Dart Solution

I decided to use the quadratic formula to solve part 1 which slowed me down while I struggled to remember how it went, but meant that part 2 was a one line change.

This year really is a roller coaster…

int countGoodDistances(int time, int targetDistance) {
  var det = sqrt(time * time - 4 * targetDistance);
  return (((time + det) / 2).ceil() - 1) -
      (max(((time - det) / 2).floor(), 0) + 1) +
      1;
}

solve(List> data, [param]) {
  var distances = data.first
      .indices()
      .map((ix) => countGoodDistances(data[0][ix], data[1][ix]));
  return distances.reduce((s, t) => s * t);
}

getNums(l) => l.split(RegExp(r'\s+')).skip(1);

part1(List lines) =>
    solve([for (var l in lines) getNums(l).map(int.parse).toList()]);

part2(List lines) => solve([
      for (var l in lines) [int.parse(getNums(l).join(''))]
    ]);