use std::future::Future;

use std::{fs::File, marker::PhantomData, path::Path};

use anyhow::{anyhow, Error};
use serde::{de::DeserializeOwned, Serialize};

use wasi_common::{dir, file};
use wasmtime::Memory;
use wasmtime::{
    AsContext, AsContextMut, Caller, Config, Engine, Extern, Instance, Linker, Module, Store, Trap,
    TypedFunc,
};
use wasmtime_wasi::{Dir, WasiCtx, WasiCtxBuilder};

/// Represents a resource currently managed by the plugin, like a file descriptor.
pub struct PluginResource(u32);

/// This is the buffer that is used Host side.
/// Note that it mirrors the functionality of
/// the `__Buffer` found in the `plugin/src/lib.rs` prelude.
struct WasiBuffer {
    ptr: u32,
    len: u32,
}

impl WasiBuffer {
    pub fn into_u64(self) -> u64 {
        ((self.ptr as u64) << 32) | (self.len as u64)
    }

    pub fn from_u64(packed: u64) -> Self {
        WasiBuffer {
            ptr: (packed >> 32) as u32,
            len: packed as u32,
        }
    }
}

/// Represents a typed WebAssembly function.
pub struct WasiFn<A: Serialize, R: DeserializeOwned> {
    function: TypedFunc<u64, u64>,
    _function_type: PhantomData<fn(A) -> R>,
}

impl<A: Serialize, R: DeserializeOwned> Copy for WasiFn<A, R> {}

impl<A: Serialize, R: DeserializeOwned> Clone for WasiFn<A, R> {
    fn clone(&self) -> Self {
        Self {
            function: self.function,
            _function_type: PhantomData,
        }
    }
}

pub struct Metering {
    initial: u64,
    refill: u64,
}

impl Default for Metering {
    fn default() -> Self {
        Metering {
            initial: 1000,
            refill: 1000,
        }
    }
}

/// This struct is used to build a new [`Plugin`], using the builder pattern.
/// Create a new default plugin with `PluginBuilder::new_with_default_ctx`,
/// and add host-side exported functions using `host_function` and `host_function_async`.
/// Finalize the plugin by calling [`init`].
pub struct PluginBuilder {
    wasi_ctx: WasiCtx,
    engine: Engine,
    linker: Linker<WasiCtxAlloc>,
    metering: Metering,
}

/// Creates an engine with the default configuration.
/// N.B. This must create an engine with the same config as the one
/// in `plugin_runtime/build.rs`.
fn create_default_engine() -> Result<Engine, Error> {
    let mut config = Config::default();
    config.async_support(true);
    config.consume_fuel(true);
    Engine::new(&config)
}

impl PluginBuilder {
    /// Create a new [`PluginBuilder`] with the given WASI context.
    /// Using the default context is a safe bet, see [`new_with_default_context`].
    /// This plugin will yield after a configurable amount of fuel is consumed.
    pub fn new(wasi_ctx: WasiCtx, metering: Metering) -> Result<Self, Error> {
        let engine = create_default_engine()?;
        let linker = Linker::new(&engine);

        Ok(PluginBuilder {
            wasi_ctx,
            engine,
            linker,
            metering,
        })
    }

    /// Create a new `PluginBuilder` with the default `WasiCtx` (see [`default_ctx`]).
    /// This plugin will yield after a configurable amount of fuel is consumed.
    pub fn new_default() -> Result<Self, Error> {
        let default_ctx = WasiCtxBuilder::new()
            .inherit_stdout()
            .inherit_stderr()
            .build();
        let metering = Metering::default();
        Self::new(default_ctx, metering)
    }

