From 40479b07959786dbda39f5a901e2e9e72b833563 Mon Sep 17 00:00:00 2001
From: Forest Belton <forest@homolo.gy>
Date: Wed, 14 Jul 2021 14:52:21 -0400
Subject: [PATCH] Generalize Y coordinate computation for wrapping

---
 inc/util.inc | 12 ++++++++
 src/map.s    | 80 ++++++++++++++++++++++++++++++++++------------------
 2 files changed, 65 insertions(+), 27 deletions(-)

diff --git a/inc/util.inc b/inc/util.inc
index a913c0f..d67cce3 100644
--- a/inc/util.inc
+++ b/inc/util.inc
@@ -65,3 +65,15 @@ MACRO SLA16
     sla LOW(\1)
     rl HIGH(\1)
 ENDM
+
+; Check \1 - \2
+; \1 16-bit register
+; \2 16-bit immediate/register
+MACRO CP16
+    ld a, HIGH(\1)
+    cp HIGH(\2)
+    jr nz, .done\@
+    ld a, LOW(\1)
+    cp LOW(\1)
+.done\@
+ENDM
diff --git a/src/map.s b/src/map.s
index 991d0e9..7fc0194 100644
--- a/src/map.s
+++ b/src/map.s
@@ -120,6 +120,46 @@ MACRO LOAD_MAPX
 .done\@:
 ENDM
 
+; Loads the map Y-coordinate into C
+MACRO LOAD_MAPY
+    ; if SCY <= 0x7f
+    ld a, [rSCY]
+    cp $80
+    jr nc, .wrap\@
+
+    ; C = CAMERA_Y - SCY/8
+    ld a, [rSCY]
+    ld c, a
+    ld a, [CURRENT_CAMERA_Y]
+    srln c, 3
+    sub c
+    ld c, a
+    jr .done\@
+
+    ; else SCY > 0x7f
+.wrap\@:
+    ; C = CAMERA_Y - PAGE_Y/8
+    ld a, [PAGEY]
+    ld c, a
+    ld a, [CURRENT_CAMERA_Y]
+    srln c, 3
+    sub c
+    ld c, a
+
+    ; HL += 32 * PAGE_Y/8
+    ld a, [PAGEY]
+    srln a, 3
+    ld d, a
+    ld e, 32
+.adjust_hl\@:
+    ld a, d
+    ADD16 hl
+    dec e
+    jr nz, .adjust_hl\@
+
+.done\@:
+ENDM
+
 ; Update map state based on SCX/SCY
 Map_Scroll::
     ; If SCY = PAGEY, write map row
@@ -149,7 +189,7 @@ Map_Scroll::
     sub 8
     ld [PAGEY], a
 
-    jp .done
+    ret
 
 .scroll_down_check:
     ; Check SCY + (SCRN_Y + 16) = PAGEY
@@ -160,9 +200,7 @@ Map_Scroll::
     jr nz, .scroll_left_check
 
     ld a, [PAGEY]
-    srl a
-    srl a
-    srl a
+    srln a, 3
     dec a
     call get_row_ptr
 
@@ -179,7 +217,7 @@ Map_Scroll::
     add 8
     ld [PAGEY], a
 
-    jr .done
+    ret
 
 .scroll_left_check:
     ; If SCX = PAGEX, write map col
@@ -191,9 +229,7 @@ Map_Scroll::
     ; HL = VRAM + PAGEX/8
     ld hl, _SCRN0
     ld a, [PAGEX]
-    srl a
-    srl a
-    srl a
+    srln a, 3
     dec a
     ADD16 hl
 
@@ -203,14 +239,7 @@ Map_Scroll::
     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
-    ld c, a
+    LOAD_MAPY
 
     call enqueue_col_write
 
@@ -225,7 +254,7 @@ Map_Scroll::
     sub SCRN_X + 16
     ld hl, rSCX
     cp [hl]
-    jr nz, .done
+    ret nz
 
     ; HL = VRAM + PAGEX/8
     ld hl, _SCRN0
@@ -242,14 +271,7 @@ Map_Scroll::
     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
-    ld c, a
+    LOAD_MAPY
 
     call enqueue_col_write
 
@@ -257,8 +279,6 @@ Map_Scroll::
     add 8
     ld [PAGEX], a
 
-.done:
-    ; TODO: Turn jumps here into rets
     ret
 
 Map_Update::
@@ -312,6 +332,12 @@ Map_Update::
     ld a, SCRN_VX_B
     ADD16 bc
 
+    ; If BC = 9c00, set BC = 9800
+    CP16 bc, _SCRN0 + SCRN_VY_B * SCRN_VX_B
+    jr nz, .update_col_loop_next
+    ld b, HIGH(_SCRN0)
+
+.update_col_loop_next:
     dec d
     jr nz, .update_col_loop