+/*
+ * If |pad| is zero, the operation is effectively reduced to memcpy,
+ * and if |pad| is 0xff, then it performs two's complement, ~dst + 1.
+ * Note that in latter case sequence of zeros yields itself, and so
+ * does 0x80 followed by any number of zeros. These properties are
+ * used elsewhere below...
+ */
+static void twos_complement(unsigned char *dst, const unsigned char *src,
+ size_t len, unsigned char pad)
+{
+ unsigned int carry = pad & 1;
+
+ /* Begin at the end of the encoding */
+ dst += len;
+ src += len;
+ /* two's complement value: ~value + 1 */
+ while (len-- != 0) {
+ *(--dst) = (unsigned char)(carry += *(--src) ^ pad);
+ carry >>= 8;
+ }
+}
+