    /// Add an `async` host function. See [`host_function`] for details.
    pub fn host_function_async<F, A, R, Fut>(
        mut self,
        name: &str,
        function: F,
    ) -> Result<Self, Error>
    where
        F: Fn(A) -> Fut + Send + Sync + 'static,
        Fut: Future<Output = R> + Send + 'static,
        A: DeserializeOwned + Send + 'static,
        R: Serialize + Send + Sync + 'static,
    {
        self.linker.func_wrap1_async(
            "env",
            &format!("__{}", name),
            move |mut caller: Caller<'_, WasiCtxAlloc>, packed_buffer: u64| {
                // TODO: use try block once available
                let result: Result<(WasiBuffer, Memory, _), Trap> = (|| {
                    // grab a handle to the memory
                    let plugin_memory = match caller.get_export("memory") {
                        Some(Extern::Memory(mem)) => mem,
                        _ => return Err(Trap::new("Could not grab slice of plugin memory"))?,
                    };

                    let buffer = WasiBuffer::from_u64(packed_buffer);

                    // get the args passed from Guest
                    let args =
                        Plugin::buffer_to_bytes(&plugin_memory, caller.as_context(), &buffer)?;

                    let args: A = Plugin::deserialize_to_type(args)?;

                    // Call the Host-side function
                    let result = function(args);

                    Ok((buffer, plugin_memory, result))
                })();

                Box::new(async move {
                    let (buffer, mut plugin_memory, future) = result?;

                    let result: R = future.await;
                    let result: Result<Vec<u8>, Error> = Plugin::serialize_to_bytes(result)
                        .map_err(|_| {
                            Trap::new("Could not serialize value returned from function").into()
                        });
                    let result = result?;

                    Plugin::buffer_to_free(caller.data().free_buffer(), &mut caller, buffer)
                        .await?;

                    let buffer = Plugin::bytes_to_buffer(
                        caller.data().alloc_buffer(),
                        &mut plugin_memory,
                        &mut caller,
                        result,
                    )
                    .await?;

                    Ok(buffer.into_u64())
                })
            },
        )?;
        Ok(self)
    }

    /// Add a new host function to the given `PluginBuilder`.
    /// A host function is a function defined host-side, in Rust,
    /// that is accessible guest-side, in WebAssembly.
    /// You can specify host-side functions to import using
    /// the `#[input]` macro attribute:
    /// ```ignore
    /// #[input]
    /// fn total(counts: Vec<f64>) -> f64;
    /// ```
    /// When loading a plugin, you need to provide all host functions the plugin imports:
    /// ```ignore
    /// let plugin = PluginBuilder::new_with_default_context()
    ///     .host_function("total", |counts| counts.iter().fold(0.0, |tot, n| tot + n))
    ///     // and so on...
    /// ```
    /// And that's a wrap!
    pub fn host_function<A, R>(
        mut self,
        name: &str,
        function: impl Fn(A) -> R + Send + Sync + 'static,
    ) -> Result<Self, Error>
    where
        A: DeserializeOwned + Send,
        R: Serialize + Send + Sync,
    {
        self.linker.func_wrap1_async(
            "env",
            &format!("__{}", name),
            move |mut caller: Caller<'_, WasiCtxAlloc>, packed_buffer: u64| {
                // TODO: use try block once available
                let result: Result<(WasiBuffer, Memory, Vec<u8>), Trap> = (|| {
                    // grab a handle to the memory
                    let plugin_memory = match caller.get_export("memory") {
                        Some(Extern::Memory(mem)) => mem,
                        _ => return Err(Trap::new("Could not grab slice of plugin memory"))?,
                    };

                    let buffer = WasiBuffer::from_u64(packed_buffer);

                    // get the args passed from Guest
                    let args = Plugin::buffer_to_type(&plugin_memory, &mut caller, &buffer)?;

                    // Call the Host-side function
                    let result: R = function(args);

                    // Serialize the result back to guest
                    let result = Plugin::serialize_to_bytes(result).map_err(|_| {
                        Trap::new("Could not serialize value returned from function")
                    })?;

                    Ok((buffer, plugin_memory, result))
                })();

                Box::new(async move {
                    let (buffer, mut plugin_memory, result) = result?;

                    Plugin::buffer_to_free(caller.data().free_buffer(), &mut caller, buffer)
                        .await?;

                    let buffer = Plugin::bytes_to_buffer(
                        caller.data().alloc_buffer(),
                        &mut plugin_memory,
                        &mut caller,
                        result,
                    )
                    .await?;

