use std::{
    io::Read,
    sync::{atomic::AtomicBool, Arc},
};

use ahash::HashMap;
use once_cell::sync::Lazy;

use crate::{DataLoader, LoadedData};

// ---

/// To register a new external data loader, simply add an executable in your $PATH whose name
/// starts with this prefix.
// NOTE: this constant is duplicated in `rerun` to avoid an extra dependency there.
pub const EXTERNAL_DATA_LOADER_PREFIX: &str = "rerun-loader-";

/// When an external [`crate::DataLoader`] is asked to load some data that it doesn't know
/// how to load, it should exit with this exit code.
// NOTE: Always keep in sync with other languages.
// NOTE: this constant is duplicated in `rerun` to avoid an extra dependency there.
pub const EXTERNAL_DATA_LOADER_INCOMPATIBLE_EXIT_CODE: i32 = 66;

/// Keeps track of the paths all external executable [`crate::DataLoader`]s.
///
/// Lazy initialized the first time a file is opened by running a full scan of the `$PATH`.
///
/// External loaders are _not_ registered on a per-extension basis: we want users to be able to
/// filter data on a much more fine-grained basis that just file extensions (e.g. checking the file
/// itself for magic bytes).
pub static EXTERNAL_LOADER_PATHS: Lazy<Vec<std::path::PathBuf>> = Lazy::new(|| {
    re_tracing::profile_scope!("initialize-external-loaders");

    let dir_separator = if cfg!(target_os = "windows") {
        ';'
    } else {
        ':'
    };

    let dirpaths = std::env::var("PATH")
        .ok()
        .into_iter()
        .flat_map(|paths| {
            paths
                .split(dir_separator)
                .map(ToOwned::to_owned)
                .collect::<Vec<_>>()
        })
        .map(std::path::PathBuf::from);

    let mut executables = HashMap::<String, Vec<std::path::PathBuf>>::default();
    for dirpath in dirpaths {
        re_tracing::profile_scope!("dir", dirpath.to_string_lossy());
        let Ok(dir) = std::fs::read_dir(dirpath) else {
            continue;
        };
        let paths = dir.into_iter().filter_map(|entry| {
            let Ok(entry) = entry else {
                return None;
            };
            let filepath = entry.path();
            let is_rerun_loader = filepath.file_name().map_or(false, |filename| {
                filename
                    .to_string_lossy()
                    .starts_with(EXTERNAL_DATA_LOADER_PREFIX)
            });
            (filepath.is_file() && is_rerun_loader).then_some(filepath)
        });

        for path in paths {
            if let Some(filename) = path.file_name() {
                let exe_paths = executables
                    .entry(filename.to_string_lossy().to_string())
                    .or_default();
                exe_paths.push(path.clone());
            }
        }
    }

    executables
        .into_iter()
        .filter_map(|(name, paths)| {
            if paths.len() > 1 {
                re_log::debug!(name, ?paths, "Found duplicated data-loader in $PATH");
            }

            // Only keep the first entry according to PATH order.
            paths.into_iter().next()
        })
        .collect()
});

/// Iterator over all registered external [`crate::DataLoader`]s.
#[inline]
pub fn iter_external_loaders() -> impl ExactSizeIterator<Item = std::path::PathBuf> {
    re_tracing::profile_wait!("EXTERNAL_LOADER_PATHS");
    EXTERNAL_LOADER_PATHS.iter().cloned()
}

