//! MPEG-1/2/2.5 audio (MP1/MP2/MP3) decodability gate.
//!
//! Per ISO/IEC 11172-3 / ISO/IEC 13818-3, an MPEG-audio frame is validated by
//! header sanity + framing resync, not a payload CRC (the optional 16-bit CRC in
//! the header protects only the side-information and is absent unless the
//! protection bit says otherwise). The gate mirrors that header-only check and
//! ACCEPTS free-format (`bitrate_index == 0`) as a legal decodable mode — it
//! deliberately does NOT apply the stricter free-format reject that a full
//! decoder would (see the note at the `bitrate_index` check). So the gate rejects
//! only the truly invalid headers: a packet that begins with the 11-bit
//! MPEG-audio sync but whose version / layer / sample-rate fields (or the
//! reserved bitrate index 15) are reserved/invalid is undecodable → drop it (a
//! silence gap; each packet keeps its own PTS). A packet with no leading sync is
//! not a frame we can validate (raw payload / continuation), so it passes through
//! unchanged — never false-dropped.

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

/// Decoded validity of a candidate MPEG-audio header.
enum MpaVerdict {
    /// No 11-bit sync at the packet head — not a frame we can validate.
    NoSync,
    /// Sync present and every field is legal — decodable.
    Valid,
    /// Sync present but a field is reserved/invalid — a conformant header parser
    /// rejects this exactly.
    Invalid,
}

/// Header-only validity check per ISO/IEC 11172-3 / ISO/IEC 13818-3 (which
/// ACCEPTS free-format, `bitrate_index == 0`) — deliberately NOT the stricter
/// free-format reject a full decoder applies. A dropped MPEG-audio frame has a
/// corrupt header, so no duration is computed (the fields it would come from are
/// the invalid ones).
fn mpa_verdict(data: &[u8]) -> MpaVerdict {
    if data.len() < 4 {
        return MpaVerdict::NoSync;
    }
    let h = u32::from_be_bytes([data[0], data[1], data[2], data[3]]);
    // 11-bit sync (0x7FF at the top).
    if (h & 0xffe0_0000) != 0xffe0_0000 {
        return MpaVerdict::NoSync;
    }
    // Reject per spec: version field 01, layer field 00, bitrate_index 15,
    // sample-rate field 3.
    if (h & (3 << 19)) == (1 << 19)
        || (h & (3 << 17)) == 0
        || (h & (0xf << 12)) == (0xf << 12)
        || (h & (3 << 10)) == (3 << 10)
    {
        return MpaVerdict::Invalid;
    }
    // NOTE: bitrate_index == 0 (free format) is NOT rejected. It is a legal,
    // decodable MPEG-audio mode (the spec permits it and a decoder derives the
    // frame size from the sync spacing). Dropping it would be a false positive on
    // a clean stream, so it passes the gate.
    MpaVerdict::Valid
}

pub struct MpegAudioParser {
    tally: DropTally,
    /// Last emitted PTS (ns), carried forward across a PES with no PTS rather than
    /// resetting the timeline to 0 (see the AC-3/DTS parsers) — preserves A/V sync.
    last_pts_ns: i64,
}

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

impl MpegAudioParser {
    pub fn new() -> Self {
        Self {
            tally: DropTally::new("mpegaudio"),
            last_pts_ns: 0,
        }
    }

    pub fn dropped_frames(&self) -> u64 {
        self.tally.dropped_frames()
    }

    pub fn dropped_duration_ns(&self) -> u64 {
        self.tally.dropped_duration_ns()
    }
}

impl CodecParser for MpegAudioParser {
    fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
        if pes.data.is_empty() {
            return Vec::new();
        }
        let pts_ns = pes
            .pts
            .or(pes.dts)
            .map(pts_to_ns)
            .unwrap_or(self.last_pts_ns);
        self.last_pts_ns = pts_ns;

        let drop =
            self.tally.is_poisoned() || matches!(mpa_verdict(&pes.data), MpaVerdict::Invalid);
        if drop {
            let reason = if self.tally.is_poisoned() {
                "track-poisoned"
            } else {
                "header"
            };
            self.tally.record_drop(pts_ns, 0, pes.data.len(), reason);
            return Vec::new();
        }

        self.tally.record_kept();
        vec![Frame {
            discontinuity: pes.discontinuity,
            coding: None,
            source: None,
            pts_ns,
            keyframe: true,
            data: pes.data.clone(),
            duration_ns: None,
        }]
    }

    fn flush(&mut self) -> Vec<Frame> {
        self.tally.log_summary();
        Vec::new()
    }

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

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

    fn make_pes(data: Vec<u8>, pts: Option<i64>) -> PesPacket {
        PesPacket {
            source: None,
            pid: 0x1100,
            pts,
            dts: None,
            data,
            discontinuity: false,
        }
    }

    /// A valid MPEG-1 Layer III header: sync 0xFFF, version MPEG-1 (11), layer
    /// III (01), bitrate_index 9, sample-rate 0 (44.1 kHz), no CRC. Bytes:
    /// 0xFF 0xFB 0x90 0x00 — the canonical MP3 frame header.
    fn mp3_frame(payload: usize) -> Vec<u8> {
        let mut f = vec![0xFF, 0xFB, 0x90, 0x00];
        f.extend(std::iter::repeat(0xAA).take(payload));
        f
    }

    #[test]
    fn valid_header_is_kept() {
        let mut p = MpegAudioParser::new();
        let f = p.parse(&make_pes(mp3_frame(400), Some(90000)));
        assert_eq!(f.len(), 1);
        assert_eq!(f[0].pts_ns, pts_to_ns(90000));
        assert_eq!(p.dropped_frames(), 0);
    }

