use std::cell::{Cell, RefCell};
use std::collections::HashMap;
use std::ffi::c_void;
use std::path::{Path, PathBuf};
use std::rc::Rc;
use std::sync::Arc;

use avril_caps::{Capabilities, PathTarget};
use avril_core::serde_v8::ToJsBuffer;
use avril_core::{Extension, Op, PublicModule, v8};
use futures::channel::oneshot;
use libffi::middle::{Arg, Cif, CodePtr, Ret, Type};

#[cfg(not(all(target_pointer_width = "64", target_endian = "little")))]
compile_error!("avril-ffi currently requires a 64-bit little-endian target");

// Resource IDs cross into JS as Numbers, so they must stay exact safe integers.
const MAX_ID: u64 = 9_007_199_254_740_991;

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
enum Kind {
    Void,
    Bool,
    I8,
    U8,
    I16,
    U16,
    I32,
    U32,
    I64,
    U64,
    Isize,
    Usize,
    F32,
    F64,
    Pointer,
    Buffer,
    Function,
}

impl Kind {
    fn parse(name: &str) -> Result<Self, String> {
        Ok(match name {
            "void" => Self::Void,
            "bool" => Self::Bool,
            "i8" => Self::I8,
            "u8" => Self::U8,
            "i16" => Self::I16,
            "u16" => Self::U16,
            "i32" => Self::I32,
            "u32" => Self::U32,
            "i64" => Self::I64,
            "u64" => Self::U64,
            "isize" => Self::Isize,
            "usize" => Self::Usize,
            "f32" => Self::F32,
            "f64" => Self::F64,
            "pointer" => Self::Pointer,
            "buffer" => Self::Buffer,
            "function" => Self::Function,
            _ => return Err(format!("Unknown native type: {name}")),
        })
    }

    fn ffi_type(self) -> Type {
        match self {
            Self::Void => Type::void(),
            Self::Bool | Self::U8 => Type::u8(),
            Self::I8 => Type::i8(),
            Self::U16 => Type::u16(),
            Self::I16 => Type::i16(),
            Self::U32 => Type::u32(),
            Self::I32 => Type::i32(),
            Self::U64 => Type::u64(),
            Self::I64 => Type::i64(),
            Self::Usize => Type::usize(),
            Self::Isize => Type::isize(),
            Self::F32 => Type::f32(),
            Self::F64 => Type::f64(),
            Self::Pointer | Self::Buffer | Self::Function => Type::pointer(),
        }
    }
}

#[derive(Clone, Debug)]
struct Def {
    params: Vec<Kind>,
    result: Kind,
}

#[derive(serde::Deserialize)]
struct DefWire {
    symbol: String,
    params: Vec<String>,
    result: String,
    nonblocking: bool,
}

struct SymbolState {
    address: usize,
    def: Def,
}

struct LibraryState {
    _library: libloading::Library,
    symbols: Vec<SymbolState>,
}

#[derive(Clone, Debug)]
enum Value {
    Void,
    Bool(bool),
    I64(i64),
    U64(u64),
    F32(f32),
    F64(f64),
    Ptr(u64),
}

struct Completed {
    outcome: Result<Value, String>,
    buffers: Vec<Vec<u8>>,
    originals: Vec<Vec<u8>>,
    targets: Vec<v8::Global<v8::Value>>,
}

struct PendingTargets {
    targets: Vec<v8::Global<v8::Value>>,
    originals: Vec<Vec<u8>>,
}

type CallReceiver = oneshot::Receiver<Result<(Value, Vec<Vec<u8>>), String>>;

struct Registry {
    caps: Arc<Capabilities>,
    libraries: RefCell<HashMap<u64, Arc<LibraryState>>>,
    next_library: Cell<u64>,
    receivers: RefCell<HashMap<u64, CallReceiver>>,
    targets: RefCell<HashMap<u64, PendingTargets>>,
    completed: Rc<RefCell<HashMap<u64, Completed>>>,
    next_call: Cell<u64>,
}

enum CallError {
    Type(String),
    Error(String),
}

impl CallError {
    fn type_(message: impl Into<String>) -> Self {
        Self::Type(message.into())
    }
    fn error(message: impl Into<String>) -> Self {
        Self::Error(message.into())
    }
}

