typefind: speed up mxf_type_find over 300 times for worst case scenarios

* memcmp is expensive and was being abused, reduce calling it by checking
  the first byte.
* iterating one byte at at time over 64 kbites introduces a certain overhead,
  therefore we now do it in chunks of 1024 bytes

And I do mean over 300 times. The average instruction call per mxf_type_find
was previously 785685 and it's now down to 2458 :)

1 files changed, 21 insertions, 14 deletions

diff --git a/gst/typefind/gsttypefindfunctions.c b/gst/typefind/gsttypefindfunctions.c
index 77edb7e58..d394de006 100644
--- a/gst/typefind/gsttypefindfunctions.c
+++ b/gst/typefind/gsttypefindfunctions.c
@@ -2697,28 +2697,35 @@ mxf_type_find (GstTypeFind * tf, gpointer ununsed)
   DataScanCtx c = { 0, NULL, 0 };
 
   while (c.offset <= MXF_MAX_PROBE_LENGTH) {
-    if (G_UNLIKELY (!data_scan_ctx_ensure_data (tf, &c, 16)))
+    guint i;
+    if (G_UNLIKELY (!data_scan_ctx_ensure_data (tf, &c, 1024)))
       break;
 
-    if (memcmp (c.data, partition_pack_key, 13) == 0) {
-      /* Header partition pack? */
-      if (c.data[13] != 0x02)
-        goto advance;
+    /* look over in chunks of 1kbytes to avoid too much overhead */
 
-      /* Partition status */
-      if (c.data[14] >= 0x05)
-        goto advance;
+    for (i = 0; i < 1024 - 16; i++) {
+      /* Check first byte before calling more expensive memcmp function */
+      if (G_UNLIKELY (c.data[0] == 0x06
+              && memcmp (c.data + i, partition_pack_key, 13) == 0)) {
+        /* Header partition pack? */
+        if (c.data[i + 13] != 0x02)
+          goto advance;
 
-      /* Reserved, must be 0x00 */
-      if (c.data[15] != 0x00)
-        goto advance;
+        /* Partition status */
+        if (c.data[i + 14] >= 0x05)
+          goto advance;
 
-      gst_type_find_suggest (tf, GST_TYPE_FIND_MAXIMUM, MXF_CAPS);
-      return;
+        /* Reserved, must be 0x00 */
+        if (c.data[i + 15] != 0x00)
+          goto advance;
+
+        gst_type_find_suggest (tf, GST_TYPE_FIND_MAXIMUM, MXF_CAPS);
+        return;
+      }
     }
 
   advance:
-    data_scan_ctx_advance (tf, &c, 1);
+    data_scan_ctx_advance (tf, &c, 1024 - 16);
   }
 }