shuffle

shuffle:
;(seed) is a non-zero 16-bit value
;array is 52 bytes that will be populated with integers 0..51, shuffled.
    xor a
loop:
    ld e,a

;This xorhift routine was written by John Metcalf
;   http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html
    ld hl,(seed)
    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 (seed),hl
; End of the xorshift routine
    ld a,r
    add a,h
    ld h,a

    ld a,e
    push af
    inc a
    ex de,hl

    ld hl,0
    ld b,8
    ld c,l
    add hl,hl
    rla
    jr nc,$+4
    add hl,de
    adc a,c
    djnz $-6
;(array+A) ==> (array+index)
;index ==> (array+A)
;add A to array
    ld e,a
    ld d,b
    pop af
    ld l,a
    ld h,b
    ld bc,array
    add hl,bc
    ex de,hl
    add hl,bc
    ldi
    dec hl
    ld (hl),a
    inc a
    cp 52
    jr nz,loop
    ret