telfax/src/fax/t4.rs

use crate::document::convert::Page;
use crate::error::{FaxError, Result};

pub struct T4Codec;

/// EOL marker: 11 zero bits followed by a 1 bit.
const EOL_BIT_LEN: u8 = 12;
const EOL_MASK: u64 = (1u64 << EOL_BIT_LEN) - 1; // 12 bits: 0xFFF

/// Find the bit offset of the first valid EOL marker (000000000001) in the data.
/// Returns None if no EOL is found.
fn find_first_eol_bit(data: &[u8]) -> Option<usize> {
    if data.len() < 2 {
        return None;
    }
    // Use a sliding 12-bit window across the bitstream
    let mut window: u64 = 0;
    let mut bits_in_window = 0u8;

    for (byte_idx, &byte) in data.iter().enumerate() {
        for bit_idx in (0..8u8).rev() {
            let bit = ((byte >> bit_idx) & 1) as u64;
            window = ((window << 1) | bit) & EOL_MASK;
            if bits_in_window < EOL_BIT_LEN {
                bits_in_window += 1;
            }
            if bits_in_window == EOL_BIT_LEN && (window & EOL_MASK) == 1 {
                let bit_pos = byte_idx * 8 + (7 - bit_idx as usize);
                return Some(bit_pos - (EOL_BIT_LEN as usize - 1));
            }
        }
    }
    None
}

/// Extract bytes from `data` starting at `bit_offset`, producing a byte-aligned buffer.
fn extract_from_bit(data: &[u8], bit_offset: usize) -> Vec<u8> {
    if bit_offset >= data.len() * 8 {
        return Vec::new();
    }
    let start_byte = bit_offset / 8;
    let start_bit = (bit_offset % 8) as u8;

    if start_bit == 0 {
        return data[start_byte..].to_vec();
    }

    let mut out = Vec::with_capacity(data.len() - start_byte);
    for i in start_byte..data.len() - 1 {
        let b = (data[i] << start_bit) | (data[i + 1] >> (8 - start_bit));
        out.push(b);
    }
    // Last byte: shift left, lower bits become zero
    let last = data[data.len() - 1] << start_bit;
    out.push(last);
    out
}

#[derive(Debug, Clone)]
pub struct T4Page {
    pub data: Vec<u8>,
    pub width_pels: u32,
    pub rows: u32,
    pub encoding: T4Encoding,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum T4Encoding {
    Group3MH,
    Group3MR,
    Group4MMR,
}

impl T4Codec {
    /// Decode T.4 Group 3 MH data into a Page, determining height from the data.
    /// Automatically strips leading non-MH bytes (modem overhead bytes) by scanning
    /// for the first valid EOL marker.
    pub fn decode_g3_page(data: &[u8], width_pels: u32) -> Result<Page> {
        // Strip non-MH bytes before the first EOL marker
        let clean_data = match find_first_eol_bit(data) {
            Some(bit_offset) => extract_from_bit(data, bit_offset),
            None => data.to_vec(),
        };

        let bytes_per_row = width_pels.div_ceil(8) as usize;
        let mut pixels = Vec::new();

        let mut has_content = false;
        let mut line_count = 0u32;
        let result = fax::decoder::decode_g3(clean_data.iter().copied(), |transitions: &[u16]| {
            has_content = true;
            line_count += 1;
            if line_count <= 3 {
                tracing::info!("G3 line {}: {} transitions, first few: {:?}", line_count, transitions.len(), &transitions[..transitions.len().min(10)]);
            }
            let start = pixels.len();
            pixels.resize(start + bytes_per_row, 0);
            if !transitions.is_empty() {
                Self::transitions_to_pixels(transitions, width_pels, &mut pixels[start..]);
            }
        });

        if result.is_some() || has_content {
            let rows = pixels.len() / bytes_per_row;
            tracing::info!("G3 decode done: {} lines, {} rows, result={:?}", line_count, rows, result);
            if rows == 0 {
                return Err(FaxError::t4("Group 3 decode returned empty page"));
            }
            Ok(Page {
                pixels,
                width_pels,
                rows: rows as u32,
            })
        } else {
            Err(FaxError::t4("Group 3 decode returned None"))
        }
    }

