new-state)
          (@block-set! rng 0 (%rng-to-uint16 new-state))))

(def %rng-rotate-right-8
     (fun (x y)
          (%rng-to-uint8 (int-bitwise-or (int-shift-right x y)
                                         (int-shift-left x (- 8 y))))))

(def %rng-step
     (fun (rng)
          (%rng-set-state! rng (+ (* (%rng-get-state rng) 12829) 47989))))

(def %rng-output
     (fun (rng)
          (let ((state (%rng-get-state rng)))
            (%rng-rotate-right-8
             (%rng-to-uint8 (int-shift-right
                             (int-bitwise-xor state (int-shift-right state 5))
                             5))
             (int-shift-right state 13)))))

;; Return the next 8-bit unsigned integer (0 to 255, included)
(def rng-next-int8
     (fun (rng)
          (let ((i (%rng-output rng)))
            (%rng-step rng)
            i)))

;; FIXME: this is hackish, find a better way to do it (probably using
;; multiple streams, see sample/pcg32x2-demo.c in the PCG source).
(def rng-next-int
     (fun (rng)
          (let ((b0 (rng-next-int8 rng))
                (b1 (rng-next-int8 rng))
                (b2 (rng-next-int8 rng))
                (b3 (rng-next-int8 rng)))
            (int-bitwise-or (int-shift-left b0 24)
                            (int-shift-left b1 16)
                            (int-shift-left b2 8)
                            b3))))