par2-rs 0.3.0

pub mod creator;
pub mod file_desc;
pub mod file_verify;
pub mod header;
pub mod main;
pub mod recovery;

use std::fs::File;
use std::io::{self, BufRead, BufReader, Read, Seek, SeekFrom};
use std::path::Path;

use tracing::{debug, trace, warn};

use crate::checksum::Md5State;
use crate::error::{Par2Error, Result};
use crate::types::RecoverySetId;

const MAX_MAIN_BODY_BYTES: usize = 12 + 32_768 * 16;
const MAX_FILE_DESC_BODY_BYTES: usize = 56 + 100_000;
const MAX_IFSC_BODY_BYTES: usize = 16 + 32_768 * 20;
const MAX_CREATOR_BODY_BYTES: usize = 100_000;

pub use creator::CreatorPacket;
pub use file_desc::FileDescriptionPacket;
pub use file_verify::IfscPacket;
pub use header::{HEADER_SIZE, MAGIC, PacketHeader, PacketType};
pub use main::MainPacket;
pub use recovery::{RecoverySliceData, RecoverySlicePacket};

/// A parsed PAR2 packet (any type).
#[derive(Debug, Clone)]
pub enum Packet {
    Main(MainPacket),
    FileDescription(FileDescriptionPacket),
    InputFileSliceChecksum(IfscPacket),
    RecoverySlice(RecoverySlicePacket),
    Creator(CreatorPacket),
    Unknown {
        packet_type: [u8; 16],
        body: Vec<u8>,
    },
}

#[derive(Debug, Clone)]
pub struct ScannedPacket {
    pub packet: Packet,
    pub offset: u64,
    pub recovery_set_id: RecoverySetId,
}

/// Parse a single packet from a byte slice that starts at the packet header.
///
/// Returns the parsed packet and the number of bytes consumed.
/// `offset` is used for error reporting (position in the file/stream).
fn parse_packet_internal(
    data: &[u8],
    offset: u64,
    recovery_path: Option<&Path>,
) -> Result<(Packet, usize)> {
    let header = PacketHeader::parse(data, offset)?;
    let total_len =
        usize::try_from(header.length).map_err(|_| Par2Error::ResourceLimitExceeded {
            reason: format!("packet length {} exceeds addressable memory", header.length),
        })?;

    if data.len() < total_len {
        return Err(Par2Error::PacketTooShort {
            expected: header.length,
            actual: data.len() as u64,
        });
    }

    // Validate packet hash
    header.validate_hash(&data[..total_len], offset)?;

    let body = &data[HEADER_SIZE..total_len];

    let packet = match header.packet_type {
        PacketType::Main => {
            debug!("parsed Main packet at offset {offset}");
            Packet::Main(MainPacket::parse(body, header.recovery_set_id)?)
        }
        PacketType::FileDescription => {
            debug!("parsed FileDescription packet at offset {offset}");
            Packet::FileDescription(FileDescriptionPacket::parse(body)?)
        }
        PacketType::InputFileSliceChecksum => {
            debug!("parsed IFSC packet at offset {offset}");
            Packet::InputFileSliceChecksum(IfscPacket::parse(body)?)
        }
        PacketType::RecoverySlice => {
            debug!("parsed RecoverySlice packet at offset {offset}");
            if let Some(path) = recovery_path {
                if body.len() <= 4 {
                    return Err(Par2Error::InvalidRecoveryPacket {
                        reason: format!("body too short: {} bytes, need more than 4", body.len()),
                    });
                }
                let exponent = u32::from_le_bytes(body[0..4].try_into().unwrap());
                Packet::RecoverySlice(RecoverySlicePacket {
                    exponent,
                    data: RecoverySliceData::file_backed(
                        path.to_path_buf(),
                        offset + HEADER_SIZE as u64 + 4,
                        body.len() - 4,
                    ),
                })
            } else {
                Packet::RecoverySlice(RecoverySlicePacket::parse(body)?)
            }
        }
        PacketType::Creator => {
            debug!("parsed Creator packet at offset {offset}");
            Packet::Creator(CreatorPacket::parse(body)?)
        }
        PacketType::Unknown(sig) => {
            debug!("parsed Unknown packet type at offset {offset}");
            Packet::Unknown {
                packet_type: sig,
                body: body.to_vec(),
            }
        }
    };

