avformat/cdg: add probe

libavformat/cdg.c

@@ -26,6 +26,22 @@
 #define CDG_COMMAND        0x09
 #define CDG_MASK           0x3F
 
+static int read_probe(const AVProbeData *p)
+{
+    const int cnt = p->buf_size / CDG_PACKET_SIZE;
+    int score = 0;
+
+    for (int i = 0; i < cnt; i++) {
+        const int x = p->buf[i * CDG_PACKET_SIZE] & CDG_MASK;
+
+        score += x == CDG_COMMAND;
+        if (x != CDG_COMMAND && x != 0)
+            return 0;
+    }
+
+    return FFMIN(score, AVPROBE_SCORE_MAX);
+}
+
 static int read_header(AVFormatContext *s)
 {
     AVStream *vst;
@@ -70,6 +86,7 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
 const AVInputFormat ff_cdg_demuxer = {
     .name           = "cdg",
     .long_name      = NULL_IF_CONFIG_SMALL("CD Graphics"),
+    .read_probe     = read_probe,
     .read_header    = read_header,
     .read_packet    = read_packet,
     .flags          = AVFMT_GENERIC_INDEX,