explore: fix map rendering (compute the view pointer ourselves)

Root cause of the 'maps look wrong' bug: LoadMapData renders the view from
wCurrentTileBlockMapViewPointer, which the engine ONLY ever sets from warp /
connection / walk data (LoadDestinationWarpPosition, the connected-map handler,
advance_player_sprite) -- never from wXCoord/wYCoord. My old code set coords and
called LoadMapData, so every map was drawn from a STALE pointer left over from
the previous map: arbitrary, inconsistent windows.

Fix: after LoadMapData loads the block map into wOverworldMap, compute a centered
pointer directly --
  stride = wCurMapWidth + MAP_BORDER*2
  ptr = wOverworldMap + (height/2 + 1)*stride + width/2
-- clear wYBlockCoord/wXBlockCoord, and call LoadCurrentMapView to redraw. On
exit, save/restore the origin's real view pointer (+ block coords) before
reloading so the overworld comes back correctly.

Verified by DFS-traversing the whole connection graph and stitching 36 centered
maps into a coherent Kanto overworld image.

1 files changed, 51 insertions, 12 deletions

diff --git a/engine/slop_menu.asm b/engine/slop_menu.asm
index 69ec2142..fcac7d10 100644
--- a/engine/slop_menu.asm
+++ b/engine/slop_menu.asm
@@ -227,21 +227,51 @@ SlopExplore:
 	ld b, a
 	ld a, [wXCoord]
 	ld c, a
-	push bc
+	push bc                        ; origin Y,X
+	ld a, [wCurrentTileBlockMapViewPointer]
+	ld b, a
+	ld a, [wCurrentTileBlockMapViewPointer + 1]
+	ld c, a
+	push bc                        ; origin view pointer
 	ld a, [wCurMap]
-	push af
+	push af                        ; origin map
 .render
-; seed safe coords (0,0 is in-bounds for any map), load the map to get its real
-; dimensions + connections, then re-render centered on the map.
-	xor a
-	ld [wYCoord], a
-	ld [wXCoord], a
+; LoadMapData loads the map header + block map into wOverworldMap, but it renders
+; using a STALE wCurrentTileBlockMapViewPointer -- the game only ever sets that
+; from warp / connection / walk data, never from wXCoord/wYCoord. So load the
+; map, then compute a centered view pointer ourselves and redraw the view.
 	call LoadMapData
+; stride = wCurMapWidth + MAP_BORDER*2 ; bufRow = height/2 + 1 ; bufCol = width/2
+; wCurrentTileBlockMapViewPointer = wOverworldMap + bufRow*stride + bufCol
+	ld a, [wCurMapWidth]
+	add MAP_BORDER * 2
+	ld c, a
+	ld b, 0                        ; bc = row stride (blocks)
+	ld hl, 0
 	ld a, [wCurMapHeight]
-	ld [wYCoord], a
+	srl a
+	inc a                          ; bufRow (>= 1)
+.viewRowLoop
+	add hl, bc
+	dec a
+	jr nz, .viewRowLoop
 	ld a, [wCurMapWidth]
-	ld [wXCoord], a
-	call LoadMapData               ; render centered
+	srl a                          ; bufCol = width/2
+	add l
+	ld l, a
+	jr nc, .viewNoCarry
+	inc h
+.viewNoCarry
+	ld bc, wOverworldMap
+	add hl, bc
+	ld a, l
+	ld [wCurrentTileBlockMapViewPointer], a
+	ld a, h
+	ld [wCurrentTileBlockMapViewPointer + 1], a
+	xor a
+	ld [wYBlockCoord], a
+	ld [wXBlockCoord], a
+	call LoadCurrentMapView        ; redraw wTileMap centered on the map
 	ldh a, [rLCDC]                 ; hide ALL sprites (player + actors) via LCDC
 	res B_LCDC_OBJS, a
 	ldh [rLCDC], a
@@ -318,12 +348,21 @@ SlopExplore:
 .exit
 	pop af
 	ld [wCurMap], a               ; restore origin map
-	pop bc
+	pop bc                        ; restore origin view pointer
+	ld a, b
+	ld [wCurrentTileBlockMapViewPointer], a
+	ld a, c
+	ld [wCurrentTileBlockMapViewPointer + 1], a
+	pop bc                        ; restore origin Y,X
 	ld a, b
 	ld [wYCoord], a
+	and $1
+	ld [wYBlockCoord], a
 	ld a, c
 	ld [wXCoord], a
-	call LoadMapData              ; reload the origin map
+	and $1
+	ld [wXBlockCoord], a
+	call LoadMapData              ; reload origin, rendered from the restored pointer
 	and a                         ; carry clear -> back to SL0P menu
 	ret