1 files changed, 149 insertions, 0 deletions
diff --git a/engine/menu/swap_items.asm b/engine/menu/swap_items.asm
new file mode 100644
index 00000000..b1fa78be
--- /dev/null
+++ b/engine/menu/swap_items.asm
@@ -0,0 +1,149 @@
+HandleItemListSwapping:
+	ld a,[wListMenuID]
+	cp a,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 a,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 a,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 a,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