type CallResult<T> = Result<T, CallError>;

fn throw<T>(scope: &mut v8::PinScope, error: &CallError) -> Option<T> {
    let (message, type_error) = match error {
        CallError::Type(message) => (message, true),
        CallError::Error(message) => (message, false),
    };
    let message = v8::String::new(scope, message)?;
    let exception = if type_error {
        v8::Exception::type_error(scope, message)
    } else {
        v8::Exception::error(scope, message)
    };
    scope.throw_exception(exception);
    None
}

fn bigint_u64(scope: &mut v8::PinScope, value: v8::Local<v8::Value>) -> CallResult<u64> {
    if !value.is_big_int() {
        return Err(CallError::type_("Expected a bigint"));
    }
    let bigint = value.to_big_int(scope).unwrap();
    let (value, lossless) = bigint.u64_value();
    if lossless {
        return Ok(value);
    }
    Err(CallError::type_("Bigint is out of range"))
}

fn bigint_i64(scope: &mut v8::PinScope, value: v8::Local<v8::Value>) -> CallResult<i64> {
    if !value.is_big_int() {
        return Err(CallError::type_("Expected a bigint"));
    }
    let bigint = value.to_big_int(scope).unwrap();
    let (value, lossless) = bigint.i64_value();
    if lossless {
        return Ok(value);
    }
    Err(CallError::type_("Bigint is out of range"))
}

fn number(scope: &mut v8::PinScope, value: v8::Local<v8::Value>) -> CallResult<f64> {
    if !value.is_number() {
        return Err(CallError::type_("Expected a number"));
    }
    value
        .number_value(scope)
        .ok_or_else(|| CallError::type_("Expected a number"))
}

fn integer(
    scope: &mut v8::PinScope,
    value: v8::Local<v8::Value>,
    min: f64,
    max: f64,
) -> CallResult<i64> {
    if value.is_big_int() {
        let value = bigint_i64(scope, value)?;
        if (value as f64) < min || (value as f64) > max {
            return Err(CallError::Type("Integer argument is out of range".into()));
        }
        return Ok(value);
    }
    let value = number(scope, value)?;
    if value.fract() != 0.0 || value < min || value > max {
        return Err(CallError::Type("Integer argument is out of range".into()));
    }
    Ok(value as i64)
}

fn view_bytes(
    scope: &mut v8::PinScope,
    value: v8::Local<v8::Value>,
) -> CallResult<(*mut u8, usize)> {
    if let Ok(view) = v8::Local::<v8::ArrayBufferView>::try_from(value) {
        return Ok((view.data() as *mut u8, view.byte_length()));
    }
    if let Ok(buffer) = v8::Local::<v8::ArrayBuffer>::try_from(value) {
        let pointer = buffer
            .data()
            .map(|pointer| pointer.as_ptr() as *mut u8)
            .unwrap_or(std::ptr::null_mut());
        return Ok((pointer, buffer.byte_length()));
    }
    let _ = scope;
    Err(CallError::type_(
        "Expected an ArrayBuffer or an ArrayBuffer view",
    ))
}

// One 8-byte slot per argument; libffi reads only the bytes of the slot's type.
#[derive(Clone, Copy)]
struct Slot(u64);

impl Slot {
    fn from_i64(value: i64) -> Self {
        Self(value as u64)
    }
    fn from_u64(value: u64) -> Self {
        Self(value)
    }
    fn from_f32(value: f32) -> Self {
        Self(f32::to_bits(value) as u64)
    }
    fn from_f64(value: f64) -> Self {
        Self(f64::to_bits(value))
    }
    fn from_ptr(value: *mut c_void) -> Self {
        Self(value as usize as u64)
    }
}

