gasm_host/

host.rs

//! The gasm ABI v0 host: instantiates a guest and implements the "gasm",
//! "gasm:gfx", "gasm:net" and "gasm:storage" imports, plus the WASI subset in
//! [`crate::wasi`].

use std::collections::{HashMap, HashSet};
use std::future::Future;
use std::pin::Pin;
use std::sync::{Arc, OnceLock};
use std::task::Poll;
use std::time::{Duration, Instant};

use wasmtime::{Caller, Config, Engine, Instance, Linker, Memory, Module, Store, TypedFunc, bail, format_err};

use crate::assets::Assets;
use crate::audio::AudioOut;
use crate::gfx::Gfx;
use crate::net::Net;
use crate::present::letterbox;
use crate::storage::Storage;
use crate::switching::{self, Cmd, Exchange, Job};
use crate::wasi::{self, Splitmix};

pub const ABI_VERSION: i32 = 0;

/// FNV-1a 32-bit; mirrored in runners/web/gasm-host.js for cross-runner checks.
#[derive(Clone, Copy)]
pub struct Fnv(pub u32);

impl Default for Fnv {
    fn default() -> Self {
        Fnv(0x811c_9dc5)
    }
}

impl Fnv {
    pub fn new() -> Self {
        Self::default()
    }
    pub fn update(&mut self, bytes: &[u8]) {
        let mut h = self.0;
        for &b in bytes {
            h ^= b as u32;
            h = h.wrapping_mul(0x0100_0193);
        }
        self.0 = h;
    }
}

/// Headless virtual time: derived from the frame number, monotonic when the
/// guest changes its frame rate. Same arithmetic in gasm-host.js
/// (`VirtualClock`) and the `gasm::native` stub.
#[derive(Clone, Copy)]
pub struct VirtualClock {
    base_ms: f64,
    base_frame: u64,
    rate: f64,
}

impl Default for VirtualClock {
    fn default() -> Self {
        VirtualClock { base_ms: 0.0, base_frame: 0, rate: 60.0 }
    }
}

impl VirtualClock {
    /// Time at the start of `frame` (frames are counted from 0) at frame rate `rate`.
    pub fn at(&mut self, frame: u64, rate: f64) -> f64 {
        if rate != self.rate {
            self.base_ms += (frame - self.base_frame) as f64 * 1000.0 / self.rate;
            self.base_frame = frame;
            self.rate = rate;
        }
        self.base_ms + (frame - self.base_frame) as f64 * 1000.0 / self.rate
    }
}

/// Raw input for one frame, set by the runner before each `gasm_frame`.
/// `None` fields mean the runner has no such device (the import returns -1).
#[derive(Default, Clone)]
pub struct RawInput {
    /// held keys, bit k%8 of byte k/8 = GASM_KEY code k
    pub keys: Option<[u8; KEY_STATE_BYTES]>,
    /// (code, down) since the previous frame, in order
    pub key_events: Vec<(u16, bool)>,
    pub pointer: Option<Pointer>,
    pub gamepads: Option<[Gamepad; 4]>,
}

pub const KEY_STATE_BYTES: usize = 32;
pub const POINTER_BYTES: usize = 48;
pub const GAMEPAD_BYTES: usize = 204;
pub const GAMEPAD_BUTTONS: usize = 32;
pub const GAMEPAD_AXES: usize = 16;

#[derive(Default, Clone, Copy)]
pub struct Pointer {
    /// position in drawable pixels; the frame position is derived by the host
    pub x: f32,
    pub y: f32,
    pub dx: f32,
    pub dy: f32,
    pub wheel_x: f32,
    pub wheel_y: f32,
    pub buttons: u32,
    pub pressed: u32,
    pub released: u32,
    /// GASM_POINTER_INSIDE | IS_HIDDEN | IS_LOCKED
    pub flags: u32,
    /// drawable size the position refers to
    pub drawable: (f32, f32),
    /// the runner shows 2D frames at whole multiples (`--integer-scale`)
    pub integer_scale: bool,
}

#[derive(Default, Clone)]
pub struct Gamepad {
    pub connected: bool,
    pub standard: bool,
    pub buttons: Vec<f32>,
    pub axes: Vec<f32>,
    pub name: String,
}

/// Map a drawable position into the last video_present frame (letterboxed as the
/// runners display it, `present::letterbox`). Same arithmetic in runners/web/lib/input.js.
pub fn frame_position(x: f32, y: f32, drawable: (f32, f32), frame: (usize, usize), integer_scale: bool, aspect: Option<(u32, u32)>) -> (f32, f32) {
    if frame.0 == 0 || frame.1 == 0 || drawable.0 <= 0.0 || drawable.1 <= 0.0 {
        return (x, y);
    }
    let (ox, oy, sx, sy) = letterbox((drawable.0 as f64, drawable.1 as f64), (frame.0 as f64, frame.1 as f64), integer_scale, aspect);
    (((x as f64 - ox) / sx) as f32, ((y as f64 - oy) / sy) as f32)
}

/// Longest title `set_title` keeps, in UTF-8 bytes.
pub const TITLE_MAX_BYTES: usize = 256;

/// A guest's `set_title` text as runners show it: control characters (Unicode
/// Cc) and bidi controls (U+202A–U+202E, U+2066–U+2069) removed, then cut to
/// 256 bytes at a character boundary. None: empty, the runner's default.
/// Same rules in runners/web/lib/host.js `cleanTitle`.
pub fn clean_title(text: &str) -> Option<String> {
    let mut out = String::new();
    for c in text.chars().filter(|&c| !c.is_control() && !matches!(c, '\u{202a}'..='\u{202e}' | '\u{2066}'..='\u{2069}')) {
        if out.len() + c.len_utf8() > TITLE_MAX_BYTES {
            break;
        }
        out.push(c);
    }
    (!out.is_empty()).then_some(out)
}

/// The payload of custom section `name` in a wasm binary (None for a .cwasm or
/// a malformed module: the loader reports those).
pub fn custom_section<'a>(wasm: &'a [u8], name: &str) -> Option<&'a [u8]> {
    fn leb(b: &[u8], at: &mut usize) -> Option<usize> {
        let (mut v, mut shift) = (0usize, 0);
        loop {
            let byte = *b.get(*at)?;
            *at += 1;
            v |= ((byte & 0x7f) as usize).checked_shl(shift)?;
            if byte & 0x80 == 0 {
                return Some(v);
            }
            shift += 7;
            if shift > 28 {
                return None;
            }
        }
    }
    if wasm.get(..4)? != b"\0asm" {
        return None;
    }
    let mut at = 8;
    while at < wasm.len() {
        let id = wasm[at];
        at += 1;
        let size = leb(wasm, &mut at)?;
        let body = wasm.get(at..at.checked_add(size)?)?;
        at += size;
        if id == 0 {
            let mut p = 0;
            let n = leb(body, &mut p)?;
            if body.get(p..p.checked_add(n)?)? == name.as_bytes() {
                return body.get(p + n..);
            }
        }
    }
    None
}

