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

typedef struct {
  bool has_exactly_two;
  bool has_exactly_three;
} ChecksumResult;

bool is_off_by_one(const char *id1, const char *id2) {
  bool miss = false;
  while(*id1 > 0 && *id2 > 0) {
    if (*id1 != *id2) {
      if (miss) return false;
      miss = true;
    }
    id1++;
    id2++;
  }

  return miss;
}

void off_by_one_intersection(const char *id1, const char *id2, char *intersection) {
  int i = 0;
  while(*id1 > 0 && *id2 > 0) {
    if (*id1 == *id2) {
      intersection[i] = *id1;
      i++;
    }
    id1++;
    id2++;
  }
  intersection[i] = 0;
}

void checksum(const char *id, ChecksumResult *result) {
  result->has_exactly_two = false;
  result->has_exactly_three = false;

  int char_counts[26] = {0};
  while(*id > 0) {
    char_counts[*id - 'a'] ++;
    id++;
  }

  for (int i = 0; i < 26; i++) {
    if(char_counts[i] == 2) result->has_exactly_two = true;
    if(char_counts[i] == 3) result->has_exactly_three = true;
  }
}

typedef char Line[27];
int part_1(FILE *input) {
  rewind(input);
  ChecksumResult result = {0};
  int twos = 0;
  int threes = 0;
  Line line_buffer;
  while(fscanf(input, "%s", line_buffer) > 0) {
    checksum(line_buffer, &result);
    if (result.has_exactly_two) twos++;
    if (result.has_exactly_three) threes++;
  }
  return twos * threes;
}

void part_2(FILE *input) {
  rewind(input);
  Line lines[250] = {0};
  Line buffer;
  int line_number = 0;
  while(fscanf(input, "%s", buffer) > 0) {
    strcpy(lines[line_number++], buffer);
  }

  bool found = false;
  for(int i = 0; !found && i < 250; i++) {
    for(int j = i; !found && j < 250; j++) {
      if(is_off_by_one(lines[i], lines[j])) {
	found = true;
	off_by_one_intersection(lines[i], lines[j], buffer);
      }
    }
  }
  printf("Part 2: %s\n", buffer);
}

int main() {
  char *input_file_path = "../data/2018/2/input.txt";
  FILE *input = fopen(input_file_path, "r");
  if (input == NULL) {
    printf("Can't open file %s\n", input_file_path);
    return 1;
  }

  printf("Part 1: %d\n", part_1(input));
  part_2(input); // This is a string and I don't want to malloc it
  fclose(input);
}