//! Decoding of farbfeld images
//!
//! farbfeld is a lossless image format which is easy to parse, pipe and compress.
//!
//! It has the following format:
//!
//! | Bytes  | Description                                             |
//! |--------|---------------------------------------------------------|
//! | 8      | "farbfeld" magic value                                  |
//! | 4      | 32-Bit BE unsigned integer (width)                      |
//! | 4      | 32-Bit BE unsigned integer (height)                     |
//! | [2222] | 4⋅16-Bit BE unsigned integers [RGBA] / pixel, row-major |
//!
//! The RGB-data should be sRGB for best interoperability and not alpha-premultiplied.
//!
//! # Related Links
//! * <https://tools.suckless.org/farbfeld/> - the farbfeld specification

use std::convert::TryFrom;
use std::i64;
use std::io::{self, Seek, SeekFrom, Read, Write, BufReader, BufWriter};

use byteorder::{BigEndian, ByteOrder, NativeEndian};

use crate::color::ColorType;
use crate::error::{DecodingError, ImageError, ImageResult, UnsupportedError, UnsupportedErrorKind};
use crate::image::{self, ImageDecoder, ImageDecoderExt, ImageEncoder, ImageFormat, Progress};

/// farbfeld Reader
pub struct FarbfeldReader<R: Read> {
    width: u32,
    height: u32,
    inner: BufReader<R>,
    /// Relative to the start of the pixel data
    current_offset: u64,
    cached_byte: Option<u8>,
}

impl<R: Read> FarbfeldReader<R> {
    fn new(reader: R) -> ImageResult<FarbfeldReader<R>> {
        fn read_dimm<R: Read>(from: &mut R) -> ImageResult<u32> {
            let mut buf = [0u8; 4];
            from.read_exact(&mut buf).map_err(|err|
                ImageError::Decoding(DecodingError::new(
                    ImageFormat::Farbfeld.into(),
                    err,
                )))?;
            Ok(BigEndian::read_u32(&buf))
        }

        let mut inner = BufReader::new(reader);

        let mut magic = [0u8; 8];
        inner.read_exact(&mut magic).map_err(|err|
            ImageError::Decoding(DecodingError::new(
                ImageFormat::Farbfeld.into(),
                err,
            )))?;
        if &magic != b"farbfeld" {
            return Err(ImageError::Decoding(DecodingError::new(
                ImageFormat::Farbfeld.into(),
                format!("Invalid magic: {:02x?}", magic),
            )));
        }

        Ok(FarbfeldReader {
            width: read_dimm(&mut inner)?,
            height: read_dimm(&mut inner)?,
            inner,
            current_offset: 0,
            cached_byte: None,
        })
    }
}

impl<R: Read> Read for FarbfeldReader<R> {
    fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
        let mut bytes_written = 0;
        if let Some(byte) = self.cached_byte.take() {
            buf[0] = byte;
            buf = &mut buf[1..];
            bytes_written = 1;
            self.current_offset += 1;
        }

        if buf.len() == 1 {
            buf[0] = cache_byte(&mut self.inner, &mut self.cached_byte)?;
            bytes_written += 1;
            self.current_offset += 1;
        } else {
            for channel_out in buf.chunks_exact_mut(2) {
                consume_channel(&mut self.inner, channel_out)?;
                bytes_written += 2;
                self.current_offset += 2;
            }
        }

        Ok(bytes_written)
    }
}

impl<R: Read + Seek> Seek for FarbfeldReader<R> {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
        fn parse_offset(original_offset: u64, end_offset: u64, pos: SeekFrom) -> Option<i64> {
            match pos {
                SeekFrom::Start(off) =>
                    i64::try_from(off).ok()?.checked_sub(i64::try_from(original_offset).ok()?),
                SeekFrom::End(off) =>
                    if off < i64::try_from(end_offset).unwrap_or(i64::MAX) {
                        None
                    } else {
                        Some(i64::try_from(end_offset.checked_sub(original_offset)?).ok()? + off)
                    },
                SeekFrom::Current(off) =>
                    if off < i64::try_from(original_offset).unwrap_or(i64::MAX) {
                        None
                    } else {
                        Some(off)
                    },
            }
        }

        let original_offset = self.current_offset;
        let end_offset = self.width as u64 * self.height as u64 * 2;
        let offset_from_current = parse_offset(original_offset, end_offset, pos)
            .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, "invalid seek to a negative or overflowing position"))?;