    /// Decode T.4/T.6 data into raw 1-bit pixel data (MSB-first, row-major).
    /// `transitions_to_pixels` converts run-length transitions to pixel data.
    fn transitions_to_pixels(
        transitions: &[u16],
        width_pels: u32,
        row_buf: &mut [u8],
    ) {
        row_buf.fill(0);
        let mut is_black = false;
        let mut prev = 0u32;

        for &t in transitions {
            let t = t as u32;
            if t > width_pels {
                break;
            }
            if is_black {
                for p in prev..t.min(width_pels) {
                    let byte_idx = (p / 8) as usize;
                    let bit_idx = 7 - (p % 8);
                    if byte_idx < row_buf.len() {
                        row_buf[byte_idx] |= 1 << bit_idx;
                    }
                }
            }
            prev = t;
            is_black = !is_black;
        }
        // If ends on black, fill to end
        if is_black {
            for p in prev..width_pels {
                let byte_idx = (p / 8) as usize;
                let bit_idx = 7 - (p % 8);
                if byte_idx < row_buf.len() {
                    row_buf[byte_idx] |= 1 << bit_idx;
                }
            }
        }
    }

    pub fn decode(data: &[u8], width_pels: u32, rows: u32, encoding: T4Encoding) -> Result<Vec<u8>> {
        let bytes_per_row = width_pels.div_ceil(8) as usize;
        let mut pixels = vec![0u8; bytes_per_row * rows as usize];

        let data_owned = data.to_vec();

        match encoding {
            T4Encoding::Group3MH | T4Encoding::Group3MR => {
                let clean_data = match find_first_eol_bit(data) {
                    Some(bit_offset) => extract_from_bit(data, bit_offset),
                    None => data.to_vec(),
                };
                let mut row_idx = 0u32;
                let result = fax::decoder::decode_g3(clean_data.into_iter(), |transitions: &[u16]| {
                    if (row_idx as usize) < pixels.len() / bytes_per_row {
                        let start = row_idx as usize * bytes_per_row;
                        let end = (start + bytes_per_row).min(pixels.len());
                        let row_buf = &mut pixels[start..end];
                        Self::transitions_to_pixels(transitions, width_pels, row_buf);
                    }
                    row_idx += 1;
                });
                if result.is_some() || row_idx > 0 {
                    Ok(pixels)
                } else {
                    Err(FaxError::t4("Group 3 decode returned None"))
                }
            }
            T4Encoding::Group4MMR => {
                let mut row_idx = 0u32;
                let result = fax::decoder::decode_g4(
                    data_owned.into_iter(),
                    width_pels as u16,
                    None,
                    |transitions: &[u16]| {
                        if (row_idx as usize) < pixels.len() / bytes_per_row {
                            let start = row_idx as usize * bytes_per_row;
                            let end = (start + bytes_per_row).min(pixels.len());
                            let row_buf = &mut pixels[start..end];
                            Self::transitions_to_pixels(transitions, width_pels, row_buf);
                        }
                        row_idx += 1;
                    },
                );
                match result {
                    Some(()) => Ok(pixels),
                    None => Err(FaxError::t4("Group 4 decode returned None")),
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::fax::encoder::mh::MhEncoder;

    #[test]
    fn test_find_first_eol_at_start() {
        let data = vec![0x00, 0x10, 0xFF, 0xFF];
        assert_eq!(find_first_eol_bit(&data), Some(0));
    }

    #[test]
    fn test_find_first_eol_with_offset() {
        let data = vec![0xFF, 0x00, 0x10, 0xFF];
        assert_eq!(find_first_eol_bit(&data), Some(8));
    }

    #[test]
    fn test_find_first_eol_none() {
        let data = vec![0xFF, 0xFF, 0xFF];
        assert_eq!(find_first_eol_bit(&data), None);
    }

    #[test]
    fn test_find_first_eol_short_data() {
        assert_eq!(find_first_eol_bit(&[0x00]), None);
        assert_eq!(find_first_eol_bit(&[]), None);
    }

    #[test]
    fn test_extract_from_bit_aligned() {
        let data = vec![0xAA, 0xBB, 0xCC];
        let result = extract_from_bit(&data, 8);
        assert_eq!(result, vec![0xBB, 0xCC]);
    }

    #[test]
    fn test_extract_from_bit_unaligned() {
        let data = vec![0b00000000, 0b00010001, 0xFF];
        // Start at bit 12 (byte 1, bit 4):
        // byte 1 bits remaining: 0001 (bits 12-15)
        // byte 2 bits: 11111111 (bits 16-23)
        // As bytes: [00011111, 11110000] = [0x1F, 0xF0]
        let result = extract_from_bit(&data, 12);
        assert_eq!(result.len(), 2);
        assert_eq!(result[0], 0x1F);
        assert_eq!(result[1], 0xF0);
    }

    #[test]
    fn test_encoder_roundtrip() {
        // Create a simple 2-row, 8-pixel wide image: alternating white/black
        let width = 8u32;
        let rows = 2u32;
        let bytes_per_row = 1usize;
        let mut pixels = vec![0u8; bytes_per_row * rows as usize];
        // Row 0: 10101010 (black, white, black, white, black, white, black, white)
        pixels[0] = 0b10101010;
        // Row 1: 01010101
        pixels[1] = 0b01010101;

        let encoded = MhEncoder::encode(&pixels, width, rows).unwrap();
        // Should be non-empty
        assert!(!encoded.is_empty());
        // Should start with EOL (0x00 0x10)
        assert_eq!(encoded[0], 0x00);
        assert!(encoded[1] >= 0x10 && encoded[1] < 0x20);

