island-story/src/collision.s

INCLUDE "oam.inc"

SECTION "Collision", ROM0

DEF COLLF_WALK EQU (1 << 0)
DEF COLLF_LADDER EQU (1 << 1)
DEF COLLF_PORTAL EQU (1 << 2)

; Determines if player has hit the background
; @return NZ = false, Z = true
player_bg_collides::
    ; C = MAP X = PLAYER X / 8 + CAMERA X
    ld a, [PLAYER_X]
    call get_tile_coord
    ld c, a

    ld a, [CURRENT_CAMERA_X]
    add c
    ld c, a

    ; C < 0
    ld a, c
    cpl
    bit 7, a
    ret z

    ; C >= MAP WIDTH
    ld a, [CURRENT_MAP_WIDTH]
    cp c
    jr nz, .load_y

    ret

.load_y:
    ; B = MAP Y = PLAYER Y / 8 + CAMERA Y
    ld a, [PLAYER_Y]
    call get_tile_coord
    ld b, a

    ld a, [CURRENT_CAMERA_Y]
    add b
    ld b, a

    ; B < 0
    ld a, b
    cpl
    bit 7, a
    ret z

    ; B >= MAP HEIGHT
    ld a, [CURRENT_MAP_HEIGHT]
    cp b
    jr nz, .check_tiles

    ; Set NZ
    or $ff
    ret

.check_tiles:
    ; top left
    call can_move_to
    ret z

    ; top right
    inc c
    call can_move_to
    ret z

    ; bottom left
    dec c
    inc b
    call can_move_to
    ret z

    ; bottom right
    inc c
    call can_move_to

    ret

; Check if the player is in mid-air
; @note NZ = true, Z = false
player_in_air::
    ; c = x % 8 == 0 ? x/8 : x/8 + 1
    ld a, [PLAYER_X]
    call get_tile_coord
    ld c, a

    ld a, [CURRENT_CAMERA_X]
    add c
    ld c, a

    ld a, [PLAYER_Y]
    call get_tile_coord
    ld b, a

    ld a, [CURRENT_CAMERA_Y]
    add b
    ld b, a

    ; check underneath player (2 tiles down from top-left sprite)
    inc b
    inc b

    call can_move_to
    ret z

    inc c
    call can_move_to

    ret

; Check whether the location can be moved to or not
; @param b y-coordinate
; @param c x-coordinate
; @destroy a, d, e
; @note NZ = true, Z = false
can_move_to:
    push bc
    push hl

    ld a, [CURRENT_MAP_COLLISION]
    ld l, a
    ld a, [CURRENT_MAP_COLLISION + 1]
    ld h, a

    ld a, [CURRENT_MAP_WIDTH]
    ld d, 0
    ld e, a

    ; compute map index (HL + B * (CURRENT_MAP_WIDTH + 1) + C)
.mul_y_width:
    add hl, de
    dec b
    jr nz, .mul_y_width
    add hl, bc

    ld a, [hl]
    and COLLF_WALK

    pop hl
    pop bc

    ret

; Convert from pixel space to tile space
; @param a Screen pixel coordinate
; @destroys hl
; @returns Screen tile coordinate in A
get_tile_coord:
    ld hl, sp - 1
    ld [hl], a

    and %111
    ld a, [hl]

    jr z, .skip_inc
    add 8

.skip_inc:
    srl a
    srl a
    srl a

    ret