BizHawkClient: Add README (ArchipelagoMW#2689)

Co-authored-by: Nicholas Saylor <[email protected]>

worlds/_bizhawk/README.md

@@ -0,0 +1,279 @@
+# BizHawk Client
+
+`BizHawkClient` is an abstract base class for a client that can access the memory of a ROM running in BizHawk. It does
+the legwork of connecting Python to a Lua connector script, letting you focus on the loop of checking locations and
+making on-the-fly modifications based on updates from the server. It also provides the same experience to users across
+multiple games that use it, and was built in response to a growing number of similar but separate bespoke game clients
+which are/were largely exclusive to BizHawk anyway.
+
+It's similar to `SNIClient`, but where `SNIClient` is designed to work for specifically SNES games across different
+emulators/hardware, `BizHawkClient` is designed to work for specifically BizHawk across the different systems BizHawk
+supports.
+
+The idea is that `BizHawkClient` connects to and communicates with a Lua script running in BizHawk. It provides an API
+that will call BizHawk functions for you to do things like read and write memory. And on an interval, control will be
+handed to a function you write for your game (`game_watcher`) which should interact with the game's memory to check what
+locations have been checked, give the player items, detect and send deathlinks, etc...
+
+Table of Contents:
+- [Connector Requests](#connector-requests)
+    - [Requests that depend on other requests](#requests-that-depend-on-other-requests)
+- [Implementing a Client](#implementing-a-client)
+    - [Example](#example)
+- [Tips](#tips)
+
+## Connector Requests
+
+Communication with BizHawk is done through `connector_bizhawk_generic.lua`. The client sends requests to the Lua script
+via sockets; the Lua script processes the request and sends the corresponding responses.
+
+The Lua script includes its own documentation, but you probably don't need to worry about the specifics. Instead, you'll
+be using the functions in `worlds/_bizhawk/__init__.py`. If you do need more control over the specific requests being
+sent or their order, you can still use `send_requests` to directly communicate with the connector script.
+
+It's not necessary to use the UI or client context if you only want to interact with the connector script. You can
+import and use just `worlds/_bizhawk/__init__.py`, which only depends on default modules.
+
+Here's a list of the included classes and functions. I would highly recommend looking at the actual function signatures
+and docstrings to learn more about each function.
+
+```
+class ConnectionStatus
+class BizHawkContext
+
+class NotConnectedError
+class RequestFailedError
+class ConnectorError
+class SyncError
+
+async def read(ctx, read_list) -> list[bytes]
+async def write(ctx, write_list) -> None:
+async def guarded_read(ctx, read_list, guard_list) -> (list[bytes] | None)
+async def guarded_write(ctx, write_list, guard_list) -> bool
+
+async def lock(ctx) -> None
+async def unlock(ctx) -> None
+
+async def get_hash(ctx) -> str
+async def get_system(ctx) -> str
+async def get_cores(ctx) -> dict[str, str]
+async def ping(ctx) -> None
+
+async def display_message(ctx, message: str) -> None
+async def set_message_interval(ctx, value: float) -> None
+
+async def connect(ctx) -> bool
+def disconnect(ctx) -> None
+
+async def get_script_version(ctx) -> int
+async def send_requests(ctx, req_list) -> list[dict[str, Any]]
+```
+
+`send_requests` is what actually communicates with the connector, and any functions like `guarded_read` will build the
+requests and then call `send_requests` for you. You can call `send_requests` yourself for more direct control, but make
+sure to read the docs in `connector_bizhawk_generic.lua`.
+
+A bundle of requests sent by `send_requests` will all be executed on the same frame, and by extension, so will any
+helper that calls `send_requests`. For example, if you were to call `read` with 3 items on your `read_list`, all 3
+addresses will be read on the same frame and then sent back.
+
+It also means that, by default, the only way to run multiple requests on the same frame is for them to be included in
+the same `send_requests` call. As soon as the connector finishes responding to a list of requests, it will advance the
+frame before checking for the next batch.
+
+### Requests that depend on other requests
+
+The fact that you have to wait at least a frame to act on any response may raise concerns. For example, Pokemon
+Emerald's save data is at a dynamic location in memory; it moves around when you load a new map. There is a static
+variable that holds the address of the save data, so we want to read the static variable to get the save address, and
+then use that address in a `write` to send the player an item. But between the `read` that tells us the address of the
+save data and the `write` to save data itself, an arbitrary number of frames have been executed, and the player may have
+loaded a new map, meaning we've written data to who knows where.
