day 17 generic† solution

† mostly, subject to a few broad assumptions

day17/sol3.c

@@ -0,0 +1,85 @@
+#include <stdio.h>
+
+int prog[] = {2,4,1,1,7,5,4,4,1,4,0,3,5,5,3,0};
+int plen = sizeof(prog) /  sizeof(int);
+
+int run(long a, int i, int print) {
+	long b = 0;
+	long c = 0;
+	int ip;
+	for (ip = 0; ip+2 <= plen; ip += 2) {
+		int op = prog[ip];
+		long r = prog[ip+1];
+
+		if (op != 1 && op != 3 && op != 4 && r > 3) {
+			// combo op
+			if (r == 4) {
+				r = a;
+			} else if (r == 5) {
+				r = b;
+			} else if (r == 6) {
+				r = c;
+			} else if (r >= 7) {
+				return -1; // invalid
+			}
+		}
+
+		switch(op) {
+		case 0: a = a >> r; break; // adv
+		case 6: b = a >> r; break; // bdv
+		case 7: c = a >> r; break; // cdv
+		case 1: b ^= r;     break; // bxl
+		case 2: b = r % 8;  break; // bst
+		case 4: b ^= c;     break; // bxc
+		case 3: if (a != 0) ip = r-2; break; // jnz
+		case 5: // out
+			int o = r % 8;
+			if (print) {
+				printf("%d ", o);
+			} else {
+				if (o != prog[i++]) {
+					return -2;
+				}
+			}
+			break;
+		default:
+			return -1; //invalid
+		}
+	}
+	if (print) printf("\b\n");
+	return 0;
+}
+
+// assumptions:
+// - the whole program is a loop
+// - 'a' is decoded 3 bits at a time
+// - output only affected by higher bits, not lower bits
+long reverse(long init, int i) {
+	if (i < 0) {
+		return init;
+	}
+
+	for (int b = 0; b < 8; b++) {
+		long a = init<<3 | b;
+		if (run(a, i, 0) == 0) {
+			printf("# %lo\n", a);
+			long ret = reverse(a, i-1);
+			if (ret >= 0) {
+				return ret;
+			}
+		}
+	}
+	//error
+	return -3;
+}
+
+int main() {
+	run(46337277, 0, 1);
+	run(202991746427434L, 0, 1);
+	long a = reverse(0, plen-1);
+	printf("%ld\n", a);
+	if (a >= 0) {
+		printf("%d\n", run(a, 0, 1));
+	}
+	return 0;
+}