                    Ok(buffer.into_u64())
                })
            },
        )?;
        Ok(self)
    }

    /// Initializes a [`Plugin`] from a given compiled Wasm module.
    /// Both binary (`.wasm`) and text (`.wat`) module formats are supported.
    pub async fn init(self, binary: PluginBinary<'_>) -> Result<Plugin, Error> {
        Plugin::init(binary, self).await
    }
}

#[derive(Copy, Clone)]
struct WasiAlloc {
    alloc_buffer: TypedFunc<u32, u32>,
    free_buffer: TypedFunc<u64, ()>,
}

struct WasiCtxAlloc {
    wasi_ctx: WasiCtx,
    alloc: Option<WasiAlloc>,
}

impl WasiCtxAlloc {
    fn alloc_buffer(&self) -> TypedFunc<u32, u32> {
        self.alloc
            .expect("allocator has been not initialized, cannot allocate buffer!")
            .alloc_buffer
    }

    fn free_buffer(&self) -> TypedFunc<u64, ()> {
        self.alloc
            .expect("allocator has been not initialized, cannot free buffer!")
            .free_buffer
    }

    fn init_alloc(&mut self, alloc: WasiAlloc) {
        self.alloc = Some(alloc)
    }
}

pub enum PluginBinary<'a> {
    Wasm(&'a [u8]),
    Precompiled(&'a [u8]),
}

/// Represents a WebAssembly plugin, with access to the WebAssembly System Interface.
/// Build a new plugin using [`PluginBuilder`].
pub struct Plugin {
    store: Store<WasiCtxAlloc>,
    instance: Instance,
}

impl Plugin {
    /// Dumps the *entirety* of Wasm linear memory to `stdout`.
    /// Don't call this unless you're debugging a memory issue!
    pub fn dump_memory(data: &[u8]) {
        for (i, byte) in data.iter().enumerate() {
            if i % 32 == 0 {
                println!();
            }
            if i % 4 == 0 {
                print!("|");
            }
            if *byte == 0 {
                print!("__")
            } else {
                print!("{:02x}", byte);
            }
        }
        println!();
    }

    async fn init(binary: PluginBinary<'_>, plugin: PluginBuilder) -> Result<Self, Error> {
        // initialize the WebAssembly System Interface context
        let engine = plugin.engine;
        let mut linker = plugin.linker;
        wasmtime_wasi::add_to_linker(&mut linker, |s| &mut s.wasi_ctx)?;

        // create a store, note that we can't initialize the allocator,
        // because we can't grab the functions until initialized.
        let mut store: Store<WasiCtxAlloc> = Store::new(
            &engine,
            WasiCtxAlloc {
                wasi_ctx: plugin.wasi_ctx,
                alloc: None,
            },
        );

        let module = match binary {
            PluginBinary::Precompiled(bytes) => unsafe { Module::deserialize(&engine, bytes)? },
            PluginBinary::Wasm(bytes) => Module::new(&engine, bytes)?,
        };

        // set up automatic yielding based on configuration
        store.add_fuel(plugin.metering.initial).unwrap();
        store.out_of_fuel_async_yield(u64::MAX, plugin.metering.refill);

        // load the provided module into the asynchronous runtime
        linker.module_async(&mut store, "", &module).await?;
        let instance = linker.instantiate_async(&mut store, &module).await?;

        // now that the module is initialized,
        // we can initialize the store's allocator
        let alloc_buffer = instance.get_typed_func(&mut store, "__alloc_buffer")?;
        let free_buffer = instance.get_typed_func(&mut store, "__free_buffer")?;
        store.data_mut().init_alloc(WasiAlloc {
            alloc_buffer,
            free_buffer,
        });

        Ok(Plugin { store, instance })
    }

    /// Attaches a file or directory the the given system path to the runtime.
    /// Note that the resource must be freed by calling `remove_resource` afterwards.
    pub fn attach_path<T: AsRef<Path>>(&mut self, path: T) -> Result<PluginResource, Error> {
        // grab the WASI context
        let ctx = self.store.data_mut();

