advent-of-code/2022/2.c

#include <stdio.h>
#include <string.h>

struct RPS_Game {
  // Part 1 Rock / Part 2 Lose
  const int X;
  // Part 1 Paper / Part 2 Draw
  const int Y;
  // Part 1 Scissors / Part 2 Win
  const int Z;
  // Outcomes
  const int draw;
  const int win;
};

// Part 1 - Calculate score
void process_round(struct RPS_Game *game, int *score, const char opponent_move,
                   const char my_move) {

  switch (my_move) {
  case 'X': // Rock
    *score += game->X;
    if (opponent_move == 'A') {
      *score += game->draw;
    } else if (opponent_move == 'C') {
      *score += game->win;
    }
    break;
  case 'Y': // Paper
    *score += game->Y;
    if (opponent_move == 'A') {
      *score += game->win;
    } else if (opponent_move == 'B') {
      *score += game->draw;
    }
    break;
  case 'Z': // Scissors
    *score += game->Z;
    if (opponent_move == 'B') {
      *score += game->win;
    } else if (opponent_move == 'C') {
      *score += game->draw;
    }
    break;
  }
}

// Part 2 - Find move given a desired result, then calculate score
void find_move(struct RPS_Game *game, int *score, const char opponent_move,
               const char result) {

  char my_move = '\0';

  switch (result) {
  case 'X': // need to lose
    if (opponent_move == 'A') {
      my_move = 'Z';
    } else if (opponent_move == 'B') {
      my_move = 'X';
    } else if (opponent_move == 'C') {
      my_move = 'Y';
    }
    break;
  case 'Y': // need a draw
    if (opponent_move == 'A') {
      my_move = 'X';
    } else if (opponent_move == 'B') {
      my_move = 'Y';
    } else if (opponent_move == 'C') {
      my_move = 'Z';
    }
    break;
  case 'Z': // need to win
    if (opponent_move == 'A') {
      my_move = 'Y';
    } else if (opponent_move == 'B') {
      my_move = 'Z';
    } else if (opponent_move == 'C') {
      my_move = 'X';
    }
    break;
  }
  process_round(game, score, opponent_move, my_move);
}

int main(int argc, char *argv[]) {

  if (argc != 2) {
    fprintf(stderr, "USAGE: %s input_file\n", argv[0]);
    return 1;
  }

  FILE *file = fopen(argv[1], "r");
  if (!file) {
    fprintf(stderr, "Error opening file %s\n", argv[1]);
    return 1;
  }

  int part1_score = 0;
  int part2_score = 0;
  char round[5] = {0};
  struct RPS_Game game = {.X = 1, .Y = 2, .Z = 3, .draw = 3, .win = 6};

  while (fgets(round, sizeof round / sizeof *round, file) != NULL) {
    if (strlen(round) == 1) {
      break;
    }
    process_round(&game, &part1_score, round[0], round[2]);
    find_move(&game, &part2_score, round[0], round[2]);
  }

  fclose(file);
  printf("My part 1 score: %d\n", part1_score);
  printf("My part 2 score: %d\n", part2_score);

  return 0;
}