AOC2021 Day21

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

var rolls = 0;
Iterable<int> roll1to100() sync* {
  while (true) {
    for (var d in 1.to(101)) {
      rolls += 1;
      yield d;
    }
  }
}

Iterator<int>? g;
int roll3_100() {
  g ??= roll1to100().iterator;
  var s = 0;
  for (var _ in 0.to(3)) {
    g!.moveNext();
    s += g!.current;
  }
  return s;
}

play1(int a, int b) {
  var ascore = 0, bscore = 0;
  while (true) {
    var d = roll3_100();
    a = (a + d + 9) % 10 + 1;
    ascore += a;
    if (ascore >= 1000) break;
    d = roll3_100();
    b = (b + d + 9) % 10 + 1;
    bscore += b;
    if (bscore >= 1000) break;
  }
  return (((a > b) ? bscore : ascore) * rolls);
}

class State {
  int posA, posB, scoreA, scoreB;
  State(this.posA, this.posB, this.scoreA, this.scoreB);
  factory State.copy(State s) => State(s.posA, s.posB, s.scoreA, s.scoreB);

  @override
  int get hashCode => Object.hash(posA, posB, scoreA, scoreB);

  @override
  bool operator ==(Object o) =>
      o is State &&
      posA == o.posA &&
      posB == o.posB &&
      scoreA == o.scoreA &&
      scoreB == o.scoreB;
}

// Number of times each score arises from 3 throws.
var throws = {3: 1, 4: 3, 5: 6, 6: 7, 7: 6, 8: 3, 9: 1};

play2(int a, int b) {
  // Counts the number of times each state has occurred so far.
  var tally = {State(a, b, 0, 0): 1};
  var winsA = 0;
  var winsB = 0;
  var turnA = false;
  while (tally.isNotEmpty) {
    turnA = !turnA;
    var newTally = <State, int>{};
    for (var e in tally.entries) {
      for (var t in throws.entries) {
        // Get the current state afresh each time
        var ns = State.copy(e.key);
        // Number of times this dice-throw occurs, times number of times the
        // original state occurred.
        var count = e.value * t.value;
        if (turnA) {
          ns.posA = (ns.posA + t.key + 9) % 10 + 1;
          ns.scoreA += ns.posA;
          if (ns.scoreA >= 21) {
            // update the counts, and leave this reality :-)
            winsA += count;
            continue;
          }
        } else {
          ns.posB = (ns.posB + t.key + 9) % 10 + 1;
          ns.scoreB += ns.posB;
          if (ns.scoreB >= 21) {
            winsB += count;
            continue;
          }
        }
        newTally[ns] = (newTally[ns] ?? 0) + count;
      }
    }
    tally = newTally;
  }
  return max(winsA, winsB);
}

main() {
  var t = DateTime.now();
  print(play1(1, 6));
  print(play2(1, 6));
  print('${DateTime.now().difference(t).inMilliseconds} milliseconds');
}