/// The guest's built-in title (custom section `gasm.title`, UTF-8), cleaned like
/// set_title: the default title before (or without) set_title.
pub fn static_title(wasm: &[u8]) -> Option<String> {
    clean_title(std::str::from_utf8(custom_section(wasm, "gasm.title")?).ok()?)
}

pub struct Host {
    memory: Option<Memory>,
    start: Instant,
    /// Headless runs use frame-derived time so results are reproducible (also
    /// the WASI clocks).
    pub virtual_time_ms: Option<f64>,
    /// Headless runs: a fixed `random_get` sequence; None = OS randomness.
    pub random: Option<Splitmix>,
    pub frame_rate: f64,
    pub pads: [u32; 4],
    /// UTF-8 typed since the previous frame (set by the runner before each frame);
    /// None: no keyboard, text_input returns -1
    pub text: Option<String>,
    pub input: RawInput,
    /// GASM_INPUT_* flags requested by the guest (input_mode)
    pub input_mode: u32,
    pub assets: Assets,
    pub params: HashMap<String, String>,
    pub audio: Option<Box<dyn AudioOut>>,
    pub gfx: Gfx,
    /// gasm:gl state (the null GL: natively, gl guests run headless)
    pub gl: crate::gl::Gl,
    /// frames run so far (gl queries and syncs are ready from the next frame on)
    pub frame_index: u64,
    pub net: Net,
    pub storage: Storage,
    /// false during catch-up frames: gfx begin_frame returns 0
    pub show_frame: bool,
    audio_rate: u32,
    audio_channels: u32,
    /// imports this runner provides ("module" and "module.function"), for `has`
    provided: HashSet<String>,
    /// Last presented frame as RGBA8, tightly packed.
    pub rgba: Vec<u8>,
    pub width: usize,
    pub height: usize,
    pub frames_presented: u64,
    /// video_set_aspect (num, den); None: square pixels
    pub aspect: Option<(u32, u32)>,
    /// gasm_run guests: what the runner asks of the suspended guest (yield_frame)
    pub(crate) run: Option<Arc<Exchange>>,
    /// set_title (cleaned); None: the runner's default
    pub title: Option<String>,
    /// set_title changed `title` since the runner last looked (it clears this)
    pub title_changed: bool,
    pub hashing: bool,
    pub video_hash: Fnv,
    pub audio_hash: Fnv,
    pub audio_frames: u64,
}

impl Host {
    pub fn new(
        assets: impl Into<Assets>,
        params: HashMap<String, String>,
        audio: Option<Box<dyn AudioOut>>,
        gfx: Gfx,
        net: Net,
        storage: Storage,
    ) -> Self {
        Host {
            memory: None,
            start: Instant::now(),
            virtual_time_ms: None,
            random: None,
            frame_rate: 60.0,
            pads: [0; 4],
            text: None,
            input: RawInput::default(),
            input_mode: 0,
            assets: assets.into(),
            params,
            audio,
            gfx,
            gl: Default::default(),
            frame_index: 0,
            net,
            storage,
            show_frame: true,
            audio_rate: 44100,
            audio_channels: 2,
            provided: HashSet::new(),
            title: None,
            title_changed: false,
            aspect: None,
            run: None,
            rgba: Vec::new(),
            width: 0,
            height: 0,
            frames_presented: 0,
            hashing: true,
            video_hash: Fnv::new(),
            audio_hash: Fnv::new(),
            audio_frames: 0,
        }
    }

    /// Reproducible mode for headless runs: virtual time (starting at 0) and a
    /// fixed random sequence.
    pub fn set_reproducible(&mut self) {
        self.virtual_time_ms = Some(0.0);
        self.random = Some(Splitmix(0));
    }

    pub(crate) fn started(&self) -> Instant {
        self.start
    }
}

/// Borrow `len` bytes at guest offset `ptr`, trapping the guest if out of bounds.
pub(crate) fn guest_slice(mem: &[u8], ptr: u32, len: u64) -> wasmtime::Result<&[u8]> {
    let start = ptr as u64;
    let end = start.checked_add(len).ok_or_else(|| format_err!("guest pointer overflow"))?;
    if end > mem.len() as u64 {
        bail!("guest access out of bounds: {start:#x}..{end:#x} (memory is {:#x})", mem.len());
    }
    Ok(&mem[start as usize..end as usize])
}

/// Mutable [`guest_slice`].
pub(crate) fn guest_slice_mut(mem: &mut [u8], ptr: u32, len: u64) -> wasmtime::Result<&mut [u8]> {
    let start = ptr as u64;
    let end = start.checked_add(len).ok_or_else(|| format_err!("guest pointer overflow"))?;
    if end > mem.len() as u64 {
        bail!("guest access out of bounds: {start:#x}..{end:#x} (memory is {:#x})", mem.len());
    }
    Ok(&mut mem[start as usize..end as usize])
}

/// The "copied only if it fits" convention: copy `src` to `dst` if `src.len() <= cap`;
/// returns the full length either way.
fn copy_if_fits(mem: &mut [u8], dst: u32, cap: u32, src: &[u8]) -> wasmtime::Result<i32> {
    if src.len() <= cap as usize {
        guest_slice_mut(mem, dst, src.len() as u64)?.copy_from_slice(src);
    }
    Ok(src.len() as i32)
}

pub(crate) fn memory(caller: &Caller<'_, Host>) -> wasmtime::Result<Memory> {
    caller.data().memory.ok_or_else(|| format_err!("guest memory not yet available"))
}

fn guest_str(caller: &Caller<'_, Host>, ptr: u32, len: u32) -> wasmtime::Result<String> {
    let mem = memory(caller)?;
    let bytes = guest_slice(mem.data(caller), ptr, len as u64)?;
    std::str::from_utf8(bytes).map(str::to_owned).map_err(|e| format_err!("string argument is not UTF-8: {e}"))
}