    Ok((packet, total_len))
}

pub fn parse_packet(data: &[u8], offset: u64) -> Result<(Packet, usize)> {
    parse_packet_internal(data, offset, None)
}

fn parse_packet_body(header: &PacketHeader, body: Vec<u8>) -> Result<Packet> {
    Ok(match header.packet_type {
        PacketType::Main => Packet::Main(MainPacket::parse(&body, header.recovery_set_id)?),
        PacketType::FileDescription => {
            Packet::FileDescription(FileDescriptionPacket::parse(&body)?)
        }
        PacketType::InputFileSliceChecksum => {
            Packet::InputFileSliceChecksum(IfscPacket::parse(&body)?)
        }
        PacketType::RecoverySlice => Packet::RecoverySlice(RecoverySlicePacket::parse(&body)?),
        PacketType::Creator => Packet::Creator(CreatorPacket::parse(&body)?),
        PacketType::Unknown(sig) => Packet::Unknown {
            packet_type: sig,
            body,
        },
    })
}

/// Scan a byte stream for PAR2 packets.
///
/// Scans through `data` looking for valid PAR2 packets. When a valid packet is
/// found, it is parsed and added to the result. When invalid data is encountered,
/// we scan forward byte-by-byte looking for the next magic sequence.
///
/// `base_offset` is the offset of `data[0]` in the original file (for error reporting).
pub fn scan_packets(data: &[u8], base_offset: u64) -> Vec<(Packet, u64)> {
    scan_packets_internal(data, base_offset, None)
}

fn scan_packets_internal(
    data: &[u8],
    base_offset: u64,
    recovery_path: Option<&Path>,
) -> Vec<(Packet, u64)> {
    let mut packets = Vec::new();
    let mut pos = 0;

    while pos + HEADER_SIZE <= data.len() {
        // Try to parse a packet at the current position
        let offset = base_offset + pos as u64;

        match parse_packet_internal(&data[pos..], offset, recovery_path) {
            Ok((packet, consumed)) => {
                trace!("packet at offset {offset}, size {consumed}");
                packets.push((packet, offset));
                pos += consumed;
            }
            Err(_) => {
                // Scan forward to find the next magic sequence
                match find_next_magic(&data[pos + 1..]) {
                    Some(skip) => {
                        let skipped = skip + 1;
                        warn!("skipped {skipped} bytes at offset {offset} looking for next packet");
                        pos += skipped;
                    }
                    None => {
                        // No more magic sequences found
                        break;
                    }
                }
            }
        }
    }

    packets
}

fn find_next_magic_in_reader(
    reader: &mut BufReader<File>,
    offset: &mut u64,
) -> io::Result<Option<u64>> {
    let mut matched = 0usize;

    loop {
        let mut found = None;
        let mut consumed = 0usize;

        {
            let buf = reader.fill_buf()?;
            if buf.is_empty() {
                return Ok(None);
            }

            while consumed < buf.len() {
                let byte = buf[consumed];
                if byte == MAGIC[matched] {
                    matched += 1;
                    if matched == MAGIC.len() {
                        found = Some(*offset + consumed as u64 + 1 - MAGIC.len() as u64);
                        consumed += 1;
                        break;
                    }
                } else {
                    matched = if byte == MAGIC[0] { 1 } else { 0 };
                }
                consumed += 1;
            }
        }

        reader.consume(consumed);
        *offset += consumed as u64;

        if let Some(found) = found {
            return Ok(Some(found));
        }
    }
}

fn validate_streamed_hash(
    header: &PacketHeader,
    header_bytes: &[u8; HEADER_SIZE],
    body: &[u8],
    offset: u64,
) -> Result<()> {
    let mut hasher = Md5State::new();
    hasher.update(&header_bytes[32..HEADER_SIZE]);
    hasher.update(body);
    let computed = hasher.finalize();
    if computed != header.packet_hash {
        return Err(Par2Error::PacketHashMismatch { offset });
    }
    Ok(())
}