+
+There are two solutions to this problem.
+
+1. Use `guarded_write` instead of `write`. We can include a guard against the address changing, and the script will only
+perform the write if the data in memory matches what's in the guard. In the below example, `write_result` will be `True`
+if the guard validated and the data was written, and `False` if the guard failed to validate.
+
+```py
+# Get the address of the save data
+read_result: bytes = (await _bizhawk.read(ctx, [(0x3001111, 4, "System Bus")]))[0]
+save_data_address = int.from_bytes(read_result, "little")
+
+# Write to `save_data_address` if it hasn't changed
+write_result: bool = await _bizhawk.guarded_write(
+    ctx,
+    [(save_data_address, [0xAA, 0xBB], "System Bus")],
+    [(0x3001111, read_result, "System Bus")]
+)
+
+if write_result:
+    # The data at 0x3001111 was still the same value as
+    # what was returned from the first `_bizhawk.read`,
+    # so the data was written.
+    ...
+else:
+    # The data at 0x3001111 has changed since the
+    # first `_bizhawk.read`, so the data was not written.
+    ...
+```
+
+2. Use `lock` and `unlock` (discouraged if not necessary). When you call `lock`, you tell the emulator to stop advancing
+frames and just process requests until it receives an unlock request. This means you can lock, read the address, write
+the data, and then unlock on a single frame. **However**, this is _slow_. If you can't get in and get out quickly
+enough, players will notice a stutter in the emulation.
+
+```py
+# Pause emulation
+await _bizhawk.lock(ctx)
+
+# Get the address of the save data
+read_result: bytes = (await _bizhawk.read(ctx, [(0x3001111, 4, "System Bus")]))[0]
+save_data_address = int.from_bytes(read_result, "little")
+
+# Write to `save_data_address`
+await _bizhawk.write(ctx, [(save_data_address, [0xAA, 0xBB], "System Bus")])
+
+# Resume emulation
+await _bizhawk.unlock(ctx)
+```
+
+You should always use `guarded_read` and `guarded_write` instead of locking the emulator if possible. It may be
+unreliable, but that's by design. Most of the time you should have no problem giving up and retrying. Data that is
+volatile but only changes occasionally is the perfect use case.
+
+If data is almost guaranteed to change between frames, locking may be the better solution. You can lower the time spent
+locked by using `send_requests` directly to include as many requests alongside the `LOCK` and `UNLOCK` requests as
+possible. But in general it's probably worth doing some extra asm hacking and designing to make guards work instead.
+
+## Implementing a Client
+
+`BizHawkClient` itself is built on `CommonClient` and inspired heavily by `SNIClient`. Your world's client should
+inherit from `BizHawkClient` in `worlds/_bizhawk/client.py`. It must implement `validate_rom` and `game_watcher`, and
+must define values for `system` and `game`.
+
+As with the functions and classes in the previous section, I would highly recommend looking at the types and docstrings
+of the code itself.
+
+`game` should be the same value you use for your world definition.
+
+`system` can either be a string or a tuple of strings. This is the system (or systems) that your client is intended to
+handle games on (SNES, GBA, etc.). It's used to prevent validators from running on unknown systems and crashing. The
+actual abbreviation corresponds to whatever BizHawk returns from `emu.getsystemid()`.
+
+`patch_suffix` is an optional `ClassVar` meant to specify the file extensions you want to register. It can be a string
+or tuple of strings. When a player clicks "Open Patch" in a launcher, the suffix(es) will be whitelisted in the file
+select dialog and they will be associated with BizHawkClient. This does not affect whether the user's computer will
+associate the file extension with Archipelago.
+
+`validate_rom` is called to figure out whether a given ROM belongs to your client. It will only be called when a ROM is
+running on a system you specified in your `system` class variable. In most cases, that will be a single system and you
+can be sure that you're not about to try to read from nonexistent domains or out of bounds. If you decide to claim this
+ROM as yours, this is where you should do setup for things like `items_handling`.
+
+`game_watcher` is the "main loop" of your client where you should be checking memory and sending new items to the ROM.
+`BizHawkClient` will make sure that your `game_watcher` only runs when your client has validated the ROM, and will do
+its best to make sure you're connected to the connector script before calling your watcher. It runs this loop either
+immediately once it receives a message from the server, or a specified amount of time after the last iteration of the
+loop finished.
