// server_udp_third.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>

#define BUF_SIZE 4096
#define END_MARKER "END_OF_TRANSMISSION"

static void process_block(char *buf,
                          ssize_t len,
                          long long *pos_counter,
                          long long *repl_counter) {
    if (!buf || !pos_counter || !repl_counter) {
        return;
    }

    for (ssize_t i = 0; i < len; i++) {
        char c = buf[i];
        long long pos = *pos_counter;

        if (((pos + 1) % 3 == 0) && c != '\0') {
            buf[i] = ' ';
            (*repl_counter)++;
        }

        (*pos_counter)++;
    }
}

int main(int argc, char *argv[]) {
    if (argc < 3) {
        fprintf(stderr,
                "Usage: %s <port> <output_file>\n",
                argv[0]);
        fprintf(stderr,
                "Example: %s 5000 out.txt\n",
                argv[0]);
        return -1;
    }

    char *endptr = NULL;
    errno = 0;
    long port_long = strtol(argv[1], &endptr, 10);
    if (errno != 0 || endptr == argv[1] || *endptr != '\0' ||
        port_long <= 0 || port_long > 65535) {
        fprintf(stderr, "ERROR: invalid port '%s'\n", argv[1]);
        return -1;
    }
    int port = (int) port_long;

    const char *out_path = argv[2];

    FILE *fout = fopen(out_path, "w");
    if (!fout) {
        perror("fopen(output_file)");
        return -1;
    }

    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket");
        fclose(fout);
        return -1;
    }

    struct sockaddr_in servaddr;
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(port);

    if (bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {
        perror("bind");
        close(sockfd);
        fclose(fout);
        return -1;
    }

    printf("UDP server (third-char) listening on port %d, output file: %s\n",
           port, out_path);

    char buf[BUF_SIZE];
    long long pos_counter = 0;
    long long repl_counter = 0;
    int done = 0;

    while (!done) {
        struct sockaddr_in cliaddr;
        socklen_t cli_len = sizeof(cliaddr);

        ssize_t n = recvfrom(sockfd,
                             buf,
                             sizeof(buf) - 1,
                             0,
                             (struct sockaddr *) &cliaddr,
                             &cli_len);
        if (n < 0) {
            perror("recvfrom");
            repl_counter = -1;
            break;
        }

        buf[n] = '\0';

        if (strcmp(buf, END_MARKER) == 0) {
            printf("Received END marker from %s:%d, finishing.\n",
                   inet_ntoa(cliaddr.sin_addr),
                   ntohs(cliaddr.sin_port));
            done = 1;
            break;
        }

        process_block(buf, n, &pos_counter, &repl_counter);

        if (fwrite(buf, 1, (size_t) n, fout) != (size_t) n) {
            perror("fwrite");
            repl_counter = -1;
            done = 1;
            break;
        }
    }

    if (fclose(fout) == EOF) {
        perror("fclose(output_file)");
        repl_counter = -1;
    }

    close(sockfd);

    printf("Total replacements (every 3rd char): %lld\n", repl_counter);
    if (repl_counter < 0) {
        return -1;
    }

    return 0;
}