Wire embedded z-machine interpreter into MUD mode stack

EmbeddedIFSession runs the hybrid interpreter in a daemon thread,
bridged to the async MUD loop via threading.Event synchronization.
.z3 files use the embedded path; other formats fall back to dfrotz.

- MUD ZUI components: MudScreen (buffered output), MudInputStream
  (thread-safe input), MudFilesystem (quetzal saves), NullAudio
- save/restore via QuetzalWriter/QuetzalParser and :: escape commands
- state inspection: get_location_name(), get_room_objects()
- error reporting for interpreter crashes
- fix quetzal parser bit slice bug: _parse_stks used [0:3] (3 bits,
  max 7 locals) instead of [0:4] (4 bits, max 15) — Zork uses 15

src/mudlib/commands/play.py

@@ -3,6 +3,7 @@
 import pathlib
 
 from mudlib.commands import CommandDefinition, register
+from mudlib.embedded_if_session import EmbeddedIFSession
 from mudlib.if_session import IFSession, broadcast_to_spectators
 from mudlib.player import Player
 
@@ -60,8 +61,20 @@ async def cmd_play(player: Player, args: str) -> None:
         await player.send(msg)
         return
 
-    # Create and start IF session
+    # Ensure story_path is a Path object (for mocking compatibility)
-    session = IFSession(player, str(story_path), game_name)
+    if not isinstance(story_path, pathlib.Path):
+        story_path = pathlib.Path(story_path)
+
+    # Use embedded interpreter for z3 files, dfrotz for others
+    if story_path.suffix == ".z3":
+        try:
+            session = EmbeddedIFSession(player, str(story_path), game_name)
+        except (FileNotFoundError, OSError) as e:
+            await player.send(f"error starting game: {e}\r\n")
+            return
+    else:
+        session = IFSession(player, str(story_path), game_name)
+
     try:
         intro = await session.start()
     except FileNotFoundError:
@@ -78,8 +91,8 @@ async def cmd_play(player: Player, args: str) -> None:
 
     await player.send("(type ::help for escape commands)\r\n")
 
-    # Check for saved game
+    # Check for saved game (both session types now support _do_restore)
-    if session.save_path.exists():
+    if hasattr(session, "_do_restore") and session.save_path.exists():
         await player.send("restoring saved game...\r\n")
         restored_text = await session._do_restore()
         if restored_text:

