//! Encoding and decoding QR codes.

#![allow(clippy::expect_fun_call)]

use keyfork_bug as bug;

use bug::POISONED_MUTEX;
use image::{ImageBuffer, ImageReader, Luma};
use std::{
    io::{Cursor, Write},
    process::{Command, Stdio},
    sync::{mpsc::channel, Arc, Condvar, Mutex},
    time::{Duration, Instant},
};
use v4l::{
    buffer::Type,
    io::{traits::CaptureStream, userptr::Stream},
    video::Capture,
    Device, FourCC,
};

type Image = ImageBuffer<Luma<u8>, Vec<u8>>;

/// A QR code could not be generated.
#[derive(thiserror::Error, Debug)]
pub enum QRGenerationError {
    /// The resulting QR coode could not be read from the generator program.
    #[error("{0}")]
    Io(#[from] std::io::Error),

    /// The generator program produced invalid data.
    #[error("Could not decode output of qrencode (this is a bug!): {0}")]
    StringParse(#[from] std::string::FromUtf8Error),
}

/// An error occurred while scanning for a QR code.
#[derive(thiserror::Error, Debug)]
pub enum QRCodeScanError {
    /// The camera could not load the requested format.
    #[error("Camera could not use {expected} format, instead used {actual}")]
    CameraGaveBadFormat {
        /// The expected format, in FourCC format.
        expected: String,

        /// The actual format, in FourCC format.
        actual: String,
    },

    /// Interfacing with the camera resulted in an error.
    #[error("Unable to interface with camera: {0}")]
    CameraIO(#[from] std::io::Error),

    /// Decoding an image from the camera resulted in an error.
    #[error("Could not decode image: {0}")]
    ImageDecode(#[from] image::ImageError),
}

/// The level of error correction when generating a QR code.
#[derive(Default)]
pub enum ErrorCorrection {
    /// 7% of the QR code can be recovered.
    #[default]
    Lowest,

    /// 15% of the QR code can be recovered.
    Medium,

    /// 25% of the QR code can be recovered.
    Quartile,

    /// 30% of the QR code can be recovered.
    Highest,
}

/// Generate a terminal-printable QR code for a given string. Uses the `qrencode` CLI utility.
///
/// # Errors
/// The function may return an error if interacting with the QR code generation program fails.
pub fn qrencode(
    text: &str,
    error_correction: impl Into<Option<ErrorCorrection>>,
) -> Result<String, QRGenerationError> {
    let error_correction_arg = match error_correction.into().unwrap_or_default() {
        ErrorCorrection::Lowest => "L",
        ErrorCorrection::Medium => "M",
        ErrorCorrection::Quartile => "Q",
        ErrorCorrection::Highest => "H",
    };

    let mut qrencode = Command::new("qrencode")
        .arg("-t")
        .arg("ansiutf8")
        .arg("-m")
        .arg("2")
        .arg("-l")
        .arg(error_correction_arg)
        .stdin(Stdio::piped())
        .stdout(Stdio::piped())
        .spawn()?;
    if let Some(stdin) = qrencode.stdin.as_mut() {
        stdin.write_all(text.as_bytes())?;
    }
    let output = qrencode.wait_with_output()?;
    let result = String::from_utf8(output.stdout)?;
    Ok(result)
}

const VIDEO_FORMAT_READ_ERROR: &str = "Failed to read video device format";

trait Scanner {
    fn scan_image(&mut self, image: Image) -> Option<String>;
}

#[cfg(feature = "decode-backend-zbar")]
mod zbar {
    use super::{Image, Scanner};

    pub struct Zbar {
        scanner: keyfork_zbar::image_scanner::ImageScanner,
    }

    impl Zbar {
        #[allow(dead_code)]
        pub fn new() -> Self {
            Self::default()
        }
    }

    impl Default for Zbar {
        fn default() -> Self {
            Self {
                scanner: keyfork_zbar::image_scanner::ImageScanner::new(),
            }
        }
    }

    impl Scanner for Zbar {
        fn scan_image(&mut self, image: Image) -> Option<String> {
            let image = keyfork_zbar::image::Image::from(image);
            self.scanner
                .scan_image(&image)
                .into_iter()
                .next()
                .map(|symbol| String::from_utf8_lossy(symbol.data()).into())
        }
    }
}

