explore: direction arrows showing where you can move

Overlay real ▲▼◀▶ arrow sprites at the screen edges for every direction the
camera can currently page: either there's more map that way, or the map has a
connection that way. Arrows update live -- e.g. at a connectionless edge that
direction's arrow disappears.

Implementation: the font has only a down-arrow ($ee) and right-arrow ($ed), so
up/left are made with CGB OBJ Y/X-flip (the emulator supports OBJ flips). Drawn
as 4 sprites in wShadowOAM (8x8 OBJ mode, font loaded so $ed/$ee are OBJ-
reachable at 0x8ed0/0x8ee0). Set wUpdateSpritesEnabled=$ff after building them so
the vblank PrepareOAMData leaves shadow OAM alone instead of repopulating the
player sprite over the arrows.

Verified: Pallet mid-map shows all 4; Viridian shows N/S/W (no E); paging to a
connectionless edge drops that arrow and it returns on the way back.

1 files changed, 79 insertions, 2 deletions

diff --git a/engine/slop_menu.asm b/engine/slop_menu.asm
index 66a8a482..258770f5 100644
--- a/engine/slop_menu.asm
+++ b/engine/slop_menu.asm
@@ -280,8 +280,85 @@ SlopExplore:
 	call LoadCurrentMapView
 	ld b, HIGH(vBGMap0)
 	call CopyScreenTileBufferToVRAM
-	ldh a, [rLCDC]                 ; hide sprites for a clean camera
-	res B_LCDC_OBJS, a
+	call LoadFontTilePatterns      ; arrow glyphs ($ed/$ee) into VRAM for the OBJ layer
+; draw a direction-arrow sprite for each direction the camera can currently move:
+; either there's more map that way, or the map has a connection that way.
+	call ClearSprites
+	ld hl, wShadowOAM
+; up: movable unless at the top edge (wYCoord == 0) with no north connection
+	ld a, [wYCoord]
+	and a
+	jr nz, .putUp
+	ld a, [wNorthConnectedMap]
+	inc a
+	jr z, .skipUp
+.putUp
+	ld de, .arrUp
+	call .putArrow
+.skipUp
+; down: movable unless at the bottom edge with no south connection
+	ld a, [wCurrentMapHeight2]
+	dec a
+	ld b, a
+	ld a, [wYCoord]
+	cp b
+	jr nz, .putDown
+	ld a, [wSouthConnectedMap]
+	inc a
+	jr z, .skipDown
+.putDown
+	ld de, .arrDown
+	call .putArrow
+.skipDown
+; left: movable unless at the left edge with no west connection
+	ld a, [wXCoord]
+	and a
+	jr nz, .putLeft
+	ld a, [wWestConnectedMap]
+	inc a
+	jr z, .skipLeft
+.putLeft
+	ld de, .arrLeft
+	call .putArrow
+.skipLeft
+; right: movable unless at the right edge with no east connection
+	ld a, [wCurrentMapWidth2]
+	dec a
+	ld b, a
+	ld a, [wXCoord]
+	cp b
+	jr nz, .putRight
+	ld a, [wEastConnectedMap]
+	inc a
+	jr z, .skipRight
+.putRight
+	ld de, .arrRight
+	call .putArrow
+.skipRight
+	jr .arrowsDone
+.putArrow                        ; de -> Y, X, tile, attr; write to [hl], advance hl
+	ld a, [de]
+	inc de
+	ld [hli], a
+	ld a, [de]
+	inc de
+	ld [hli], a
+	ld a, [de]
+	inc de
+	ld [hli], a
+	ld a, [de]
+	ld [hli], a
+	ret
+.arrUp    db  18,  80, $ee, $40   ; down-arrow glyph, Y-flipped -> up
+.arrDown  db 144,  80, $ee, $00
+.arrLeft  db  80,  12, $ed, $20   ; right-arrow glyph, X-flipped -> left
+.arrRight db  80, 156, $ed, $00
+.arrowsDone
+	ld a, $ff
+	ld [wUpdateSpritesEnabled], a  ; freeze shadow OAM so PrepareOAMData keeps our arrows
+	ldh a, [rLCDC]
+	res B_LCDC_OBJ_SIZE, a         ; 8x8 sprites -> single-tile arrows
+	set B_LCDC_OBJS, a             ; show the arrow sprites
 	ldh [rLCDC], a
 	ld a, $90
 	ldh [hWY], a