//! A C API for benchmarking Wasmtime's WebAssembly compilation, instantiation,
//! and execution.
//!
//! The API expects sequential calls to:
//!
//!  - `wasm_bench_create`
//!  - `wasm_bench_compile`
//!  - `wasm_bench_instantiate`
//!  - `wasm_bench_execute`
//!  - `wasm_bench_free`
//!
//! You may repeat this sequence of calls multiple times to take multiple
//! measurements of compilation, instantiation, and execution time within a
//! single process.
//!
//! All API calls must happen on the same thread.
//!
//! Functions which return pointers use null as an error value. Function which
//! return `int` use `0` as OK and non-zero as an error value.
//!
//! # Example
//!
//! ```
//! use wasmtime_bench_api::*;
//!
//! let engine = unsafe { wasm_bench_create() };
//! assert!(!engine.is_null());
//!
//! let wasm = wat::parse_bytes(br#"
//!     (module
//!         (func $bench_start (import "bench" "start"))
//!         (func $bench_end (import "bench" "end"))
//!         (func $start (export "_start")
//!             call $bench_start
//!             i32.const 1
//!             i32.const 2
//!             i32.add
//!             drop
//!             call $bench_end
//!         )
//!     )
//! "#).unwrap();
//!
//! // Start your compilation timer here.
//! let code = unsafe { wasm_bench_compile(engine, wasm.as_ptr(), wasm.len()) };
//! // End your compilation timer here.
//! assert_eq!(code, OK);
//!
//! // The Wasm benchmark will expect us to provide functions to start ("bench"
//! // "start") and stop ("bench" "stop") the measurement counters/timers during
//! // execution.
//! extern "C" fn bench_start() {
//!     // Start your execution timer here.
//! }
//! extern "C" fn bench_stop() {
//!     // End your execution timer here.
//! }
//!
//! // Start your instantiation timer here.
//! let code = unsafe { wasm_bench_instantiate(engine, bench_start, bench_stop) };
//! // End your instantiation timer here.
//! assert_eq!(code, OK);
//!
//! // No need to start timers for the execution since, by convention, the timer
//! // functions we passed during instantiation will be called by the benchmark
//! // at the appropriate time (before and after the benchmarked section).
//! let code = unsafe { wasm_bench_execute(engine) };
//! assert_eq!(code, OK);
//!
//! unsafe {
//!     wasm_bench_free(engine);
//! }
//! ```

use anyhow::{anyhow, Context, Result};
use std::os::raw::{c_int, c_void};
use std::slice;
use wasi_common::WasiCtxBuilder;
use wasmtime::{Config, Engine, Instance, Linker, Module, Store};
use wasmtime_wasi::Wasi;

pub type ExitCode = c_int;
pub const OK: ExitCode = 0;
pub const ERR: ExitCode = -1;

/// Exposes a C-compatible way of creating the engine from the bytes of a single
/// Wasm module.
///
/// This function returns a pointer to a structure that contains the engine's
/// initialized state.
#[no_mangle]
pub extern "C" fn wasm_bench_create() -> *mut c_void {
    let state = Box::new(BenchState::new());
    Box::into_raw(state) as _
}

/// Free the engine state allocated by this library.
#[no_mangle]
pub extern "C" fn wasm_bench_free(state: *mut c_void) {
    assert!(!state.is_null());
    unsafe {
        Box::from_raw(state as *mut BenchState);
    }
}

/// Compile the Wasm benchmark module.
#[no_mangle]
pub extern "C" fn wasm_bench_compile(
    state: *mut c_void,
    wasm_bytes: *const u8,
    wasm_bytes_length: usize,
) -> ExitCode {
    let state = unsafe { (state as *mut BenchState).as_mut().unwrap() };
    let wasm_bytes = unsafe { slice::from_raw_parts(wasm_bytes, wasm_bytes_length) };
    let result = state.compile(wasm_bytes).context("failed to compile");
    to_exit_code(result)
}

/// Instantiate the Wasm benchmark module.
#[no_mangle]
pub extern "C" fn wasm_bench_instantiate(
    state: *mut c_void,
    bench_start: extern "C" fn(),
    bench_end: extern "C" fn(),
) -> ExitCode {
    let state = unsafe { (state as *mut BenchState).as_mut().unwrap() };
    let result = state
        .instantiate(bench_start, bench_end)
        .context("failed to instantiate");
    to_exit_code(result)
}

/// Execute the Wasm benchmark module.
#[no_mangle]
pub extern "C" fn wasm_bench_execute(state: *mut c_void) -> ExitCode {
    let state = unsafe { (state as *mut BenchState).as_mut().unwrap() };
    let result = state.execute().context("failed to execute");
    to_exit_code(result)
}

/// Helper function for converting a Rust result to a C error code.
///
/// This will print an error indicating some information regarding the failure.
fn to_exit_code<T>(result: impl Into<Result<T>>) -> ExitCode {
    match result.into() {
        Ok(_) => OK,
        Err(error) => {
            eprintln!("{:?}", error);
            ERR
        }
    }
}

/// This structure contains the actual Rust implementation of the state required
/// to manage the Wasmtime engine between calls.
struct BenchState {
    engine: Engine,
    store: Store,
    module: Option<Module>,
    instance: Option<Instance>,
    did_execute: bool,
}

impl BenchState {
    fn new() -> Self {
        let mut config = Config::new();
        config.wasm_simd(true);
        // NB: do not configure a code cache.

        let engine = Engine::new(&config);
        let store = Store::new(&engine);
        Self {
            engine,
            store,
            module: None,
            instance: None,
            did_execute: false,
        }
    }

    fn compile(&mut self, bytes: &[u8]) -> Result<()> {
        assert!(
            self.module.is_none(),
            "create a new engine to repeat compilation"
        );
        self.module = Some(Module::from_binary(&self.engine, bytes)?);
        Ok(())
    }

    fn instantiate(
        &mut self,
        bench_start: extern "C" fn(),
        bench_end: extern "C" fn(),
    ) -> Result<()> {
        assert!(
            self.instance.is_none(),
            "create a new engine to repeat instantiation"
        );
        let module = self
            .module
            .as_mut()
            .expect("compile the module before instantiating it");

        let mut linker = Linker::new(&self.store);

        // Import a very restricted WASI environment.
        let mut cx = WasiCtxBuilder::new();
        cx.inherit_stdio();
        let cx = cx.build()?;
        let wasi = Wasi::new(linker.store(), cx);
        wasi.add_to_linker(&mut linker)?;

        // Import the specialized benchmarking functions.
        linker.func("bench", "start", move || bench_start())?;
        linker.func("bench", "end", move || bench_end())?;

        self.instance = Some(linker.instantiate(&module)?);
        Ok(())
    }

    fn execute(&mut self) -> Result<()> {
        assert!(!self.did_execute, "create a new engine to repeat execution");
        self.did_execute = true;

        let instance = self
            .instance
            .as_ref()
            .expect("instantiate the module before executing it");

        let start_func = instance.get_func("_start").expect("a _start function");
        let runnable_func = start_func.get0::<()>()?;
        match runnable_func() {
            Ok(_) => Ok(()),
            Err(trap) => {
                // Since _start will likely return by using the system `exit` call, we must
                // check the trap code to see if it actually represents a successful exit.
                match trap.i32_exit_status() {
                    Some(0) => Ok(()),
                    Some(n) => Err(anyhow!("_start exited with a non-zero code: {}", n)),
                    None => Err(anyhow!(
                        "executing the benchmark resulted in a trap: {}",
                        trap
                    )),
                }
            }
        }
    }
}