From 12604b6d65cce7c1c994ce9dc18a4ad94691737c Mon Sep 17 00:00:00 2001
From: Forest Belton <65484+forestbelton@users.noreply.github.com>
Date: Mon, 12 Jul 2021 03:11:47 -0400
Subject: [PATCH] Implement scrolling right

---
 src/main.s |   3 +-
 src/map.s  | 244 +++++++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 210 insertions(+), 37 deletions(-)

diff --git a/src/main.s b/src/main.s
index 198ec9b..20605c3 100644
--- a/src/main.s
+++ b/src/main.s
@@ -158,8 +158,7 @@ scroll_update:
     ld a, [CURRENT_MAP_WIDTH]
     ld b, a
     ld a, [CURRENT_CAMERA_X]
-    inc a
-    ; add 18
+    add SCRN_X_B
     cp b
     ret z
 
diff --git a/src/map.s b/src/map.s
index 8a675e3..3209d7d 100644
--- a/src/map.s
+++ b/src/map.s
@@ -101,52 +101,35 @@ Map_Scroll::
 
     ; B = CAMERA_X - SCX/8
     ld a, [rSCX]
+    srl a
+    srl a
+    srl a
     ld b, a
     ld a, [CURRENT_CAMERA_X]
-    srl b
-    srl b
-    srl b
     sub b
     ld b, a
 
-    ; TODO: WTF does this shit even do...
+    ; C = CAMERA_Y - 2
     ld a, [CURRENT_CAMERA_Y]
-    cp 2
-    jr c, .done
-
     sub 2
     ld c, a
 
-    ; a = scy/8 + (scy/8 < 2 ? 30 : -2)
-
+    ; HL = _SCRN0 + 32 * (SCY/8 - 2)
+.find_vram_ptr:
     ld a, [rSCY]
+    sub 16
     srl a
     srl a
     srl a
-
-    cp 2
-    jr nc, .loop0
-    add 32
-
-.loop0:
-    sub 2
-
-    ; HL = _SCRN0 + 32 * (32 - page_y/8 - 2)
-    ; HL = _SCRN0 + 32 * A
-    ld e, a
+    ld d, a
+    ld e, 32
     ld hl, _SCRN0
-.loop:
-    ld a, e
-    or a
-    jr z, .write_up
-
-    ld a, 32
+.0:
+    ld a, d
     ADD16 hl
-
     dec e
-    jr .loop
+    jr nz, .0
 
-.write_up:
     call enqueue_row_write
 
     ld a, [PAGEY]
@@ -161,7 +144,7 @@ Map_Scroll::
     sub SCRN_Y + 16
     ld hl, rSCY
     cp [hl]
-    jr nz, .done
+    jr nz, .scroll_right_check
 
     ld a, [PAGEY]
     srl a
@@ -182,8 +165,6 @@ Map_Scroll::
     ld b, a
 
     ; C = CAMERA_Y + 18 + 1
-    ; TODO: Figure out if a similar method to this one is sufficient for
-    ; whatever the fuck the scroll up check is doing
     ld a, [CURRENT_CAMERA_Y]
     add SCRN_Y_B + 1
     ld c, a
@@ -197,7 +178,47 @@ Map_Scroll::
     jr .done
 
     ; If SCX = PAGEX, write map col
+.scroll_left_check:
+
     ; If SCX = SCRN_X - PAGEX, write map col
+.scroll_right_check:
+    ; Check SCX + (SCRN_X + 16) = PAGEX
+    ld a, [PAGEX]
+    sub SCRN_X + 16
+    ld hl, rSCX
+    cp [hl]
+    jr nz, .done
+
+    ; HL = VRAM + PAGEX/8
+    ld hl, _SCRN0
+    ld a, [PAGEX]
+    srl a
+    srl a
+    srl a
+    dec a
+    ADD16 hl
+
+    ; B = CAMERA_X + 20
+    ld a, [CURRENT_CAMERA_X]
+    add SCRN_X_B + 1
+    ld b, a
+
+    ; C = CAMERA_Y - SCY/8
+    ld a, [rSCY]
+    ld c, a
+    ld a, [CURRENT_CAMERA_Y]
+    srl c
+    srl c
+    srl c
+    sub c
+    ; dec a
+    ld c, a
+
+    call enqueue_col_write
+
+    ld a, [PAGEX]
+    add 8
+    ld [PAGEX], a
 
 .done:
     ld hl, LAST_SCY
@@ -220,7 +241,7 @@ Map_Update::
     MEMCPY bc, hl, d
 
