//! A downloader that supports HTTP range requests and resuming downloads

use std::{
    error::Error,
    path::Path,
    pin::Pin,
    task::{Poll, ready},
    time::Duration,
};

use reqwest::header::{CONTENT_LENGTH, HeaderValue, RANGE};
use tokio::{
    fs::{self, File},
    io::{self, AsyncSeek, AsyncSeekExt, AsyncWrite, AsyncWriteExt, BufWriter},
};

use thiserror::Error;

/// Start value of the read timeout. This is doubled on each retry.
const READ_TIMEOUT: Duration = Duration::from_secs(1);
const CONNECT_TIMEOUT: Duration = Duration::from_secs(30);
// Maximum number of retry attempts for timeouts
const MAX_RETRY_ATTEMPTS: u32 = 4;

/// Custom error type for download operations
#[derive(Error, Debug)]
pub enum DownloadError {
    /// Failed to initialize client
    #[error("Failed to initialize HTTP client")]
    ClientInitialization(#[source] reqwest::Error),

    /// Failed to get content length
    #[error("Failed to request download")]
    HeadRequest(#[source] reqwest::Error),

    /// Server returned error status
    #[error("Download failed: {0}")]
    HttpStatus(reqwest::StatusCode),

    /// Invalid content length header
    #[error("Invalid content length header: {0}")]
    InvalidContentLength(&'static str),

    /// Failed to make range request
    #[error("Failed to retrieve range")]
    RangeRequest(#[source] reqwest::Error),

    /// Failed to read chunk
    #[error("Failed to read chunk")]
    ChunkRead(#[source] reqwest::Error),

    /// Failed to write chunk
    #[error("Failed to write chunk")]
    ChunkWrite(#[source] io::Error),

    /// Failed to get stream position
    #[error("Failed to get existing file size")]
    StreamPosition(#[source] io::Error),

    /// Failed to flush writer
    #[error("Failed to flush writer")]
    Flush(#[source] io::Error),

    /// Size validation error
    #[error("Size validation failed: {0}")]
    SizeValidation(String),

    /// File operation error
    #[error("File operation failed: {0}")]
    FileOperation(#[source] io::Error),

    /// Other error
    #[error("{0}")]
    Other(&'static str),
}

impl DownloadError {
    /// Checks if the error is caused by a timeout or network issue that can be retried
    pub fn should_retry(&self) -> bool {
        match self {
            DownloadError::HeadRequest(e)
            | DownloadError::RangeRequest(e)
            | DownloadError::ChunkRead(e)
            | DownloadError::ClientInitialization(e) => is_network_error(e),
            DownloadError::HttpStatus(status) => {
                // Retry server errors and timeout status
                status.is_server_error() || *status == reqwest::StatusCode::REQUEST_TIMEOUT
            }
            // Don't retry other types of errors
            _ => false,
        }
    }
}

/// Checks if the error is a network-related error that can be retried
fn is_network_error(error: &reqwest::Error) -> bool {
    // Retry on timeout errors
    // Retry on connection errors (which often happen when switching networks)
    // Retry on request errors (like "connection reset")
    if error.is_timeout() || error.is_connect() || error.is_request() {
        return true;
    }

    let mut error = error as &dyn Error;
    loop {
        if let Some(io_err) = error.downcast_ref::<std::io::Error>() {
            // Check if the error is a timeout or connection error
            if io_err.kind() == io::ErrorKind::TimedOut
                || io_err.kind() == io::ErrorKind::ConnectionReset
            {
                return true;
            }
        }
        if let Some(source) = error.source() {
            error = source;
        } else {
            break false;
        }
    }
}

/// Receiver of the current progress so far
pub trait ProgressUpdater: Send + 'static {
    /// Progress so far
    fn set_progress(&mut self, fraction_complete: f32);

    /// Clear progress so far
    fn clear_progress(&mut self);

    /// URL that is being downloaded
    fn set_url(&mut self, url: &str);
}

/// This describes how to handle files that do not match an expected size
#[derive(Debug, Clone, Copy)]
pub enum SizeHint {
    /// Fail if the resulting file does not exactly match the expected size.
    Exact(usize),
    /// Fail if the resulting file is larger than the specified limit.
    Maximum(usize),
}

impl SizeHint {
    /// This function succeeds if `actual` is allowed according to the [SizeHint]. Otherwise, it
    /// returns an error.
    fn check_size(&self, actual: usize) -> anyhow::Result<()> {
        match *self {
            SizeHint::Exact(expected) if actual != expected => {
                anyhow::bail!("File size mismatch: expected {expected} bytes, served {actual}")
            }
            SizeHint::Maximum(limit) if actual > limit => {
                anyhow::bail!(
                    "File size exceeds limit: expected at most {limit} bytes, served {actual}"
                )
            }
            _ => Ok(()),
        }
    }
}