fn convert_arg(
    scope: &mut v8::PinScope,
    value: v8::Local<v8::Value>,
    kind: Kind,
    nonblocking: bool,
    buffers: &mut Vec<Vec<u8>>,
) -> CallResult<Slot> {
    Ok(match kind {
        Kind::Bool => {
            if !value.is_boolean() {
                return Err(CallError::type_("Expected a boolean"));
            }
            Slot::from_u64(value.boolean_value(scope) as u64)
        }
        Kind::I8 => Slot::from_i64(integer(scope, value, i8::MIN as f64, i8::MAX as f64)?),
        Kind::U8 => Slot::from_i64(integer(scope, value, 0.0, u8::MAX as f64)?),
        Kind::I16 => Slot::from_i64(integer(scope, value, i16::MIN as f64, i16::MAX as f64)?),
        Kind::U16 => Slot::from_i64(integer(scope, value, 0.0, u16::MAX as f64)?),
        Kind::I32 => Slot::from_i64(integer(scope, value, i32::MIN as f64, i32::MAX as f64)?),
        Kind::U32 => Slot::from_i64(integer(scope, value, 0.0, u32::MAX as f64)?),
        Kind::I64 | Kind::Isize => {
            if value.is_big_int() {
                Slot::from_i64(bigint_i64(scope, value)?)
            } else {
                Slot::from_i64(integer(
                    scope,
                    value,
                    -9_007_199_254_740_992.0,
                    9_007_199_254_740_992.0,
                )?)
            }
        }
        Kind::U64 | Kind::Usize => {
            if value.is_big_int() {
                Slot::from_u64(bigint_u64(scope, value)?)
            } else {
                Slot::from_i64(integer(scope, value, 0.0, 9_007_199_254_740_992.0)?)
            }
        }
        Kind::F32 => Slot::from_f32(number(scope, value)? as f32),
        Kind::F64 => Slot::from_f64(number(scope, value)?),
        Kind::Pointer | Kind::Function => {
            if value.is_null_or_undefined() {
                Slot::from_ptr(std::ptr::null_mut())
            } else if value.is_big_int() {
                Slot::from_u64(bigint_u64(scope, value)?)
            } else if let Ok((pointer, _)) = view_bytes(scope, value) {
                if nonblocking {
                    return Err(CallError::type_(
                        "A pointer into a view cannot be used in a nonblocking call; pass an UnsafePointer allocation",
                    ));
                }
                Slot::from_ptr(pointer as *mut c_void)
            } else {
                return Err(CallError::type_("Expected a pointer"));
            }
        }
        Kind::Buffer => {
            let (pointer, length) = view_bytes(scope, value)?;
            if nonblocking {
                let copy = if length == 0 {
                    Vec::new()
                } else {
                    // SAFETY: pointer/length describe this view's live bytes; we copy them out now.
                    unsafe { std::slice::from_raw_parts(pointer, length) }.to_vec()
                };
                buffers.push(copy);
                let copy = buffers.last_mut().unwrap();
                Slot::from_ptr(copy.as_mut_ptr() as *mut c_void)
            } else {
                Slot::from_ptr(pointer as *mut c_void)
            }
        }
        Kind::Void => return Err(CallError::type_("void is not a parameter type")),
    })
}

fn call_native(address: usize, def: &Def, slots: &mut [Slot]) -> Value {
    let cif = Cif::new(
        def.params.iter().map(|kind| kind.ffi_type()),
        def.result.ffi_type(),
    );
    let args: Vec<Arg> = slots.iter().map(|slot| Arg::new(&slot.0)).collect();
    let function = CodePtr::from_ptr(address as *const c_void);
    // SAFETY: the caller checked address against def when the library (or the
    // UnsafeFnPointer's creator) was opened; slot bytes match def's types.
    unsafe {
        match def.result {
            Kind::Void => {
                cif.call_return_into(function, &args, Ret::void());
                Value::Void
            }
            Kind::Bool => Value::Bool(cif.call::<u8>(function, &args) != 0),
            Kind::I8 => Value::I64(cif.call::<i8>(function, &args) as i64),
            Kind::U8 => Value::U64(cif.call::<u8>(function, &args) as u64),
            Kind::I16 => Value::I64(cif.call::<i16>(function, &args) as i64),
            Kind::U16 => Value::U64(cif.call::<u16>(function, &args) as u64),
            Kind::I32 => Value::I64(cif.call::<i32>(function, &args) as i64),
            Kind::U32 => Value::U64(cif.call::<u32>(function, &args) as u64),
            Kind::I64 | Kind::Isize => Value::I64(cif.call::<i64>(function, &args)),
            Kind::U64 | Kind::Usize => Value::U64(cif.call::<u64>(function, &args)),
            Kind::F32 => Value::F32(cif.call::<f32>(function, &args)),
            Kind::F64 => Value::F64(cif.call::<f64>(function, &args)),
            Kind::Pointer | Kind::Function => {
                Value::Ptr(cif.call::<*mut c_void>(function, &args) as usize as u64)
            }
            Kind::Buffer => Value::Ptr(cif.call::<*mut c_void>(function, &args) as usize as u64),
        }
    }
}

