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

static char *strrev(char *to_reverse) {
    size_t length = strlen(to_reverse);
    int position = length - 1;
    char *reversed = malloc((1 + length) * sizeof(char));

    while (position >= 0) {
        *reversed = to_reverse[position];
        reversed++;
        position--;
    }
    *reversed = '\0';
    reversed = reversed - length;

    return reversed;
}

static char first_digit(char *line) {
    while (*line != '\0' && !isdigit(*line)) {
        line++;
    }

    if (*line != '\0') {
        return *line;
    } else {
        return 0;
    }
}

static char *read_line(FILE *input_file) {
    unsigned int max_length = 16;
    unsigned int current_length = 0;
    char *line = malloc(max_length * sizeof(char));
    int current_char = fgetc(input_file);

    while (current_char != EOF && current_char != '\n') {
        if (current_length == max_length) {
            max_length *= 2;
            line = realloc(line, max_length * sizeof(char));
        }

        line[current_length] = current_char;
        current_length++;
        current_char = fgetc(input_file);
    }

    if (current_length == max_length) {
        line = realloc(line, (max_length + 1) * sizeof(char));
    }
    line[current_length] = '\0';

    return line;
}

unsigned int get_calibration_sum(FILE *input_file) {
    unsigned int sum = 0;
    char *current_line, *reversed_line;
    char intermediate_buffer[3];
    intermediate_buffer[2] = '\0';

    while (!feof(input_file)) {
        current_line = read_line(input_file);
        reversed_line = strrev(current_line);
        if (*current_line != '\0') {
            intermediate_buffer[0] = first_digit(current_line);
            intermediate_buffer[1] = first_digit(reversed_line);
            sum += atoi(intermediate_buffer);
        }
        free(reversed_line);
        free(current_line);
    }

    return sum;
}