/// Download `url` to `file`. If the file already exists, this appends to it, as long
/// as the file pointed to by `url` is larger than it.
///
/// Make sure that `file` is stored in a secure directory.
///
/// # Arguments
/// - `progress_updater` - This interface is notified of download progress.
/// - `size_hint` - File size restrictions.
pub async fn get_to_file(
    file: impl AsRef<Path>,
    url: &str,
    progress_updater: &mut impl ProgressUpdater,
    size_hint: SizeHint,
) -> anyhow::Result<()> {
    let file = create_or_append(file)
        .await
        .map_err(DownloadError::FileOperation)?;
    let mut file = BufWriter::new(file);
    let mut attempts = 0;
    let mut read_timeout = READ_TIMEOUT;
    while let Err(err) =
        get_to_writer(&mut file, url, progress_updater, size_hint, read_timeout).await
    {
        if !err.should_retry() {
            anyhow::bail!(err);
        }
        attempts += 1;
        read_timeout *= 2;
        if attempts >= MAX_RETRY_ATTEMPTS {
            anyhow::bail!("Max retry attempts reached: {err}");
        }
        log::warn!("Download failed: {err}. Retrying in with timeout: {read_timeout:?}");
    }
    Ok(())
}

/// Download `url` to `writer`.
///
/// # Arguments
/// - `progress_updater` - This interface is notified of download progress.
/// - `size_hint` - File size restrictions.
pub async fn get_to_writer(
    mut writer: impl AsyncWrite + AsyncSeek + Unpin,
    url: &str,
    progress_updater: &mut impl ProgressUpdater,
    size_hint: SizeHint,
    read_timeout: Duration,
) -> Result<(), DownloadError> {
    // Create a new client for each download attempt to prevent stale connections
    let client = reqwest::Client::builder()
        .read_timeout(read_timeout)
        .connect_timeout(CONNECT_TIMEOUT)
        .build()
        .map_err(DownloadError::ClientInitialization)?;

    progress_updater.set_url(url);

    // Fetch content length first
    let response = client
        .head(url)
        .send()
        .await
        .map_err(DownloadError::HeadRequest)?;

    if !response.status().is_success() {
        return Err(DownloadError::HttpStatus(response.status()));
    }

    let total_size = response
        .headers()
        .get(CONTENT_LENGTH)
        .ok_or_else(|| DownloadError::InvalidContentLength("Missing file size"))?;

    let total_size: usize = total_size
        .to_str()
        .map_err(|_| DownloadError::InvalidContentLength("Invalid content length header"))?
        .parse()
        .map_err(|_| DownloadError::InvalidContentLength("Invalid size format"))?;

    match size_hint.check_size(total_size) {
        Ok(_) => {}
        Err(e) => return Err(DownloadError::SizeValidation(e.to_string())),
    }

    let already_fetched_bytes = writer
        .stream_position()
        .await
        .map_err(DownloadError::StreamPosition)?
        .try_into()
        .map_err(|_| DownloadError::Other("Invalid file position"))?;

    progress_updater.set_progress(already_fetched_bytes as f32 / total_size as f32);
    if total_size == already_fetched_bytes {
        return Ok(());
    }
    if already_fetched_bytes > total_size {
        return Err(DownloadError::SizeValidation(
            "Found existing file that was larger".to_string(),
        ));
    }

    // Fetch content, one range at a time
    let mut writer = WriterWithProgress {
        writer,
        progress_updater,
        written_nbytes: already_fetched_bytes,
        total_nbytes: total_size,
    };

    for range in RangeIter::new(already_fetched_bytes, total_size) {
        let mut response = client
            .get(url)
            .header(RANGE, range)
            .send()
            .await
            .map_err(DownloadError::RangeRequest)?;

        let status = response.status();
        if !status.is_success() {
            return Err(DownloadError::HttpStatus(status));
        }

        let mut bytes_read = 0;

        while let Some(chunk) = response.chunk().await.map_err(DownloadError::ChunkRead)? {
            bytes_read += chunk.len();
            if bytes_read > total_size - already_fetched_bytes {
                // Protect against servers responding with more data than expected
                return Err(DownloadError::SizeValidation(
                    "Server returned more than requested bytes".to_string(),
                ));
            }

            writer
                .write_all(&chunk)
                .await
                .map_err(DownloadError::ChunkWrite)?;
        }
    }

    writer.shutdown().await.map_err(DownloadError::Flush)?;

    Ok(())
}

/// If a file exists, append to it. Otherwise, create a new file
async fn create_or_append(path: impl AsRef<Path>) -> io::Result<File> {
    match fs::File::create_new(&path).await {
        // New file created
        Ok(file) => Ok(file),
        // Append to an existing file
        Err(_err) => {
            let mut file = fs::OpenOptions::new().append(true).open(path).await?;
            // Seek to end, or else the seek position might be wrong
            file.seek(io::SeekFrom::End(0)).await?;
            Ok(file)
        }
    }
}

