libfreemkv/src/mux/codec/ac3.rs

//! AC3 (Dolby Digital) / EAC3 (Dolby Digital Plus) frame parser.
//!
//! AC3 frames are self-contained and always start with syncword 0x0B77.
//! Buffers across PES boundaries so frames that span two PES packets
//! are emitted complete, not truncated.

use super::{CodecParser, Frame, PesPacket, pts_to_ns};

/// Hard cap on the carry-over buffer. An AC-3/E-AC-3 frame is at most 8192
/// bytes (the `frame_size > 8192` reject below), so a single straddling frame
/// plus a little slack never needs more than this. If the buffer grows past
/// the cap without yielding a frame (pathological / never-syncing input) we
/// drop it and resync rather than accumulate one PES worth of data per call
/// for the whole title.
const MAX_AC3_BUF: usize = 64 * 1024;

pub struct Ac3Parser {
    /// Leftover bytes from previous PES (incomplete frame at end).
    buf: Vec<u8>,
}

impl Default for Ac3Parser {
    fn default() -> Self {
        Self::new()
    }
}

impl Ac3Parser {
    pub fn new() -> Self {
        Self {
            buf: Vec::with_capacity(4096),
        }
    }
}

impl CodecParser for Ac3Parser {
    fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
        if pes.data.is_empty() {
            return Vec::new();
        }

        let pts_ns = pes.pts.map(pts_to_ns).unwrap_or(0);

        // Prepend leftover from previous PES
        self.buf.extend_from_slice(&pes.data);

        let data = &self.buf;
        let mut frames = Vec::new();
        let mut pos = 0;

        while pos < data.len() {
            let sync = find_ac3_sync(&data[pos..]);
            let start = match sync {
                Some(offset) => pos + offset,
                None => break,
            };

            let remaining = &data[start..];

            if remaining.len() < 6 {
                // Not enough data to determine frame size — keep for next PES
                break;
            }

            let bsid = get_bsid(remaining);
            let frame_size = if bsid >= 11 {
                eac3_frame_size(remaining)
            } else {
                ac3_frame_size(remaining)
            };

            if frame_size == 0 || frame_size > 8192 {
                // Invalid frame size — skip this sync word
                pos = start + 2;
                continue;
            }

            if start + frame_size > data.len() {
                // Incomplete frame — keep for next PES
                break;
            }

            frames.push(Frame {
                pts_ns,
                keyframe: true,
                data: data[start..start + frame_size].to_vec(),
                duration_ns: None,
            });
            pos = start + frame_size;
        }

        // Keep unconsumed data for the next call. `pos` is the start of the
        // unconsumed region: either a partial frame that straddles this PES
        // boundary — which, by construction, begins at a syncword (every byte
        // before `pos` was emitted as a frame or skipped as pre-sync junk) — or
        // trailing bytes too short to size/complete a frame. Carry from `pos`,
        // NOT from the next syncword: discarding bytes between `pos` and the
        // next sync would drop the partial frame we are deliberately keeping
        // across the boundary.
        let keep_from = if pos < data.len() {
            // A syncword at/after `pos` marks the carry-over start (anything
            // before it is junk with no sync). With no full sync, retain the
            // whole tail — including a lone trailing 0x0B that may be the first
            // half of a syncword split across the PES boundary.
            match find_ac3_sync(&data[pos..]) {
                Some(o) => pos + o,
                None if data.last() == Some(&0x0B) => data.len() - 1,
                None => data.len(),
            }
        } else {
            data.len()
        };

        if keep_from < data.len() {
            let tail = &data[keep_from..];
            if tail.len() > MAX_AC3_BUF {
                // No frame could be parsed out of a buffer this large — this is
                // not valid AC-3 here. Drop it and resync on the next PES rather
                // than grow without bound on pathological input.
                self.buf.clear();
            } else {
                self.buf = tail.to_vec();
            }
        } else {
            self.buf.clear();
        }

        frames
    }

    fn codec_private(&self) -> Option<Vec<u8>> {
        None
    }
}

/// Find AC3/E-AC-3 syncword (0x0B77) in data.
fn find_ac3_sync(data: &[u8]) -> Option<usize> {
    (0..data.len().saturating_sub(1)).find(|&i| data[i] == 0x0B && data[i + 1] == 0x77)
}

