aoc2021/day21

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

int part1(int start_p1, int start_p2) {
  // player positions
  int p1 = start_p1;
  int p2 = start_p2;
  // player scores
  int p1_score = 0;
  int p2_score = 0;
  // die stuff
  int die = 0;
  int roll_count = 0;
  // turn switch
  bool p1_goes = true;
  while (p1_score < 1000 && p2_score < 1000) {
    // Roll the die 3 times
    int total_roll = (++die % 100) + (++die % 100) + (++die % 100);
    total_roll %= 10;
    roll_count += 3;
    // move the correct player
    if (p1_goes) {
      p1 += total_roll;
      if (p1 > 10) p1 -= 10;
      p1_score += p1;
      p1_goes = false;
    } else {
      p2 += total_roll;
      if (p2 > 10) p2 -= 10;
      p2_score += p2;
      p1_goes = true;
    }
  }
  // printf("p1 score: %d, p2 score: %d, roll count: %d\n", p1_score, p2_score, roll_count);
  if (p1_score >= 1000) {
    return p2_score * roll_count;
  } else {
    return p1_score * roll_count;
  }
}

// roll combos are:
// 1-1-1, 1-1-2, 1-1-3, 1-2-1, 1-2-2, 1-2-3, 1-3-1, 1-3-2, 1-3-3,
// 2-1-1, 2-1-2, 2-1-3, 2-2-1, 2-2-2, 2-2-3, 2-3-1, 2-3-2, 2-3-3,
// 3-1-1, 3-1-2, 3-1-3, 3-2-1, 3-2-2, 3-2-3, 3-3-1, 3-3-2, 3-3-3
int rolls[27] = {
  3, 4, 5, 4, 5, 6, 5, 6, 7, // 1x3, 2x4, 3x5, 2x6, 1x7
  4, 5, 6, 5, 6, 7, 6, 7, 8, //      1x4, 2x5, 3x6, 2x7, 1x8
  5, 6, 7, 6, 7, 8, 7, 8, 9  //           1x5, 2x6, 3x7, 2x8, 1x9
};
// TODO: uses this to collapse keep_moving to use 7 iterations, rather than 27.
// 1x3, 3x4, 6x5, 7x6, 6x7, 3x8, 1x9

unsigned long long invocations = 0;
unsigned long long overflows = 0;

void keep_moving(bool p1_moves, unsigned long long* p1_wins, unsigned long long*
  p2_wins, int p1, int p2, int p1_score, int p2_score, int depth) {
  assert(p1_score < 21);
  assert(1 <= p1 && p1 <= 10);
  assert(p2_score < 21);
  assert(1 <= p2 && p2 <= 10);
  ++invocations;
  printf("Depth: %d ", depth);

  for (int i = 0; i < 27; ++i) {
    if (p1_moves) {
      p1 += rolls[i];
      while (p1 > 10) {
        p1 -= 10;
      }
      p1_score += p1;
      // printf("moving p1 by %d (roll %d) to position %d with score %d\n", rolls[i], i, p1, p1_score);
      if (p1_score >= 21) {
        ++(*p1_wins);
      } else {
        p1_moves = false;
        keep_moving(p1_moves, p1_wins, p2_wins, p1, p2, p1_score, p2_score,
        (depth+1));
      }
    } else {
      p2 += rolls[i];
      while (p2 > 10) {
        p2 -= 10;
      }
      p2_score += p2;
      if (p2_score >= 21) {
        ++(*p2_wins);
      } else {
        p1_moves = true;
        keep_moving(p1_moves, p1_wins, p2_wins, p1, p2, p1_score, p2_score,
        (depth+1));
      }
    }
  }
}

unsigned long long part2(int start_p1, int start_p2) {
  unsigned long long p1_wins = 0;
  unsigned long long p2_wins = 0;
  int p1 = start_p1;
  int p2 = start_p2;
  int p1_score = 0;
  int p2_score = 0;
  bool p1_moves = true;
  keep_moving(p1_moves, &p1_wins, &p2_wins, p1, p2, p1_score, p2_score, 0);
  printf("Player 1 wins %llu times.\nPlayer 2 wins %llu times\n", p1_wins, p2_wins);
  return p1_wins > p2_wins ? p1_wins : p2_wins;
}

int main() {
  int p1, p2;
  int ret = scanf("Player 1 starting position: %d ", &p1);
  if (ret < 1) {
    fprintf(stderr, "player 1's input not found");
  }
  ret = scanf("Player 2 starting position: %d ", &p2);
  if (ret < 1) {
    fprintf(stderr, "player 2's input not found");
  }
  printf("P1: %d, p2: %d\n", p1, p2);
  printf("%d\n", part1(p1, p2));
  printf("%llu\n", part2(p1, p2));
  printf("Invocations: %llu\n", invocations);
}