/// The pointer() record (GASM_POINTER_OFF_* layout).
fn pointer_bytes(p: &Pointer, frame: (usize, usize), aspect: Option<(u32, u32)>) -> [u8; POINTER_BYTES] {
    let (fx, fy) = frame_position(p.x, p.y, p.drawable, frame, p.integer_scale, aspect);
    let mut b = [0u8; POINTER_BYTES];
    for (i, v) in [p.x, p.y, fx, fy, p.dx, p.dy, p.wheel_x, p.wheel_y].into_iter().enumerate() {
        b[i * 4..i * 4 + 4].copy_from_slice(&v.to_le_bytes());
    }
    for (i, v) in [p.buttons, p.pressed, p.released, p.flags].into_iter().enumerate() {
        b[32 + i * 4..36 + i * 4].copy_from_slice(&v.to_le_bytes());
    }
    b
}

/// The gamepad() record (GASM_GAMEPAD_OFF_* layout).
fn gamepad_bytes(g: &Gamepad) -> [u8; GAMEPAD_BYTES] {
    let mut b = [0u8; GAMEPAD_BYTES];
    let nb = g.buttons.len().min(GAMEPAD_BUTTONS);
    let na = g.axes.len().min(GAMEPAD_AXES);
    let flags = if g.connected { 1 | if g.standard { 2 } else { 0 } } else { 0u32 };
    for (i, v) in [flags, nb as u32, na as u32].into_iter().enumerate() {
        b[i * 4..i * 4 + 4].copy_from_slice(&v.to_le_bytes());
    }
    for (i, v) in g.buttons.iter().take(nb).enumerate() {
        b[12 + i * 4..16 + i * 4].copy_from_slice(&v.to_le_bytes());
    }
    for (i, v) in g.axes.iter().take(na).enumerate() {
        b[140 + i * 4..144 + i * 4].copy_from_slice(&v.to_le_bytes());
    }
    b
}

fn add_gasm_imports(linker: &mut Linker<Host>) -> wasmtime::Result<()> {
    linker.func_wrap("gasm", "log", |caller: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<()> {
        let mem = memory(&caller)?;
        let msg = guest_slice(mem.data(&caller), ptr, len as u64)?;
        eprintln!("[guest] {}", String::from_utf8_lossy(msg));
        Ok(())
    })?;

    linker.func_wrap("gasm", "has", |caller: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<i32> {
        let name = guest_str(&caller, ptr, len)?;
        Ok(caller.data().provided.contains(&name) as i32)
    })?;

    linker.func_wrap("gasm", "video_set_aspect", |mut caller: Caller<'_, Host>, num: u32, den: u32| -> wasmtime::Result<()> {
        let aspect = match (num, den) {
            (0, 0) => None,
            (1..=65535, 1..=65535) if num as u64 * 8 >= den as u64 && den as u64 * 8 >= num as u64 => Some((num, den)),
            _ => bail!("video_set_aspect: invalid ratio {num}:{den}"),
        };
        caller.data_mut().aspect = aspect;
        Ok(())
    })?;

    linker.func_wrap("gasm", "set_title", |mut caller: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<()> {
        let title = clean_title(&guest_str(&caller, ptr, len)?);
        let h = caller.data_mut();
        if h.title != title {
            h.title = title;
            h.title_changed = true;
        }
        Ok(())
    })?;

    linker.func_wrap("gasm", "time_ms", |caller: Caller<'_, Host>| -> f64 {
        let h = caller.data();
        h.virtual_time_ms.unwrap_or_else(|| h.start.elapsed().as_secs_f64() * 1000.0)
    })?;

    linker.func_wrap("gasm", "set_frame_rate", |mut caller: Caller<'_, Host>, hz: f64| {
        if hz.is_finite() && (1.0..=1000.0).contains(&hz) {
            caller.data_mut().frame_rate = hz;
        }
    })?;

    linker.func_wrap(
        "gasm",
        "video_present",
        |mut caller: Caller<'_, Host>, ptr: u32, w: u32, h: u32, stride: u32| -> wasmtime::Result<()> {
            let (w, h, stride) = (w as usize, h as usize, stride as usize);
            if w == 0 || h == 0 || w > 4096 || h > 4096 || stride < w * 4 {
                bail!("video_present: bad geometry {w}x{h} stride {stride}");
            }
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let src = guest_slice(data, ptr, (stride * (h - 1) + w * 4) as u64)?;
            let row = w * 4;
            host.rgba.resize(row * h, 0);
            if stride == row {
                host.rgba.copy_from_slice(src);
            } else {
                for (y, dst) in host.rgba.chunks_exact_mut(row).enumerate() {
                    dst.copy_from_slice(&src[y * stride..y * stride + row]);
                }
            }
            if host.hashing {
                host.video_hash.update(&host.rgba);
            }
            host.width = w;
            host.height = h;
            host.frames_presented += 1;
            Ok(())
        },
    )?;

    linker.func_wrap("gasm", "audio_config", |mut caller: Caller<'_, Host>, rate: u32, channels: u32| {
        let h = caller.data_mut();
        if (8000..=192_000).contains(&rate) && (channels == 1 || channels == 2) {
            h.audio_rate = rate;
            h.audio_channels = channels;
            if let Some(a) = &mut h.audio {
                a.configure(rate, channels);
            }
        }
    })?;

    linker.func_wrap(
        "gasm",
        "audio_push",
        |mut caller: Caller<'_, Host>, ptr: u32, frames: u32| -> wasmtime::Result<()> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let len = frames as u64 * host.audio_channels as u64 * 4;
            let bytes = guest_slice(data, ptr, len)?;
            if host.hashing {
                host.audio_hash.update(bytes);
            }
            host.audio_frames += frames as u64;
            if let Some(a) = &mut host.audio {
                a.push_le_f32(bytes);
            }
            Ok(())
        },
    )?;

    linker.func_wrap("gasm", "input_pad", |caller: Caller<'_, Host>, player: u32| -> u32 {
        caller.data().pads.get(player as usize).copied().unwrap_or(0)
    })?;

    linker.func_wrap(
        "gasm",
        "text_input",
        |mut caller: Caller<'_, Host>, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(t) = &host.text else { return Ok(-1) };
            copy_if_fits(data, dst, cap, t.as_bytes())
        },
    )?;

    linker.func_wrap("gasm", "input_mode", |mut caller: Caller<'_, Host>, flags: u32| {
        caller.data_mut().input_mode = flags & 7;
    })?;