fn value_to_v8<'s>(
    scope: &mut v8::PinScope<'s, '_>,
    value: &Value,
    result: Kind,
) -> Option<v8::Local<'s, v8::Value>> {
    Some(match (value, result) {
        (Value::Void, _) => v8::undefined(scope).into(),
        (Value::Bool(value), _) => v8::Boolean::new(scope, *value).into(),
        (Value::I64(value), Kind::I64 | Kind::Isize) => {
            v8::BigInt::new_from_i64(scope, *value).into()
        }
        (Value::U64(value), Kind::U64 | Kind::Usize) => {
            v8::BigInt::new_from_u64(scope, *value).into()
        }
        (Value::I64(value), _) => v8::Number::new(scope, *value as f64).into(),
        (Value::U64(value), _) => v8::Number::new(scope, *value as f64).into(),
        (Value::F32(value), _) => v8::Number::new(scope, *value as f64).into(),
        (Value::F64(value), _) => v8::Number::new(scope, *value).into(),
        (Value::Ptr(value), _) => v8::BigInt::new_from_u64(scope, *value).into(),
    })
}

fn parse_def(wire: DefWire) -> Result<(String, Def), String> {
    let params = wire
        .params
        .iter()
        .map(|name| Kind::parse(name))
        .collect::<Result<Vec<_>, _>>()?;
    if params.contains(&Kind::Void) {
        return Err("void is not a parameter type".into());
    }
    let result = Kind::parse(&wire.result)?;
    if result == Kind::Buffer {
        return Err("buffer is not a result type".into());
    }
    let _ = wire.nonblocking;
    Ok((wire.symbol, Def { params, result }))
}

fn resolve_library(caps: &Capabilities, specifier: &str) -> Result<libloading::Library, CallError> {
    // A bare name goes through the loader's search path; only unrestricted
    // ffi authority covers names whose resolution we cannot check beforehand.
    let bare = !specifier.contains(['/', '\\']) && Path::new(specifier).components().count() == 1;
    if let Ok(url) = url::Url::parse(specifier)
        && url.scheme() == "file"
    {
        let path = url
            .to_file_path()
            .map_err(|_| CallError::error("Invalid file URL"))?;
        return load_checked(caps, &path);
    }
    if bare && !specifier.starts_with('.') {
        if !caps.unrestricted_ffi() {
            return Err(CallError::error(format!(
                "PermissionDenied: ffi {specifier}"
            )));
        }
        // SAFETY: loading a library runs its initializers; unrestricted ffi
        // authority is exactly permission to run native code in-process.
        return unsafe { libloading::Library::new(specifier) }
            .map_err(|error| CallError::error(error.to_string()));
    }
    let path = if Path::new(specifier).is_absolute() {
        PathBuf::from(specifier)
    } else {
        caps.cwd().join(specifier)
    };
    load_checked(caps, &path)
}

fn load_checked(caps: &Capabilities, path: &Path) -> Result<libloading::Library, CallError> {
    let target =
        PathTarget::for_executable(path).map_err(|error| CallError::error(error.to_string()))?;
    caps.check_ffi(&target)
        .map_err(|error| CallError::error(error.to_string()))?;
    // SAFETY: see resolve_library.
    unsafe { libloading::Library::new(target.as_path()) }
        .map_err(|error| CallError::error(error.to_string()))
}

fn id<T>(scope: &mut v8::PinScope, value: v8::Local<v8::Value>) -> CallResult<T>
where
    T: TryFrom<u64>,
{
    let number = number(scope, value).map_err(|_| CallError::type_("Expected an id"))?;
    T::try_from(number as u64).map_err(|_| CallError::type_("Id is out of range"))
}

fn snapshot_args<'s>(
    scope: &mut v8::PinScope<'s, '_>,
    values: &v8::Local<'s, v8::Array>,
) -> CallResult<Vec<v8::Local<'s, v8::Value>>> {
    (0..values.length())
        .map(|index| values.get_index(scope, index))
        .collect::<Option<Vec<_>>>()
        .ok_or_else(|| CallError::type_("Could not read the arguments array"))
}