#[cfg(feature = "decode-backend-rqrr")]
mod rqrr {
    use super::{Image, Scanner};

    pub struct Rqrr;

    impl Scanner for Rqrr {
        fn scan_image(&mut self, image: Image) -> Option<String> {
            let mut image = rqrr::PreparedImage::prepare(image);
            for grid in image.detect_grids() {
                if let Ok((_, content)) = grid.decode() {
                    return Some(content);
                }
            }
            None
        }
    }
}

#[allow(dead_code)]
fn dbg_elapsed(count: u64, instant: Instant) {
    let elapsed = instant.elapsed().as_secs();
    let framerate = count as f64 / elapsed as f64;
    eprintln!("framerate: {count}/{elapsed} = {framerate}");
    std::thread::sleep(std::time::Duration::from_secs(5));
}

#[derive(Debug)]
struct ScanQueue {
    shutdown: bool,
    images: Vec<Image>,
}

/// Continuously scan the `index`-th camera for a QR code.
///
/// # Errors
///
/// The function may return an error if the hardware is unable to scan video or if an image could
/// not be decoded.
///
/// # Panics
///
/// The function may panic if a mutex is poisoned by a thread panicking, which should
/// only happen during a mutex, or if it can't send a message over the mpsc channel.
pub fn scan_camera(timeout: Duration, index: usize) -> Result<Option<String>, QRCodeScanError> {
    let device = Device::new(index)?;
    let mut fmt = device
        .format()
        .unwrap_or_else(bug::panic!(VIDEO_FORMAT_READ_ERROR));
    fmt.fourcc = FourCC::new(b"MPG1");
    device.set_format(&fmt)?;
    let mut stream = Stream::with_buffers(&device, Type::VideoCapture, 4)?;
    let start = Instant::now();

    #[allow(unused)]
    let mut count = 0;

    let thread_count = 4;

    std::thread::scope(|scope| {
        let scan_queue = ScanQueue {
            shutdown: false,
            images: vec![],
        };

        let arced = Arc::new((Mutex::new(scan_queue), Condvar::new()));
        let (tx, rx) = channel();

        for _ in 0..thread_count {
            let tx = tx.clone();
            let arced = arced.clone();

            scope.spawn(move || {
                cfg_if::cfg_if! {
                    if #[cfg(feature = "decode-backend-zbar")] {
                        let mut scanner = zbar::Zbar::default();
                    } else if #[cfg(feature = "decode-backend-rqrr")] {
                        let mut scanner = rqrr::Rqrr;
                    } else {
                        unimplemented!("neither decode-backend-zbar nor decode-backend-rqrr were selected")
                    }
                };

                let (queue_mutex, condvar) = &*arced;
                loop {
                    // NOTE: Carrying the `queue` variable through the loop, so we can
                    // pass it through without re-locking, means that we don't drop the
                    // lock on the mutex. Therefore, we unlock, then immediately
                    // re-lock when we pass the value to wait_while().
                    //
                    // By holding onto the queue until we pass it back to the Condvar,
                    // and checking shutdown, we ensure that there's no way we miss the
                    // shutdown being set before we release the guard on the queue.
                    let queue = queue_mutex.lock().expect(bug::bug!(POISONED_MUTEX));
                    if queue.shutdown {
                        break;
                    }
                    let mut queue = condvar
                        .wait_while(queue, |queue| {
                            queue.images.is_empty() && !queue.shutdown
                        })
                        .expect(bug::bug!(POISONED_MUTEX));
                    if let Some(image) = queue.images.pop() {
                        // drop the queue here since this is what's going to take time
                        drop(queue);
                        if let Some(content) = scanner.scan_image(image) {
                            if tx.send(content).is_err() {
                                break;
                            }
                        }
                    }
                }
            });
        }

        while Instant::now().duration_since(start) < timeout {
            if let Ok(content) = rx.try_recv() {
                arced.0.lock().expect(bug::bug!(POISONED_MUTEX)).shutdown = true;
                arced.1.notify_all();
                return Ok(Some(content));
            }

            count += 1;
            let (buffer, _) = stream.next()?;
            let image = ImageReader::new(Cursor::new(buffer))
                .with_guessed_format()?
                .decode()?
                .to_luma8();

            arced
                .0
                .lock()
                .expect(bug::bug!(POISONED_MUTEX))
                .images
                .push(image);
            arced.1.notify_one();
        }

        // dbg_elapsed(count, start);

        arced.0.lock().expect(bug::bug!(POISONED_MUTEX)).shutdown = true;
        arced.1.notify_all();

        Ok(None)
    })
}