        // TODO: convert to seek_relative() once that gets stabilised
        self.inner.seek(SeekFrom::Current(offset_from_current))?;
        self.current_offset = if offset_from_current < 0 {
                                  original_offset.checked_sub(offset_from_current.wrapping_neg() as u64)
                              } else {
                                  original_offset.checked_add(offset_from_current as u64)
                              }.expect("This should've been checked above");

        if self.current_offset < end_offset && self.current_offset % 2 == 1 {
            let curr = self.inner.seek(SeekFrom::Current(-1))?;
            cache_byte(&mut self.inner, &mut self.cached_byte)?;
            self.inner.seek(SeekFrom::Start(curr))?;
        } else {
            self.cached_byte = None;
        }

        Ok(original_offset)
    }
}

fn consume_channel<R: Read>(from: &mut R, to: &mut [u8]) -> io::Result<()> {
    let mut ibuf = [0u8; 2];
    from.read_exact(&mut ibuf)?;
    NativeEndian::write_u16(to, BigEndian::read_u16(&ibuf));
    Ok(())
}

fn cache_byte<R: Read>(from: &mut R, cached_byte: &mut Option<u8>) -> io::Result<u8> {
    let mut obuf = [0u8; 2];
    consume_channel(from, &mut obuf)?;
    *cached_byte = Some(obuf[1]);
    Ok(obuf[0])
}

/// farbfeld decoder
pub struct FarbfeldDecoder<R: Read> {
    reader: FarbfeldReader<R>,
}

impl<R: Read> FarbfeldDecoder<R> {
    /// Creates a new decoder that decodes from the stream ```r```
    pub fn new(r: R) -> ImageResult<FarbfeldDecoder<R>> {
        Ok(FarbfeldDecoder { reader: FarbfeldReader::new(r)? })
    }
}

impl<'a, R: 'a + Read> ImageDecoder<'a> for FarbfeldDecoder<R> {
    type Reader = FarbfeldReader<R>;

    fn dimensions(&self) -> (u32, u32) {
        (self.reader.width, self.reader.height)
    }

    fn color_type(&self) -> ColorType {
        ColorType::Rgba16
    }

    fn into_reader(self) -> ImageResult<Self::Reader> {
        Ok(self.reader)
    }

    fn scanline_bytes(&self) -> u64 {
        2
    }
}

impl<'a, R: 'a + Read + Seek> ImageDecoderExt<'a> for FarbfeldDecoder<R> {
    fn read_rect_with_progress<F: Fn(Progress)>(
        &mut self,
        x: u32,
        y: u32,
        width: u32,
        height: u32,
        buf: &mut [u8],
        progress_callback: F,
    ) -> ImageResult<()> {
        // A "scanline" (defined as "shortest non-caching read" in the doc) is just one channel in this case

        let start = self.reader.seek(SeekFrom::Current(0))?;
        image::load_rect(x, y, width, height, buf, progress_callback, self,
                         |s, scanline| s.reader.seek(SeekFrom::Start(scanline * 2)).map(|_| ()),
                         |s, buf| s.reader.read_exact(buf))?;
        self.reader.seek(SeekFrom::Start(start))?;
        Ok(())
    }
}

/// farbfeld encoder
pub struct FarbfeldEncoder<W: Write> {
    w: BufWriter<W>,
}

impl<W: Write> FarbfeldEncoder<W> {
    /// Create a new encoder that writes its output to ```w```
    pub fn new(w: W) -> FarbfeldEncoder<W> {
        FarbfeldEncoder { w: BufWriter::new(w) }
    }

    /// Encodes the image ```data``` (native endian)
    /// that has dimensions ```width``` and ```height```
    pub fn encode(self, data: &[u8], width: u32, height: u32) -> ImageResult<()> {
        self.encode_impl(data, width, height)?;
        Ok(())
    }

    fn encode_impl(mut self, data: &[u8], width: u32, height: u32) -> io::Result<()> {
        self.w.write_all(b"farbfeld")?;

        let mut buf = [0u8; 4];
        BigEndian::write_u32(&mut buf, width);
        self.w.write_all(&buf)?;

        BigEndian::write_u32(&mut buf, height);
        self.w.write_all(&buf)?;

        for channel in data.chunks_exact(2) {
            BigEndian::write_u16(&mut buf, NativeEndian::read_u16(channel));
            self.w.write_all(&buf[..2])?;
        }

