17! ::: spoiler p1 discussion Simultaneously very fun and also the fucking worst. Fun: Ooooh, I get to simulate a computer, exciting! Worst: Literally 8 edge cases where fucking up even just one can fuck up your hour. ::: ::: spoiler p2 discussion I did this by hand. sort of. I mean I didn't code up something that found the answer. Basically I looked at the program in the input and wrote it out, and realised that A was essentially a loop variable, where the number of iterations was the number of octal digits A would take to represent. The most significant octal digits (octits?) would determine the tail end of the output sequence, so to find the smallest A you can do a DFS starting from the MS octit. I did this by hand. ::: ::: spoiler EDIT: code. Not gonna explain any of it. ``` class Comp { List reg; List prog; int ip = 0; List output = []; late List<(int, bool) Function()> ops; int get combo => prog[ip + 1] < 4 ? prog[ip + 1] : reg[prog[ip + 1] - 4]; Comp(this.reg, this.prog) { ops = [ () => (reg[0] = (reg[0] >> combo), false), () => (reg[1] ^= prog[ip + 1], false), () => (reg[1] = combo % 8, false), () => (reg[0] != 0) ? (ip = prog[ip + 1], true) : (0, false), () => (reg[1] ^= reg[2], false), () { output.add(combo % 8); return (0, false); }, () => (reg[1] = (reg[0] >> combo), false), () => (reg[2] = (reg[0] >> combo), false) ]; } compute() { output.clear(); while (ip < prog.length) { if (!ops[prog[ip]]().$2) { ip += 2; } } } reset(int A) { ip = 0; reg[0] = A; reg[1] = 0; reg[2] = 0; } } void d17(bool sub) { List input = getLines(); Comp c = Comp( input.take(3).map((s) => s.split(" ").last).map(int.parse).toList(), input.last.split(" ").last.split(",").map(int.parse).toList()) ..compute(); print("Part a: ${c.output.join(",")}"); if (!sub) return; List sols = []; bool dfs(int cur) { bool found = false; sols.add(cur); int sol = sols.reduce((a, b) => 8 * a + b); c..reset(sol)..compute(); if (c.prog .whereIndexed((i, e) => i >= c.prog.length - c.output.length) .foldIndexed(true, (i, p, e) => p && c.output[i] == e)) { if (found = c.output.length == c.prog.length) { print("Part b: $sol"); } else { for (int i = 0; i < 8 && !(found = found || dfs(i)); i++) {} } } sols.removeLast(); return found; } for (int a = 0; a < 8 && !dfs(a); a++) {} } ``` :::