    linker.func_wrap(
        "gasm",
        "key_state",
        |mut caller: Caller<'_, Host>, dst: u32, len: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(keys) = host.input.keys else { return Ok(-1) };
            let n = (len as usize).min(KEY_STATE_BYTES);
            guest_slice_mut(data, dst, n as u64)?.copy_from_slice(&keys[..n]);
            Ok(KEY_STATE_BYTES as i32)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "key_events",
        |mut caller: Caller<'_, Host>, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            if host.input.keys.is_none() {
                return Ok(-1);
            }
            let events = &host.input.key_events;
            let len = events.len() * 4;
            if len <= cap as usize {
                let out = guest_slice_mut(data, dst, len as u64)?;
                for (b, &(c, d)) in out.chunks_exact_mut(4).zip(events) {
                    let c = c.to_le_bytes();
                    b.copy_from_slice(&[c[0], c[1], d as u8, 0]);
                }
            }
            Ok(len as i32)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "pointer",
        |mut caller: Caller<'_, Host>, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(p) = &host.input.pointer else { return Ok(-1) };
            if cap as usize >= POINTER_BYTES {
                let b = pointer_bytes(p, (host.width, host.height), host.aspect);
                guest_slice_mut(data, dst, POINTER_BYTES as u64)?.copy_from_slice(&b);
            }
            Ok(POINTER_BYTES as i32)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "gamepad",
        |mut caller: Caller<'_, Host>, slot: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let (Some(pads), true) = (&host.input.gamepads, slot < 4) else { return Ok(-1) };
            if cap as usize >= GAMEPAD_BYTES {
                let b = gamepad_bytes(&pads[slot as usize]);
                guest_slice_mut(data, dst, GAMEPAD_BYTES as u64)?.copy_from_slice(&b);
            }
            Ok(GAMEPAD_BYTES as i32)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "gamepad_name",
        |mut caller: Caller<'_, Host>, slot: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(g) = host.input.gamepads.as_ref().and_then(|p| p.get(slot as usize)).filter(|g| g.connected) else { return Ok(-1) };
            copy_if_fits(data, dst, cap, g.name.as_bytes())
        },
    )?;

    linker.func_wrap(
        "gasm",
        "param",
        |mut caller: Caller<'_, Host>, ptr: u32, len: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let name = guest_str(&caller, ptr, len)?;
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(v) = host.params.get(&name) else { return Ok(-1) };
            copy_if_fits(data, dst, cap, v.as_bytes())
        },
    )?;

    // Assets: n = bytes available from offset (clamped to the request) is bounds-checked
    // against guest memory before anything is read.
    fn read_asset(caller: &mut Caller<'_, Host>, ptr: u32, len: u32, offset: u64, dst: u32, cap: u32) -> wasmtime::Result<i32> {
        let name = guest_str(caller, ptr, len)?;
        let mem = memory(caller)?;
        let (data, host) = mem.data_and_store_mut(caller);
        let Some(asset) = host.assets.get(&name) else { return Ok(-1) };
        let n = asset.size().saturating_sub(offset).min(cap as u64);
        let out = guest_slice_mut(data, dst, n)?;
        Ok(asset.read_at(offset, out) as i32)
    }

    linker.func_wrap(
        "gasm",
        "asset_read_at",
        |mut caller: Caller<'_, Host>, ptr: u32, len: u32, offset: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            read_asset(&mut caller, ptr, len, offset as u64, dst, cap)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "asset_read_at64",
        |mut caller: Caller<'_, Host>, ptr: u32, len: u32, offset: u64, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            read_asset(&mut caller, ptr, len, offset, dst, cap)
        },
    )?;

    linker.func_wrap(
        "gasm",
        "asset_read",
        |mut caller: Caller<'_, Host>, ptr: u32, len: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            read_asset(&mut caller, ptr, len, 0, dst, cap)
        },
    )?;

    linker.func_wrap("gasm", "asset_count", |caller: Caller<'_, Host>| -> u32 { caller.data().assets.names().len() as u32 })?;

    linker.func_wrap(
        "gasm",
        "asset_name",
        |mut caller: Caller<'_, Host>, index: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
            let mem = memory(&caller)?;
            let (data, host) = mem.data_and_store_mut(&mut caller);
            let Some(n) = host.assets.names().get(index as usize) else { return Ok(-1) };
            copy_if_fits(data, dst, cap, n.as_bytes())
        },
    )?;

    linker.func_wrap(
        "gasm",
        "asset_size",
        |caller: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<i32> {
            let name = guest_str(&caller, ptr, len)?;
            Ok(match caller.data().assets.size(&name) {
                None => -1,
                Some(n) if n > i32::MAX as u64 => -2,
                Some(n) => n as i32,
            })
        },
    )?;

    linker.func_wrap(
        "gasm",
        "asset_size64",
        |caller: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<i64> {
            let name = guest_str(&caller, ptr, len)?;
            Ok(caller.data().assets.size(&name).map_or(-1, |n| n as i64))
        },
    )?;
    Ok(())
}

