1 files changed, 53 insertions, 0 deletions
diff --git a/src/lib/compute_ip_checksum.c b/src/lib/compute_ip_checksum.c
new file mode 100644
index 0000000000..0b8eb90f85
--- /dev/null
+++ b/src/lib/compute_ip_checksum.c
@@ -0,0 +1,53 @@
+#include <stdint.h>
+#include <ip_checksum.h>
+
+unsigned long compute_ip_checksum(void *addr, unsigned long length)
+{
+	uint8_t *ptr;
+	volatile union {
+		uint8_t  byte[2];
+		uint16_t word;
+	} value;
+	unsigned long sum;
+	unsigned long i;
+	/* In the most straight forward way possible,
+	 * compute an ip style checksum.
+	 */
+	sum = 0;
+	ptr = addr;
+	for(i = 0; i < length; i++) {
+		unsigned long value;
+		value = ptr[i];
+		if (i & 1) {
+			value <<= 8;
+		}
+		/* Add the new value */
+		sum += value;
+		/* Wrap around the carry */
+		if (sum > 0xFFFF) {
+			sum = (sum + (sum >> 16)) & 0xFFFF;
+		}
+	}
+	value.byte[0] = sum & 0xff;
+	value.byte[1] = (sum >> 8) & 0xff;
+	return (~value.word) & 0xFFFF;
+}
+
+unsigned long add_ip_checksums(unsigned long offset, unsigned long sum, unsigned long new)
+{
+	unsigned long checksum;
+	sum = ~sum & 0xFFFF;
+	new = ~new & 0xFFFF;
+	if (offset & 1) {
+		/* byte swap the sum if it came from an odd offset 
+		 * since the computation is endian independant this
+		 * works.
+		 */
+		new = ((new >> 8) & 0xff) | ((new << 8) & 0xff00);
+	}
+	checksum = sum + new;
+	if (checksum > 0xFFFF) {
+		checksum -= 0xFFFF;
+	}
+	return (~checksum) & 0xFFFF;
+}