Apply gravity if player walks off of platform

src/collision.s

@@ -54,6 +54,49 @@ player_bg_collides::
 
     ret
 
+; Check if the player is in mid-air
+; @note NZ = false, Z = true
+player_in_air::
+    ; c = x % 8 == 0 ? x/8 : x/8 + 1
+    ld hl, PLAYER_X
+    ld a, [hl]
+    ld c, a
+    and %111
+    jr z, .skip_inc_c
+    ld a, c
+    add 8
+    ld c, a
+.skip_inc_c:
+    srl c
+    srl c
+    srl c
+
+    ; b = y % 8 == 0 ? y/8 : y/8 + 1
+    inc hl
+    ld a, [hl]
+    ld b, a
+    and %111
+    jr z, .skip_inc_b
+    ld a, b
+    add 8
+    ld b, a
+.skip_inc_b:
+    srl b
+    srl b
+    srl b
+
+    ; check underneath player (2 tiles down from top-left sprite)
+    inc b
+    inc b
+
+    call can_move_to
+    ret nz
+
+    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

src/player.s

@@ -29,6 +29,7 @@ DEF PLAYER_SPEED EQU 2
 
 DEF GRAVITY EQU (0 << 8) | $2e ; 0.18
 DEF INIT_VY EQU (3 << 8) | $99 ; 3.60
+DEF INIT_FALL_VY EQU ($ff << 8) | $1a ; a.b
 
 Player_Init::
     ; Clear player data
@@ -76,9 +77,7 @@ Player_Update::
     ld [hl], 1
 
     ld hl, PLAYER_VY
-    ld [hl], HIGH(INIT_VY)
-    inc hl
-    ld [hl], LOW(INIT_VY)
+    STORE16 INIT_VY
 
 ; todo: deduplicate
 .jump_update_check:
@@ -188,13 +187,13 @@ Player_Update::
     ld hl, keys
     ld a, [hl]
     and BTN_LEFT
-    ret z
+    jr z, .fall
 
     ; check for left boundary
     ld hl, PLAYER_X
     ld a, [hl]
     or a
-    ret z
+    jr z, .fall
 
     sub PLAYER_SPEED
     ld [hl], a
@@ -206,12 +205,28 @@ Player_Update::
     ld [hl], $ff
 
     call update_oam
-    ret
+    jr .fall
 
 .left_rollback:
     ld hl, PLAYER_X
     inc [hl]
     inc [hl]
+
+.fall:
+    call player_in_air
+    ret nz
+
+    ; only set jump state if not already jumping
+    ld hl, PLAYER_JUMPING
+    ld a, [hl]
+    or a
+    ret nz
+
+    ld [hl], 1
+
+    ld hl, PLAYER_VY
+    STORE16 INIT_FALL_VY
+
     ret
 
 ; Update player metasprite with positional data