/// Used to download partial content
struct RangeIter {
    current: usize,
    end: usize,
}

impl RangeIter {
    fn new(current: usize, end: usize) -> Self {
        Self { current, end }
    }
}

impl Iterator for RangeIter {
    type Item = HeaderValue;

    fn next(&mut self) -> Option<Self::Item> {
        if self.current > self.end {
            return None;
        }
        let prev = self.current;

        let read_n = self.end.saturating_sub(self.current);
        if read_n == 0 {
            return None;
        }

        self.current += read_n;

        // NOTE: Subtracting 1 because range includes final byte
        let end = self.current - 1;

        Some(HeaderValue::from_str(&format!("bytes={prev}-{end}")).expect("valid range/str"))
    }
}

struct WriterWithProgress<'a, PU: ProgressUpdater, Writer> {
    writer: Writer,
    progress_updater: &'a mut PU,
    written_nbytes: usize,
    /// Actual or estimated total number of bytes
    total_nbytes: usize,
}

impl<PU: ProgressUpdater, Writer: AsyncWrite + Unpin> AsyncWrite
    for WriterWithProgress<'_, PU, Writer>
{
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
        buf: &[u8],
    ) -> Poll<Result<usize, io::Error>> {
        let file = Pin::new(&mut self.writer);
        let nbytes = ready!(file.poll_write(cx, buf))?;

        let total_nbytes = self.total_nbytes;
        let total_written = self.written_nbytes + nbytes;

        self.written_nbytes = total_written;
        self.progress_updater
            .set_progress(total_written as f32 / total_nbytes as f32);

        Poll::Ready(Ok(nbytes))
    }

    fn poll_flush(
        mut self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> Poll<Result<(), std::io::Error>> {
        Pin::new(&mut self.writer).poll_flush(cx)
    }

    fn poll_shutdown(
        mut self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> Poll<Result<(), std::io::Error>> {
        Pin::new(&mut self.writer).poll_shutdown(cx)
    }
}

#[cfg(test)]
mod test {
    use std::io::Cursor;

    use anyhow::Context;
    use async_tempfile::TempDir;
    use rand::RngCore;
    use tokio::{fs, io::AsyncWriteExt};

    use super::*;

    #[tokio::test]
    async fn test_create_or_append() -> anyhow::Result<()> {
        let temp_dir = TempDir::new().await?;
        let file_path = temp_dir.join("test");

        // Write to a new file
        const CONTENT: &[u8] = b"very important file";

        let mut file = create_or_append(&file_path).await?;
        file.write_all(CONTENT).await?;
        file.flush().await?;
        drop(file);

        assert_eq!(fs::read(&file_path).await?, CONTENT);

        // Verify that we can trust the stream position
        let mut file = create_or_append(&file_path).await?;
        let content_len: u64 = CONTENT.len().try_into()?;
        assert_eq!(file.stream_position().await?, content_len);
        drop(file);

        // Append some more stuff
        const EXTRA: &[u8] = b"my addition";

        let mut file = create_or_append(&file_path).await?;
        file.write_all(EXTRA).await?;
        file.flush().await?;
        drop(file);

        // Append occurred correctly
        const COMPLETE_STRING: &[u8] = b"very important filemy addition";
        assert_eq!(fs::read(file_path).await?, COMPLETE_STRING);

        Ok(())
    }

    #[derive(Default)]
    struct FakeProgressUpdater {
        complete: f32,
        url: String,
    }

    impl ProgressUpdater for FakeProgressUpdater {
        fn set_progress(&mut self, fraction_complete: f32) {
            self.complete = fraction_complete;
        }

        fn clear_progress(&mut self) {
            self.complete = 0.;
        }

        fn set_url(&mut self, url: &str) {
            self.url = url.to_owned();
        }
    }

    /// Test that [get_to_writer] correctly downloads new files
    #[tokio::test]
    async fn test_fetch_complete() -> anyhow::Result<()> {
        // Generate random data
        let file_data = Box::leak(Box::new(vec![0u8; 1024 * 1024 + 1]));
        rand::rng().fill_bytes(file_data);

        // Start server
        let mut server = mockito::Server::new_async().await;
        let file_url = format!("{}/my_file", server.url());
        add_file_server_mock(&mut server, "/my_file", file_data);

