Using Patrik Rak's 32-bit xorshift in the rand routine

Author: Zeda

common/rand.z80

@@ -1,38 +1,44 @@
 #ifndef included_rand
 #define included_rand
+
 rand:
-;Tested and passes all CAcert tests
-;Uses a very simple 32-bit LCG and 32-bit LFSR
-;it has a period of 18,446,744,069,414,584,320
-;roughly 18.4 quintillion.
-;LFSR taps: 0,2,6,7 = 11000101
-;323cc
-;Thanks to Runer112 for his help on optimizing the LCG and suggesting to try the much simpler LCG. On their own, the two are terrible, but together they are great.
-;Uses 64 bits of state
+; This rand routine combines Patrik Rak's fantastic 32-bit xorshift
+; (https://gist.github.com/raxoft/c074743ea3f926db0037) with a simple lcg for
+; extra smoothing.
+; It has a period of 281,474,976,645,120 and uses 48 bits of state
+; 42 bytes
+; 210cc
  ld hl,(seed0)
- ld de,(seed0+2)
  ld b,h
  ld c,l
- add hl,hl \ rl e \ rl d
- add hl,hl \ rl e \ rl d
+ add hl,hl
+ add hl,hl
  inc l
  add hl,bc
  ld (seed0),hl
- ld hl,(seed0+2)
- adc hl,de
- ld (seed0+2),hl
- ex de,hl
-;lfsr
- ld hl,(seed1)
- ld bc,(seed1+2)
- add hl,hl \ rl c \ rl b
- ld (seed1+2),bc
- sbc a,a
- and %11000101
+
+; xorshift
+ ld hl,(seed1) ; yw -> zt
+ ld de,(seed1+2) ; xz -> yw
+ ld (seed1+2),hl ; x = y, z = w
+ ld a,l ; w = w ^ ( w << 3 )
+ add a,a
+ add a,a
+ add a,a
  xor l
  ld l,a
+ ld a,d ; t = x ^ (x << 1)
+ add a,a
+ xor d
+ ld h,a
+ rra ; t = t ^ (t >> 1) ^ w
+ xor h
+ xor l
+ ld h,e ; y = z
+ ld l,a ; w = t
  ld (seed1),hl
- ex de,hl
+
+; Mix the xorshift and the lcg
  add hl,bc
  ret
 #endif

common/randinit.z80

@@ -2,16 +2,16 @@
 #define included_randinit
 
 randinit:
- ld a,r
- ld hl,seed0
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- xor (hl) \ ld (hl),a \ inc hl
- or 97 ;no particular reason
- or (hl) \ ld (hl),a
+; need to make sure seed1 is non-zero
+ ld hl,seed1
+ ld a,(hl)
+ inc hl
+ or (hl)
+ inc hl
+ or (hl)
+ inc hl
+ or (hl)
+ ret nz
+ dec (hl)
  ret
 #endif