        Ok(())
    }
}

impl<W: Write> ImageEncoder for FarbfeldEncoder<W> {
    fn write_image(
        self,
        buf: &[u8],
        width: u32,
        height: u32,
        color_type: ColorType,
    ) -> ImageResult<()> {
        if color_type != ColorType::Rgba16 {
            return Err(ImageError::Unsupported(UnsupportedError::from_format_and_kind(
                ImageFormat::Farbfeld.into(),
                UnsupportedErrorKind::Color(color_type.into()),
            )));
        }

        self.encode(buf, width, height)
    }
}

#[cfg(test)]
mod tests {
    use crate::farbfeld::FarbfeldDecoder;
    use crate::ImageDecoderExt;
    use std::io::{Cursor, Seek, SeekFrom};
    use byteorder::{ByteOrder, NativeEndian};

    static RECTANGLE_IN: &[u8] =     b"farbfeld\
                                       \x00\x00\x00\x02\x00\x00\x00\x03\
                                       \xFF\x01\xFE\x02\xFD\x03\xFC\x04\xFB\x05\xFA\x06\xF9\x07\xF8\x08\
                                       \xF7\x09\xF6\x0A\xF5\x0B\xF4\x0C\xF3\x0D\xF2\x0E\xF1\x0F\xF0\x10\
                                       \xEF\x11\xEE\x12\xED\x13\xEC\x14\xEB\x15\xEA\x16\xE9\x17\xE8\x18";

    #[test]
    fn read_rect_1x2() {
        static RECTANGLE_OUT: &[u16] =                                 &[0xF30D, 0xF20E, 0xF10F, 0xF010,
                                                                         0xEB15, 0xEA16, 0xE917, 0xE818];

        read_rect(1, 1, 1, 2, RECTANGLE_OUT);
    }

    #[test]
    fn read_rect_2x2() {
        static RECTANGLE_OUT: &[u16] = &[0xFF01, 0xFE02, 0xFD03, 0xFC04, 0xFB05, 0xFA06, 0xF907, 0xF808,
                                         0xF709, 0xF60A, 0xF50B, 0xF40C, 0xF30D, 0xF20E, 0xF10F, 0xF010];

        read_rect(0, 0, 2, 2, RECTANGLE_OUT);
    }

    #[test]
    fn read_rect_2x1() {
        static RECTANGLE_OUT: &[u16] = &[0xEF11, 0xEE12, 0xED13, 0xEC14, 0xEB15, 0xEA16, 0xE917, 0xE818];

        read_rect(0, 2, 2, 1, RECTANGLE_OUT);
    }

    #[test]
    fn read_rect_2x3() {
        static RECTANGLE_OUT: &[u16] = &[0xFF01, 0xFE02, 0xFD03, 0xFC04, 0xFB05, 0xFA06, 0xF907, 0xF808,
                                         0xF709, 0xF60A, 0xF50B, 0xF40C, 0xF30D, 0xF20E, 0xF10F, 0xF010,
                                         0xEF11, 0xEE12, 0xED13, 0xEC14, 0xEB15, 0xEA16, 0xE917, 0xE818];

        read_rect(0, 0, 2, 3, RECTANGLE_OUT);
    }

    #[test]
    fn read_rect_in_stream() {
        static RECTANGLE_OUT: &[u16] = &[0xEF11, 0xEE12, 0xED13, 0xEC14];

        let mut input = vec![];
        input.extend_from_slice(b"This is a 31-byte-long prologue");
        input.extend_from_slice(RECTANGLE_IN);
        let mut input_cur = Cursor::new(input);
        input_cur.seek(SeekFrom::Start(31)).unwrap();

        let mut out_buf = [0u8; 64];
        FarbfeldDecoder::new(input_cur)
            .unwrap()
            .read_rect(0, 2, 1, 1, &mut out_buf)
            .unwrap();
        let exp = degenerate_pixels(RECTANGLE_OUT);
        assert_eq!(&out_buf[..exp.len()], &exp[..]);
    }

    fn read_rect(x: u32, y: u32, width: u32, height: u32, exp_wide: &[u16]) {
        let mut out_buf = [0u8; 64];
        FarbfeldDecoder::new(Cursor::new(RECTANGLE_IN))
            .unwrap()
            .read_rect(x, y, width, height, &mut out_buf)
            .unwrap();
        let exp = degenerate_pixels(exp_wide);
        assert_eq!(&out_buf[..exp.len()], &exp[..]);
    }

    fn degenerate_pixels(exp_wide: &[u16]) -> Vec<u8> {
        let mut exp = vec![0u8; exp_wide.len() * 2];
        NativeEndian::write_u16_into(exp_wide, &mut exp);
        exp
    }
}