aoc_omni/c/2020/1/main.c

#include <stddef.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "../lib/util.h"

char *example =
"1721\n"
"979\n"
"366\n"
"299\n"
"675\n"
"1456\n";

#define INPUT_NUMBER_BUFFER_LEN 5

#define TARGET 2020
#define NUMBER_BUFFER_LEN TARGET + 1

bool isDigit(char c) {
  return c >= '0' && c <= '9';
}

long part_1() {
  bool number_buffer[NUMBER_BUFFER_LEN] = {0};
  char input_number_buffer[INPUT_NUMBER_BUFFER_LEN] = {0};

  char *data_buffer = load_input();
  char *data = data_buffer;

  do {
    for (int i = 0; i < INPUT_NUMBER_BUFFER_LEN; i++) {
      if (isDigit(*data)) {
	input_number_buffer[i] = *data;
	data++;
      } else {
	input_number_buffer[i] = 0;
      }
    }
    
    int number = atoi(input_number_buffer);

    // Check for TARGET matches
    if (number_buffer[TARGET - number]) {
      long x = number;
      long y = TARGET - number;
      free(data_buffer);
      return x * y;
    }

    number_buffer[number] = true;

    data++;
  } while (*data != 0);

  return 0;
}

long part_2() {
  int number_buffer[NUMBER_BUFFER_LEN] = {0};
  char input_number_buffer[INPUT_NUMBER_BUFFER_LEN] = {0};
  int previously_seen_numbers[200] = {0};
  size_t previously_seen_numbers_index = 0;

  size_t number_buffer_index = 0;

  char *data_buffer = load_input();
  char *data = data_buffer;

  do {
    for (int i = 0; i < INPUT_NUMBER_BUFFER_LEN; i++) {
      if (isDigit(*data)) {
	input_number_buffer[i] = *data;
	data++;
      } else {
	input_number_buffer[i] = 0;
      }
    }

    int number = atoi(input_number_buffer);

    // Check for TARGET matches
    int x = number_buffer[TARGET - number];
    if (x > 0) {
      free(data_buffer);
      return number * x * (TARGET - number - x);
    }

    for (int i = 0; i < previously_seen_numbers_index; i++) {
      size_t index = number + previously_seen_numbers[i];
      if (index < NUMBER_BUFFER_LEN) number_buffer[index] = number;
    }

    previously_seen_numbers[previously_seen_numbers_index] = number;
    previously_seen_numbers_index++;

    data++;
  } while (*data != 0);

  return 0;
}

int main() {
  printf("Part 1: %d\n", part_1());
  printf("Part 2: %d\n", part_2());
}