        // open the file we want, and convert it into the right type
        // this is a footgun and a half
        let file = File::open(&path).unwrap();
        let dir = Dir::from_std_file(file);
        let dir = Box::new(wasmtime_wasi::dir::Dir::from_cap_std(dir));

        // grab an empty file descriptor, specify capabilities
        let fd = ctx.wasi_ctx.table().push(Box::new(()))?;
        let caps = dir::DirCaps::all();
        let file_caps = file::FileCaps::all();

        // insert the directory at the given fd,
        // return a handle to the resource
        ctx.wasi_ctx
            .insert_dir(fd, dir, caps, file_caps, path.as_ref().to_path_buf());
        Ok(PluginResource(fd))
    }

    /// Returns `true` if the resource existed and was removed.
    /// Currently the only resource we support is adding scoped paths (e.g. folders and files)
    /// to plugins using [`attach_path`].
    pub fn remove_resource(&mut self, resource: PluginResource) -> Result<(), Error> {
        self.store
            .data_mut()
            .wasi_ctx
            .table()
            .delete(resource.0)
            .ok_or_else(|| anyhow!("Resource did not exist, but a valid handle was passed in"))?;
        Ok(())
    }

    // So this call function is kinda a dance, I figured it'd be a good idea to document it.
    // the high level is we take a serde type, serialize it to a byte array,
    // (we're doing this using bincode for now)
    // then toss that byte array into webassembly.
    // webassembly grabs that byte array, does some magic,
    // and serializes the result into yet another byte array.
    // we then grab *that* result byte array and deserialize it into a result.
    //
    // phew...
    //
    // now the problem is, webassambly doesn't support buffers.
    // only really like i32s, that's it (yeah, it's sad. Not even unsigned!)
    // (ok, I'm exaggerating a bit).
    //
    // the Wasm function that this calls must have a very specific signature:
    //
    // fn(pointer to byte array: i32, length of byte array: i32)
    //     -> pointer to (
    //            pointer to byte_array: i32,
    //            length of byte array: i32,
    //     ): i32
    //
    // This pair `(pointer to byte array, length of byte array)` is called a `Buffer`
    // and can be found in the cargo_test plugin.
    //
    // so on the wasm side, we grab the two parameters to the function,
    // stuff them into a `Buffer`,
    // and then pray to the `unsafe` Rust gods above that a valid byte array pops out.
    //
    // On the flip side, when returning from a wasm function,
    // we convert whatever serialized result we get into byte array,
    // which we stuff into a Buffer and allocate on the heap,
    // which pointer to we then return.
    // Note the double indirection!
    //
    // So when returning from a function, we actually leak memory *twice*:
    //
    // 1) once when we leak the byte array
    // 2) again when we leak the allocated `Buffer`
    //
    // This isn't a problem because Wasm stops executing after the function returns,
    // so the heap is still valid for our inspection when we want to pull things out.

    /// Serializes a given type to bytes.
    fn serialize_to_bytes<A: Serialize>(item: A) -> Result<Vec<u8>, Error> {
        // serialize the argument using bincode
        let bytes = bincode::serialize(&item)?;
        Ok(bytes)
    }

    /// Deserializes a given type from bytes.
    fn deserialize_to_type<R: DeserializeOwned>(bytes: &[u8]) -> Result<R, Error> {
        // serialize the argument using bincode
        let bytes = bincode::deserialize(bytes)?;
        Ok(bytes)
    }

    // fn deserialize<R: DeserializeOwned>(
    //     plugin_memory: &mut Memory,
    //     mut store: impl AsContextMut<Data = WasiCtxAlloc>,
    //     buffer: WasiBuffer,
    // ) -> Result<R, Error> {
    //     let buffer_start = buffer.ptr as usize;
    //     let buffer_end = buffer_start + buffer.len as usize;

    //     // read the buffer at this point into a byte array
    //     // deserialize the byte array into the provided serde type
    //     let item = &plugin_memory.data(store.as_context())[buffer_start..buffer_end];
    //     let item = bincode::deserialize(bytes)?;
    //     Ok(item)
    // }