        // Download the file to `writer` and compare it to `file_data`
        let mut writer = Cursor::new(vec![]);
        let mut progress_updater = FakeProgressUpdater::default();

        get_to_writer(
            &mut writer,
            &file_url,
            &mut progress_updater,
            SizeHint::Exact(file_data.len()),
            READ_TIMEOUT,
        )
        .await
        .context("Complete download failed")?;

        assert_eq!(progress_updater.url, file_url);
        assert_eq!(progress_updater.complete, 1.);
        assert_eq!(&mut writer.into_inner(), file_data);

        Ok(())
    }

    /// Test that [get_to_writer] correctly downloads partial files
    #[tokio::test]
    async fn test_fetch_interrupted() -> anyhow::Result<()> {
        // Generate random data
        let file_data = Box::leak(Box::new(vec![0u8; 1024 * 1024]));
        rand::rng().fill_bytes(file_data);

        // Start server
        let mut server = mockito::Server::new_async().await;
        let file_url = format!("{}/my_file", server.url());
        add_file_server_mock(&mut server, "/my_file", file_data);

        // Interrupt after exactly half the file has been downloaded
        let mut buffer = vec![0u8; file_data.len() / 2];
        let mut limited_writer = Cursor::new(&mut buffer[..]);

        let mut progress_updater = FakeProgressUpdater::default();

        get_to_writer(
            &mut limited_writer,
            &file_url,
            &mut progress_updater,
            SizeHint::Exact(file_data.len()),
            READ_TIMEOUT,
        )
        .await
        .expect_err("Expected interrupted download");

        assert_eq!(progress_updater.url, file_url);

        let completed = progress_updater.complete;
        assert!(
            (completed - 0.5).abs() < f32::EPSILON,
            "expected half to be completed, got {completed}"
        );

        assert_eq!(
            &*buffer,
            &file_data[..buffer.len()],
            "partial download incorrect"
        );

        // Download the remainder
        let partial_len = buffer.len();
        let mut writer = Cursor::new(buffer);
        writer.set_position(partial_len as u64);

        let mut progress_updater = FakeProgressUpdater::default();

        get_to_writer(
            &mut writer,
            &file_url,
            &mut progress_updater,
            SizeHint::Exact(file_data.len()),
            READ_TIMEOUT,
        )
        .await
        .context("Partial download failed")?;

        assert_eq!(progress_updater.url, file_url);
        assert_eq!(progress_updater.complete, 1.);
        assert_eq!(&mut writer.into_inner(), file_data);

        Ok(())
    }

    /// Create endpoints that serve a file at `url_path` using HTTP range requests
    fn add_file_server_mock(server: &mut mockito::Server, url_path: &str, data: &'static [u8]) {
        // Respond to head requests with file size
        server
            .mock("HEAD", url_path)
            .with_header(CONTENT_LENGTH, &data.len().to_string())
            .create();

        // Respond to HTTP range requests with file
        server
            .mock("GET", url_path)
            .with_body_from_request(|request| {
                let range = request.header(RANGE);
                let range = range[0].to_str().expect("expected str");
                let (begin, end) = parse_http_range(range).expect("invalid range");

                data[begin..=end].to_vec()
            })
            .create();
    }

    /// Parse a range header value, e.g. "bytes=0-31"
    fn parse_http_range(val: &str) -> anyhow::Result<(usize, usize)> {
        // parse: bytes=0-31
        let (_, val) = val.split_once('=').context("invalid range header")?;
        let (begin, end) = val.split_once('-').context("invalid range")?;

        let begin: usize = begin.parse().context("invalid range begin")?;
        let end: usize = end.parse().context("invalid range end")?;

        Ok((begin, end))
    }

    /// Make sure unexpectedly large files are rejected
    #[tokio::test]
    async fn test_nefarious_sizes() -> anyhow::Result<()> {
        // Head length is too large
        let mut server = mockito::Server::new_async().await;
        let file_url = format!("{}/my_file", server.url());
        server
            .mock("HEAD", "/my_file")
            .with_header(CONTENT_LENGTH, "2")
            .create();

        get_to_writer(
            Cursor::new(vec![]),
            &file_url,
            &mut FakeProgressUpdater::default(),
            SizeHint::Exact(1),
            READ_TIMEOUT,
        )
        .await
        .expect_err("Reject unexpected content length");

        // Reject larger than expected files
        let file_data = vec![0u8; 2];

        let mut server = mockito::Server::new_async().await;
        let file_url = format!("{}/my_file", server.url());
        server
            .mock("HEAD", "/my_file")
            // Lie about size in header
            .with_header(CONTENT_LENGTH, "1")
            .create();
        server
            .mock("GET", "/my_file")
            .with_body(&file_data)
            .create();

        get_to_writer(
            Cursor::new(vec![]),
            &file_url,
            &mut FakeProgressUpdater::default(),
            SizeHint::Exact(file_data.len()),
            READ_TIMEOUT,
        )
        .await
        .expect_err("Reject unexpected chunk sizes");

        Ok(())
    }
}