day2part1.c

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

int main(int argc, char *argv[])
{
    FILE* fptr = fopen("puzzle_input.txt", "r");
    char buffer[4096];

    long int result = 0;

    if (fptr != NULL) {
        while (fgets(buffer, sizeof(buffer), fptr)) {
            char* range;
            char* rest = buffer;

            while ((range = strtok_r(rest, ",", &rest))) {
                long int start = atol(strtok(range, "-"));
                long int end = atol(strtok(NULL, "-"));

                for (long int i = start; i <= end; i++) {
                    char id[256];
                    sprintf(id, "%ld", i);
                    
                    int length = strlen(id);
                    if (length % 2 != 0) {
                        continue;
                    }

                    if (strncmp(id, id + length / 2, length / 2) == 0) {
                        result += i;
                    }
                }
            }
        }

        fclose(fptr);
    }
    
    printf("Result: %ld\n", result);

    return EXIT_SUCCESS;
}