net: don't use get_message() when constructing datagram header

This is causing an unnecessary copy operation.

panda/src/net/datagramTCPHeader.cxx

@@ -24,23 +24,23 @@
  */
 DatagramTCPHeader::
 DatagramTCPHeader(const NetDatagram &datagram, int header_size) {
-  const string &str = datagram.get_message();
+  size_t length = datagram.get_length();
   switch (header_size) {
   case 0:
     break;
 
   case datagram_tcp16_header_size:
     {
-      uint16_t size = str.length();
-      nassertv(size == str.length());
+      uint16_t size = (uint16_t)length;
+      nassertv((size_t)size == length);
       _header.add_uint16(size);
     }
     break;
 
   case datagram_tcp32_header_size:
     {
-      uint32_t size = str.length();
-      nassertv(size == str.length());
+      uint32_t size = (uint32_t)length;
+      nassertv((size_t)size == length);
       _header.add_uint32(size);
     }
     break;
@@ -93,8 +93,7 @@ verify_datagram(const NetDatagram &datagram, int header_size) const {
     return true;
   }
 
-  const string &str = datagram.get_message();
-  int actual_size = str.length();
+  int actual_size = (int)datagram.get_length();
   int expected_size = get_datagram_size(header_size);
   if (actual_size == expected_size) {
     return true;

panda/src/net/datagramUDPHeader.cxx

@@ -24,10 +24,11 @@
  */
 DatagramUDPHeader::
 DatagramUDPHeader(const NetDatagram &datagram) {
-  const string &str = datagram.get_message();
+  const unsigned char *begin = (const unsigned char *)datagram.get_data();
+  const unsigned char *end = begin + datagram.get_length();
   uint16_t checksum = 0;
-  for (size_t p = 0; p < str.size(); p++) {
-    checksum += (uint16_t)(uint8_t)str[p];
+  for (const unsigned char *p = begin; p != end; ++p) {
+    checksum += (uint16_t)(uint8_t)*p;
   }
 
   // Now pack the header.
@@ -49,11 +50,11 @@ DatagramUDPHeader(const void *data) : _header(data, datagram_udp_header_size) {
  */
 bool DatagramUDPHeader::
 verify_datagram(const NetDatagram &datagram) const {
-  const string &str = datagram.get_message();
-
+  const unsigned char *begin = (const unsigned char *)datagram.get_data();
+  const unsigned char *end = begin + datagram.get_length();
   uint16_t checksum = 0;
-  for (size_t p = 0; p < str.size(); p++) {
-    checksum += (uint16_t)(uint8_t)str[p];
+  for (const unsigned char *p = begin; p != end; ++p) {
+    checksum += (uint16_t)(uint8_t)*p;
   }
 
   if (checksum == get_datagram_checksum()) {