fn authorize_any(registry: &Registry) -> CallResult<()> {
    registry
        .caps
        .check_ffi_any()
        .map_err(|error| CallError::error(error.to_string()))
}

pub fn init(caps: Arc<Capabilities>) -> Extension {
    let registry = Rc::new(Registry {
        caps,
        libraries: RefCell::new(HashMap::new()),
        next_library: Cell::new(1),
        receivers: RefCell::new(HashMap::new()),
        targets: RefCell::new(HashMap::new()),
        completed: Rc::new(RefCell::new(HashMap::new())),
        next_call: Cell::new(1),
    });

    let dlopen_registry = registry.clone();
    let close_registry = registry.clone();
    let call_registry = registry.clone();
    let begin_registry = registry.clone();
    let await_registry = registry.clone();
    let finish_registry = registry.clone();
    let ptr_registry = registry.clone();
    let address_registry = registry.clone();
    let read_registry = registry.clone();
    let write_registry = registry.clone();
    let bytes_registry = registry.clone();
    let write_bytes_registry = registry.clone();
    let cstring_registry = registry.clone();

    Extension {
        name: "ext:ffi",
        dependencies: &[],
        ops: vec![
            Op::raw("op_ffi_dlopen", move |scope, args| {
                let specifier = args.get(0).to_rust_string_lossy(scope);
                let wires: Vec<DefWire> = match avril_core::serde_v8::from_v8(scope, args.get(1)) {
                    Ok(wires) => wires,
                    Err(error) => {
                        return throw(scope, &CallError::type_(error.to_string()));
                    }
                };
                let library = match resolve_library(&dlopen_registry.caps, &specifier) {
                    Ok(library) => library,
                    Err(error) => return throw(scope, &error),
                };
                let mut symbols = Vec::new();
                for wire in wires {
                    let (name, def) = match parse_def(wire) {
                        Ok(parsed) => parsed,
                        Err(error) => return throw(scope, &CallError::type_(error)),
                    };
                    // SAFETY: we only record the symbol's address; it is
                    // invoked later through libffi with def's signature.
                    let address = unsafe {
                        match library.get::<*const c_void>(name.as_bytes()) {
                            Ok(symbol) => match symbol.try_as_raw_ptr() {
                                Some(address) if !address.is_null() => address as usize,
                                Some(_) => {
                                    return throw(
                                        scope,
                                        &CallError::error("Symbol resolved to a null address"),
                                    );
                                }
                                None => {
                                    return throw(
                                        scope,
                                        &CallError::error("Could not resolve symbol"),
                                    );
                                }
                            },
                            Err(error) => {
                                return throw(scope, &CallError::error(error.to_string()));
                            }
                        }
                    };
                    symbols.push(SymbolState { address, def });
                }
                let library_id = dlopen_registry.next_library.get();
                if library_id > MAX_ID {
                    return throw(scope, &CallError::error("FFI library IDs exhausted"));
                }
                dlopen_registry.next_library.set(library_id + 1);
                dlopen_registry.libraries.borrow_mut().insert(
                    library_id,
                    Arc::new(LibraryState { _library: library, symbols }),
                );
                Some(v8::Number::new(scope, library_id as f64).into())
            }),
            Op::raw("op_ffi_close", move |scope, args| {
                if let Ok(library_id) = id::<u64>(scope, args.get(0)) {
                    close_registry.libraries.borrow_mut().remove(&library_id);
                }
                Some(v8::undefined(scope).into())
            }),
            Op::raw("op_ffi_call", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    let library_id = id::<u64>(scope, args.get(0))?;
                    let index = id::<usize>(scope, args.get(1))?;
                    let library = call_registry
                        .libraries
                        .borrow()
                        .get(&library_id)
                        .cloned()
                        .ok_or_else(|| CallError::error("Library is closed"))?;
                    let symbol = library
                        .symbols
                        .get(index)
                        .ok_or_else(|| CallError::error("Unknown symbol"))?;
                    let values = v8::Local::<v8::Array>::try_from(args.get(2))
                        .map_err(|_| CallError::type_("Expected an arguments array"))?;
                    if values.length() as usize != symbol.def.params.len() {
                        return Err(CallError::type_("Wrong number of arguments"));
                    }
                    // Read every element before converting any: an accessor
                    // could run JS, and no view address may be captured yet.
                    let elements = snapshot_args(scope, &values)?;
