;Xorshift Random Number Generator
;
; 16-bit xorshift pseudorandom number generator
; 20 bytes, 96 cycles

; returns   hl = pseudorandom number
; corrupts   a

org 5b00h

xrnd16:
  ld hl,1       ; seed must not be 0

  ld a,h
  rra
  ld a,l
  rra
  xor h
  ld h,a
  ld a,l
  rra
  ld a,h
  rra
  xor l
  ld l,a
  xor h
  ld h,a

  ld (xrnd16+1),hl

  ret
  
xrnd8:
  ld hl,0001h
  ld de,4000h
lp0:
  ld a,h
  rra
  ld a,l
  rra
  xor h
  ld h,a
  ld a,l
  rra
  ld a,h
  rra
  xor l
  ld l,a
  ld (de),a
  xor h
  ld h,a

  inc de
  ld a,d
  cp 58h
  jr nz,lp0

  ret