/// Extract bsid from an AC-3/E-AC-3 frame starting at the syncword.
/// bsid is at byte 5, bits 7..3.
pub fn get_bsid(data: &[u8]) -> u8 {
    if data.len() < 6 {
        return 0;
    }
    (data[5] >> 3) & 0x1F
}

/// Calculate E-AC-3 frame size in bytes from the frmsiz field.
fn eac3_frame_size(data: &[u8]) -> usize {
    if data.len() < 4 {
        return 0;
    }
    let frmsiz = ((data[2] as usize & 0x07) << 8) | data[3] as usize;
    (frmsiz + 1) * 2
}

/// Calculate AC-3 frame size in bytes from fscod and frmsizecod.
fn ac3_frame_size(data: &[u8]) -> usize {
    if data.len() < 5 {
        return 0;
    }
    let fscod = (data[4] >> 6) & 0x03;
    let frmsizecod = (data[4] & 0x3F) as usize;
    if frmsizecod >= AC3_FRAME_SIZES.len() {
        return 0;
    }
    let words = AC3_FRAME_SIZES[frmsizecod];
    match fscod {
        0 => words[0] * 2,
        1 => words[1] * 2,
        2 => words[2] * 2,
        _ => 0,
    }
}

/// AC-3 frame size table: [frmsizecod] -> [48kHz words, 44.1kHz words, 32kHz words]
const AC3_FRAME_SIZES: [[usize; 3]; 38] = [
    [64, 69, 96],
    [64, 70, 96],
    [80, 87, 120],
    [80, 88, 120],
    [96, 104, 144],
    [96, 105, 144],
    [112, 121, 168],
    [112, 122, 168],
    [128, 139, 192],
    [128, 140, 192],
    [160, 174, 240],
    [160, 175, 240],
    [192, 208, 288],
    [192, 209, 288],
    [224, 243, 336],
    [224, 244, 336],
    [256, 278, 384],
    [256, 279, 384],
    [320, 348, 480],
    [320, 349, 480],
    [384, 417, 576],
    [384, 418, 576],
    [448, 487, 672],
    [448, 488, 672],
    [512, 557, 768],
    [512, 558, 768],
    [640, 696, 960],
    [640, 697, 960],
    [768, 835, 1152],
    [768, 836, 1152],
    [896, 975, 1344],
    [896, 976, 1344],
    [1024, 1114, 1536],
    [1024, 1115, 1536],
    [1152, 1253, 1728],
    [1152, 1254, 1728],
    [1280, 1393, 1920],
    [1280, 1394, 1920],
];

#[cfg(test)]
mod tests {
    use super::*;

    fn make_ac3_frame(fscod: u8, frmsizecod: u8) -> Vec<u8> {
        let size = AC3_FRAME_SIZES[frmsizecod as usize][fscod as usize] * 2;
        let mut frame = vec![0u8; size];
        frame[0] = 0x0B;
        frame[1] = 0x77;
        frame[4] = (fscod << 6) | frmsizecod;
        frame[5] = 0x08 << 3; // bsid = 8 (AC-3)
        frame
    }

    #[test]
    fn parse_empty_pes() {
        let mut parser = Ac3Parser::new();
        let pes = PesPacket {
            pid: 0,
            pts: None,
            dts: None,
            data: vec![],
        };
        assert!(parser.parse(&pes).is_empty());
    }

    #[test]
    fn parse_single_frame() {
        let mut parser = Ac3Parser::new();
        let frame_data = make_ac3_frame(0, 2); // 48kHz, 80 words = 160 bytes
        let pes = PesPacket {
            pid: 0,
            pts: Some(90000),
            dts: None,
            data: frame_data.clone(),
        };
        let frames = parser.parse(&pes);
        assert_eq!(frames.len(), 1);
        assert_eq!(frames[0].data.len(), 160);
    }

    #[test]
    fn parse_frame_spanning_two_pes() {
        let mut parser = Ac3Parser::new();
        let frame_data = make_ac3_frame(0, 2); // 160 bytes
        let mid = 80;

        // First PES: first half of frame
        let pes1 = PesPacket {
            pid: 0,
            pts: Some(90000),
            dts: None,
            data: frame_data[..mid].to_vec(),
        };
        let frames1 = parser.parse(&pes1);
        assert!(frames1.is_empty(), "partial frame should not emit");

        // Second PES: second half
        let pes2 = PesPacket {
            pid: 0,
            pts: Some(93000),
            dts: None,
            data: frame_data[mid..].to_vec(),
        };
        let frames2 = parser.parse(&pes2);
        assert_eq!(frames2.len(), 1);
        assert_eq!(frames2[0].data.len(), 160);
    }

