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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
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