C
Surprise, O(n^12) solutions don't scale! But then it was delightful when the realization hit that the solution is actually very simple to implement - just keep removing the first digit that is followed by a higher one.
static uint64_t joltage(char *s, int len, int target) {
int i;
for (; len > target; len--) {
for (i=0; i<len-1 && s[i] >= s[i+1]; i++) ;
memmove(s+i, s+i+1, len-i);
}
return strtoul(s, NULL, 10);
}
int main() {
char buf[1024];
uint64_t p1=0,p2=0;
int len;
while (fgets(buf, sizeof(buf), stdin)) {
for (len=0; isdigit(buf[len]); len++) ;
buf[len] = '\0';
p2 += joltage(buf, len, 12);
p1 += joltage(buf, 12, 2);
}
printf("03: %lu %lu\n", p1, p2);
}
I'm still having to finish yesterday's x86-16 assembly implementation, for which I had to write some support code to deal with large numbers as strings. That will come in useful today, too!