    /// Takes an item, allocates a buffer, serializes the argument to that buffer,
    /// and returns a (ptr, len) pair to that buffer.
    async fn bytes_to_buffer(
        alloc_buffer: TypedFunc<u32, u32>,
        plugin_memory: &mut Memory,
        mut store: impl AsContextMut<Data = WasiCtxAlloc>,
        item: Vec<u8>,
    ) -> Result<WasiBuffer, Error> {
        // allocate a buffer and write the argument to that buffer
        let len = item.len() as u32;
        let ptr = alloc_buffer.call_async(&mut store, len).await?;
        plugin_memory.write(&mut store, ptr as usize, &item)?;
        Ok(WasiBuffer { ptr, len })
    }

    /// Takes a `(ptr, len)` pair and returns the corresponding deserialized buffer.
    fn buffer_to_type<R: DeserializeOwned>(
        plugin_memory: &Memory,
        store: impl AsContext<Data = WasiCtxAlloc>,
        buffer: &WasiBuffer,
    ) -> Result<R, Error> {
        let buffer_start = buffer.ptr as usize;
        let buffer_end = buffer_start + buffer.len as usize;

        // read the buffer at this point into a byte array
        // deserialize the byte array into the provided serde type
        let result = &plugin_memory.data(store.as_context())[buffer_start..buffer_end];
        let result = bincode::deserialize(result)?;

        Ok(result)
    }

    /// Takes a `(ptr, len)` pair and returns the corresponding deserialized buffer.
    fn buffer_to_bytes<'a>(
        plugin_memory: &'a Memory,
        store: wasmtime::StoreContext<'a, WasiCtxAlloc>,
        buffer: &'a WasiBuffer,
    ) -> Result<&'a [u8], Error> {
        let buffer_start = buffer.ptr as usize;
        let buffer_end = buffer_start + buffer.len as usize;

        // read the buffer at this point into a byte array
        // deserialize the byte array into the provided serde type
        let result = &plugin_memory.data(store)[buffer_start..buffer_end];
        Ok(result)
    }

    async fn buffer_to_free(
        free_buffer: TypedFunc<u64, ()>,
        mut store: impl AsContextMut<Data = WasiCtxAlloc>,
        buffer: WasiBuffer,
    ) -> Result<(), Error> {
        // deallocate the argument buffer
        Ok(free_buffer
            .call_async(&mut store, buffer.into_u64())
            .await?)
    }

    /// Retrieves the handle to a function of a given type.
    pub fn function<A: Serialize, R: DeserializeOwned, T: AsRef<str>>(
        &mut self,
        name: T,
    ) -> Result<WasiFn<A, R>, Error> {
        let fun_name = format!("__{}", name.as_ref());
        let fun = self
            .instance
            .get_typed_func::<u64, u64, _>(&mut self.store, &fun_name)?;
        Ok(WasiFn {
            function: fun,
            _function_type: PhantomData,
        })
    }

    /// Asynchronously calls a function defined Guest-side.
    pub async fn call<A: Serialize, R: DeserializeOwned>(
        &mut self,
        handle: &WasiFn<A, R>,
        arg: A,
    ) -> Result<R, Error> {
        let mut plugin_memory = self
            .instance
            .get_memory(&mut self.store, "memory")
            .ok_or_else(|| anyhow!("Could not grab slice of plugin memory"))?;

        // write the argument to linear memory
        // this returns a (ptr, length) pair
        let arg_buffer = Self::bytes_to_buffer(
            self.store.data().alloc_buffer(),
            &mut plugin_memory,
            &mut self.store,
            Self::serialize_to_bytes(arg)?,
        )
        .await?;

        // call the function, passing in the buffer and its length
        // this returns a ptr to a (ptr, length) pair
        let result_buffer = handle
            .function
            .call_async(&mut self.store, arg_buffer.into_u64())
            .await?;

        Self::buffer_to_type(
            &plugin_memory,
            &mut self.store,
            &WasiBuffer::from_u64(result_buffer),
        )
    }
}