fn validate_streamed_packet_from_reader(
    reader: &mut BufReader<File>,
    header: &PacketHeader,
    header_bytes: &[u8; HEADER_SIZE],
    body_len: usize,
    offset: u64,
) -> Result<()> {
    let mut hasher = Md5State::new();
    hasher.update(&header_bytes[32..HEADER_SIZE]);

    let mut remaining = body_len;
    let mut buf = [0u8; 64 * 1024];
    while remaining > 0 {
        let take = remaining.min(buf.len());
        reader.read_exact(&mut buf[..take]).map_err(Par2Error::Io)?;
        hasher.update(&buf[..take]);
        remaining -= take;
    }

    let computed = hasher.finalize();
    if computed != header.packet_hash {
        return Err(Par2Error::PacketHashMismatch { offset });
    }
    Ok(())
}

fn max_buffered_non_recovery_body_len(packet_type: PacketType) -> Option<usize> {
    match packet_type {
        PacketType::Main => Some(MAX_MAIN_BODY_BYTES),
        PacketType::FileDescription => Some(MAX_FILE_DESC_BODY_BYTES),
        PacketType::InputFileSliceChecksum => Some(MAX_IFSC_BODY_BYTES),
        PacketType::Creator => Some(MAX_CREATOR_BODY_BYTES),
        PacketType::RecoverySlice | PacketType::Unknown(_) => None,
    }
}

fn parse_non_recovery_packet_from_reader(
    reader: &mut BufReader<File>,
    header: &PacketHeader,
    header_bytes: &[u8; HEADER_SIZE],
    offset: u64,
) -> Result<Option<Packet>> {
    let body_len =
        usize::try_from(header.body_length()).map_err(|_| Par2Error::ResourceLimitExceeded {
            reason: format!(
                "packet body length {} exceeds addressable memory",
                header.body_length()
            ),
        })?;
    let Some(max_body_len) = max_buffered_non_recovery_body_len(header.packet_type) else {
        validate_streamed_packet_from_reader(reader, header, header_bytes, body_len, offset)?;
        return Ok(None);
    };
    if body_len > max_body_len {
        validate_streamed_packet_from_reader(reader, header, header_bytes, body_len, offset)?;
        return Ok(None);
    }

    let mut body = vec![0u8; body_len];
    reader.read_exact(&mut body).map_err(Par2Error::Io)?;
    validate_streamed_hash(header, header_bytes, &body, offset)?;
    parse_packet_body(header, body).map(Some).or(Ok(None))
}

fn parse_recovery_packet_from_reader(
    reader: &mut BufReader<File>,
    header: &PacketHeader,
    offset: u64,
    path: &Path,
) -> Result<Packet> {
    let body_len =
        usize::try_from(header.body_length()).map_err(|_| Par2Error::ResourceLimitExceeded {
            reason: format!(
                "packet body length {} exceeds addressable memory",
                header.body_length()
            ),
        })?;
    if body_len <= 4 {
        return Err(Par2Error::InvalidRecoveryPacket {
            reason: format!("body too short: {body_len} bytes, need more than 4"),
        });
    }

    let mut exponent_bytes = [0u8; 4];
    reader
        .read_exact(&mut exponent_bytes)
        .map_err(Par2Error::Io)?;
    let exponent = u32::from_le_bytes(exponent_bytes);
    let payload_len = body_len - 4;
    let payload_offset = offset + HEADER_SIZE as u64 + 4;
    reader
        .seek(SeekFrom::Start(payload_offset + payload_len as u64))
        .map_err(Par2Error::Io)?;

    Ok(Packet::RecoverySlice(RecoverySlicePacket {
        exponent,
        // The streaming scanner seeks past recovery payloads without hashing
        // them, so keep the packet hash around for lazy validation at repair
        // time (damaged .vol files are routine on Usenet).
        data: RecoverySliceData::file_backed_with_hash(
            path.to_path_buf(),
            payload_offset,
            payload_len,
            header.packet_hash,
        ),
    }))
}