fn trap(e: String) -> wasmtime::Error {
    format_err!("{e}")
}

fn add_gfx_imports(linker: &mut Linker<Host>) -> wasmtime::Result<()> {
    const M: &str = "gasm:gfx";
    linker.func_wrap(M, "width", |c: Caller<'_, Host>| -> u32 { c.data().gfx.size().0 })?;
    linker.func_wrap(M, "height", |c: Caller<'_, Host>| -> u32 { c.data().gfx.size().1 })?;
    linker.func_wrap(M, "create_shader", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let src = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_shader(&src).map_err(trap)
    })?;
    linker.func_wrap(M, "create_buffer", |mut c: Caller<'_, Host>, size: u32, usage: u32| -> wasmtime::Result<u32> {
        c.data_mut().gfx.create_buffer(size, usage).map_err(trap)
    })?;
    linker.func_wrap(M, "create_pipeline", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let json = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_pipeline(&json).map_err(trap)
    })?;
    linker.func_wrap(M, "create_bind_group", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let json = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_bind_group(&json).map_err(trap)
    })?;
    linker.func_wrap(
        M,
        "write_buffer",
        |mut c: Caller<'_, Host>, buf: u32, offset: u32, ptr: u32, len: u32| -> wasmtime::Result<()> {
            let mem = memory(&c)?;
            let (data, host) = mem.data_and_store_mut(&mut c);
            let bytes = guest_slice(data, ptr, len as u64)?;
            host.gfx.write_buffer(buf, offset, bytes).map_err(trap)?;
            if host.hashing {
                host.video_hash.update(bytes);
            }
            Ok(())
        },
    )?;
    linker.func_wrap(M, "create_bind_group_layout", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let json = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_bind_group_layout(&json).map_err(trap)
    })?;
    linker.func_wrap(M, "create_texture", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let json = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_texture(&json).map_err(trap)
    })?;
    linker.func_wrap(M, "create_sampler", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<u32> {
        let json = guest_str(&c, ptr, len)?;
        c.data_mut().gfx.create_sampler(&json).map_err(trap)
    })?;
    linker.func_wrap(
        M,
        "write_texture",
        |mut c: Caller<'_, Host>, tex: u32, mip: u32, x: u32, y: u32, w: u32, h: u32, ptr: u32, len: u32| -> wasmtime::Result<()> {
            let mem = memory(&c)?;
            let (data, host) = mem.data_and_store_mut(&mut c);
            let bytes = guest_slice(data, ptr, len as u64)?;
            host.gfx.write_texture(tex, mip, x, y, w, h, bytes).map_err(trap)?;
            if host.hashing {
                // header (little-endian u32s) + payload, so the target region counts too
                for v in [tex, mip, x, y, w, h] {
                    host.video_hash.update(&v.to_le_bytes());
                }
                host.video_hash.update(bytes);
            }
            Ok(())
        },
    )?;
    linker.func_wrap(M, "begin_frame", |mut c: Caller<'_, Host>, r: f32, g: f32, b: f32, a: f32| -> wasmtime::Result<u32> {
        let h = c.data_mut();
        let show = h.show_frame;
        h.gfx.begin_frame([r, g, b, a], show).map(u32::from).map_err(trap)
    })?;
    linker.func_wrap(M, "set_pipeline", |mut c: Caller<'_, Host>, p: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.set_pipeline(p).map_err(trap)
    })?;
    linker.func_wrap(M, "set_bind_group", |mut c: Caller<'_, Host>, i: u32, bg: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.set_bind_group(i, bg).map_err(trap)
    })?;
    linker.func_wrap(
        M,
        "set_bind_group_offsets",
        |mut c: Caller<'_, Host>, i: u32, bg: u32, ptr: u32, count: u32| -> wasmtime::Result<()> {
            let mem = memory(&c)?;
            let (data, host) = mem.data_and_store_mut(&mut c);
            let bytes = guest_slice(data, ptr, count as u64 * 4)?;
            let mut offsets = [0u32; 32];
            if count as usize > offsets.len() {
                bail!("gfx.set_bind_group_offsets: {count} offsets (a bind group has at most {})", offsets.len());
            }
            for (o, b) in offsets.iter_mut().zip(bytes.chunks_exact(4)) {
                *o = u32::from_le_bytes([b[0], b[1], b[2], b[3]]);
            }
            host.gfx.set_bind_group_offsets(i, bg, &offsets[..count as usize]).map_err(trap)
        },
    )?;
    linker.func_wrap(
        M,
        "set_viewport",
        |mut c: Caller<'_, Host>, x: f32, y: f32, w: f32, h: f32, min: f32, max: f32| -> wasmtime::Result<()> {
            c.data_mut().gfx.set_viewport(x, y, w, h, min, max).map_err(trap)
        },
    )?;
    linker.func_wrap(M, "set_scissor_rect", |mut c: Caller<'_, Host>, x: u32, y: u32, w: u32, h: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.set_scissor_rect(x, y, w, h).map_err(trap)
    })?;
    linker.func_wrap(M, "set_vertex_buffer", |mut c: Caller<'_, Host>, slot: u32, b: u32, off: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.set_vertex_buffer(slot, b, off).map_err(trap)
    })?;
    linker.func_wrap(M, "set_index_buffer", |mut c: Caller<'_, Host>, b: u32, fmt: u32, off: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.set_index_buffer(b, fmt, off).map_err(trap)
    })?;
    linker.func_wrap(M, "draw", |mut c: Caller<'_, Host>, vc: u32, ic: u32, fv: u32, fi: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.draw(vc, ic, fv, fi).map_err(trap)
    })?;
    linker.func_wrap(M, "draw_indexed", |mut c: Caller<'_, Host>, ic: u32, n: u32, first: u32, base: i32, fi: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.draw_indexed(ic, n, first, base, fi).map_err(trap)
    })?;
    linker.func_wrap(M, "end_frame", |mut c: Caller<'_, Host>| -> wasmtime::Result<()> {
        let h = c.data_mut();
        h.frames_presented += 1;
        h.gfx.end_frame().map_err(trap)
    })?;
    linker.func_wrap(M, "destroy", |mut c: Caller<'_, Host>, handle: u32| -> wasmtime::Result<()> {
        c.data_mut().gfx.destroy(handle).map_err(trap)
    })?;
    Ok(())
}

fn add_net_imports(linker: &mut Linker<Host>) -> wasmtime::Result<()> {
    const M: &str = "gasm:net";
    linker.func_wrap(M, "open", |mut c: Caller<'_, Host>, ptr: u32, len: u32| -> wasmtime::Result<i32> {
        let url = guest_str(&c, ptr, len)?;
        Ok(c.data_mut().net.open(&url))
    })?;
    linker.func_wrap(M, "state", |c: Caller<'_, Host>, h: i32| -> wasmtime::Result<u32> { c.data().net.state(h).map_err(trap) })?;
    linker.func_wrap(M, "send", |mut c: Caller<'_, Host>, h: i32, ptr: u32, len: u32| -> wasmtime::Result<i32> {
        let mem = memory(&c)?;
        let msg = guest_slice(mem.data(&c), ptr, len as u64)?.to_vec();
        if msg.is_empty() {
            c.data().net.state(h).map_err(trap)?; // still validates the handle
            return Ok(-1);
        }
        c.data_mut().net.send(h, msg).map_err(trap)
    })?;
    linker.func_wrap(M, "recv", |mut c: Caller<'_, Host>, h: i32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
        let mem = memory(&c)?;
        let (data, host) = mem.data_and_store_mut(&mut c);
        let msg = match host.net.peek(h).map_err(trap)? {
            Ok(m) => m,
            Err(code) => return Ok(code),
        };
        let n = copy_if_fits(data, dst, cap, msg)?;
        if n as usize <= cap as usize {
            host.net.pop(h);
        }
        Ok(n)
    })?;
    linker.func_wrap(M, "close", |mut c: Caller<'_, Host>, h: i32| -> wasmtime::Result<()> { c.data_mut().net.close(h).map_err(trap) })?;
    Ok(())
}

