Game Boy snippet

get_map_position.asm

get_map_position:
; from a sprite's pixel position, get the BG map address.
; d: Y pixel position
; e: X pixel position
; hl: returned map address
    ld h, HIGH(_SCRN0) >> 2

    ; Y
    ld a, [rSCY] ; account for scroll
    sub a, 16    ; account for base sprite offset
    add a, d
    and $F8      ; snap to grid
    add a, a
    rl h
    add a, a
    rl h
    ld l, a

    ; X
    ld a, [rSCX] ; account for scroll
    sub a, 8     ; account for base sprite offset
    add a, e
    and $F8      ; snap to grid
    rrca
    rrca
    rrca
    add a, l
    ld l, a

    ret