src/mudlib/embedded_if_session.py

@@ -0,0 +1,166 @@
+import asyncio
+import logging
+import re
+import threading
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+from mudlib.if_session import IFResponse
+from mudlib.zmachine.mud_ui import create_mud_ui
+from mudlib.zmachine.quetzal import QuetzalParser, QuetzalWriter
+from mudlib.zmachine.zcpu import ZCpuQuit, ZCpuRestart
+from mudlib.zmachine.zmachine import ZMachine
+
+if TYPE_CHECKING:
+    from mudlib.player import Player
+
+logger = logging.getLogger(__name__)
+
+
+class EmbeddedIFSession:
+    """Wraps z-machine interpreter for MUD integration."""
+
+    def __init__(self, player: "Player", story_path: str, game_name: str = ""):
+        self.player = player
+        self.story_path = story_path
+        self.game_name = game_name or Path(story_path).stem
+        self._data_dir = Path(__file__).resolve().parents[2] / "data"
+        self._thread: threading.Thread | None = None
+        self._done = False
+        self._error: str | None = None
+
+        story_bytes = Path(story_path).read_bytes()
+        save_path = self.save_path
+        self._ui, self._screen, self._keyboard = create_mud_ui(save_path)
+        self._zmachine = ZMachine(story_bytes, self._ui)
+        self._filesystem = self._ui.filesystem
+
+    @property
+    def save_path(self) -> Path:
+        safe_name = re.sub(r"[^a-zA-Z0-9_-]", "_", self.player.name)
+        return self._data_dir / "if_saves" / safe_name / f"{self.game_name}.qzl"
+
+    async def start(self) -> str:
+        self._thread = threading.Thread(target=self._run_interpreter, daemon=True)
+        self._thread.start()
+
+        loop = asyncio.get_running_loop()
+        await loop.run_in_executor(None, self._keyboard._waiting.wait)
+
+        intro = self._screen.flush()
+        return intro
+
+    async def handle_input(self, text: str) -> IFResponse:
+        if text.lower() == "::quit":
+            await self._do_save()
+            return IFResponse(output="game saved.", done=True)
+
+        if text.lower() == "::help":
+            help_text = """escape commands:
+::quit - exit the game
+::save - save game progress
+::help - show this help"""
+            return IFResponse(output=help_text, done=False)
+
+        if text.lower() == "::save":
+            confirmation = await self._do_save()
+            return IFResponse(output=confirmation, done=False)
+
+        self._keyboard._waiting.clear()
+        self._keyboard.feed(text)
+
+        loop = asyncio.get_running_loop()
+
+        def wait_for_next_input():
+            while not self._done and not self._keyboard._waiting.is_set():
+                self._keyboard._waiting.wait(timeout=0.1)
+
+        await loop.run_in_executor(None, wait_for_next_input)
+
+        output = self._screen.flush()
+        if self._done and self._error:
+            output = f"{output}\r\n{self._error}" if output else self._error
+        return IFResponse(output=output, done=self._done)
+
+    async def stop(self):
+        self._done = True
+        if self._keyboard._waiting.is_set():
+            self._keyboard.feed("")
+
+    def _run_interpreter(self):
+        try:
+            self._zmachine.run()
+        except ZCpuQuit:
+            logger.debug("Interpreter quit normally")
+        except ZCpuRestart:
+            logger.debug("Interpreter restart requested")
+        except Exception as e:
+            logger.error(f"Interpreter crashed: {e}")
+            self._error = f"interpreter error: {e}"
+        finally:
+            self._done = True
+            self._keyboard._waiting.set()
+
+    async def _do_save(self) -> str:
+        try:
+            writer = QuetzalWriter(self._zmachine)
+            save_data = writer.generate_save_data()
+            success = self._filesystem.save_game(save_data)
+            if success:
+                return "saved."
+            return "error: save failed"
+        except Exception as e:
+            return f"error: save failed ({e})"
+
+    async def _do_restore(self) -> str:
+        """Restore game state from disk. Returns status message."""
+        if not self.save_path.exists():
+            return ""
+        try:
+            save_data = self.save_path.read_bytes()
+            parser = QuetzalParser(self._zmachine)
+            parser.load_from_bytes(save_data)
+            # Flush stale intro text from screen buffer
+            self._screen.flush()
+            # Feed a blank line to continue execution from restored state
+            self._keyboard._waiting.clear()
+            self._keyboard.feed("")
+            # Wait for interpreter to process and generate output
+            loop = asyncio.get_running_loop()
+
+            def wait_for_output():
+                while not self._done and not self._keyboard._waiting.is_set():
+                    self._keyboard._waiting.wait(timeout=0.1)
+
+            await loop.run_in_executor(None, wait_for_output)
+            # Get the output from restored state
+            output = self._screen.flush()
+            return f"restored.\r\n{output}" if output else "restored."
+        except Exception as e:
+            logger.debug(f"Restore failed: {e}")
+            return ""
+
+    def get_location_name(self) -> str | None:
+        try:
+            location_obj = self._zmachine._mem.read_global(0)
+            if location_obj == 0:
+                return None
+            return self._zmachine._objectparser.get_shortname(location_obj)
+        except Exception:
+            return None
+
+    def get_room_objects(self) -> list[str]:
+        try:
+            location_obj = self._zmachine._mem.read_global(0)
+            if location_obj == 0:
+                return []
+
+            objects = []
+            child = self._zmachine._objectparser.get_child(location_obj)
+            while child != 0:
+                name = self._zmachine._objectparser.get_shortname(child)
+                objects.append(name)
+                child = self._zmachine._objectparser.get_sibling(child)
+            return objects
+        except Exception:
+            return []

src/mudlib/player.py

@@ -10,6 +10,7 @@ from mudlib.entity import Entity
 
 if TYPE_CHECKING:
     from mudlib.editor import Editor
+    from mudlib.embedded_if_session import EmbeddedIFSession
     from mudlib.if_session import IFSession
 
 
@@ -23,7 +24,7 @@ class Player(Entity):
     mode_stack: list[str] = field(default_factory=lambda: ["normal"])
     caps: ClientCaps = field(default_factory=lambda: ClientCaps(ansi=True))
     editor: Editor | None = None
-    if_session: IFSession | None = None
+    if_session: IFSession | EmbeddedIFSession | None = None
 