                    let mut buffers = Vec::new();
                    let mut slots = Vec::new();
                    for (value, kind) in elements.iter().zip(&symbol.def.params) {
                        slots.push(convert_arg(scope, *value, *kind, false, &mut buffers)?);
                    }
                    let value = call_native(symbol.address, &symbol.def, &mut slots);
                    value_to_v8(scope, &value, symbol.def.result)
                        .ok_or_else(|| CallError::error("Could not convert the result"))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_call_ptr", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&ptr_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let wire: DefWire = avril_core::serde_v8::from_v8(scope, args.get(1))
                        .map_err(|error| CallError::type_(error.to_string()))?;
                    let (_, def) = parse_def(wire).map_err(CallError::type_)?;
                    let values = v8::Local::<v8::Array>::try_from(args.get(2))
                        .map_err(|_| CallError::type_("Expected an arguments array"))?;
                    if values.length() as usize != def.params.len() {
                        return Err(CallError::type_("Wrong number of arguments"));
                    }
                    let elements = snapshot_args(scope, &values)?;
                    let mut buffers = Vec::new();
                    let mut slots = Vec::new();
                    for (value, kind) in elements.iter().zip(&def.params) {
                        slots.push(convert_arg(scope, *value, *kind, false, &mut buffers)?);
                    }
                    let value = call_native(address, &def, &mut slots);
                    value_to_v8(scope, &value, def.result)
                        .ok_or_else(|| CallError::error("Could not convert the result"))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_call_async_begin", move |scope, args| {
                let result = (|| -> CallResult<u64> {
                    let library_id = id::<u64>(scope, args.get(0))?;
                    let index = id::<usize>(scope, args.get(1))?;
                    let library = begin_registry
                        .libraries
                        .borrow()
                        .get(&library_id)
                        .cloned()
                        .ok_or_else(|| CallError::error("Library is closed"))?;
                    let symbol = library
                        .symbols
                        .get(index)
                        .ok_or_else(|| CallError::error("Unknown symbol"))?;
                    let values = v8::Local::<v8::Array>::try_from(args.get(2))
                        .map_err(|_| CallError::type_("Expected an arguments array"))?;
                    if values.length() as usize != symbol.def.params.len() {
                        return Err(CallError::type_("Wrong number of arguments"));
                    }
                    let elements = snapshot_args(scope, &values)?;
                    let mut buffers: Vec<Vec<u8>> = Vec::new();
                    let mut targets = Vec::new();
                    let mut originals = Vec::new();
                    let mut slots = Vec::new();
                    for (value, kind) in elements.iter().zip(&symbol.def.params) {
                        if *kind == Kind::Buffer {
                            targets.push(v8::Global::new(scope, *value));
                        }
                        slots.push(convert_arg(scope, *value, *kind, true, &mut buffers)?);
                        if *kind == Kind::Buffer {
                            originals.push(buffers.last().unwrap().clone());
                        }
                    }
                    let call_id = begin_registry.next_call.get();
                    if call_id > MAX_ID {
                        return Err(CallError::error("FFI call IDs exhausted"));
                    }
                    begin_registry.next_call.set(call_id + 1);
                    let (sender, receiver) = oneshot::channel();
                    begin_registry
                        .receivers
                        .borrow_mut()
                        .insert(call_id, receiver);
                    begin_registry
                        .targets
                        .borrow_mut()
                        .insert(call_id, PendingTargets { targets, originals });
                    let address = symbol.address;
                    let def = symbol.def.clone();
                    let library = library.clone();
                    let spawned =
                        std::thread::Builder::new()
                            .name("avril:ffi".into())
                            .spawn(move || {
                                // Keeps the library loaded for the call's duration.
                                let _library = &library;
                                let mut slots = slots;
                                let outcome =
                                    std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                                        (call_native(address, &def, &mut slots), buffers)
                                    }));
                                let result = match outcome {
                                    Ok((value, buffers)) => Ok((value, buffers)),
                                    Err(_) => Err("Native call panicked".to_string()),
                                };
                                let _ = sender.send(result);
                            });
                    if let Err(error) = spawned {