        // Decode back
        let decoded = T4Codec::decode(&encoded, width, rows, T4Encoding::Group3MH).unwrap();
        assert_eq!(decoded.len(), pixels.len());
        assert_eq!(decoded, pixels);
    }

    #[test]
    fn test_decode_g3_page_with_garbage_prefix() {
        let width = 8u32;
        let rows = 1u32;
        let bytes_per_row = 1usize;
        let mut pixels = vec![0u8; bytes_per_row * rows as usize];
        pixels[0] = 0xFF;

        let encoded = MhEncoder::encode(&pixels, width, rows).unwrap();

        // Prepend garbage (simulating modem overhead)
        let garbage: Vec<u8> = vec![0x0d, 0xc2, 0x10, 0x5c, 0x40].into_iter().cycle().take(25).collect();
        let mut with_garbage = garbage.clone();
        with_garbage.extend_from_slice(&encoded);

        let page = T4Codec::decode_g3_page(&with_garbage, width).unwrap();
        assert_eq!(page.width_pels, width);
        assert_eq!(page.rows, rows);
        assert_eq!(&page.pixels, &pixels);
    }

    #[test]
    fn test_strip_garbage_decodes_cleanly() {
        let width = 100u32;
        let rows = 10u32;
        let bytes_per_row = ((width + 7) / 8) as usize;
        let mut pixels = vec![0u8; bytes_per_row * rows as usize];
        // Fill with checkerboard pattern
        for y in 0..rows {
            for x in 0..width {
                let byte_idx = (x / 8) as usize;
                let bit_idx = 7 - (x % 8);
                if (x + y) % 2 == 0 {
                    pixels[(y as usize) * bytes_per_row + byte_idx] |= 1 << bit_idx;
                }
            }
        }

        let encoded = MhEncoder::encode(&pixels, width, rows).unwrap();

        // Prepend modem-like garbage
        let garbage: Vec<u8> = vec![0x0d, 0xc2, 0x10, 0x5c, 0x40].into_iter().cycle().take(100).collect();
        let mut with_garbage = garbage;
        with_garbage.extend_from_slice(&encoded);

        let decoded = T4Codec::decode_g3_page(&with_garbage, width).unwrap();
        assert_eq!(decoded.rows, rows);
        assert_eq!(decoded.pixels, pixels);
    }

    #[test]
    fn test_decode_g3_page_no_rtc() {
        // Encode a page, then strip the trailing RTC (6 EOLs = 72 bits).
        // decode_g3_page should still succeed because the line data is valid
        // even without the end-of-document RTC marker.
        let width = 100u32;
        let rows = 5u32;
        let bytes_per_row = ((width + 7) / 8) as usize;
        let mut pixels = vec![0u8; bytes_per_row * rows as usize];
        for y in 0..rows {
            for x in 0..width {
                let byte_idx = (x / 8) as usize;
                let bit_idx = 7 - (x % 8);
                pixels[(y as usize) * bytes_per_row + byte_idx] |= 1 << bit_idx;
            }
        }

        let encoded = MhEncoder::encode(&pixels, width, rows).unwrap();

        // Convert to bitstring and strip the last 6 EOLs (72 bits)
        let bits: String = encoded.iter().map(|b| format!("{:08b}", b)).collect();
        let eol = "000000000001";

        // Find all EOL positions
        let mut pos = 0;
        let mut eol_pos = Vec::new();
        while let Some(p) = bits[pos..].find(eol) {
            eol_pos.push(pos + p);
            pos += p + 12;
        }
        assert!(eol_pos.len() >= 7, "need at least 7 EOLs (1 leading + rows + 6 RTC)");

        // The RTC is the last 6 consecutive EOLs with exactly 12-bit spacing
        // Check last 7 EOLs: the last 6 should be consecutive, preceded by
        // the last line's EOL (which won't have 12-bit spacing to the RTC).
        let rtc_start = eol_pos[eol_pos.len() - 6];
        let truncated_bits = &bits[..rtc_start];
        let truncated_bytes = (truncated_bits.len() + 7) / 8;

        let mut truncated = vec![0u8; truncated_bytes];
        for (i, bit) in truncated_bits.chars().enumerate() {
            if bit == '1' {
                truncated[i / 8] |= 1 << (7 - (i % 8));
            }
        }

        let decoded = T4Codec::decode_g3_page(&truncated, width).unwrap();
        assert_eq!(decoded.rows, rows);
        assert_eq!(decoded.width_pels, width);
    }

    #[test]
    fn test_decode_g3_page_invalid_data_still_fails() {
        // All-zero data has no EOL marker (needs a '1' bit), so decode should fail
        let result = T4Codec::decode_g3_page(&[0x00; 100], 100);
        assert!(result.is_err(), "expected error, got {:?}", result);
    }
}