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

#define MIN_TCP_PORT 1
#define TCP_TABLE "/proc/net/tcp"
#define TCP_TABLE_LINE_LENGTH 151
#define LOCALHOST_HEX "0100007F"
#define WILDCARD_HEX "00000000"
#define LISTENING_HEX "0A"
#define PORTS_BLOCKS_TO_ALLOW 4

/**
 * Get all the listening TCP ports
 */
static unsigned short *get_listening_ports(unsigned short *listening_ports, FILE *tcp_table_fptr) {
    char line[TCP_TABLE_LINE_LENGTH];
    char delimiter[] = " ";
    unsigned short len = 0;
    char *address;
    char *state;
    char *field;
    size_t allowed_for_ports = PORTS_BLOCKS_TO_ALLOW;

    // Skip first line (header)
    fgets(line, sizeof(line), tcp_table_fptr);

    // Tokenize lines one by one
    while (fgets(line, sizeof(line), tcp_table_fptr) != NULL) {
        strtok(line, delimiter);

        address = strtok(NULL, delimiter);
        strtok(NULL, delimiter);
        state = strtok(NULL, delimiter);
        field = strtok(NULL, delimiter);
        
        while (field != NULL) {
            field = strtok(NULL, delimiter);
        }

        if ((!strncmp(address, LOCALHOST_HEX, 8) || !strncmp(address, WILDCARD_HEX, 8)) && !strncmp(state, LISTENING_HEX, 2)) {
            if (len == allowed_for_ports) {
                allowed_for_ports = allowed_for_ports + PORTS_BLOCKS_TO_ALLOW;
                listening_ports = realloc(listening_ports, allowed_for_ports * sizeof(unsigned short));
            }
            listening_ports[len] = strtol(address + strlen(address) - 4, NULL, 16);
            len++;
        }
    }

    if (len == allowed_for_ports) {
        allowed_for_ports = allowed_for_ports + PORTS_BLOCKS_TO_ALLOW;
        listening_ports = realloc(listening_ports, allowed_for_ports * sizeof(unsigned short));
    }
    listening_ports[len] = 0;

    return listening_ports;
}

static unsigned char is_port_available(const unsigned short port, const unsigned short *listening_ports) {
    unsigned short i = 0;
    while (listening_ports[i] != 0 && listening_ports[i] != port) {
        i++;
    }
    return listening_ports[i] == 0;
}

static inline unsigned char is_valid_tcp_port(const unsigned short tcp_port) {
    return tcp_port >= MIN_TCP_PORT;
}

int main(int argc, char *argv[]) {
    unsigned short current_port;

    if (argc >= 2) {
        current_port = atoi(argv[1]);
    } else {
        current_port = 0;
    }

    if (!is_valid_tcp_port(current_port)) {
        fprintf(stderr, "Provide a valid TCP port number as first argument.\n");
        return EX_USAGE;
    }

    // Open TCP table
    FILE *tcp_table_fptr = fopen(TCP_TABLE, "r");
    if (tcp_table_fptr == NULL) {
        fprintf(stderr, "Error opening the TCP table.\n");
        return EX_OSFILE;
    }

    unsigned short *listening_ports = malloc(PORTS_BLOCKS_TO_ALLOW * sizeof(unsigned short));
    listening_ports = get_listening_ports(listening_ports, tcp_table_fptr);

    // Check if the current port is available, add
    while (!is_port_available(current_port, listening_ports)) {
        current_port++;
    }

    free(listening_ports);

    if (is_valid_tcp_port(current_port)) {
        printf("%d\n", current_port);
        return EXIT_SUCCESS;
    } else {
        fprintf(stderr, "No more ports available. How did you fuck up that bad ???\n");
        return EX_TEMPFAIL;
    }
}