Update world api.md

Co-authored-by: Nicholas Saylor <79181893+nicholassaylor@users.noreply.github.com>

BaseClasses.py

@@ -692,17 +692,25 @@ class CollectionState():
 
     def update_reachable_regions(self, player: int):
         self.stale[player] = False
+        world: AutoWorld.World = self.multiworld.worlds[player]
         reachable_regions = self.reachable_regions[player]
-        blocked_connections = self.blocked_connections[player]
         queue = deque(self.blocked_connections[player])
-        start = self.multiworld.get_region("Menu", player)
+        start: Region = world.get_region(world.origin_region_name)
 
         # init on first call - this can't be done on construction since the regions don't exist yet
         if start not in reachable_regions:
             reachable_regions.add(start)
-            blocked_connections.update(start.exits)
+            self.blocked_connections[player].update(start.exits)
             queue.extend(start.exits)
 
+        if world.explicit_indirect_conditions:
+            self._update_reachable_regions_explicit_indirect_conditions(player, queue)
+        else:
+            self._update_reachable_regions_auto_indirect_conditions(player, queue)
+
+    def _update_reachable_regions_explicit_indirect_conditions(self, player: int, queue: deque):
+        reachable_regions = self.reachable_regions[player]
+        blocked_connections = self.blocked_connections[player]
         # run BFS on all connections, and keep track of those blocked by missing items
         while queue:
             connection = queue.popleft()
@@ -722,6 +730,29 @@ class CollectionState():
                     if new_entrance in blocked_connections and new_entrance not in queue:
                         queue.append(new_entrance)
 
+    def _update_reachable_regions_auto_indirect_conditions(self, player: int, queue: deque):
+        reachable_regions = self.reachable_regions[player]
+        blocked_connections = self.blocked_connections[player]
+        new_connection: bool = True
+        # run BFS on all connections, and keep track of those blocked by missing items
+        while new_connection:
+            new_connection = False
+            while queue:
+                connection = queue.popleft()
+                new_region = connection.connected_region
+                if new_region in reachable_regions:
+                    blocked_connections.remove(connection)
+                elif connection.can_reach(self):
+                    assert new_region, f"tried to search through an Entrance \"{connection}\" with no Region"
+                    reachable_regions.add(new_region)
+                    blocked_connections.remove(connection)
+                    blocked_connections.update(new_region.exits)
+                    queue.extend(new_region.exits)
+                    self.path[new_region] = (new_region.name, self.path.get(connection, None))
+                    new_connection = True
+            # sweep for indirect connections, mostly Entrance.can_reach(unrelated_Region)
+            queue.extend(blocked_connections)
+
     def copy(self) -> CollectionState:
         ret = CollectionState(self.multiworld)
         ret.prog_items = {player: counter.copy() for player, counter in self.prog_items.items()}

docs/world api.md

@@ -288,8 +288,8 @@ like entrance randomization in logic.
 
 Regions have a list called `exits`, containing `Entrance` objects representing transitions to other regions.
 
-There must be one special region, "Menu", from which the logic unfolds. AP assumes that a player will always be able to
+There must be one special region (Called "Menu" by default, but configurable using [origin_region_name](https://github.com/ArchipelagoMW/Archipelago/blob/main/worlds/AutoWorld.py#L295-L296)),
-return to the "Menu" region by resetting the game ("Save and quit").
+from which the logic unfolds. AP assumes that a player will always be able to return to this starting region by resetting the game ("Save and quit").
 
 ### Entrances
 
@@ -303,6 +303,34 @@ generation (entrance randomization).
 An access rule is a function that returns `True` or `False` for a `Location` or `Entrance` based on the current `state`
 (items that have been collected).
 
+The two possible ways to make a [CollectionRule](https://github.com/ArchipelagoMW/Archipelago/blob/main/worlds/generic/Rules.py#L10) are:
+- `def rule(state: CollectionState) -> bool:`
+- `lambda state: ... boolean expression ...`
+
+An access rule can be assigned through `set_rule(location, rule)`.
+
+Access rules usually check for one of two things.
+- Items that have been collected (e.g. `state.has("Sword", player)`)
+- Locations, Regions or Entrances that have been reached (e.g. `state.can_reach_region("Boss Room")`)
+
+Keep in mind that entrances and locations implicitly check for the accessibility of their parent region, so you do not need to check explicitly for it.
+
+#### An important note on Entrance access rules:
+When using `state.can_reach` within an entrance access condition, you must also use `multiworld.register_indirect_condition`.
+
+For efficiency reasons, every time reachable regions are searched, every entrance is only checked once in a somewhat non-deterministic order.
+This is fine when checking for items using `state.has`, because items do not change during a region sweep.
+However, `state.can_reach` checks for the very same thing we are updating: Regions.
+This can lead to non-deterministic behavior and, in the worst case, even generation failures.
+Even doing `state.can_reach_location` or `state.can_reach_entrance` is problematic, as these functions call `state.can_reach_region` on the respective parent region.
+
+**Therefore, it is considered unsafe to perform `state.can_reach` from within an access condition for an entrance**, unless you are checking for something that sits in the source region of the entrance.
+You can use `multiworld.register_indirect_condition(region, entrance)` to explicitly tell the generator that, when a given region becomes accessible, it is necessary to re-check a specific entrance.
+You **must** use `multiworld.register_indirect_condition` if you perform this kind of `can_reach` from an entrance access rule, unless you have a **very** good technical understanding of the relevant code and can reason why it will never lead to problems in your case.
+
+Alternatively, you can set [world.explicit_indirect_conditions = False](https://github.com/ArchipelagoMW/Archipelago/blob/main/worlds/AutoWorld.py#L298-L301),
+avoiding the need for indirect conditions at the expense of performance.
+
 ### Item Rules
 