fn add_storage_imports(linker: &mut Linker<Host>) -> wasmtime::Result<()> {
    const M: &str = "gasm:storage";
    linker.func_wrap(M, "get", |mut c: Caller<'_, Host>, kp: u32, kl: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
        let key = guest_str(&c, kp, kl)?;
        let mem = memory(&c)?;
        let (data, host) = mem.data_and_store_mut(&mut c);
        let Some(v) = host.storage.get(&key) else { return Ok(-1) };
        copy_if_fits(data, dst, cap, v)
    })?;
    linker.func_wrap(M, "set", |mut c: Caller<'_, Host>, kp: u32, kl: u32, vp: u32, vl: u32| -> wasmtime::Result<i32> {
        let key = guest_str(&c, kp, kl)?;
        let mem = memory(&c)?;
        let (data, host) = mem.data_and_store_mut(&mut c);
        let value = guest_slice(data, vp, vl as u64)?;
        Ok(match host.storage.set(&key, value) {
            Ok(()) => 0,
            Err(e) => {
                eprintln!("[gasm] storage: {key}: {e}");
                e.code()
            }
        })
    })?;
    linker.func_wrap(M, "count", |mut c: Caller<'_, Host>| -> u32 { c.data_mut().storage.keys().len() as u32 })?;
    linker.func_wrap(M, "key", |mut c: Caller<'_, Host>, index: u32, dst: u32, cap: u32| -> wasmtime::Result<i32> {
        let mem = memory(&c)?;
        let (data, host) = mem.data_and_store_mut(&mut c);
        let Some(k) = host.storage.keys().get(index as usize) else { return Ok(-1) };
        copy_if_fits(data, dst, cap, k.as_bytes())
    })?;
    linker.func_wrap(M, "delete", |mut c: Caller<'_, Host>, kp: u32, kl: u32| -> wasmtime::Result<i32> {
        let key = guest_str(&c, kp, kl)?;
        Ok(if c.data_mut().storage.delete(&key) { 0 } else { -1 })
    })?;
    Ok(())
}

/// Why a guest call ended.
#[derive(Debug)]
pub enum Stop {
    /// The guest called proc_exit(code).
    Exit(i32),
    Trap(String),
}

impl std::fmt::Display for Stop {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Stop::Exit(code) => write!(f, "guest exited with code {code}"),
            Stop::Trap(msg) => write!(f, "guest trapped: {msg}"),
        }
    }
}

impl std::error::Error for Stop {}

fn classify(e: wasmtime::Error) -> Stop {
    if let Some(exit) = e.downcast_ref::<wasi::Exit>() {
        return Stop::Exit(exit.0);
    }
    if e.downcast_ref::<switching::Quit>().is_some() {
        return Stop::Exit(0);
    }
    if e.downcast_ref::<wasmtime::Trap>() == Some(&wasmtime::Trap::Interrupt) {
        return Stop::Trap(format!("a guest call ran longer than the time limit (stuck in a loop?); see --call-timeout\n{e:?}"));
    }
    Stop::Trap(format!("{e:?}"))
}

/// Epoch ticks per second (the watchdog thread's rate).
const TICKS_PER_SEC: u64 = 10;

/// The process-wide engine: the same configuration for compiling, precompiling
/// and loading (`.cwasm` files must match it). Epoch interruption lets a guest
/// call that never returns be stopped (`LoadOptions::call_timeout`). Calls can be
/// sync (`gasm_frame` guests) or async (`gasm_run` guests: stack switching).
pub fn engine() -> &'static Engine {
    static ENGINE: OnceLock<Engine> = OnceLock::new();
    ENGINE.get_or_init(new_engine)
}

fn new_engine() -> Engine {
    let mut config = Config::new();
    config.epoch_interruption(true);
    // a gasm_run guest's whole run is on this stack: the default wasm stack limit plus room
    config.async_stack_size(4 << 20);
    let engine = Engine::new(&config).expect("wasmtime engine");
    let ticker = engine.clone();
    std::thread::Builder::new()
        .name("gasm-epoch".into())
        .spawn(move || {
            loop {
                std::thread::sleep(Duration::from_millis(1000 / TICKS_PER_SEC));
                ticker.increment_epoch();
            }
        })
        .expect("epoch thread");
    engine
}

/// Ahead-of-time compile a guest to native code for this host (no JIT needed at load).
pub fn precompile(wasm: &[u8]) -> wasmtime::Result<Vec<u8>> {
    engine().precompile_module(wasm)
}

/// How to load a guest.
#[derive(Clone, Copy)]
pub struct LoadOptions {
    /// Accept a precompiled `.cwasm` (native code: only files you made yourself
    /// with `gasm-run --compile`). Otherwise only `.wasm` modules load.
    pub allow_precompiled: bool,
    /// Trap a single guest call (init, a frame, exit) that runs longer than this.
    pub call_timeout: Option<Duration>,
    /// Run guests that export `gasm_run` that way (the runner switches stacks).
    /// Off: always `gasm_frame` (tests of the Asyncify path).
    pub stack_switching: bool,
}

impl Default for LoadOptions {
    fn default() -> Self {
        LoadOptions { allow_precompiled: false, call_timeout: Some(Duration::from_secs(30)), stack_switching: true }
    }
}

/// The running `gasm_run` call: owns the store until the run ends.
type Running = Pin<Box<dyn Future<Output = (Store<Host>, wasmtime::Result<i32>)>>>;

/// A running guest. `gasm_frame` guests are called once per frame; `gasm_run`
/// guests run in one suspended call (stack switching) that each [`Game::frame`]
/// resumes until its next `yield_frame`.
pub struct Game {
    /// None while a `gasm_run` call is running (the call owns it): use [`Game::with_host`]
    store: Option<Store<Host>>,
    frame: TypedFunc<(), ()>,
    exit: Option<TypedFunc<(), ()>>,
    deadline: u64,
    /// `gasm_run` (async engine)
    run: Option<TypedFunc<(), i32>>,
    running: Option<Running>,
    exchange: Option<Arc<Exchange>>,
    /// the guest exited or trapped: it is never called again
    ended: bool,
    /// the guest imports gasm:gl (the native window has no GL backend yet)
    gl: bool,
}

impl Game {
    pub fn load(wasm: &[u8], host: Host, opts: LoadOptions) -> Result<Game, Stop> {
        Self::load_inner(wasm, host, opts).map_err(classify)
    }

