The "states" (A, B, C) correspond to goto targets. The "colors" (0, 1, 2, 3) are runtime data. At each state, the current color is read, and an instruction is executed (print some color, move left or right, goto some state) based on which color is there. Transliterated into C it looks like this:
#include "machine.h"
int main(void)
{
A:
switch (SCAN) {
case 0:
WRITE(1); RIGHT; goto B;
case 1:
WRITE(3); LEFT; goto B;
case 2:
WRITE(1); RIGHT; goto H;
case 3:
WRITE(2); RIGHT; goto A;
}
B:
switch (SCAN) {
case 0:
WRITE(2); LEFT; goto C;
case 1:
WRITE(3); RIGHT; goto B;
case 2:
WRITE(1); LEFT; goto C;
case 3:
WRITE(2); RIGHT; goto A;
}
C:
switch (SCAN) {
case 0:
WRITE(3); RIGHT; goto B;
case 1:
WRITE(1); LEFT; goto B;
case 2:
WRITE(3); LEFT; goto C;
case 3:
WRITE(2); RIGHT; goto C;
}
H:
HALT;
}
Question: are there any opportunities to rewrite this logic in a more "structured" style, or to make any other optimizations?
I love these puzzles. GNU C supports a label as value for computed goto. This is useful for direct threaded dispatch. You trade off a branch instruction for an address lookup, but it makes the code more structured.
Sometimes, the fact that one implementation includes it can make it actually more difficult to standardize, if there are some implementation details that are disagreeable (see GNU's nested functions)