gdppay: optimise payloading of buffers with multiple memories

Add function to calculate a payload CRC across multiple memories
so we don't have to merge buffers with multiple memories just to
calculate the CRC. Also make CRC calculation function static,
since it's not used outside dataprotocol.h and move special-casing
of length = 0 -> CRC = 0 into CRC function (from caller).

Perhaps more importantly, since payload CRC is off by default:
don't map buffer (and possibly merge memories in the process)
if we are not going to use it to calculate a CRC anyway.

gst/gdp/dataprotocol.c

@@ -125,6 +125,10 @@ G_STMT_START {							\
 #define POLY       0x1021
 #define CRC_INIT   0xFFFF
 
+static guint16 gst_dp_crc (const guint8 * buffer, guint length);
+static guint16 gst_dp_crc_from_memory_maps (const GstMapInfo * maps,
+    guint n_maps);
+
 /*** HELPER FUNCTIONS ***/
 
 static gboolean
@@ -133,7 +137,8 @@ gst_dp_header_from_buffer_any (GstBuffer * buffer, GstDPHeaderFlag flags,
 {
   guint8 *h;
   guint16 flags_mask;
-  GstMapInfo map;
+  guint16 header_crc = 0, crc = 0;
+  gsize buffer_size;
 
   g_return_val_if_fail (GST_IS_BUFFER (buffer), FALSE);
   g_return_val_if_fail (length, FALSE);
@@ -145,10 +150,35 @@ gst_dp_header_from_buffer_any (GstBuffer * buffer, GstDPHeaderFlag flags,
   /* version, flags, type */
   GST_DP_INIT_HEADER (h, version, flags, GST_DP_PAYLOAD_BUFFER);
 
-  gst_buffer_map ((GstBuffer *) buffer, &map, GST_MAP_READ);
+  if ((flags & GST_DP_HEADER_FLAG_CRC_PAYLOAD)) {
+    GstMapInfo *maps;
+    guint n_maps, i;
+
+    buffer_size = 0;
+
+    n_maps = gst_buffer_n_memory (buffer);
+    if (n_maps > 0) {
+      maps = g_newa (GstMapInfo, n_maps);
+
+      for (i = 0; i < n_maps; ++i) {
+        GstMemory *mem;
+
+        mem = gst_buffer_peek_memory (buffer, i);
+        gst_memory_map (mem, &maps[i], GST_MAP_READ);
+        buffer_size += maps[i].size;
+      }
+
+      crc = gst_dp_crc_from_memory_maps (maps, n_maps);
+
+      for (i = 0; i < n_maps; ++i)
+        gst_memory_unmap (maps[i].memory, &maps[i]);
+    }
+  } else {
+    buffer_size = gst_buffer_get_size (buffer);
+  }
 
   /* buffer properties */
-  GST_WRITE_UINT32_BE (h + 6, map.size);
+  GST_WRITE_UINT32_BE (h + 6, buffer_size);
   GST_WRITE_UINT64_BE (h + 10, GST_BUFFER_TIMESTAMP (buffer));
   GST_WRITE_UINT64_BE (h + 18, GST_BUFFER_DURATION (buffer));
   GST_WRITE_UINT64_BE (h + 26, GST_BUFFER_OFFSET (buffer));
@@ -164,9 +194,17 @@ gst_dp_header_from_buffer_any (GstBuffer * buffer, GstDPHeaderFlag flags,
   /* from gstreamer 1.x, buffers also have the DTS */
   GST_WRITE_UINT64_BE (h + 44, GST_BUFFER_DTS (buffer));
 
-  GST_DP_SET_CRC (h, flags, map.data, map.size);
+  /* header CRC */
+  if ((flags & GST_DP_HEADER_FLAG_CRC_HEADER))
+    /* we don't crc the last four bytes since they are crc's */
+    header_crc = gst_dp_crc (h, 58);
+  else
+    header_crc = 0;
 
-  gst_buffer_unmap ((GstBuffer *) buffer, &map);
+  GST_WRITE_UINT16_BE (h + 58, header_crc);
+
+  /* payload CRC */
+  GST_WRITE_UINT16_BE (h + 60, crc);
 
   GST_MEMDUMP ("created header from buffer", h, GST_DP_HEADER_LENGTH);
   *header = h;
@@ -259,12 +297,15 @@ static const guint16 gst_dp_crc_table[256] = {
  *
  * Returns: a two-byte CRC checksum.
  */
-guint16
+static guint16
 gst_dp_crc (const guint8 * buffer, guint length)
 {
   guint16 crc_register = CRC_INIT;
 
-  g_return_val_if_fail (buffer != NULL || length == 0, 0);
+  if (length == 0)
+    return 0;
+
+  g_assert (buffer != NULL);
 
   /* calc CRC */
   for (; length--;) {
@@ -274,6 +315,38 @@ gst_dp_crc (const guint8 * buffer, guint length)
   return (0xffff ^ crc_register);
 }
 
+static guint16
+gst_dp_crc_from_memory_maps (const GstMapInfo * maps, guint n_maps)
+{
+  guint16 crc_register = CRC_INIT;
+  gsize total_length = 0;
+
+  if (n_maps == 0)
+    return 0;
+
+  g_assert (maps != NULL);
+
+  /* calc CRC */
+  while (n_maps > 0) {
+    guint8 *buffer = maps->data;
+    gsize length = maps->size;
+
+    total_length += length;
+
+    while (length-- > 0) {
+      crc_register = (guint16) ((crc_register << 8) ^
+          gst_dp_crc_table[((crc_register >> 8) & 0x00ff) ^ *buffer++]);
+    }
+    --n_maps;
+    ++maps;
+  }
+
+  if (G_UNLIKELY (total_length == 0))
+    return 0;
+
+  return (0xffff ^ crc_register);
+}
+
 GType
 gst_dp_version_get_type (void)
 {

gst/gdp/dataprotocol.h

@@ -140,10 +140,6 @@ GstDPPacketizer *
                 gst_dp_packetizer_new           (GstDPVersion version);
 void            gst_dp_packetizer_free          (GstDPPacketizer *packetizer);
 
-/* crc checksum */
-guint16         gst_dp_crc                      (const guint8 * buffer,
-                                                 guint length);
-
 /* payload information from header */
 guint32         gst_dp_header_payload_length    (const guint8 * header);
 GstDPPayloadType