Day1.c

#include <stdio.h>

enum MODES
{
    ADDING,
    SUBTRACTING
};

int map_character_to_int(int character)
{
    int result = character - '0';
    if (result < 0 || result > 9)
    {
        return -1;
    }
    else
    {
        return result;
    }
}

int main()
{
    const char* FILE_PATH = "Advent.txt";
    FILE* fp;
    int error = fopen_s(&fp, FILE_PATH, "r");
    if (fp == NULL)
    {
        printf("Could not open file %s. Error code %d", FILE_PATH, error);
        return 1;
    }

    int character;

    int total = 0;
    int current_value = 0;
    int mode = ADDING;
    for(;;)
    {
        character = fgetc(fp);
        if (character == '+')
        {
            mode = ADDING;
            continue;
        }

        if (character == '-')
        {
            mode = SUBTRACTING;
            continue;
        }

        if (character == '\n' || character == EOF)
        {
            if (mode == SUBTRACTING)
            {
                current_value *= -1;
            }
            total += current_value;
            current_value = 0;

            if (character == EOF)
            {
                break;
            }

            continue;
        }

        int character_value = map_character_to_int(character);
        if (character_value != -1)
        {
            current_value *= 10;
            current_value += character_value;
            continue;
        }
    }

    fclose(fp);

    printf("Total: %d\n", total);
    return 0;
}