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    use alloc::{
        borrow::Cow,
        rc::{Rc, Weak},
        vec::Vec,
    };
    use core::fmt;
    use host::Externals;
    use isa;
    use module::ModuleInstance;
    use parity_wasm::elements::Local;
    use runner::{check_function_args, Interpreter, InterpreterState, StackRecycler};
    use types::ValueType;
    use value::RuntimeValue;
    use {Signature, Trap};

    /// Reference to a function (See [`FuncInstance`] for details).
    ///
    /// This reference has a reference-counting semantics.
    ///
    /// [`FuncInstance`]: struct.FuncInstance.html
    #[derive(Clone, Debug)]
    pub struct FuncRef(Rc<FuncInstance>);

    impl ::core::ops::Deref for FuncRef {
        type Target = FuncInstance;
        fn deref(&self) -> &FuncInstance {
            &self.0
        }
    }

    /// Runtime representation of a function.
    ///
    /// Functions are the unit of organization of code in WebAssembly. Each function takes a sequence of values
    /// as parameters and either optionally return a value or trap.
    /// Functions can call other function including itself (i.e recursive calls are allowed) and imported functions
    /// (i.e functions defined in another module or by the host environment).
    ///
    /// Functions can be defined either:
    ///
    /// - by a wasm module,
    /// - by the host environment and passed to a wasm module as an import.
    ///   See more in [`Externals`].
    ///
    /// [`Externals`]: trait.Externals.html
    pub struct FuncInstance(FuncInstanceInternal);

    #[derive(Clone)]
    pub(crate) enum FuncInstanceInternal {
        Internal {
            signature: Rc<Signature>,
            module: Weak<ModuleInstance>,
            body: Rc<FuncBody>,
        },
        Host {
            signature: Signature,
            host_func_index: usize,
        },
    }

    impl fmt::Debug for FuncInstance {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match self.as_internal() {
                &FuncInstanceInternal::Internal { ref signature, .. } => {
                    // We can't write description of self.module here, because it generate
                    // debug string for function instances and this will lead to infinite loop.
                    write!(f, "Internal {{ signature={:?} }}", signature,)
                }
                &FuncInstanceInternal::Host { ref signature, .. } => {
                    write!(f, "Host {{ signature={:?} }}", signature)
                }
            }
        }
    }

    impl FuncInstance {
        /// Allocate a function instance for a host function.
        ///
        /// When this function instance will be called by the wasm code,
        /// the instance of [`Externals`] will be invoked by calling `invoke_index`
        /// with specified `host_func_index` here.
        /// This call will be made with the `signature` provided here.
        ///
        /// [`Externals`]: trait.Externals.html
        pub fn alloc_host(signature: Signature, host_func_index: usize) -> FuncRef {
            let func = FuncInstanceInternal::Host {
                signature,
                host_func_index,
            };
            FuncRef(Rc::new(FuncInstance(func)))
        }

        /// Returns [signature] of this function instance.
        ///
        /// This function instance can only be called with matching signatures.
        ///
        /// [signature]: struct.Signature.html
        pub fn signature(&self) -> &Signature {
            match *self.as_internal() {
                FuncInstanceInternal::Internal { ref signature, .. } => signature,
                FuncInstanceInternal::Host { ref signature, .. } => signature,
            }
        }

        pub(crate) fn as_internal(&self) -> &FuncInstanceInternal {
            &self.0
        }

        pub(crate) fn alloc_internal(
            module: Weak<ModuleInstance>,
            signature: Rc<Signature>,
            body: FuncBody,
        ) -> FuncRef {
            let func = FuncInstanceInternal::Internal {
                signature,
                module: module,
                body: Rc::new(body),
            };
            FuncRef(Rc::new(FuncInstance(func)))
        }

        pub(crate) fn body(&self) -> Option<Rc<FuncBody>> {
            match *self.as_internal() {
                FuncInstanceInternal::Internal { ref body, .. } => Some(Rc::clone(body)),
                FuncInstanceInternal::Host { .. } => None,
            }
        }