+
+`validate_rom`, `game_watcher`, and other methods will be passed an instance of `BizHawkClientContext`, which is a
+subclass of `CommonContext`. It additionally includes `slot_data` (if you are connected and asked for slot data),
+`bizhawk_ctx` (the instance of `BizHawkContext` that you should be giving to functions like `guarded_read`), and
+`watcher_timeout` (the amount of time in seconds between iterations of the game watcher loop).
+
+### Example
+
+A very simple client might look like this. All addresses here are made up; you should instead be using addresses that
+make sense for your specific ROM. The `validate_rom` here tries to read the name of the ROM. If it gets the value it
+wanted, it sets a couple values on `ctx` and returns `True`. The `game_watcher` reads some data from memory and acts on
+it by sending messages to AP. You should be smarter than this example, which will send `LocationChecks` messages even if
+there's nothing new since the last loop.
+
+```py
+from typing import TYPE_CHECKING
+
+from NetUtils import ClientStatus
+
+import worlds._bizhawk as bizhawk
+from worlds._bizhawk.client import BizHawkClient
+
+if TYPE_CHECKING:
+    from worlds._bizhawk.context import BizHawkClientContext
+
+
+class MyGameClient(BizHawkClient):
+    game = "My Game"
+    system = "GBA"
+    patch_suffix = ".apextension"
+
+    async def validate_rom(self, ctx: "BizHawkClientContext") -> bool:
+        try:
+            # Check ROM name/patch version
+            rom_name = ((await bizhawk.read(ctx.bizhawk_ctx, [(0x100, 6, "ROM")]))[0]).decode("ascii")
+            if rom_name != "MYGAME":
+                return False  # Not a MYGAME ROM
+        except bizhawk.RequestFailedError:
+            return False  # Not able to get a response, say no for now
+
+        # This is a MYGAME ROM
+        ctx.game = self.game
+        ctx.items_handling = 0b001
+        ctx.want_slot_data = True
+
+        return True
+
+    async def game_watcher(self, ctx: "BizHawkClientContext") -> None:
+        try:
+            # Read save data
+            save_data = await bizhawk.read(
+                ctx.bizhawk_ctx,
+                [(0x3000100, 20, "System Bus")]
+            )[0]
+
+            # Check locations
+            if save_data[2] & 0x04:
+                await ctx.send_msgs([{
+                    "cmd": "LocationChecks",
+                    "locations": [23]
+                }])
+
+            # Send game clear
+            if not ctx.finished_game and (save_data[5] & 0x01):
+                await ctx.send_msgs([{
+                    "cmd": "StatusUpdate",
+                    "status": ClientStatus.CLIENT_GOAL
+                }])
+
+        except bizhawk.RequestFailedError:
+            # The connector didn't respond. Exit handler and return to main loop to reconnect
+            pass
+```
+
+### Tips
+
+- Make sure your client gets imported when your world is imported. You probably don't need to actually use anything in
+your `client.py` elsewhere, but you still have to import the file for your client to register itself.
+- When it comes to performance, there are two directions to optimize:
+  1. If you need to execute multiple commands on the same frame, do as little work as possible. Only read and write necessary data,
+  and if you have to use locks, unlock as soon as it's okay to advance frames. This is probably the obvious one.
+  2. Multiple things that don't have to happen on the same frame should be split up if they're likely to be slow.
+  Remember, the game watcher runs only a few times per second. Extra function calls on the client aren't that big of a
+  deal; the player will not notice if your `game_watcher` is slow. But the emulator has to be done with any given set of
+  commands in 1/60th of a second to avoid hiccups (faster still if your players use speedup). Too many reads of too much
+  data at the same time is more likely to cause a bad user experience.
+- Your `game_watcher` will be called regardless of the status of the client's connection to the server. Double-check the
+server connection before trying to interact with it.
+- By default, the player will be asked to provide their slot name after connecting to the server and validating, and
+that input will be used to authenticate with the `Connect` command. You can override `set_auth` in your own client to
+set it automatically based on data in the ROM or on your client instance.
+- You can override `on_package` in your client to watch raw packages, but don't forget you also have access to a
+subclass of `CommonContext` and its API.
+- You can import `BizHawkClientContext` for type hints using `typing.TYPE_CHECKING`. Importing it without conditions at
+the top of the file will probably cause a circular dependency.
+- Your game's system may have multiple usable cores in BizHawk. You can use `get_cores` to try to determine which one is
+currently loaded (it's the best we can do). Some cores may differ in the names of memory domains. It's good to check all
+the available cores to find differences before your users do.
+- The connector script includes a DEBUG variable that you can use to log requests/responses. (Be aware that as the log
+grows in size in BizHawk, it begins to stutter while trying to print it.)