    #[test]
    fn skip_garbage_before_sync() {
        let mut parser = Ac3Parser::new();
        let frame_data = make_ac3_frame(0, 2);
        let mut data = vec![0xDE, 0xAD, 0xBE, 0xEF]; // garbage
        data.extend_from_slice(&frame_data);
        let pes = PesPacket {
            pid: 0,
            pts: None,
            dts: None,
            data,
        };
        let frames = parser.parse(&pes);
        assert_eq!(frames.len(), 1);
        assert_eq!(frames[0].data.len(), 160);
    }

    #[test]
    fn sync_word_split_across_pes_is_preserved() {
        // A frame whose 0x0B77 syncword straddles the PES boundary (0x0B at the
        // tail of PES 1, 0x77 at the head of PES 2) must still be emitted whole.
        // Previously the lone trailing 0x0B was dropped and the frame lost.
        let mut parser = Ac3Parser::new();
        let frame_data = make_ac3_frame(0, 2); // 160 bytes, starts with 0x0B 0x77

        // PES 1: a complete frame, then a single 0x0B (first half of next sync).
        let mut pes1_data = frame_data.clone();
        pes1_data.push(0x0B);
        let pes1 = PesPacket {
            pid: 0,
            pts: Some(90000),
            dts: None,
            data: pes1_data,
        };
        let frames1 = parser.parse(&pes1);
        assert_eq!(frames1.len(), 1, "first complete frame emitted");

        // PES 2: 0x77 (second half of sync) + rest of the second frame.
        let mut pes2_data = vec![0x77];
        pes2_data.extend_from_slice(&frame_data[2..]);
        let pes2 = PesPacket {
            pid: 0,
            pts: Some(93000),
            dts: None,
            data: pes2_data,
        };
        let frames2 = parser.parse(&pes2);
        assert_eq!(frames2.len(), 1, "split-sync frame must be recovered");
        assert_eq!(frames2[0].data.len(), 160);
    }

    #[test]
    fn buffer_stays_bounded_across_many_garbage_pes() {
        // Finding 14: the carry-over buffer must never grow without bound. Feed
        // many large PES packets that contain no usable frame and assert the
        // retained buffer stays tiny — carry-from-`pos` drops all pre-sync junk,
        // and a never-completing frame is bounded by the 8192-byte frame cap and
        // the MAX_AC3_BUF resync guard.
        let mut parser = Ac3Parser::new();
        for i in 0..256 {
            // Vary the trailing byte so we also exercise the lone-0x0B retain.
            let mut data = vec![0x55u8; 8192];
            if i % 3 == 0 {
                *data.last_mut().unwrap() = 0x0B;
            }
            let pes = PesPacket {
                pid: 0,
                pts: None,
                dts: None,
                data,
            };
            let frames = parser.parse(&pes);
            assert!(frames.is_empty());
            assert!(
                parser.buf.len() <= MAX_AC3_BUF,
                "buffer grew to {} (cap {})",
                parser.buf.len(),
                MAX_AC3_BUF
            );
        }
        // After all that garbage the retained tail is at most a single partial
        // syncword byte — never an accumulation of whole PES packets.
        assert!(parser.buf.len() <= 1, "retained {} bytes", parser.buf.len());
    }

    #[test]
    fn split_sync_below_cap_is_still_retained() {
        // The cap must not break the normal split-sync straddle: a short tail
        // ending in 0x0B (well under the cap) is retained so the next PES can
        // complete the syncword.
        let mut parser = Ac3Parser::new();
        let data = vec![0x00, 0x00, 0x0B];
        let pes = PesPacket {
            pid: 0,
            pts: None,
            dts: None,
            data,
        };
        assert!(parser.parse(&pes).is_empty());
        assert_eq!(parser.buf, vec![0x0B], "lone trailing 0x0B retained");
    }

    #[test]
    fn ac3_frame_size_table() {
        // fscod=0 (48kHz), frmsizecod=0: 64 words = 128 bytes
        assert_eq!(ac3_frame_size(&[0x0B, 0x77, 0, 0, 0x00, 0x40]), 128);
        // fscod=0 (48kHz), frmsizecod=2: 80 words = 160 bytes
        assert_eq!(ac3_frame_size(&[0x0B, 0x77, 0, 0, 0x02, 0x40]), 160);
    }
}