pub fn scan_packets_from_path_with_set_ids(path: &Path) -> Result<Vec<ScannedPacket>> {
    let file = File::open(path).map_err(Par2Error::Io)?;
    let file_len = file.metadata().map_err(Par2Error::Io)?.len();
    crate::file_cache::advise_sequential(&file, path, file_len);
    let mut reader = BufReader::with_capacity(256 * 1024, file);
    let mut packets = Vec::new();
    let mut offset = 0u64;

    while let Some(packet_offset) =
        find_next_magic_in_reader(&mut reader, &mut offset).map_err(Par2Error::Io)?
    {
        let mut header_bytes = [0u8; HEADER_SIZE];
        header_bytes[..MAGIC.len()].copy_from_slice(MAGIC);

        match reader.read_exact(&mut header_bytes[MAGIC.len()..]) {
            Ok(()) => {}
            Err(e) if e.kind() == io::ErrorKind::UnexpectedEof => break,
            Err(e) => return Err(Par2Error::Io(e)),
        }

        let header = match PacketHeader::parse(&header_bytes, packet_offset) {
            Ok(header) => header,
            Err(_) => {
                reader
                    .seek(SeekFrom::Start(packet_offset + 1))
                    .map_err(Par2Error::Io)?;
                offset = packet_offset + 1;
                continue;
            }
        };
        if packet_offset
            .checked_add(header.length)
            .is_none_or(|packet_end| packet_end > file_len)
        {
            reader
                .seek(SeekFrom::Start(packet_offset + 1))
                .map_err(Par2Error::Io)?;
            offset = packet_offset + 1;
            continue;
        }

        let packet = match header.packet_type {
            PacketType::RecoverySlice => {
                parse_recovery_packet_from_reader(&mut reader, &header, packet_offset, path)
                    .map(Some)
            }
            _ => parse_non_recovery_packet_from_reader(
                &mut reader,
                &header,
                &header_bytes,
                packet_offset,
            ),
        };

        match packet {
            Ok(packet) => {
                if let Some(packet) = packet {
                    packets.push(ScannedPacket {
                        packet,
                        offset: packet_offset,
                        recovery_set_id: header.recovery_set_id,
                    });
                }
                offset = packet_offset + header.length;
            }
            Err(_) => {
                reader
                    .seek(SeekFrom::Start(packet_offset + 1))
                    .map_err(Par2Error::Io)?;
                offset = packet_offset + 1;
            }
        }
    }

    crate::file_cache::drop_touched_file_cache(
        reader.get_ref(),
        path,
        file_len,
        0,
        offset.min(file_len),
    );
    Ok(packets)
}

pub fn scan_packets_from_path(path: &Path) -> Result<Vec<(Packet, u64)>> {
    scan_packets_from_path_with_set_ids(path).map(|packets| {
        packets
            .into_iter()
            .map(|packet| (packet.packet, packet.offset))
            .collect()
    })
}

/// Find the byte offset of the next PAR2 magic sequence in `data`.
fn find_next_magic(data: &[u8]) -> Option<usize> {
    if data.len() < MAGIC.len() {
        return None;
    }
    for i in 0..=data.len() - MAGIC.len() {
        if &data[i..i + MAGIC.len()] == MAGIC {
            return Some(i);
        }
    }
    None
}

/// Extract the recovery set ID from the first Main packet found in the data.
pub fn find_recovery_set_id(packets: &[(Packet, u64)]) -> Option<RecoverySetId> {
    for (packet, _) in packets {
        if let Packet::Main(main) = packet {
            return Some(main.recovery_set_id);
        }
    }
    None
}

#[cfg(test)]
mod tests {
    use super::*;
    use md5::{Digest, Md5};
    use std::io::Write;
    use tempfile::NamedTempFile;

    /// Helper to build a complete valid packet (header + body).
    fn make_full_packet(packet_type: &[u8; 16], body: &[u8], recovery_set_id: [u8; 16]) -> Vec<u8> {
        let length = (HEADER_SIZE + body.len()) as u64;