    fn load_inner(wasm: &[u8], host: Host, opts: LoadOptions) -> wasmtime::Result<Game> {
        let module = if wasm.starts_with(b"\0asm") {
            Module::new(engine(), wasm)?
        } else if opts.allow_precompiled {
            // Precompiled artifact from `gasm-run --compile`: native code, trusted
            // like the runner itself (unlike .wasm, which is sandboxed).
            unsafe { Module::deserialize(engine(), wasm)? }
        } else {
            bail!("not a wasm module (a precompiled .cwasm runs as native code: pass --allow-precompiled if you made it yourself)");
        };
        let gl = module.imports().any(|i| i.module() == "gasm:gl");
        if gl && module.imports().any(|i| i.module() == "gasm:gfx") {
            bail!("a module imports gasm:gfx or gasm:gl, not both");
        }
        // guests that export gasm_run run in one async call (stack switching)
        let has_run = module.get_export("gasm_run").is_some();
        let switching = has_run && opts.stack_switching;
        if has_run && !switching && module.imports().any(|i| i.module() == "asyncify") {
            // made without wasm-opt --asyncify: its gasm_frame can't suspend
            bail!("this module is a run build (no Asyncify): it needs stack switching (gasm_run); use the game's Asyncify build");
        }
        let engine = engine();
        let mut linker: Linker<Host> = Linker::new(engine);
        add_gasm_imports(&mut linker)?;
        if switching {
            switching::add_yield_async(&mut linker)?;
        } else {
            switching::add_yield_sync(&mut linker)?;
        }
        add_gfx_imports(&mut linker)?;
        crate::gl::add_gl_imports(&mut linker)?;
        add_net_imports(&mut linker)?;
        add_storage_imports(&mut linker)?;
        let mut store = Store::new(engine, host);
        // what `has` reports: the gasm modules and functions above (not WASI stubs or traps)
        let mut provided = HashSet::new();
        for (m, f, _) in linker.iter(&mut store) {
            provided.insert(m.to_owned());
            provided.insert(format!("{m}.{f}"));
        }
        for f in wasi::IMPLEMENTED {
            provided.insert(format!("{}.{f}", wasi::MODULE));
        }
        provided.insert(wasi::MODULE.to_owned());
        store.data_mut().provided = provided;
        wasi::add_to_linker(&mut linker, &module)?;
        // Imports this runner doesn't know (e.g. JS glue some Rust crates pull in)
        // link as traps: harmless unless the guest actually calls them.
        linker.define_unknown_imports_as_traps(&module)?;

        let deadline = opts.call_timeout.map_or(u64::MAX / 2, |t| (t.as_millis() as u64 * TICKS_PER_SEC).div_ceil(1000).max(1));
        store.epoch_deadline_trap();
        store.set_epoch_deadline(deadline);
        let exchange = switching.then(|| Exchange::new(deadline));
        store.data_mut().run = exchange.clone();
        let instance: Instance = if switching {
            switching::block_on(linker.instantiate_async(&mut store, &module))??
        } else {
            linker.instantiate(&mut store, &module)?
        };
        let memory = instance
            .get_memory(&mut store, "memory")
            .ok_or_else(|| format_err!("guest does not export `memory`"))?;
        store.data_mut().memory = Some(memory);

        // a call that must not suspend, on either engine
        fn call<R: wasmtime::WasmResults + Send + Sync>(store: &mut Store<Host>, f: TypedFunc<(), R>, switching: bool) -> wasmtime::Result<R> {
            if switching { switching::block_on(f.call_async(store, ()))? } else { f.call(store, ()) }
        }
        if let Ok(init) = instance.get_typed_func::<(), ()>(&mut store, "_initialize") {
            store.set_epoch_deadline(deadline);
            call(&mut store, init, switching)?;
        }
        let version = instance.get_typed_func::<(), i32>(&mut store, "gasm_abi_version")?;
        let version = call(&mut store, version, switching)?;
        if version != ABI_VERSION {
            bail!("guest targets gasm ABI v{version}, runner implements v{ABI_VERSION}");
        }
        store.set_epoch_deadline(deadline);
        let init = instance.get_typed_func::<(), i32>(&mut store, "gasm_init")?;
        let rc = call(&mut store, init, switching)?;
        if rc != 0 {
            bail!("gasm_init failed with code {rc}");
        }
        let frame = instance.get_typed_func::<(), ()>(&mut store, "gasm_frame")?;
        let exit = instance.get_typed_func::<(), ()>(&mut store, "gasm_exit").ok();
        let run = if switching { Some(instance.get_typed_func::<(), i32>(&mut store, "gasm_run")?) } else { None };
        Ok(Game { store: Some(store), frame, exit, deadline, run, running: None, exchange, ended: false, gl })
    }

    /// Whether the guest imports gasm:gl.
    pub fn uses_gl(&self) -> bool {
        self.gl
    }

    /// Whether the guest runs through `gasm_run` (the runner switches stacks).
    pub fn switching(&self) -> bool {
        self.run.is_some()
    }

    /// One frame: call `gasm_frame`, or resume `gasm_run` until its next `yield_frame`.
    pub fn frame(&mut self) -> Result<(), Stop> {
        if self.ended {
            return Err(Stop::Trap("the guest is not running".into()));
        }
        let Some(run) = self.run.clone() else {
            let store = self.store.as_mut().expect("store");
            store.set_epoch_deadline(self.deadline);
            let r = self.frame.call(&mut *store, ()).map_err(classify);
            store.data_mut().frame_index += 1;
            self.ended = r.is_err();
            return r;
        };
        match &self.exchange {
            Some(ex) if self.running.is_some() => ex.set(Cmd::Resume),
            _ => {
                // frame 0 starts the run; the call owns the store from now on
                let mut store = self.store.take().expect("store");
                store.set_epoch_deadline(self.deadline);
                self.running = Some(Box::pin(async move {
                    let r = run.call_async(&mut store, ()).await;
                    (store, r)
                }));
            }
        }
        self.poll()
    }

    /// Poll the running `gasm_run` call: Ok when it suspended again, the reason when it ended.
    fn poll(&mut self) -> Result<(), Stop> {
        let Some(running) = self.running.as_mut() else { return Ok(()) };
        match switching::poll_once(running.as_mut()) {
            Poll::Pending => Ok(()),
            Poll::Ready((store, r)) => {
                self.store = Some(store);
                self.running = None;
                self.ended = true;
                Err(match r {
                    Ok(code) => Stop::Exit(code),
                    Err(e) => classify(e),
                })
            }
        }
    }

