Dart

Part one was an exercise in building a simple OpCode machine. Part two was trickier. It was clear that the a register was repeatedly being divided by 8, and testing a few values showed that each 3-bits of the initial value defined one entry in the output, so I built a recursive routine that brute-forced each one in turn. Only <1ms to run though, so not that brutal.

(It's worth pointing out that at some stages multiple 3-bit values gave rise to the required value, causing a failure to resolve later on if not checked.)

(edit: for-loop in buildQuine now avoids using a 0 for the initial triplet, as this should not be a valid value, and if it turns out to generate the required digit of the quine, you will get an infinite recursion. Thanks to SteveDinn for bringing this to my attention.)

import 'dart:math';
import 'package:collection/collection.dart';
import 'package:more/more.dart';

var prog = <int>[];
typedef State = (int, int, int);
State parse(List<String> lines) {
  var regs = lines.takeTo('').map((e) => int.parse(e.split(' ').last)).toList();
  var (a, b, c) = (regs[0], regs[1], regs[2]);
  prog = lines.last.split(' ').last.split(',').map(int.parse).toList();
  return (a, b, c);
}

List<int> runProg(State rec) {
  var (int a, int b, int c) = rec;
  combo(int v) => switch (v) { 4 => a, 5 => b, 6 => c, _ => v };
  var output = <int>[], pc = 0;
  while (pc < prog.length) {
    var code = prog[pc], arg = prog[pc + 1];
    var _ = switch (code) {
      0 => a ~/= pow(2, combo(arg)),
      1 => b ^= arg,
      2 => b = combo(arg) % 8,
      3 => (a != 0) ? (pc = arg - 2) : 0,
      4 => b ^= c, //ignores arg
      5 => output.add(combo(arg) % 8),
      6 => b = a ~/ pow(2, combo(arg)),
      7 => c = a ~/ pow(2, combo(arg)),
      _ => 0
    };
    pc += 2;
  }
  return output;
}

Function eq = const ListEquality().equals;
Iterable<int> buildQuine(State rec, List<int> quine, [top = false]) sync* {
  var (int a0, int b0, int c0) = rec;
  if (top) a0 = 0;
  if (quine.length == prog.length) {
    yield a0;
    return;
  }
  for (var a in (top ? 1 : 0).to(8)) {
    var newState = (a0 * 8 + a, b0, c0);
    var newQuine = runProg(newState);
    if (eq(prog.slice(prog.length - newQuine.length), newQuine)) {
      yield* buildQuine(newState, newQuine);
    }
  }
}

part1(List<String> lines) => runProg(parse(lines)).join(',');

part2(List<String> lines) => buildQuine(parse(lines), [], true).first;