                        begin_registry.receivers.borrow_mut().remove(&call_id);
                        begin_registry.targets.borrow_mut().remove(&call_id);
                        return Err(CallError::error(error.to_string()));
                    }
                    Ok(call_id)
                })();
                match result {
                    Ok(call_id) => Some(v8::Number::new(scope, call_id as f64).into()),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::asynchronous("op_ffi_await", move |call_id: u64| {
                let receiver = await_registry.receivers.borrow_mut().remove(&call_id);
                let pending = await_registry.targets.borrow_mut().remove(&call_id);
                let (targets, originals) = match pending {
                    Some(pending) => (pending.targets, pending.originals),
                    None => (Vec::new(), Vec::new()),
                };
                let completed = await_registry.completed.clone();
                async move {
                    let outcome = match receiver {
                        Some(receiver) => match receiver.await {
                            Ok(Ok((value, buffers))) => Ok((value, buffers)),
                            Ok(Err(error)) => Err(error),
                            Err(_) => Err("Native call was dropped".to_string()),
                        },
                        None => Err("Unknown call".to_string()),
                    };
                    let (outcome, buffers) = match outcome {
                        Ok((value, buffers)) => (Ok(value), buffers),
                        Err(error) => (Err(error), Vec::new()),
                    };
                    completed.borrow_mut().insert(
                        call_id,
                        Completed {
                            outcome,
                            buffers,
                            originals,
                            targets,
                        },
                    );
                    Ok::<_, std::io::Error>(call_id)
                }
            }),
            Op::raw("op_ffi_call_async_finish", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    let call_id = id::<u64>(scope, args.get(0))?;
                    let result_kind = Kind::parse(&args.get(1).to_rust_string_lossy(scope))
                        .map_err(CallError::type_)?;
                    let completed = finish_registry
                        .completed
                        .borrow_mut()
                        .remove(&call_id)
                        .ok_or_else(|| CallError::error("Call is not complete"))?;
                    for ((target, bytes), original) in completed
                        .targets
                        .iter()
                        .zip(&completed.buffers)
                        .zip(&completed.originals)
                    {
                        let target = v8::Local::new(scope, target);
                        let (pointer, length) = view_bytes(scope, target)?;
                        if length != bytes.len() {
                            return Err(CallError::error(
                                "Buffer changed size during a nonblocking call",
                            ));
                        }
                        // Only bytes the native call changed are written
                        // back, so JS writes to other bytes during the call
                        // are not clobbered. SAFETY: pointer/length describe
                        // the target view, kept alive via this Global.
                        for (index, (byte, before)) in bytes.iter().zip(original).enumerate() {
                            if byte != before {
                                unsafe { *pointer.add(index) = *byte };
                            }
                        }
                    }
                    let value = completed.outcome.map_err(CallError::error)?;
                    value_to_v8(scope, &value, result_kind)
                        .ok_or_else(|| CallError::error("Could not convert the result"))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_address_of", move |scope, args| {
                if let Err(error) = authorize_any(&address_registry) {
                    return throw(scope, &error);
                }
                match view_bytes(scope, args.get(0)) {
                    Ok((pointer, _)) => {
                        Some(v8::BigInt::new_from_u64(scope, pointer as usize as u64).into())
                    }
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_read", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&read_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let kind = Kind::parse(&args.get(1).to_rust_string_lossy(scope))
                        .map_err(CallError::type_)?;
                    let pointer = address as *const u8;
                    // SAFETY: the caller asserts address is readable for kind;
                    // that is the documented unsafety of pointer reads.
                    let value = unsafe {
                        match kind {
                            Kind::Bool => Value::Bool(*pointer != 0),
                            Kind::I8 => Value::I64(pointer.cast::<i8>().read_unaligned() as i64),
                            Kind::U8 => Value::U64(*pointer as u64),
                            Kind::I16 => Value::I64(pointer.cast::<i16>().read_unaligned() as i64),
                            Kind::U16 => Value::U64(pointer.cast::<u16>().read_unaligned() as u64),
                            Kind::I32 => Value::I64(pointer.cast::<i32>().read_unaligned() as i64),