    /// Best-effort "the player is quitting" notification (optional `gasm_exit` export).
    /// A suspended `gasm_run` guest gets it on top of its run, which then ends.
    pub fn exit(&mut self) {
        if self.running.is_some() {
            if let Some(ex) = &self.exchange {
                ex.set(Cmd::Exit);
            }
            if let Err(Stop::Trap(t)) = self.poll() {
                eprintln!("[gasm] gasm_exit trapped: {t}");
            }
            if self.running.is_some() {
                // gasm_exit yielded: give up on the run (the store stays with it)
                self.running = None;
            }
            self.exit = None;
            return;
        }
        if let (Some(f), Some(store)) = (self.exit.take(), self.store.as_mut()) {
            store.set_epoch_deadline(self.deadline);
            let r = if self.run.is_some() { switching::block_on(f.call_async(&mut *store, ())).and_then(|r| r) } else { f.call(&mut *store, ()) };
            if let Err(e) = r {
                if let Stop::Trap(t) = classify(e) {
                    eprintln!("[gasm] gasm_exit trapped: {t}");
                }
            }
        }
    }

    /// Run `f` on the host: directly, or (a suspended `gasm_run` guest) through
    /// the guest's `yield_frame`, which owns the store meanwhile. Runners use this
    /// between frames for everything they read or set.
    pub fn with_host<R>(&mut self, f: impl FnOnce(&mut Host) -> R) -> R {
        if let Some(store) = &mut self.store {
            return f(store.data_mut());
        }
        let (Some(running), Some(ex)) = (self.running.as_mut(), &self.exchange) else {
            panic!("with_host: the guest's store is gone (gasm_exit yielded)");
        };
        let mut f = Some(f);
        let mut out = None;
        let mut job = |h: &mut Host| out = Some((f.take().expect("job runs once"))(h));
        let ptr: *mut (dyn FnMut(&mut Host) + '_) = &mut job;
        // SAFETY: the pointer is only used by yield_frame during the poll below,
        // while `job` is alive on this stack; it is dropped before we return.
        let ptr: *mut (dyn FnMut(&mut Host) + 'static) = unsafe { std::mem::transmute(ptr) };
        ex.set(Cmd::Job(Job(ptr)));
        let polled = switching::poll_once(running.as_mut());
        ex.set(Cmd::None);
        match polled {
            Poll::Pending => {}
            Poll::Ready(_) => unreachable!("yield_frame stays suspended after a job"),
        }
        out.expect("yield_frame ran the job")
    }

    /// The host of a guest that isn't suspended in `gasm_run` (every `gasm_frame`
    /// guest; a `gasm_run` guest before its first frame and after it ended).
    /// Otherwise use [`Game::with_host`].
    pub fn host(&self) -> &Host {
        self.store.as_ref().expect("Game::host: a gasm_run guest is running; use Game::with_host").data()
    }

    pub fn host_mut(&mut self) -> &mut Host {
        self.store.as_mut().expect("Game::host_mut: a gasm_run guest is running; use Game::with_host").data_mut()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn titles_are_cleaned_and_cut() {
        assert_eq!(clean_title("A\u{7}B\u{202e}C\u{2069}").as_deref(), Some("ABC"));
        assert_eq!(clean_title("\n\t"), None);
        // 200 two-byte characters: cut at 256 bytes, on a character boundary
        assert_eq!(clean_title(&"é".repeat(200)).map(|t| t.len()), Some(256));
    }

    const RUN_GUEST: &str = r#"(module
      (import "gasm" "yield_frame" (func $yield))
      (memory (export "memory") 1)
      (global $n (mut i32) (i32.const 0))
      (func (export "gasm_abi_version") (result i32) i32.const 0)
      (func (export "gasm_init") (result i32) i32.const 0)
      (func (export "gasm_frame") call $yield)
      (func (export "gasm_exit") (global.set $n (i32.const 100)))
      (func (export "frames") (result i32) global.get $n)
      (func (export "gasm_run") (result i32)
        (loop $l
          (global.set $n (i32.add (global.get $n) (i32.const 1)))
          call $yield
          (br_if $l (i32.lt_u (global.get $n) (i32.const 3))))
        i32.const 7))"#;

    fn host() -> Host {
        Host::new(Assets::new(), HashMap::new(), None, Gfx::null(), Net::new(false), Storage::memory())
    }

    #[test]
    fn gasm_run_guests_suspend_between_frames() {
        let mut g = Game::load(&wat::parse_str(RUN_GUEST).unwrap(), host(), LoadOptions::default()).unwrap();
        assert!(g.switching());
        g.with_host(|h| h.frame_rate = 30.0); // before the first frame: the store is ours
        assert!(g.frame().is_ok()); // n = 1, suspended in yield_frame
        // suspended: the host is reached through the guest's yield_frame
        assert_eq!(g.with_host(|h| h.frame_rate), 30.0);
        assert!(g.frame().is_ok()); // n = 2
        assert!(g.frame().is_ok()); // n = 3
        // the loop ends: gasm_run returns 7, the exit code
        assert!(matches!(g.frame(), Err(Stop::Exit(7))));
        assert!(g.frame().is_err(), "never called again");
    }

    #[test]
    fn exit_runs_on_top_of_a_suspended_run() {
        let mut g = Game::load(&wat::parse_str(RUN_GUEST).unwrap(), host(), LoadOptions::default()).unwrap();
        g.frame().unwrap();
        g.exit();
        // the store is back after the run ended; gasm_exit ran (n = 100)
        assert!(g.store.is_some());
    }

    #[test]
    fn yield_frame_outside_gasm_run_traps() {
        let opts = LoadOptions { stack_switching: false, ..LoadOptions::default() };
        let mut g = Game::load(&wat::parse_str(RUN_GUEST).unwrap(), host(), opts).unwrap();
        assert!(!g.switching());
        match g.frame() {
            Err(Stop::Trap(t)) => assert!(t.contains("only inside gasm_run"), "{t}"),
            other => panic!("expected a trap, got {:?}", other.err()),
        }
    }

    #[test]
    fn custom_sections_are_found() {
        // magic + version, a type section (id 1, empty), then custom "gasm.title" = "Hi\0"
        let mut wasm = b"\0asm\x01\0\0\0\x01\x01\0".to_vec();
        wasm.extend_from_slice(&[0, 14, 10]);
        wasm.extend_from_slice(b"gasm.titleHi\0");
        assert_eq!(custom_section(&wasm, "gasm.title"), Some(&b"Hi\0"[..]));
        assert_eq!(static_title(&wasm).as_deref(), Some("Hi"));
        assert_eq!(custom_section(&wasm, "other"), None);
        assert_eq!(custom_section(&wasm[..wasm.len() - 2], "gasm.title"), None); // truncated
        assert_eq!(custom_section(b"not wasm", "gasm.title"), None);
    }
}