puzzle.c

// For N = 11, a call to perms(p, N, 0) only takes 0.36 seconds.
// Why would a debugger seemingly hang forever when you try to
// step over the outermost recursive call to perms in the loop?
// Hint: How is "step over" ("next" in gdb/lldb) implemented?

u64 perms(u64 p[N], int n, u64 s) {
	if (n == 1) {
		s += signature(p);
	} else {
		for (int i = 0; i < n; i++) {
			swap(p[0], p[i]);
			s = perms(p + 1, n - 1, s);
			swap(p[0], p[i]);
		}
	}
	return s;
}

I've never bothered doing CFA but the strongest case for something like that is implementing efficient "step over" for inlined calls. I haven't thought it through in detail, though.