    #[test]
    fn pes_without_pts_carries_last_timestamp_not_zero() {
        // A PES with no PTS (legal for audio, e.g. after a discontinuity) must
        // carry the last known timestamp forward — resetting to 0 would corrupt
        // A/V sync. Mirrors the adts.rs guard test.
        let mut p = MpegAudioParser::new();
        p.parse(&make_pes(mp3_frame(400), Some(90000)));
        let f = p.parse(&make_pes(mp3_frame(400), None));
        assert_eq!(f.len(), 1);
        assert_eq!(
            f[0].pts_ns,
            pts_to_ns(90000),
            "carried forward, not reset to 0"
        );
    }

    /// Same contract as the ADTS gate: an MPEG-audio frame is dropped because
    /// its header is invalid, and the header is exactly where a frame duration
    /// (samples-per-frame for the layer, divided by the sampling rate) would
    /// have to come from — see `mpa_verdict`'s doc comment. So the drop's
    /// duration is reported as zero, not derived from rejected fields.
    #[test]
    fn dropped_frames_are_counted_but_their_duration_is_not_invented() {
        let mut parser = MpegAudioParser::new();
        // Reserved layer field (00) — rejected per ISO/IEC 11172-3.
        let mut bad = mp3_frame(32);
        bad[1] &= !0b0000_0110;
        for i in 0..3 {
            let out = parser.parse(&make_pes(bad.clone(), Some(i * 90_000)));
            assert!(out.is_empty(), "an invalid MPEG-audio frame is not emitted");
        }
        assert_eq!(parser.dropped_frames(), 3, "every drop is counted");
        assert_eq!(
            parser.dropped_duration_ns(),
            0,
            "the duration comes from the header that just failed validation, so \
             it is reported as unmeasured rather than guessed"
        );
    }

    #[test]
    fn reserved_version_field_is_dropped() {
        // version field = 01 (reserved) → rejected. byte1 = 111_01_01_1 = 0xEB
        // keeps the 11-bit sync (0xFF + top 3 bits 111) but sets version bits to 01.
        let mut p = MpegAudioParser::new();
        let mut frame = mp3_frame(400);
        frame[1] = 0xEB;
        let f = p.parse(&make_pes(frame, Some(90000)));
        assert!(f.is_empty(), "reserved version dropped");
        assert_eq!(p.dropped_frames(), 1);
    }

    #[test]
    fn reserved_sample_rate_is_dropped() {
        // Sync present but sample-rate field = 3 (reserved) → rejected per spec.
        // 0xFF 0xFB then byte2 with bits 11..10 = 11: 0x9C.
        let mut p = MpegAudioParser::new();
        let mut frame = mp3_frame(400);
        frame[2] = 0x9C; // freq field = 3
        let f = p.parse(&make_pes(frame, Some(90000)));
        assert!(f.is_empty(), "reserved sample rate dropped");
        assert_eq!(p.dropped_frames(), 1);
    }

    #[test]
    fn reserved_layer_is_dropped() {
        // Layer field 00 (reserved). byte1 bits 2..1 = 00 → 0xF9 keeps sync
        // (0xFFF needs byte1 top 3 bits set) and sets layer=00.
        let mut p = MpegAudioParser::new();
        let mut frame = mp3_frame(400);
        frame[1] = 0xF9; // 1111_1001: sync ok (top 3 =111), version 11, layer 00
        let f = p.parse(&make_pes(frame, Some(0)));
        assert!(f.is_empty(), "reserved layer dropped");
        assert_eq!(p.dropped_frames(), 1);
    }

    #[test]
    fn bad_bitrate_index_15_is_dropped() {
        let mut p = MpegAudioParser::new();
        let mut frame = mp3_frame(400);
        frame[2] = 0xF0; // bitrate_index = 1111
        assert!(p.parse(&make_pes(frame, Some(0))).is_empty());
        assert_eq!(p.dropped_frames(), 1);
    }

    #[test]
    fn free_format_bitrate_zero_is_kept() {
        // Free format (bitrate_index == 0) is legal and decodable — it must NOT
        // be dropped (that would be a false positive on a clean stream).
        let mut p = MpegAudioParser::new();
        let mut frame = mp3_frame(400);
        frame[2] = 0x00; // bitrate_index = 0000 (free format); sync/layer/rate ok
        let f = p.parse(&make_pes(frame, Some(0)));
        assert_eq!(f.len(), 1, "free-format frame kept");
        assert_eq!(p.dropped_frames(), 0);
    }

    #[test]
    fn non_sync_packet_passes_through() {
        // No 11-bit sync → not a validatable frame → keep (conservative).
        let mut p = MpegAudioParser::new();
        let f = p.parse(&make_pes(vec![0x00, 0x11, 0x22, 0x33, 0x44], Some(0)));
        assert_eq!(f.len(), 1);
        assert_eq!(p.dropped_frames(), 0);
    }

    #[test]
    fn drop_preserves_sync_via_own_pts() {
        let mut p = MpegAudioParser::new();
        let mut bad = mp3_frame(400);
        bad[2] = 0x9C; // reserved sample rate
        assert!(p.parse(&make_pes(bad, Some(90000))).is_empty());
        let f = p.parse(&make_pes(mp3_frame(400), Some(96000)));
        assert_eq!(f.len(), 1);
        assert_eq!(
            f[0].pts_ns,
            pts_to_ns(96000),
            "next frame keeps its own PTS"
        );
    }
}