Add Gen 2 party reorder API parity

src/world/gen2/WorldAPI.lua

@@ -30,6 +30,7 @@ local MapOverview = require("src.world.MapOverview")
 local Bike = require("src.world.gen2.Bike")
 local FieldMoves = require("src.world.gen2.FieldMoves")
 local Permissions = require("src.world.gen2.Permissions")
+local Mail = require("src.core.gen2.Mail")
 
 local WorldAPI = {}
 WorldAPI.__index = WorldAPI
@@ -49,6 +50,11 @@ local FIELD_ACTIONS = {
   { id = "teleport", move = "TELEPORT" },
 }
 
+local function validPartySlot(party, slot)
+  return type(slot) == "number" and slot == math.floor(slot)
+    and party[slot] ~= nil
+end
+
 function WorldAPI.new(game, modId)
   return setmetatable({ game = game, modId = modId }, WorldAPI)
 end
@@ -68,6 +74,32 @@ function WorldAPI:current()
            facing = p and p.facing }
 end
 
+-- Keep the public party-ordering contract identical across generations.
+-- Gen 2 stores mail by party slot, so it must move with the Pokemon just as
+-- the native PartyMenu's SwitchPartyMons path does.
+function WorldAPI:canReorderParty()
+  local world, game = self:overworld(), self.game
+  local party = game and game.save and game.save.party or {}
+  return #party > 1 and world ~= nil and world:acceptsMenuInput()
+end
+
+function WorldAPI:reorderParty(fromSlot, toSlot)
+  local world, game = self:overworld(), self.game
+  if not world then return nil, NO_OVERWORLD end
+  if not world:acceptsMenuInput() then return nil, "world is busy" end
+  local party = game.save and game.save.party or {}
+  if not validPartySlot(party, fromSlot)
+      or not validPartySlot(party, toSlot) then
+    return nil, "invalid party slot"
+  end
+  if fromSlot ~= toSlot then
+    party[fromSlot], party[toSlot] = party[toSlot], party[fromSlot]
+    Mail.swapSlots(game.save, fromSlot, toSlot)
+    require("src.core.Sound").play(game.data, "Sfx_SwitchPokemon")
+  end
+  return true
+end
+
 local function itemLabel(game, id)
   local def = game and game.data and game.data.items
     and game.data.items[id]

tests/modkit/cases/world_party_reorder_gen2.lua

@@ -0,0 +1,59 @@
+-- Gen 2 parity for the public party-ordering contract. The fixture also
+-- carries slot-based mail because a reorder must move that state with its mon.
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+love = love or require("tests.love_stub")
+
+local T = require("tests.harness").suite("mod world party reorder gen2")
+local WorldAPI = require("src.world.gen2.WorldAPI")
+
+local first = { species = "CHIKORITA" }
+local second = { species = "CYNDAQUIL" }
+local firstMail = { message = "FIRST" }
+local secondMail = { message = "SECOND" }
+local world = { map = { id = "NEW_BARK_TOWN" }, accepts = true }
+function world:acceptsMenuInput() return self.accepts end
+
+local game = {
+  data = { audio = { sfx = {} } },
+  save = {
+    party = { first, second },
+    mail = { party = { firstMail, secondMail }, box = {} },
+  },
+  world = world,
+}
+local api = WorldAPI.new(game, "fixture")
+
+T.check(api:canReorderParty(), "idle free roam allows party reordering")
+
+local Sound = require("src.core.Sound")
+local realPlay, played = Sound.play
+Sound.play = function(_, name) played = name end
+T.check(api:reorderParty(1, 2) == true, "valid slots reorder")
+Sound.play = realPlay
+T.check(game.save.party[1] == second and game.save.party[2] == first,
+  "the live party is swapped")
+T.check(game.save.mail.party[1] == secondMail
+    and game.save.mail.party[2] == firstMail,
+  "slot-based mail follows its Pokemon")
+T.eq(played, "Sfx_SwitchPokemon", "the native Gen 2 swap sound is used")
+
+local value, err = api:reorderParty(1.5, 2)
+T.check(value == nil and err == "invalid party slot",
+  "non-integer slots are rejected")
+value, err = api:reorderParty("1", 2)
+T.check(value == nil and err == "invalid party slot",
+  "string slots are rejected")
+
+world.accepts = false
+T.check(not api:canReorderParty(), "busy free roam blocks reordering")
+value, err = api:reorderParty(1, 2)
+T.check(value == nil and err == "world is busy",
+  "reordering refuses while the world owns input")
+
+game.world = nil
+value, err = api:reorderParty(1, 2)
+T.check(value == nil and err == "no overworld",
+  "reordering outside the overworld fails closed")
+
+T.finish()