 An item rule is a function that returns `True` or `False` for a `Location` based on a single item. It can be used to
@@ -438,7 +466,7 @@ The world has to provide the following things for generation:
 
 * the properties mentioned above
 * additions to the item pool
-* additions to the regions list: at least one called "Menu"
+* additions to the regions list: at least one named after the world class's origin_region_name ("Menu" by default)
 * locations placed inside those regions
 * a `def create_item(self, item: str) -> MyGameItem` to create any item on demand
 * applying `self.multiworld.push_precollected` for world-defined start inventory
@@ -491,7 +519,7 @@ def generate_early(self) -> None:
 
 ```python
 def create_regions(self) -> None:
-    # Add regions to the multiworld. "Menu" is the required starting point.
+    # Add regions to the multiworld. One of them must use the origin_region_name as its name ("Menu" by default).
     # Arguments to Region() are name, player, multiworld, and optionally hint_text
     menu_region = Region("Menu", self.player, self.multiworld)
     self.multiworld.regions.append(menu_region)  # or use += [menu_region...]
@@ -630,7 +658,7 @@ def set_rules(self) -> None:
 
 Custom methods can be defined for your logic rules. The access rule that ultimately gets assigned to the Location or
 Entrance should be
-a [`CollectionRule`](https://github.com/ArchipelagoMW/Archipelago/blob/main/worlds/generic/Rules.py#L9).
+a [`CollectionRule`](https://github.com/ArchipelagoMW/Archipelago/blob/main/worlds/generic/Rules.py#L10).
 Typically, this is done by defining a lambda expression on demand at the relevant bit, typically calling other
 functions, but this can also be achieved by defining a method with the appropriate format and assigning it directly.
 For an example, see [The Messenger](/worlds/messenger/rules.py).

worlds/AutoWorld.py

@@ -292,6 +292,14 @@ class World(metaclass=AutoWorldRegister):
     web: ClassVar[WebWorld] = WebWorld()
     """see WebWorld for options"""
 
+    origin_region_name: str = "Menu"
+    """Name of the Region from which accessibility is tested."""
+
+    explicit_indirect_conditions: bool = True
+    """If True, the world implementation is supposed to use MultiWorld.register_indirect_condition() correctly.
+    If False, everything is rechecked at every step, which is slower computationally, 
+    but may be desirable in complex/dynamic worlds."""
+
     multiworld: "MultiWorld"
     """autoset on creation. The MultiWorld object for the currently generating multiworld."""
     player: int

worlds/factorio/__init__.py

@@ -101,6 +101,7 @@ class Factorio(World):
     tech_tree_layout_prerequisites: typing.Dict[FactorioScienceLocation, typing.Set[FactorioScienceLocation]]
     tech_mix: int = 0
     skip_silo: bool = False
+    origin_region_name = "Nauvis"
     science_locations: typing.List[FactorioScienceLocation]
 
     settings: typing.ClassVar[FactorioSettings]
@@ -125,9 +126,6 @@ class Factorio(World):
     def create_regions(self):
         player = self.player
         random = self.multiworld.random
-        menu = Region("Menu", player, self.multiworld)
-        crash = Entrance(player, "Crash Land", menu)
-        menu.exits.append(crash)
         nauvis = Region("Nauvis", player, self.multiworld)
 
         location_count = len(base_tech_table) - len(useless_technologies) - self.skip_silo + \
@@ -184,8 +182,7 @@ class Factorio(World):
             event = FactorioItem(f"Automated {ingredient}", ItemClassification.progression, None, player)
             location.place_locked_item(event)
 
-        crash.connect(nauvis)
+        self.multiworld.regions.append(nauvis)
-        self.multiworld.regions += [menu, nauvis]
 
     def create_items(self) -> None:
         player = self.player

worlds/subnautica/__init__.py

@@ -45,7 +45,7 @@ class SubnauticaWorld(World):
     options_dataclass = options.SubnauticaOptions
     options: options.SubnauticaOptions
     required_client_version = (0, 5, 0)
-
+    origin_region_name = "Planet 4546B"
     creatures_to_scan: List[str]
 
     def generate_early(self) -> None:
@@ -66,13 +66,9 @@ class SubnauticaWorld(World):
             creature_pool, self.options.creature_scans.value)
 
     def create_regions(self):
-        # Create Regions
+        # Create Region
-        menu_region = Region("Menu", self.player, self.multiworld)
         planet_region = Region("Planet 4546B", self.player, self.multiworld)
 
-        # Link regions together
-        menu_region.connect(planet_region, "Lifepod 5")
-
         # Create regular locations
         location_names = itertools.chain((location["name"] for location in locations.location_table.values()),
                                          (creature + creatures.suffix for creature in self.creatures_to_scan))
@@ -93,11 +89,8 @@ class SubnauticaWorld(World):
                 # make the goal event the victory "item"
                 location.item.name = "Victory"
 
-        # Register regions to multiworld
+        # Register region to multiworld
-        self.multiworld.regions += [
+        self.multiworld.regions.append(planet_region)
-            menu_region,
-            planet_region
-        ]
 
     # refer to rules.py
     set_rules = set_rules