fax 0.2.3

Decoder and Encoder for CCITT Group 3 and 4 bi-level image encodings used by fax machines TIFF and PDF.
Documentation 
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use crate::{Color, BitWriter, Transitions, maps::{Mode, mode, black, white, EDFB_HALF}};

fn absdiff(a: u16, b: u16) -> u16 {
    if a > b {
        a - b
    } else {
        b - a
    }
}

pub struct Encoder<W> {
    writer: W,
    reference: Vec<u16>,
    current: Vec<u16>,
}
fn encode_color<W: BitWriter>(writer: &mut W, color: Color, mut n: u16) -> Result<(), W::Error> {
    let table = match color {
        Color::White => &white::ENTRIES,
        Color::Black => &black::ENTRIES,
    };
    let mut write = |n: u16| {
        let idx = if n >= 64 { 63 + n / 64 } else { n } as usize;
        let (v, bits) = table[idx];
        assert_eq!(v, n);
        //debug!("{}", n);
        writer.write(bits)
    };
    
    while n >= 2560 {
        write(2560)?;
        n -= 2560;
    }
    if n >= 64 {
        let d = n & !63;
        write(d)?;
        n -= d;
    }

    write(n)
}
impl<W: BitWriter> Encoder<W> {
    pub fn new(writer: W) -> Self {
        Encoder {
            writer,
            reference: vec![],
            current: vec![],
        }
    }
    pub fn encode_line(&mut self, pels: impl Iterator<Item=Color>, width: u16) -> Result<(), W::Error> {
        let mut color = Color::White;
        let mut transitions = Transitions::new(&self.reference);
        let mut a0 = 0;
        let mut start_of_line = true;
        let mut pels = pels.enumerate()
        .scan(Color::White, |state, (i, c)| {
            Some(if c != *state {
                debug!("  {i} {c:?}");
                *state = c;
                Some(i as u16)
            } else {
                None
            })
        }).filter_map(|x| x);
        let writer = &mut self.writer;
        self.current.clear();

        while a0 < width {
            let a1;
            if let Some(a1_) = pels.next() {
                self.current.push(a1_);
                a1 = a1_;
            } else {
                a1 = width;
            }
            loop {
                transitions.seek_back(a0);
                let b1 = transitions.next_color(a0, !color, start_of_line).unwrap_or(width);
                let b2 = transitions.peek();
                start_of_line = false;
                debug!("  a0={a0}, a1={a1}, b1={:?}, b2={:?}", b1, b2);
                match (b1, b2) {
                    (_b1, Some(b2)) if b2 < a1 => {
                        debug!("  Pass");
                        let bits = mode::encode(Mode::Pass).unwrap();
                        writer.write(bits)?;
                        transitions.skip(1);
                        a0 = b2;
                        continue;
                    }
                    (b1, _) if absdiff(a1, b1) <= 3 => {
                        let delta = a1 as i16 - b1 as i16;
                        debug!("  Vertical({})", delta);
                        let bits = mode::encode(Mode::Vertical(delta as i8)).unwrap();
                        writer.write(bits)?;
                        a0 = a1;
                        color = !color;
                    }
                    _ => {
                        let a2 = match pels.next() {
                            Some(a2) => {
                                self.current.push(a2);
                                a2
                            },
                            None => width
                        };
                        let a0a1 = a1 - a0;
                        let a1a2 = a2 - a1;
                        debug!("  Horizontal({}, {}) color={color:?}", a0a1, a1a2);
                        let bits = mode::encode(Mode::Horizontal).unwrap();
                        writer.write(bits)?;
                        let c = if a0 + a1 == 0 {
                            Color::White
                        } else {
                            color
                        };
                        encode_color(writer, c, a0a1)?;
                        encode_color(writer, !c, a1a2)?;
                        a0 = a2;
                    }
                }
                break;
            }
        }

        std::mem::swap(&mut self.reference, &mut self.current);
        debug!("next line");
        Ok(())
    }
    pub fn finish(mut self) -> W {
        self.writer.write(EDFB_HALF);
        self.writer.write(EDFB_HALF);
        self.writer
    }
}