plugins/id3: Add loop on frames to estimate bitrate

Estimate the bitrate and length when there's no additional header we
can rely on to get these numbers. This is done by parsing the initial
frames of the file.

This has an impact on performance, since now we are parsing much more
than before. There's a define N_FRAMES_BITRATE_ESTIMATE and
N_FRAMES_CBR_ESTIMATE that need to be tuned in order to obtain the best
compromise between performance and accuracy.

diff --git a/src/plugins/id3/id3.c b/src/plugins/id3/id3.c
index 917db54..b031964 100644 (file)
--- a/src/plugins/id3/id3.c
+++ b/src/plugins/id3/id3.c
@@ -51,9 +51,15 @@
 #define ID3V2_HEADER_SIZE       10
 #define ID3V2_FOOTER_SIZE       10
 
-/* We parse only the first 4 bytes, which are the interesting ones */
 #define MPEG_HEADER_SIZE 4
 
+/* TODO: The higher these numbers are, the more performance impact you get
+ * when parsing mp3. However the lower they are, the more imprecise bitrate
+ * _and_ length estimate will be. Investigate which would be the best numbers
+ * here. */
+#define N_FRAMES_BITRATE_ESTIMATE 512
+#define N_FRAMES_CBR_ESTIMATE 128
+
 enum mpeg_audio_version {
     MPEG_AUDIO_VERSION_1,
     MPEG_AUDIO_VERSION_2,
@@ -75,6 +81,7 @@ struct mpeg_header {
     enum mpeg_audio_layer layer;
 
     bool crc;
+    bool padding;
     uint8_t channels;
     uint8_t sampling_rate_idx;
     uint8_t codec_idx;
@@ -82,6 +89,7 @@ struct mpeg_header {
 
     unsigned int bitrate;
     unsigned int length;
+    bool cbr;
 };
 
 static const struct lms_string_size _codecs[] = {
@@ -345,6 +353,76 @@ _fill_mpeg_header(struct mpeg_header *hdr, const uint8_t b[4])
     }
 
     hdr->crc = !(b[1] & 0x1);
+    hdr->padding = b[2] & 0x2;
+
+    return 0;
+}
+
+static int
+_estimate_mp3_bitrate_from_frames(int fd, off_t mpeg_offset,
+                                  struct mpeg_header *orig_hdr)
+{
+    struct mpeg_header hdr = *orig_hdr;
+    off_t offset = mpeg_offset;
+    unsigned int sum = 0, i;
+    int r;
+    bool cbr = true;
+    /* For Layer I slot is 32 bits long, for Layer II and Layer III slot is 8
+     * bits long.
+     * [layer == 1] */
+    unsigned int padding_size_table[2] = { 1, 4 };
+    unsigned int samples_per_frame, sampling_rate;
+
+    samples_per_frame = _samples_per_frame_table[hdr.version][hdr.layer];
+    sampling_rate = _sample_rates[hdr.sampling_rate_idx];
+    assert(sampling_rate != 0);
+
+    for (i = 0; i < N_FRAMES_BITRATE_ESTIMATE;) {
+        unsigned int bitrate, padding_size;
+        unsigned int framesize;
+        uint8_t buf[4];
+
+        bitrate = _bitrate_table[hdr.version][hdr.layer][hdr.bitrate_idx];
+        if (cbr && bitrate == hdr.bitrate && i > N_FRAMES_CBR_ESTIMATE) {
+            i = 1;
+            sum = bitrate;
+            break;
+        }
+
+        sum += bitrate;
+        i++;
+
+        padding_size = hdr.padding ? padding_size_table[hdr.layer == 1] : 0;
+
+        framesize = 4;          /* mpeg header */
+        framesize += (samples_per_frame / 8) * bitrate * 1000;
+        framesize /= sampling_rate;
+        framesize += (padding_size * samples_per_frame);
+
+        offset += framesize;
+
+        lseek(fd, offset, SEEK_SET);
+        r = read(fd, buf, sizeof(buf));
+
+        if (r < 0) {
+            fprintf(stderr, "ERROR reading frame header at %#x\n",
+                    (unsigned int) offset);
+            break;
+        }
+        if (!r)
+            break;
+
+        if (buf[0] != 0xff || !_is_id3v2_second_synch_byte(buf[1]) ||
+            _fill_mpeg_header(&hdr, buf) < 0 ||
+            _fill_mp3_header(&hdr, buf) < 0) {
+            fprintf(stderr, "ERROR interpreting frame header at 0x%x:"
+                    "%#02x %#02x %#02x %#02x\n", (unsigned int) offset,
+                    buf[0], buf[1], buf[2], buf[3]);
+            break;
+        }
+    }
+
+    orig_hdr->bitrate = sum / i;
 
     return 0;
 }
@@ -359,7 +437,6 @@ _parse_vbr_headers(int fd, off_t mpeg_offset, struct mpeg_header *hdr)
     };
     uint8_t buf[18];
     off_t xing_offset;
-    bool cbr = false;
 
     /* Try Xing first since it's the most likely to be there */
     xing_offset = mpeg_offset + 4 + 2 * hdr->crc
@@ -369,8 +446,8 @@ _parse_vbr_headers(int fd, off_t mpeg_offset, struct mpeg_header *hdr)
     if (read(fd, buf, sizeof(buf)) != sizeof(buf))
         return -1;
 
-    cbr = (memcmp(buf, "Info", 4) == 0);
-    if (cbr || memcmp(buf, "Xing", 4) == 0) {
+    hdr->cbr = (memcmp(buf, "Info", 4) == 0);
+    if (hdr->cbr || memcmp(buf, "Xing", 4) == 0) {
         flags = buf[7];
 
         if (flags & 1)
@@ -461,12 +538,15 @@ found:
             (r = _parse_vbr_headers(fd, off, &hdr) < 0))
             return r;
 
-        /* assume it's cbr, otherwise it should have been set already */
-        if (!hdr.bitrate)
+        if (hdr.cbr)
             hdr.bitrate =
                 _bitrate_table[hdr.version][hdr.layer][hdr.bitrate_idx];
+        else if (!hdr.bitrate) {
+            r = _estimate_mp3_bitrate_from_frames(fd, off, &hdr);
+            if (r < 0)
+                return r;
+        }
 
-        /* estimate length from bitrate */
         if (!hdr.length)
             hdr.length =  (8 * (size - off)) / (1000 * hdr.bitrate);
     }
@@ -480,7 +560,6 @@ found:
     audio_info->bitrate = hdr.bitrate;
     audio_info->length = hdr.length;
 
-
     return 0;
 }