Prism StableRandom.

StableRandom::
    ; in: hl: pointer to 8-byte RNG state
    ; out: a: random value; other registers preserved
    push bc
    push de
    push hl
    call .advance_left_register
    call .advance_right_register
    inc hl
    inc hl
    inc hl
    call .advance_selector_register
    pop hl
    push hl
    rlca
    rlca
    ld c, a
    and 3
    ld e, a
    ld d, 0
    add hl, de
    ld b, [hl]
    pop hl
    push hl
    ld e, 5
    add hl, de
    ld a, c
    ld c, [hl]
    rlca
    rlca
    and 3
    call .combine_register_values
    pop hl
    pop de
    pop bc
    ret

.advance_left_register
    ; in: hl: pointer to left register
    ; out: hl: pointer to RIGHT register
    ld a, [hli]
    ld e, a
    ld a, [hli]
    ld d, a
    ld a, [hli]
    ld c, a
    ld a, [hld]
    ld b, a
    or c
    or d
    or e
    call z, .reseed_left_register
    ld a, e
    xor d
    ld e, a
    ld a, d
    xor c
    ld d, a
    ld a, c
    xor b
    ld c, a
    ld a, c
    ld [hld], a
    ld a, d
    ld [hld], a
    ld [hl], e
    sla e
    rl d
    rl c
    inc hl
    ld a, [hl]
    xor e
    ld [hli], a
    ld a, [hl]
    xor d
    ld [hli], a
    ld a, [hl]
    xor c
    ld [hld], a
    ld b, a
    ld c, [hl]
    sla c
    rl b
    sbc a
    and 1
    dec hl
    xor [hl]
    ld [hld], a
    ld a, [hl]
    xor b
    ld [hli], a
    inc hl
    inc hl
    inc hl
    ret

.reseed_left_register
    ; in: hl: pointer to left register + 2
    ; out: hl preserved; bcde new seed
    ld de, 5
    push hl
    add hl, de
    call .advance_selector_register
    ld b, a
    call .advance_selector_register
    ld c, a
    call .advance_selector_register
    ld d, a
    call .advance_selector_register
    ld e, a
    pop hl
    ret

.advance_right_register
    ; in: hl: pointer to right register
    ; out: hl preserved
    ld a, [hli]
    cp 210
    jr c, .right_carry_OK
    sub 210
.right_carry_OK
    ld d, a
    ld a, [hli]
    ld e, a
    ld c, [hl]
    or c
    or d
    jr z, .right_register_needs_reseed
    ld a, c
    and e
    inc a
    jr nz, .right_register_OK
    ld a, d
    cp 209
    jr nz, .right_register_OK
.right_register_needs_reseed
    call .reseed_right_register
.right_register_OK
    ld a, e
    ld [hld], a
    push hl
    ld b, 0
    ld h, b
    ld l, d
    ld a, 210
    rst AddNTimes
    ld a, l
    ld b, h
    pop hl
    ld [hld], a
    ld [hl], b
    ret

.reseed_right_register
    ; in: hl: pointer to right register + 2
    ; out: hl preserved, cde new seed
    inc hl
    call .advance_selector_register
    ld c, a
    call .advance_selector_register
    ld d, a
    call .advance_selector_register
    ld e, a
    dec hl
    ret

.advance_selector_register
    ; in: hl: pointer to selector register
    ; out: all registers but a preserved; a = new selector
    push bc
    ld a, [hl]
    ld b, 0
    rra
    rr b
    rra
    rr b
    ld a, [hl]
    swap a
    rrca
    and $f8
    add a, b
    add a, [hl]
    add a, 29
    ld [hl], a
    pop bc
    ret

.combine_register_values
    and a
    jr z, .add_registers
    dec a
    jr z, .xor_registers
    dec a
    jr z, .subtract_registers
    ld a, c
    sub b
    ret
.subtract_registers
    ld a, b
    sub c
    ret
.add_registers
    ld a, b
    add a, c
    ret
.xor_registers
    ld a, b
    xor c
    ret