                            Kind::U32 => Value::U64(pointer.cast::<u32>().read_unaligned() as u64),
                            Kind::I64 | Kind::Isize => {
                                Value::I64(pointer.cast::<i64>().read_unaligned())
                            }
                            Kind::U64 | Kind::Usize => {
                                Value::U64(pointer.cast::<u64>().read_unaligned())
                            }
                            Kind::F32 => Value::F32(pointer.cast::<f32>().read_unaligned()),
                            Kind::F64 => Value::F64(pointer.cast::<f64>().read_unaligned()),
                            Kind::Pointer | Kind::Function => {
                                Value::Ptr(pointer.cast::<usize>().read_unaligned() as u64)
                            }
                            _ => {
                                return Err(CallError::type_(
                                    "Type cannot be read through a pointer",
                                ));
                            }
                        }
                    };
                    value_to_v8(scope, &value, kind)
                        .ok_or_else(|| CallError::error("Could not convert the result"))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_write", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&write_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let kind = Kind::parse(&args.get(1).to_rust_string_lossy(scope))
                        .map_err(CallError::type_)?;
                    let mut buffers = Vec::new();
                    let slot = convert_arg(scope, args.get(2), kind, false, &mut buffers)?;
                    let size = match kind {
                        Kind::Bool | Kind::I8 | Kind::U8 => 1,
                        Kind::I16 | Kind::U16 => 2,
                        Kind::I32 | Kind::U32 | Kind::F32 => 4,
                        Kind::I64
                        | Kind::U64
                        | Kind::Isize
                        | Kind::Usize
                        | Kind::F64
                        | Kind::Pointer
                        | Kind::Function => 8,
                        _ => {
                            return Err(CallError::type_(
                                "Type cannot be written through a pointer",
                            ));
                        }
                    };
                    // SAFETY: as op_ffi_read, with the caller's writable assert.
                    unsafe {
                        std::ptr::copy_nonoverlapping(
                            (&slot.0 as *const u64).cast::<u8>(),
                            address as *mut u8,
                            size,
                        );
                    }
                    Ok(v8::undefined(scope).into())
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_read_bytes", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&bytes_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let length = id::<usize>(scope, args.get(1))?;
                    let bytes = if length == 0 {
                        Vec::new()
                    } else {
                        // SAFETY: the caller asserts address..address+length is readable.
                        unsafe { std::slice::from_raw_parts(address as *const u8, length) }.to_vec()
                    };
                    avril_core::serde_v8::to_v8(scope, ToJsBuffer::from(bytes))
                        .map_err(|error| CallError::error(error.to_string()))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_write_bytes", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&write_bytes_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let (pointer, length) = view_bytes(scope, args.get(1))?;
                    // SAFETY: source is a live view; the caller asserts the
                    // destination is writable for length bytes. Ranges may
                    // overlap, hence ptr::copy.
                    if length > 0 {
                        unsafe { std::ptr::copy(pointer, address as *mut u8, length) };
                    }
                    Ok(v8::undefined(scope).into())
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
            Op::raw("op_ffi_read_cstring", move |scope, args| {
                let result = (|| -> CallResult<v8::Local<v8::Value>> {
                    authorize_any(&cstring_registry)?;
                    let address = bigint_u64(scope, args.get(0))? as usize;
                    let max = id::<usize>(scope, args.get(1))?;
                    let mut bytes = Vec::new();
                    for offset in 0..max {
                        // SAFETY: the caller asserts a NUL-terminated string
                        // at address; we stop at max bytes regardless.
                        let byte = unsafe { *(address as *const u8).add(offset) };
                        if byte == 0 {
                            break;
                        }
                        bytes.push(byte);
                    }
                    avril_core::serde_v8::to_v8(scope, ToJsBuffer::from(bytes))
                        .map_err(|error| CallError::error(error.to_string()))
                })();
                match result {
                    Ok(value) => Some(value),
                    Err(error) => throw(scope, &error),
                }
            }),
        ],
        modules: const { &[PUBLIC_MODULES[0].module] },
        entry_point: Some("ext:ffi/main.js"),
    }
}

pub const PUBLIC_MODULES: &[PublicModule] =
    &[avril_core::public_module!("avril:ffi", "ext:ffi", "main")];