        /// Invoke this function.
        ///
        /// # Errors
        ///
        /// Returns `Err` if `args` types is not match function [`signature`] or
        /// if [`Trap`] at execution time occured.
        ///
        /// [`signature`]: #method.signature
        /// [`Trap`]: #enum.Trap.html
        pub fn invoke<E: Externals>(
            func: &FuncRef,
            args: &[RuntimeValue],
            externals: &mut E,
        ) -> Result<Option<RuntimeValue>, Trap> {
            check_function_args(func.signature(), &args)?;
            match *func.as_internal() {
                FuncInstanceInternal::Internal { .. } => {
                    let mut interpreter = Interpreter::new(func, args, None)?;
                    interpreter.start_execution(externals)
                }
                FuncInstanceInternal::Host {
                    ref host_func_index,
                    ..
                } => externals.invoke_index(*host_func_index, args.into()),
            }
        }

        /// Invoke this function using recycled stacks.
        ///
        /// # Errors
        ///
        /// Same as [`invoke`].
        ///
        /// [`invoke`]: #method.invoke
        pub fn invoke_with_stack<E: Externals>(
            func: &FuncRef,
            args: &[RuntimeValue],
            externals: &mut E,
            stack_recycler: &mut StackRecycler,
        ) -> Result<Option<RuntimeValue>, Trap> {
            check_function_args(func.signature(), &args)?;
            match *func.as_internal() {
                FuncInstanceInternal::Internal { .. } => {
                    let mut interpreter = Interpreter::new(func, args, Some(stack_recycler))?;
                    let return_value = interpreter.start_execution(externals);
                    stack_recycler.recycle(interpreter);
                    return_value
                }
                FuncInstanceInternal::Host {
                    ref host_func_index,
                    ..
                } => externals.invoke_index(*host_func_index, args.into()),
            }
        }

        /// Invoke the function, get a resumable handle. This handle can then be used to [`start_execution`]. If a
        /// Host trap happens, caller can use [`resume_execution`] to feed the expected return value back in, and then
        /// continue the execution.
        ///
        /// This is an experimental API, and this functionality may not be available in other WebAssembly engines.
        ///
        /// # Errors
        ///
        /// Returns `Err` if `args` types is not match function [`signature`].
        ///
        /// [`signature`]: #method.signature
        /// [`Trap`]: #enum.Trap.html
        /// [`start_execution`]: struct.FuncInvocation.html#method.start_execution
        /// [`resume_execution`]: struct.FuncInvocation.html#method.resume_execution
        pub fn invoke_resumable<'args>(
            func: &FuncRef,
            args: impl Into<Cow<'args, [RuntimeValue]>>,
        ) -> Result<FuncInvocation<'args>, Trap> {
            let args = args.into();
            check_function_args(func.signature(), &args)?;
            match *func.as_internal() {
                FuncInstanceInternal::Internal { .. } => {
                    let interpreter = Interpreter::new(func, &*args, None)?;
                    Ok(FuncInvocation {
                        kind: FuncInvocationKind::Internal(interpreter),
                    })
                }
                FuncInstanceInternal::Host {
                    ref host_func_index,
                    ..
                } => Ok(FuncInvocation {
                    kind: FuncInvocationKind::Host {
                        args,
                        host_func_index: *host_func_index,
                        finished: false,
                    },
                }),
            }
        }
    }

    /// A resumable invocation error.
    #[derive(Debug)]
    pub enum ResumableError {
        /// Trap happened.
        Trap(Trap),
        /// The invocation is not resumable.
        ///
        /// Invocations are only resumable if a host function is called, and the host function returns a trap of `Host` kind. For other cases, this error will be returned. This includes:
        /// - The invocation is directly a host function.
        /// - The invocation has not been started.
        /// - The invocation returns normally or returns any trap other than `Host` kind.
        ///
        /// This error is returned by [`resume_execution`].
        ///
        /// [`resume_execution`]: struct.FuncInvocation.html#method.resume_execution
        NotResumable,
        /// The invocation has already been started.
        ///
        /// This error is returned by [`start_execution`].
        ///
        /// [`start_execution`]: struct.FuncInvocation.html#method.start_execution
        AlreadyStarted,
    }

    impl From<Trap> for ResumableError {
        fn from(trap: Trap) -> Self {
            ResumableError::Trap(trap)
        }
    }