        // Build bytes 32..length for hashing
        let mut hash_input = Vec::new();
        hash_input.extend_from_slice(&recovery_set_id);
        hash_input.extend_from_slice(packet_type);
        hash_input.extend_from_slice(body);

        let packet_hash: [u8; 16] = Md5::digest(&hash_input).into();

        let mut data = Vec::new();
        data.extend_from_slice(MAGIC);
        data.extend_from_slice(&length.to_le_bytes());
        data.extend_from_slice(&packet_hash);
        data.extend_from_slice(&recovery_set_id);
        data.extend_from_slice(packet_type);
        data.extend_from_slice(body);
        data
    }

    fn make_creator_packet(creator: &str, rsid: [u8; 16]) -> Vec<u8> {
        // Pad creator to multiple of 4 for body alignment
        let mut body = creator.as_bytes().to_vec();
        while !body.len().is_multiple_of(4) {
            body.push(0);
        }
        make_full_packet(header::TYPE_CREATOR, &body, rsid)
    }

    fn make_main_packet_bytes(slice_size: u64, rsid: [u8; 16]) -> Vec<u8> {
        let mut body = Vec::new();
        body.extend_from_slice(&slice_size.to_le_bytes());
        body.extend_from_slice(&0u32.to_le_bytes()); // 0 recovery file IDs
        make_full_packet(header::TYPE_MAIN, &body, rsid)
    }

    #[test]
    fn parse_creator_packet() {
        let rsid = [0x42; 16];
        let data = make_creator_packet("TestCreator", rsid);
        let (packet, consumed) = parse_packet(&data, 0).unwrap();
        assert_eq!(consumed, data.len());
        match packet {
            Packet::Creator(c) => assert_eq!(c.creator_id, "TestCreator"),
            other => panic!("expected Creator, got {other:?}"),
        }
    }

    #[test]
    fn scan_multiple_packets() {
        let rsid = [0x11; 16];
        let mut stream = Vec::new();
        stream.extend_from_slice(&make_creator_packet("App1", rsid));
        stream.extend_from_slice(&make_main_packet_bytes(4096, rsid));
        stream.extend_from_slice(&make_creator_packet("App2", rsid));

        let packets = scan_packets(&stream, 0);
        assert_eq!(packets.len(), 3);
        assert!(matches!(&packets[0].0, Packet::Creator(_)));
        assert!(matches!(&packets[1].0, Packet::Main(_)));
        assert!(matches!(&packets[2].0, Packet::Creator(_)));
    }

    #[test]
    fn scan_skips_garbage() {
        let rsid = [0x22; 16];
        let mut stream = Vec::new();
        // Some garbage bytes before the first packet
        stream.extend_from_slice(&[0xFF; 37]);
        stream.extend_from_slice(&make_creator_packet("Found", rsid));

        let packets = scan_packets(&stream, 0);
        assert_eq!(packets.len(), 1);
        assert_eq!(packets[0].1, 37); // offset should be 37
        match &packets[0].0 {
            Packet::Creator(c) => assert_eq!(c.creator_id, "Found"),
            other => panic!("expected Creator, got {other:?}"),
        }
    }

    #[test]
    fn scan_handles_garbage_between_packets() {
        let rsid = [0x33; 16];
        let mut stream = Vec::new();
        stream.extend_from_slice(&make_creator_packet("First", rsid));
        // Garbage between packets
        stream.extend_from_slice(&[0xDE, 0xAD, 0xBE, 0xEF]);
        stream.extend_from_slice(&make_creator_packet("Second", rsid));

        let packets = scan_packets(&stream, 100);
        assert_eq!(packets.len(), 2);
    }

    #[test]
    fn scan_empty_data() {
        let packets = scan_packets(&[], 0);
        assert!(packets.is_empty());
    }

    #[test]
    fn scan_short_data() {
        let packets = scan_packets(&[0u8; 10], 0);
        assert!(packets.is_empty());
    }

    #[test]
    fn find_next_magic_works() {
        let mut data = vec![0u8; 20];
        data.extend_from_slice(MAGIC);
        data.extend_from_slice(&[0u8; 10]);
        assert_eq!(find_next_magic(&data), Some(20));
    }

    #[test]
    fn find_next_magic_at_start() {
        let mut data = Vec::new();
        data.extend_from_slice(MAGIC);
        assert_eq!(find_next_magic(&data), Some(0));
    }

    #[test]
    fn find_next_magic_not_found() {
        let data = [0u8; 100];
        assert_eq!(find_next_magic(&data), None);
    }