// ---

/// A [`crate::DataLoader`] that forwards the path to load to all executables present in
/// the user's `PATH` with a name that starts with [`EXTERNAL_DATA_LOADER_PREFIX`].
///
/// The external loaders are expected to log rrd data to their standard output.
///
/// Refer to our `external_data_loader` example for more information.
///
/// Checkout our [guide](https://www.rerun.io/docs/reference/data-loaders/overview) on
/// how to implement external loaders.
pub struct ExternalLoader;

impl crate::DataLoader for ExternalLoader {
    #[inline]
    fn name(&self) -> String {
        "rerun.data_loaders.External".into()
    }

    fn load_from_path(
        &self,
        settings: &crate::DataLoaderSettings,
        filepath: std::path::PathBuf,
        tx: std::sync::mpsc::Sender<crate::LoadedData>,
    ) -> Result<(), crate::DataLoaderError> {
        use std::process::{Command, Stdio};

        re_tracing::profile_function!(filepath.display().to_string());

        let external_loaders = {
            re_tracing::profile_wait!("EXTERNAL_LOADER_PATHS");
            EXTERNAL_LOADER_PATHS.iter()
        };

        #[derive(PartialEq, Eq)]
        struct CompatibleLoaderFound;
        let (tx_feedback, rx_feedback) = std::sync::mpsc::channel::<CompatibleLoaderFound>();

        let args = settings.to_cli_args();
        for exe in external_loaders {
            let args = args.clone();
            let filepath = filepath.clone();
            let tx = tx.clone();
            let tx_feedback = tx_feedback.clone();

            // NOTE: This is completely IO bound (spawning and waiting for child process), it must run on a
            // dedicated thread, not the shared rayon thread pool.
            _ = std::thread::Builder::new().name(exe.to_string_lossy().to_string()).spawn(move || {
                re_tracing::profile_function!(exe.to_string_lossy());

                let child = Command::new(exe)
                    // Make sure the child dataloader doesn't think it's a Rerun Viewer, otherwise
                    // it's never gonna be able to log anything.
                    .env_remove("RERUN_APP_ONLY")
                    .arg(filepath.clone())
                    .args(args)
                    .stdout(Stdio::piped())
                    .stderr(Stdio::piped())
                    .spawn();

                let mut child = match child {
                    Ok(child) => child,
                    Err(err) => {
                        re_log::error!(?filepath, loader = ?exe, %err, "Failed to execute external loader");
                        return;
                    }
                };

                let Some(stdout) = child.stdout.take() else {
                    let reason = "stdout unreachable";
                    re_log::error!(?filepath, loader = ?exe, %reason, "Failed to execute external loader");
                    return;
                };
                let Some(stderr) = child.stderr.take() else {
                    let reason = "stderr unreachable";
                    re_log::error!(?filepath, loader = ?exe, %reason, "Failed to execute external loader");
                    return;
                };

                re_log::debug!(?filepath, loader = ?exe, "Loading data from filesystem using external loader…",);

                // A single value will be sent on this channel as soon as the child process starts
                // streaming data to stdout.
                let is_sending_data = Arc::new(AtomicBool::new(false));

                let version_policy = re_log_encoding::decoder::VersionPolicy::Warn;
                let stdout = std::io::BufReader::new(stdout);
                match re_log_encoding::decoder::Decoder::new(version_policy, stdout) {
                    Ok(decoder) => {
                        let filepath = filepath.clone();
                        let tx = tx.clone();
                        // NOTE: This is completely IO bound, it must run on a dedicated thread, not the shared
                        // rayon thread pool.
                        if let Err(err) = std::thread::Builder::new()
                            .name(format!("decode_and_stream({filepath:?})"))
                            .spawn({
                                let filepath = filepath.clone();
                                let is_sending_data = Arc::clone(&is_sending_data);
                                move || decode_and_stream(&filepath, &tx, is_sending_data, decoder)
                            })
                        {
                            re_log::error!(?filepath, loader = ?exe, %err, "Failed to open spawn IO thread");
                            return;
                        }
                    }
                    Err(re_log_encoding::decoder::DecodeError::Read(_)) => {
                        // The child was not interested in that file and left without logging
                        // anything.
                        // That's fine, we just need to make sure to check its exit status further
                        // down, still.
                    }
                    Err(err) => {
                        re_log::error!(?filepath, loader = ?exe, %err, "Failed to decode external loader's output");
                        return;
                    }
                };