     @property
     def mode(self) -> str:

src/mudlib/zmachine/mud_ui.py

@@ -0,0 +1,150 @@
+import logging
+import queue
+import threading
+from pathlib import Path
+
+from . import zaudio, zfilesystem, zscreen, zstream, zui
+
+logger = logging.getLogger(__name__)
+
+
+class NullAudio(zaudio.ZAudio):
+    def __init__(self):
+        super().__init__()
+        self.features = {"has_more_than_a_bleep": False}
+
+    def play_bleep(self, bleep_type):
+        pass
+
+
+class MudScreen(zscreen.ZScreen):
+    def __init__(self):
+        super().__init__()
+        self._buffer = []
+        self._columns = 80
+        self._rows = zscreen.INFINITE_ROWS
+        self.features = {
+            "has_status_line": False,
+            "has_upper_window": False,
+            "has_graphics_font": False,
+            "has_text_colors": False,
+        }
+
+    def write(self, string):
+        self._buffer.append(string)
+
+    def flush(self) -> str:
+        result = "".join(self._buffer)
+        self._buffer.clear()
+        return result
+
+    def split_window(self, height):
+        logger.debug(f"split_window({height}) - no-op")
+
+    def select_window(self, window):
+        logger.debug(f"select_window({window}) - no-op")
+
+    def set_cursor_position(self, x, y):
+        logger.debug(f"set_cursor_position({x}, {y}) - no-op")
+
+    def erase_window(self, window=zscreen.WINDOW_LOWER, color=zscreen.COLOR_CURRENT):
+        logger.debug(f"erase_window({window}, {color}) - no-op")
+
+    def erase_line(self):
+        logger.debug("erase_line() - no-op")
+
+    def print_status_score_turns(self, text, score, turns):
+        pass
+
+    def print_status_time(self, hours, minutes):
+        pass
+
+    def set_font(self, font_number):
+        if font_number == zscreen.FONT_NORMAL:
+            return font_number
+        return None
+
+    def set_text_style(self, style):
+        pass
+
+    def set_text_color(self, foreground_color, background_color):
+        pass
+
+
+class MudInputStream(zstream.ZInputStream):
+    def __init__(self):
+        super().__init__()
+        self._input_queue = queue.Queue()
+        self._waiting = threading.Event()
+        self._ready = threading.Event()
+        self._done = False
+        self.features = {"has_timed_input": False}
+
+    def read_line(
+        self,
+        original_text=None,
+        max_length=0,
+        terminating_characters=None,
+        timed_input_routine=None,
+        timed_input_interval=0,
+    ):
+        self._waiting.set()
+        self._ready.wait()
+        self._ready.clear()
+        self._waiting.clear()
+        text = self._input_queue.get()
+        if max_length > 0:
+            text = text[:max_length]
+        return text
+
+    def read_char(self, timed_input_routine=None, timed_input_interval=0):
+        self._waiting.set()
+        self._ready.wait()
+        self._ready.clear()
+        self._waiting.clear()
+        text = self._input_queue.get()
+        if text:
+            return ord(text[0])
+        return 0
+
+    def feed(self, text: str):
+        self._input_queue.put(text)
+        self._ready.set()
+
+
+class MudFilesystem(zfilesystem.ZFilesystem):
+    def __init__(self, save_path: Path):
+        self.save_path = save_path
+
+    def save_game(self, data, suggested_filename=None):
+        try:
+            self.save_path.parent.mkdir(parents=True, exist_ok=True)
+            self.save_path.write_bytes(data)
+            return True
+        except Exception as e:
+            logger.error(f"Failed to save game: {e}")
+            return False
+
+    def restore_game(self):
+        if self.save_path.exists():
+            try:
+                return self.save_path.read_bytes()
+            except Exception as e:
+                logger.error(f"Failed to restore game: {e}")
+                return None
+        return None
+
+    def open_transcript_file_for_writing(self):
+        return None
+
+    def open_transcript_file_for_reading(self):
+        return None
+
+
+def create_mud_ui(save_path: Path) -> tuple[zui.ZUI, MudScreen, MudInputStream]:
+    audio = NullAudio()
+    screen = MudScreen()
+    keyboard = MudInputStream()
+    filesystem = MudFilesystem(save_path)
+    ui = zui.ZUI(audio, screen, keyboard, filesystem)
+    return ui, screen, keyboard

src/mudlib/zmachine/quetzal.py

@@ -216,7 +216,7 @@ class QuetzalParser:
 
             # read anywhere from 0 to 15 local vars
             local_vars = []
-            for _i in range(flags_bitfield[0:3]):
+            for _i in range(flags_bitfield[0:4]):
                 var = (bytes[ptr] << 8) + bytes[ptr + 1]
                 ptr += 2
                 local_vars.append(var)