/*
* Packs a fixed-width mac address (ascii string) into 6 bytes.
* String need not be null-terminated, but has to be fixed-width (i.e. always
* FF:FF:FF:FF:FF:FF, not 0:7:A0:F:F2:30).
* There is no error checking.
* This function merely serves an example of the used algorithm to convert
* a hexadecimal character string into a number. Here it does so two at a time.
* A pattern can be seen when examining the ASCII table:
0 48  0011 0000
1 49  0011 0001
2 50  0011 0010
3 51  0011 0011
4 52  0011 0100
5 53  0011 0101
6 54  0011 0110
7 55  0011 0111
8 56  0011 1000
9 57  0011 1001
       ^   ^^^^
A 65  0100 0001
B 66  0100 0010
C 67  0100 0011
D 68  0100 0100
E 69  0100 0101
F 70  0100 0110
       ^   ^^^^
a 97  0110 0001
b 98  0110 0010
c 99  0110 0011
d 100 0110 0100
e 101 0110 0101
f 102 0110 0110
       ^   ^^^^
The numbers 0-9 start from 0 on the lower half byte, and the second-to-last
bit is never set.
The numbers A-F and a-f start from 1 on the lower half byte, and the second-
to-last bit IS set.
It doesn't matter to us wether a character is capital or not, as long as it
can be discerned from one of the other digits.
What we do:

grab the right half:
 t = t & 0x0F;
add 9 if character is a-f or A-F:
 if (t>>6)t+=9;
OR we do it without a jump:
 t += (9*(t>>6));
So in short, we'll get:
 t = (t & 0x0F) + (9*(t>>6));
*/
static void pack_mac (char *mac, unsigned char pip[6])
{
 int i=0;
 while (i<6) {
  char c1 = *mac++;
  char c2 = *mac;
  mac+=2;
  pip[i++] =
  (              /* first character */
   ((c1 & 0xF)   /* take right half */
   +(9*(c1>>6))) /* add 9 if character is a-f or A-F */
   <<4           /* pack into the left half of the byte */
  ) | (          /* second character */
   (c2 & 0xF)
   +(9*(c2>>6))
  );             /* leave it as the left half */
 }
}