                // We have to wait in order to know whether the child process is a compatible loader.
                //
                // This can manifest itself in two distinct ways:
                // 1. If it exits immediately with an INCOMPATIBLE exit code, then we have our
                //   answer straight away.
                // - If it starts streaming data, then we immediately assume it's compatible.
                loop {
                    re_tracing::profile_scope!("waiting for compatibility");

                    match child.try_wait() {
                        Ok(Some(_)) => break,
                        Ok(None) => {
                            if is_sending_data.load(std::sync::atomic::Ordering::Relaxed) {
                                // The child process has started streaming data, it is therefore compatible.
                                // Let's get out ASAP.
                                re_log::debug!(loader = ?exe, ?filepath, "compatible external loader found");
                                tx_feedback.send(CompatibleLoaderFound).ok();
                                break; // we still want to check for errors once it finally exits!
                            }

                            // NOTE: This will busy loop if there's no work available in the native OS thread-pool.
                            std::thread::yield_now();

                            continue;
                        }
                        Err(err) => {
                            re_log::error!(?filepath, loader = ?exe, %err, "Failed to execute external loader");
                            return;
                        }
                    };
                }

                // NOTE: `try_wait` and `wait` are idempotent.
                let status = match child.wait() {
                    Ok(output) => output,
                    Err(err) => {
                        re_log::error!(?filepath, loader = ?exe, %err, "Failed to execute external loader");
                        return;
                    }
                };

                // NOTE: We assume that plugins are compatible until proven otherwise.
                let is_compatible =
                    status.code() != Some(crate::EXTERNAL_DATA_LOADER_INCOMPATIBLE_EXIT_CODE);

                if is_compatible && !status.success() {
                    let mut stderr = std::io::BufReader::new(stderr);
                    let mut reason = String::new();
                    stderr.read_to_string(&mut reason).ok();
                    re_log::error!(?filepath, loader = ?exe, %reason, "Failed to execute external loader");
                }

                if is_compatible {
                    re_log::debug!(loader = ?exe, ?filepath, "compatible external loader found");
                    tx_feedback.send(CompatibleLoaderFound).ok();
                }
            })?;
        }

        re_tracing::profile_wait!("compatible_loader");

        drop(tx_feedback);

        let any_compatible_loader = rx_feedback.recv() == Ok(CompatibleLoaderFound);
        if !any_compatible_loader {
            // NOTE: The only way to get here is if all loaders closed then sending end of the
            // channel without sending anything, i.e. none of them are compatible.
            return Err(crate::DataLoaderError::Incompatible(filepath.clone()));
        }

        Ok(())
    }

    #[inline]
    fn load_from_file_contents(
        &self,
        _settings: &crate::DataLoaderSettings,
        path: std::path::PathBuf,
        _contents: std::borrow::Cow<'_, [u8]>,
        _tx: std::sync::mpsc::Sender<crate::LoadedData>,
    ) -> Result<(), crate::DataLoaderError> {
        // TODO(#5324): You could imagine a world where plugins can be streamed rrd data via their
        // standard input… but today is not world.
        Err(crate::DataLoaderError::Incompatible(path))
    }
}

#[allow(clippy::needless_pass_by_value)]
fn decode_and_stream<R: std::io::Read>(
    filepath: &std::path::Path,
    tx: &std::sync::mpsc::Sender<crate::LoadedData>,
    is_sending_data: Arc<AtomicBool>,
    decoder: re_log_encoding::decoder::Decoder<R>,
) {
    re_tracing::profile_function!(filepath.display().to_string());

    for msg in decoder {
        is_sending_data.store(true, std::sync::atomic::Ordering::Relaxed);

        let msg = match msg {
            Ok(msg) => msg,
            Err(err) => {
                re_log::warn_once!("Failed to decode message in {filepath:?}: {err}");
                continue;
            }
        };

        let data = LoadedData::LogMsg(ExternalLoader::name(&ExternalLoader), msg);
        if tx.send(data).is_err() {
            break; // The other end has decided to hang up, not our problem.
        }
    }
}