/back implementation begin

src/main/java/uk/gemwire/bareessentials/BareEssentials.java

@@ -42,6 +42,7 @@
 import uk.gemwire.bareessentials.commands.PermissionNodes;
 import uk.gemwire.bareessentials.data.Bank;
 import uk.gemwire.bareessentials.data.Homes;
+import uk.gemwire.bareessentials.data.TeleportRewind;
 
 
 /**
@@ -112,6 +113,7 @@ public class BareEssentials {
     public static GameRules.Key<GameRules.IntegerValue> CURRENCY_SYMBOL = GameRules.register("be.currencySymbol", GameRules.Category.CHAT, GameRules.IntegerValue.create(0));
     public static GameRules.Key<GameRules.IntegerValue> STARTING_BALANCE = GameRules.register("be.bankStartingBalance", GameRules.Category.PLAYER, GameRules.IntegerValue.create(500));
     public static GameRules.Key<GameRules.IntegerValue> DAILY_INCOME = GameRules.register("be.bankDailyIncome", GameRules.Category.PLAYER, GameRules.IntegerValue.create(10));
+    // TODO
     public static GameRules.Key<GameRules.IntegerValue> MAX_HOMES = GameRules.register("be.maxHomes", GameRules.Category.PLAYER, GameRules.IntegerValue.create(1));
     public static GameRules.Key<GameRules.IntegerValue> TPA_COST = GameRules.register("be.tpaCost", GameRules.Category.PLAYER, GameRules.IntegerValue.create(0));
     public static GameRules.Key<GameRules.IntegerValue> SPAWN_COST = GameRules.register("be.spawnCost", GameRules.Category.PLAYER, GameRules.IntegerValue.create(0));
@@ -120,6 +122,9 @@ public class BareEssentials {
 
     public static GameRules.Key<GameRules.BooleanValue> OP_OVERRIDES_COOLDOWN = GameRules.register("be.opOverridesCooldowns", GameRules.Category.PLAYER, GameRules.BooleanValue.create(false));
 
+    // Whether /back will traverse the stack of back-able events. If you go to the nether and then accept a tpa back to the overworld, you can /back twice to return to the Overworld nether portal, if stacking is enabled.
+    public static GameRules.Key<GameRules.BooleanValue> BACK_STACK = GameRules.register("be.backStack", GameRules.Category.PLAYER, GameRules.BooleanValue.create(true));
+
     public static ObjectiveCriteria BANK_ACCOUNT_VALUE = ObjectiveCriteria.registerCustom("bank_value");
     public static Objective BANK_ACCOUNT_SORTED_OBJECTIVE;
 
@@ -154,6 +159,9 @@ public static void login(PlayerEvent.PlayerLoggedInEvent e) {
             // Ensure the new player has a bank account so they receive income while offline.
             Bank accts = Bank.getOrCreate(e.getEntity().getServer().overworld());
             accts.getUserBalance((ServerPlayer) e.getEntity());
+            // Ensure the player's last rewind position is available as soon as they log in.
+            TeleportRewind rewinds = TeleportRewind.getOrCreate(e.getEntity().getServer().overworld());
+
         }
 
         @SubscribeEvent

src/main/java/uk/gemwire/bareessentials/data/TeleportRewind.java

@@ -0,0 +1,103 @@
+package uk.gemwire.bareessentials.data;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.core.HolderLookup;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.saveddata.SavedData;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Stack;
+import java.util.UUID;
+
+public class TeleportRewind extends SavedData {
+
+    // Every player has a list of things they can undo.
+    public Map<UUID, Stack<TeleportEvent>> playerBacks = new HashMap<>();
+
+    private static final Factory<TeleportRewind> rewindFactory
+        = new Factory<>(TeleportRewind::new, TeleportRewind::load, null);
+
+    public TeleportRewind() {
+    }
+
+    public TeleportRewind(Map<UUID, TeleportEvent> firstBacks) {
+        for (Map.Entry<UUID, TeleportEvent> tele : firstBacks.entrySet()) {
+            playerBacks.put(tele.getKey(), new Stack<>());
+            playerBacks.get(tele.getKey()).add(tele.getValue());
+        }
+    }
+
+    @Override
+    public CompoundTag save(final CompoundTag tag, final HolderLookup.Provider registries) {
+        CompoundTag map = new CompoundTag();
+        for (var tele : playerBacks.entrySet()) {
+            TeleportEvent event = tele.getValue().pop();
+            CompoundTag entry = new CompoundTag();
+            CompoundTag source = new CompoundTag();
+            source.putInt("x", event.origin.getX());
+            source.putInt("y", event.origin.getX());
+            source.putInt("z", event.origin.getX());
+            CompoundTag destination = new CompoundTag();
+            destination.putInt("x", event.destination.getX());
+            destination.putInt("y", event.destination.getX());
+            destination.putInt("z", event.destination.getX());
+            entry.put("source", source);
+            entry.put("destination", destination);
+            entry.putString("type", event.type.getName());
+            map.put(tele.getKey().toString(), entry);
+        }
+        tag.put("be_rewinds", map);
+        return tag;
+    }
+
+    public static TeleportRewind load(CompoundTag tag, final HolderLookup.Provider prov) {
+        CompoundTag map = tag.getCompound("be_rewinds");
+        Map<UUID, TeleportEvent> teleports = new HashMap<>();
+        for (String key : map.getAllKeys()) {
+            UUID player = UUID.fromString(key);
+            CompoundTag entry = (CompoundTag) map.get(key);
+            CompoundTag source = (CompoundTag) entry.get("source");
+            BlockPos sourcePos = new BlockPos(source.getInt("x"), source.getInt("y"), source.getInt("z"));
+            CompoundTag destination = (CompoundTag) entry.get("destination");
+            BlockPos destPos = new BlockPos(destination.getInt("x"), destination.getInt("y"), destination.getInt("z"));
+            TeleportEvent evt = new TeleportEvent(EventType.fromString(entry.getString("type")), sourcePos, destPos);
+
+            teleports.put(player,evt);
+        }
+
+        return new TeleportRewind(teleports);
+    }
+
+    public static TeleportRewind getOrCreate(ServerLevel level) {
+        return level.getDataStorage().computeIfAbsent(rewindFactory, "be_back");
+    }
+
+
+    /**
+     * Teleport Events are a record of Type,Source,Destination
+     */
+    public enum EventType {
+        DEATH("DEATH"),         // Death moves you to spawn / bed
+        PORTAL("PORTAL"),       // End/Nether/Modded Portal; onTravelToDimension via Entity#changeDimension
+        COMMAND("COMMAND");     // Entity#moveto, via /tp. /tpa
+
+        String name;
+        EventType(String name) { this.name = name; }
+
+        public String getName() { return name; }
+
+        public static EventType fromString(String name) {
+            return switch (name) {
+                case "DEATH" -> DEATH;
+                case "PORTAL" -> PORTAL;
+                case "COMMAND" -> COMMAND;
+                default -> throw new IllegalStateException("Unexpected value: " + name);
+            };
+        }
+    }
+
+    public record TeleportEvent(EventType type, BlockPos origin, BlockPos destination) {
+    }
+}