EXPLORE: animate the approach to map edges instead of hard-jumping

At a map boundary, smooth-scroll the remaining (even) distance up to the
edge, then either cross into the connected map (one unavoidable instant
load) or, with no connection, stop smoothly at the edge. Previously any
edge move hard-jumped a whole screen; now only crossing from exactly on a
boundary snaps.

1 files changed, 62 insertions, 32 deletions

diff --git a/engine/slop_menu.asm b/engine/slop_menu.asm
index acad203c..36654111 100644
--- a/engine/slop_menu.asm
+++ b/engine/slop_menu.asm
@@ -349,10 +349,7 @@ SlopExplore:
 	ld a, [wYCoord]
 	sub SLOP_PAGE_Y
 	jp nc, .moveUp
-	ld hl, wNorthConnectedMap
-	ld de, wNorthConnectedMapYAlignment
-	ld a, 0                        ; clamp target = top
-	jp .crossV
+	jp .edgeUp
 .ckDown
 	bit B_PAD_DOWN, d
 	jr z, .ckLeft
@@ -364,20 +361,14 @@ SlopExplore:
 	cp b
 	jp c, .moveDown
 	jp z, .moveDown
-	ld hl, wSouthConnectedMap
-	ld de, wSouthConnectedMapYAlignment
-	ld a, b                        ; clamp target = max Y
-	jp .crossV
+	jp .edgeDown
 .ckLeft
 	bit B_PAD_LEFT, d
 	jr z, .ckRight
 	ld a, [wXCoord]
 	sub SLOP_PAGE_X
 	jp nc, .moveLeft
-	ld hl, wWestConnectedMap
-	ld de, wWestConnectedMapYAlignment
-	ld a, 0
-	jp .crossH
+	jp .edgeLeft
 .ckRight
 	bit B_PAD_RIGHT, d
 	jp z, .input
@@ -389,10 +380,7 @@ SlopExplore:
 	cp b
 	jp c, .moveRight
 	jp z, .moveRight
-	ld hl, wEastConnectedMap
-	ld de, wEastConnectedMapYAlignment
-	ld a, b
-	jp .crossH
+	jp .edgeRight
 ; in-map page moves: smooth-scroll one screen, then just redraw the arrows
 ; (the scroll already left the destination view on screen in rotated VRAM).
 .moveUp
@@ -411,13 +399,63 @@ SlopExplore:
 	ld c, SLOP_PAGE_X
 	call SlopScrollRight
 	jp .renderArrows
-; vertical cross (up/down): hl -> connected-map byte, de -> its YAlignment,
-; a = Y to clamp to when there is no connection.
+; ---- map-edge handling ----
+; A full page won't fit before the edge, so smooth-scroll the remaining (even)
+; distance right up to the edge, then either cross into the connected map (one
+; unavoidable instant load) or, if there's no connection that way, just stop at
+; the edge. This keeps the motion smooth instead of hard-jumping a whole screen.
+.edgeUp
+	ld a, [wYCoord]
+	and $fe                        ; even coords remaining to the top edge
+	jr z, .edgeUpCross
+	ld c, a
+	call SlopScrollUp
+.edgeUpCross
+	ld hl, wNorthConnectedMap
+	ld de, wNorthConnectedMapYAlignment
+	jp .crossV
+.edgeDown
+	ld a, [wCurrentMapHeight2]
+	dec a                          ; max Y
+	ld hl, wYCoord
+	sub [hl]                       ; maxY - wYCoord
+	and $fe
+	jr z, .edgeDownCross
+	ld c, a
+	call SlopScrollDown
+.edgeDownCross
+	ld hl, wSouthConnectedMap
+	ld de, wSouthConnectedMapYAlignment
+	jp .crossV
+.edgeLeft
+	ld a, [wXCoord]
+	and $fe
+	jr z, .edgeLeftCross
+	ld c, a
+	call SlopScrollLeft
+.edgeLeftCross
+	ld hl, wWestConnectedMap
+	ld de, wWestConnectedMapYAlignment
+	jp .crossH
+.edgeRight
+	ld a, [wCurrentMapWidth2]
+	dec a                          ; max X
+	ld hl, wXCoord
+	sub [hl]                       ; maxX - wXCoord
+	and $fe
+	jr z, .edgeRightCross
+	ld c, a
+	call SlopScrollRight
+.edgeRightCross
+	ld hl, wEastConnectedMap
+	ld de, wEastConnectedMapYAlignment
+	jp .crossH
+; vertical cross: hl -> connected-map byte, de -> its YAlignment. With no
+; connection we're already scrolled to the edge, so just redraw the arrows.
 .crossV
-	ld b, a
 	ld a, [hl]
 	inc a
-	jr z, .clampY                  ; $ff -> no connection
+	jr z, .clampArrows             ; $ff -> no connection
 	ld a, [de]                     ; YAlignment -> new Y (absolute)
 	ld c, a
 	inc de
@@ -432,17 +470,11 @@ SlopExplore:
 	ld [wCurMap], a
 	call LoadMapData
 	jp .renderView
-.clampY
-	ld a, b
-	ld [wYCoord], a
-	jp .renderView
-; horizontal cross (left/right): hl -> connected-map byte, de -> its YAlignment,
-; a = X to clamp to when there is no connection.
+; horizontal cross: hl -> connected-map byte, de -> its YAlignment.
 .crossH
-	ld b, a
 	ld a, [hl]
 	inc a
-	jr z, .clampX
+	jr z, .clampArrows
 	ld a, [de]                     ; YAlignment -> added to Y
 	ld c, a
 	inc de
@@ -455,10 +487,8 @@ SlopExplore:
 	ld [wCurMap], a
 	call LoadMapData
 	jp .renderView
-.clampX
-	ld a, b
-	ld [wXCoord], a
-	jp .renderView
+.clampArrows
+	jp .renderArrows
 ; ---- A: warp the player to the walkable tile nearest the screen centre ----
 ; the centre tile is wTileMap(col 8,row 9) = coord (wYCoord,wXCoord); a coord
 ; offset (dy,dx) maps to wTileMap(8+2dx, 9+2dy). Scan outward for a passable tile.