fix(webrtc): preserve H.26x headers for new sessions

Detect random-access frames from Annex-B data and cache codec headers.
Prepend missing headers to later H.264 IDR and H.265 IRAP frames.
Forward standalone headers while WebRTC waits for a decodable frame.

Signed-off-by: BigfootACA <bigfoot@radxa.com>

src/video/codec/h265_bitstream.rs

@@ -0,0 +1,112 @@
+const VPS_NAL_TYPE: u8 = 32;
+const SPS_NAL_TYPE: u8 = 33;
+const PPS_NAL_TYPE: u8 = 34;
+
+fn find_start_code(data: &[u8], from: usize) -> Option<(usize, usize)> {
+    let mut offset = from;
+    while offset + 3 <= data.len() {
+        if offset + 4 <= data.len() && data[offset..offset + 4] == [0, 0, 0, 1] {
+            return Some((offset, 4));
+        }
+        if data[offset..offset + 3] == [0, 0, 1] {
+            return Some((offset, 3));
+        }
+        offset += 1;
+    }
+    None
+}
+
+fn for_each_nal(data: &[u8], mut visit: impl FnMut(u8, &[u8])) {
+    let mut cursor = 0;
+    while let Some((start, start_code_len)) = find_start_code(data, cursor) {
+        let nal_start = start + start_code_len;
+        if nal_start + 2 > data.len() {
+            break;
+        }
+        let next_start = find_start_code(data, nal_start)
+            .map(|(offset, _)| offset)
+            .unwrap_or(data.len());
+        let mut nal_end = next_start;
+        while nal_end > nal_start && data[nal_end - 1] == 0 {
+            nal_end -= 1;
+        }
+        if nal_end >= nal_start + 2 {
+            visit((data[nal_start] >> 1) & 0x3f, &data[nal_start..nal_end]);
+        }
+        if next_start == data.len() {
+            break;
+        }
+        cursor = next_start;
+    }
+}
+
+pub fn is_keyframe(data: &[u8]) -> bool {
+    let mut keyframe = false;
+    for_each_nal(data, |nal_type, _| {
+        if (16..=23).contains(&nal_type) {
+            keyframe = true;
+        }
+    });
+    keyframe
+}
+
+pub fn extract_vps_sps_pps(data: &[u8]) -> (Option<Vec<u8>>, Option<Vec<u8>>, Option<Vec<u8>>) {
+    let mut vps = None;
+    let mut sps = None;
+    let mut pps = None;
+    for_each_nal(data, |nal_type, nal| match nal_type {
+        VPS_NAL_TYPE => vps = Some(nal.to_vec()),
+        SPS_NAL_TYPE => sps = Some(nal.to_vec()),
+        PPS_NAL_TYPE => pps = Some(nal.to_vec()),
+        _ => {}
+    });
+    (vps, sps, pps)
+}
+
+pub fn has_vps_sps_pps(data: &[u8]) -> bool {
+    let (vps, sps, pps) = extract_vps_sps_pps(data);
+    vps.is_some() && sps.is_some() && pps.is_some()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn detects_irap_but_not_trail_frame() {
+        assert!(is_keyframe(&[0, 0, 0, 1, 19 << 1, 1, 0xaa]));
+        assert!(is_keyframe(&[0, 0, 1, 21 << 1, 1, 0xbb]));
+        assert!(!is_keyframe(&[0, 0, 0, 1, 1 << 1, 1, 0xcc]));
+    }
+
+    #[test]
+    fn extracts_parameter_sets() {
+        let data = [
+            0,
+            0,
+            0,
+            1,
+            32 << 1,
+            1,
+            0xaa,
+            0,
+            0,
+            1,
+            33 << 1,
+            1,
+            0xbb,
+            0,
+            0,
+            0,
+            1,
+            34 << 1,
+            1,
+            0xcc,
+        ];
+        let (vps, sps, pps) = extract_vps_sps_pps(&data);
+        assert_eq!(vps.unwrap(), [32 << 1, 1, 0xaa]);
+        assert_eq!(sps.unwrap(), [33 << 1, 1, 0xbb]);
+        assert_eq!(pps.unwrap(), [34 << 1, 1, 0xcc]);
+        assert!(has_vps_sps_pps(&data));
+    }
+}

src/video/codec/mod.rs

@@ -8,6 +8,7 @@ pub mod convert;
 pub mod h264;
 pub mod h264_bitstream;
 pub mod h265;
+pub mod h265_bitstream;
 pub mod jpeg;
 pub mod registry;
 pub mod self_check;