    /// A resumable invocation handle. This struct is returned by `FuncInstance::invoke_resumable`.
    pub struct FuncInvocation<'args> {
        kind: FuncInvocationKind<'args>,
    }

    enum FuncInvocationKind<'args> {
        Internal(Interpreter),
        Host {
            args: Cow<'args, [RuntimeValue]>,
            host_func_index: usize,
            finished: bool,
        },
    }

    impl<'args> FuncInvocation<'args> {
        /// Whether this invocation is currently resumable.
        pub fn is_resumable(&self) -> bool {
            match &self.kind {
                &FuncInvocationKind::Internal(ref interpreter) => interpreter.state().is_resumable(),
                &FuncInvocationKind::Host { .. } => false,
            }
        }

        /// If the invocation is resumable, the expected return value type to be feed back in.
        pub fn resumable_value_type(&self) -> Option<ValueType> {
            match &self.kind {
                &FuncInvocationKind::Internal(ref interpreter) => match interpreter.state() {
                    &InterpreterState::Resumable(ref value_type) => value_type.clone(),
                    _ => None,
                },
                &FuncInvocationKind::Host { .. } => None,
            }
        }

        /// Start the invocation execution.
        pub fn start_execution<'externals, E: Externals + 'externals>(
            &mut self,
            externals: &'externals mut E,
        ) -> Result<Option<RuntimeValue>, ResumableError> {
            match self.kind {
                FuncInvocationKind::Internal(ref mut interpreter) => {
                    if interpreter.state() != &InterpreterState::Initialized {
                        return Err(ResumableError::AlreadyStarted);
                    }
                    Ok(interpreter.start_execution(externals)?)
                }
                FuncInvocationKind::Host {
                    ref args,
                    ref mut finished,
                    ref host_func_index,
                } => {
                    if *finished {
                        return Err(ResumableError::AlreadyStarted);
                    }
                    *finished = true;
                    Ok(externals.invoke_index(*host_func_index, args.as_ref().into())?)
                }
            }
        }

        /// Resume an execution if a previous trap of Host kind happened.
        ///
        /// `return_val` must be of the value type [`resumable_value_type`], defined by the host function import. Otherwise,
        /// `UnexpectedSignature` trap will be returned. The current invocation must also be resumable
        /// [`is_resumable`]. Otherwise, a `NotResumable` error will be returned.
        ///
        /// [`resumable_value_type`]: #method.resumable_value_type
        /// [`is_resumable`]: #method.is_resumable
        pub fn resume_execution<'externals, E: Externals + 'externals>(
            &mut self,
            return_val: Option<RuntimeValue>,
            externals: &'externals mut E,
        ) -> Result<Option<RuntimeValue>, ResumableError> {
            use crate::TrapKind;

            if return_val.map(|v| v.value_type()) != self.resumable_value_type() {
                return Err(ResumableError::Trap(Trap::new(
                    TrapKind::UnexpectedSignature,
                )));
            }

            match &mut self.kind {
                FuncInvocationKind::Internal(interpreter) => {
                    if interpreter.state().is_resumable() {
                        Ok(interpreter.resume_execution(return_val, externals)?)
                    } else {
                        Err(ResumableError::AlreadyStarted)
                    }
                }
                FuncInvocationKind::Host { .. } => Err(ResumableError::NotResumable),
            }
        }
    }

    #[derive(Clone, Debug)]
    pub struct FuncBody {
        pub locals: Vec<Local>,
        pub code: isa::Instructions,
    }

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