1 files changed, 149 insertions, 0 deletions
diff --git a/engine/menus/swap_items.asm b/engine/menus/swap_items.asm
new file mode 100644
index 00000000..826fe60b
--- /dev/null
+++ b/engine/menus/swap_items.asm
@@ -0,0 +1,149 @@
+HandleItemListSwapping::
+	ld a, [wListMenuID]
+	cp ITEMLISTMENU
+	jp nz, DisplayListMenuIDLoop ; only rearrange item list menus
+	push hl
+	ld hl, wListPointer
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+	inc hl ; hl = beginning of list entries
+	ld a, [wCurrentMenuItem]
+	ld b, a
+	ld a, [wListScrollOffset]
+	add b
+	add a
+	ld c, a
+	ld b, 0
+	add hl, bc ; hl = address of currently selected item entry
+	ld a, [hl]
+	pop hl
+	inc a
+	jp z, DisplayListMenuIDLoop ; ignore attempts to swap the Cancel menu item
+	ld a, [wMenuItemToSwap] ; ID of item chosen for swapping (counts from 1)
+	and a ; has the first item to swap already been chosen?
+	jr nz, .swapItems
+; if not, set the currently selected item as the first item
+	ld a, [wCurrentMenuItem]
+	inc a
+	ld b, a
+	ld a, [wListScrollOffset] ; index of top (visible) menu item within the list
+	add b
+	ld [wMenuItemToSwap], a ; ID of item chosen for swapping (counts from 1)
+	ld c, 20
+	call DelayFrames
+	jp DisplayListMenuIDLoop
+.swapItems
+	ld a, [wCurrentMenuItem]
+	inc a
+	ld b, a
+	ld a, [wListScrollOffset]
+	add b
+	ld b, a
+	ld a, [wMenuItemToSwap] ; ID of item chosen for swapping (counts from 1)
+	cp b ; is the currently selected item the same as the first item to swap?
+	jp z, DisplayListMenuIDLoop ; ignore attempts to swap an item with itself
+	dec a
+	ld [wMenuItemToSwap], a ; ID of item chosen for swapping (counts from 1)
+	ld c, 20
+	call DelayFrames
+	push hl
+	push de
+	ld hl, wListPointer
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+	inc hl ; hl = beginning of list entries
+	ld d, h
+	ld e, l ; de = beginning of list entries
+	ld a, [wCurrentMenuItem]
+	ld b, a
+	ld a, [wListScrollOffset]
+	add b
+	add a
+	ld c, a
+	ld b, 0
+	add hl, bc ; hl = address of currently selected item entry
+	ld a, [wMenuItemToSwap] ; ID of item chosen for swapping (counts from 1)
+	add a
+	add e
+	ld e, a
+	jr nc, .noCarry
+	inc d
+.noCarry ; de = address of first item to swap
+	ld a, [de]
+	ld b, a
+	ld a, [hli]
+	cp b
+	jr z, .swapSameItemType
+.swapDifferentItems
+	ld [$ff95], a ; [$ff95] = second item ID
+	ld a, [hld]
+	ld [$ff96], a ; [$ff96] = second item quantity
+	ld a, [de]
+	ld [hli], a ; put first item ID in second item slot
+	inc de
+	ld a, [de]
+	ld [hl], a ; put first item quantity in second item slot
+	ld a, [$ff96]
+	ld [de], a ; put second item quantity in first item slot
+	dec de
+	ld a, [$ff95]
+	ld [de], a ; put second item ID in first item slot
+	xor a
+	ld [wMenuItemToSwap], a ; 0 means no item is currently being swapped
+	pop de
+	pop hl
+	jp DisplayListMenuIDLoop
+.swapSameItemType
+	inc de
+	ld a, [hl]
+	ld b, a
+	ld a, [de]
+	add b ; a = sum of both item quantities
+	cp 100 ; is the sum too big for one item slot?
+	jr c, .combineItemSlots
+; swap enough items from the first slot to max out the second slot if they can't be combined
+	sub 99
+	ld [de], a
+	ld a, 99
+	ld [hl], a
+	jr .done
+.combineItemSlots
+	ld [hl], a ; put the sum in the second item slot
+	ld hl, wListPointer
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+	dec [hl] ; decrease the number of items
+	ld a, [hl]
+	ld [wListCount], a ; update number of items variable
+	cp 1
+	jr nz, .skipSettingMaxMenuItemID
+	ld [wMaxMenuItem], a ; if the number of items is only one now, update the max menu item ID
+.skipSettingMaxMenuItemID
+	dec de
+	ld h, d
+	ld l, e
+	inc hl
+	inc hl ; hl = address of item after first item to swap
+.moveItemsUpLoop ; erase the first item slot and move up all the following item slots to fill the gap
+	ld a, [hli]
+	ld [de], a
+	inc de
+	inc a ; reached the $ff terminator?
+	jr z, .afterMovingItemsUp
+	ld a, [hli]
+	ld [de], a
+	inc de
+	jr .moveItemsUpLoop
+.afterMovingItemsUp
+	xor a
+	ld [wListScrollOffset], a
+	ld [wCurrentMenuItem], a
+.done
+	xor a
+	ld [wMenuItemToSwap], a ; 0 means no item is currently being swapped
+	pop de
+	pop hl
+	jp DisplayListMenuIDLoop