    #[test]
    fn parse_packet_hash_mismatch() {
        let rsid = [0; 16];
        let mut data = make_creator_packet("test", rsid);
        // Corrupt a body byte
        let last = data.len() - 1;
        data[last] ^= 0x01;
        let err = parse_packet(&data, 5).unwrap_err();
        assert!(matches!(err, Par2Error::PacketHashMismatch { offset: 5 }));
    }

    #[test]
    fn parse_unknown_packet_type() {
        let custom_type = b"PAR 2.0\x00TestType";
        let body = [0u8; 16]; // 16 bytes body
        let rsid = [0; 16];
        let data = make_full_packet(custom_type, &body, rsid);

        let (packet, _) = parse_packet(&data, 0).unwrap();
        match packet {
            Packet::Unknown {
                packet_type,
                body: b,
            } => {
                assert_eq!(packet_type, *custom_type);
                assert_eq!(b.len(), 16);
            }
            other => panic!("expected Unknown, got {other:?}"),
        }
    }

    #[test]
    fn path_scanner_streams_and_ignores_large_unknown_packets() {
        let custom_type = b"PAR 2.0\x00TestType";
        let rsid = [0x55; 16];
        let unknown_body = vec![0xA5; 1024 * 1024 + 4];
        let mut stream = make_full_packet(custom_type, &unknown_body, rsid);
        stream.extend_from_slice(&make_main_packet_bytes(4096, rsid));

        let mut file = NamedTempFile::new().unwrap();
        file.write_all(&stream).unwrap();

        let packets = scan_packets_from_path(file.path()).unwrap();
        assert_eq!(packets.len(), 1);
        assert!(matches!(&packets[0].0, Packet::Main(_)));
    }

    #[test]
    fn path_scanner_walks_large_valid_packet_inventory() {
        let rsid = [0x5A; 16];
        let creator = make_creator_packet("stress", rsid);
        let mut stream = Vec::with_capacity(creator.len() * 70_000 + HEADER_SIZE + 12);
        for _ in 0..70_000 {
            stream.extend_from_slice(&creator);
        }
        stream.extend_from_slice(&make_main_packet_bytes(4096, rsid));

        let mut file = NamedTempFile::new().unwrap();
        file.write_all(&stream).unwrap();

        let packets = scan_packets_from_path(file.path()).unwrap();
        assert_eq!(packets.len(), 70_001);
        assert!(matches!(&packets[70_000].0, Packet::Main(_)));
    }

    #[test]
    fn path_scanner_skips_valid_hash_oversized_known_packets_by_boundary() {
        let rsid = [0x66; 16];
        let embedded_rsid = [0x99; 16];
        let embedded_main = make_main_packet_bytes(8192, embedded_rsid);
        let mut oversized_creator_body = vec![0u8; MAX_CREATOR_BODY_BYTES + 4];
        oversized_creator_body[..embedded_main.len()].copy_from_slice(&embedded_main);

        let mut stream = make_full_packet(header::TYPE_CREATOR, &oversized_creator_body, rsid);
        stream.extend_from_slice(&make_main_packet_bytes(4096, rsid));

        let mut file = NamedTempFile::new().unwrap();
        file.write_all(&stream).unwrap();

        let packets = scan_packets_from_path(file.path()).unwrap();
        assert_eq!(packets.len(), 1);
        match &packets[0].0 {
            Packet::Main(main) => assert_eq!(*main.recovery_set_id.as_bytes(), rsid),
            other => panic!("expected Main, got {other:?}"),
        }
    }

    #[test]
    fn find_recovery_set_id_works() {
        let rsid = [0x77; 16];
        let stream = make_main_packet_bytes(1024, rsid);
        let packets = scan_packets(&stream, 0);
        let found = find_recovery_set_id(&packets).unwrap();
        assert_eq!(*found.as_bytes(), rsid);
    }

    #[test]
    fn find_recovery_set_id_none() {
        let rsid = [0; 16];
        let stream = make_creator_packet("test", rsid);
        let packets = scan_packets(&stream, 0);
        assert!(find_recovery_set_id(&packets).is_none());
    }
}