 .update_col:
-    ; Skip update if PENDING_COL_PTR is 0
+    ; Skip column update if PENDING_COL_PTR is 0
     ld a, [PENDING_COL_PTR]
     ld c, a
     ld a, [PENDING_COL_PTR + 1]
@@ -228,7 +249,7 @@ Map_Update::
     or c
     ret z
 
-    ld d, SCRN_VX_B
+    ld d, SCRN_VY_B
     ld hl, PENDING_COL_DATA
 .update_col_loop:
     ld a, [hl+]
@@ -360,7 +381,7 @@ enqueue_row_write:
     ; X++, BYTES_LEFT--
     inc b
     dec c
-    jr .pad_left
+    jr .pad_right
 
 .zero_row:
     ld hl, PENDING_ROW_DATA
@@ -401,3 +422,156 @@ write_map_row:
     pop bc
 
     ret
+
+; Write a column of map data into column buffer
+; @param b Map X coordinate (signed)
+; @param c Map Y coordinate (signed)
+; @param hl Where to write the column in map VRAM
+; @destroy All registers
+enqueue_col_write:
+    ; PENDING_COL_PTR = HL
+    ld a, l
+    ld [PENDING_COL_PTR], a
+    ld a, h
+    ld [PENDING_COL_PTR + 1], a
+
+    ; If X < 0, write 0s
+    bit 7, b
+    jr nz, .zero_row
+
+    ; If X >= MAP_WIDTH, write 0s
+    ld a, [CURRENT_MAP_WIDTH]
+    dec a
+    cp c
+    jr c, .zero_row
+
+    ; HL = CURRENT_MAP_PTR
+    ld a, [CURRENT_MAP_PTR]
+    ld l, a
+    ld a, [CURRENT_MAP_PTR + 1]
+    ld h, a
+
+    ; HL = CURRENT_MAP_PTR + Y * MAP_WIDTH + X
+    ld d, 0
+    ld a, [CURRENT_MAP_WIDTH]
+    ld e, a
+    ld a, c
+
+.get_map_col_ptr:
+    or a
+    jr z, .copy_map_column
+    add hl, de
+    dec a
+    jr .get_map_col_ptr
+
+.copy_map_column:
+    ld a, b
+    ADD16 hl
+
+    ; B = BYTES_LEFT
+    ld b, SCRN_VY_B
+    ld de, PENDING_COL_DATA
+
+    ; Note: Can skip checking BYTES_LEFT > 0 in this loop. If there were
+    ; SCRN_VY_B zeros to write, then X would be 1 greater and we would have
+    ; jumped into .zero_row before reaching this code
+.pad_left:
+    ; Check Y < 0
+    bit 7, c
+    jr z, .copy_middle
+
+    ; *ROW++ = 0
+    xor a
+    ld [de], a
+    inc de
+
+    ; Y++, BYTES_LEFT--
+    inc c
+    dec b
+    jr .pad_left
+
+.copy_middle:
+    ; Check Y < MAP_HEIGHT
+    ld a, [CURRENT_MAP_HEIGHT]
+    dec a
+    cp c
+    jr c, .pad_right
+
+    ; Check BYTES_LEFT > 0
+    ld a, b
+    or a
+    ret z
+
+    ; *ROW++ = *MAP
+    ld a, [hl]
+    ld [de], a
+    inc de
+
+    ; MAP += MAP_WIDTH
+    ld a, [CURRENT_MAP_WIDTH]
+    ADD16 hl
+
+    ; Y++, BYTES_LEFT--
+    inc c
+    dec b
+    jr .copy_middle
+
+.pad_right:
+    ; Check BYTES_LEFT > 0
+    ld a, b
+    or a
+    ret z
+
+    ; *ROW++ = 0
+    xor a
+    ld [de], a
+    inc de
+
+    ; X++, BYTES_LEFT--
+    inc c
+    dec b
+    jr .pad_right
+
+.zero_row:
+    ld hl, PENDING_COL_DATA
+    xor a
+    ld c, SCRN_VY_B
+.zero_row_loop:
+    ld [hl+], a
+    dec c
+    jr nz, .zero_row_loop
+    ret
+
+; Compute the distance (modulo 32) between two values
+; @param b Value 1
+; @param c Value 2
+mmd2:
+    push bc
+
+    ld a, b
+    sub c
+
+    cpl
+    ld b, a
+    cpl
+
+    bit 7, b
+    jr z, .0
+
+    ld b, a
+
+.0:
+    ld a, 32
+    sub b
+
+    cp b
+    jr nc, .1
+
+    pop bc
+    ret
+
+.1:
+    ld a, b
+
+    pop bc
+    ret