// Initially based on https://stackoverflow.com/a/35570418

#include <netinet/in.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

static inline bool IsDigit(char c) { return c >= '0' && c <= '9'; }

static unsigned ParseAddress(char const *str, FILE *log_stream)
{
    unsigned addr = 0, byte_counter = 0, byte = 0, digits_counter = 0;
    for (size_t i = 0;; i++) {
        char const c = str[i];
        if (c == '\0') {
            if (byte_counter != 3) {
                fprintf(log_stream, "Invalid address specified: %u bytes of 4 expected are specified\n", byte_counter + 1);
                return INADDR_NONE;
            }
            if (digits_counter == 0) {
                fprintf(log_stream, "Invalid address specified: no digits on byte %u\n", byte_counter + 1);
                return INADDR_NONE;
            }
            addr |= byte << (8 * (3 - byte_counter));
            break;
        } else if (IsDigit(c)) {
            digits_counter++;
            if (digits_counter > 3) {
                fprintf(log_stream, "Invalid address specified: too many digits for byte %u\n", byte_counter + 1);
                return INADDR_NONE;
            }
            byte = byte * 10 + (c - '0');
            if (byte > UINT8_MAX) {
                fprintf(log_stream, "Invalid address specified: too big number for byte %u\n", byte_counter + 1);
                return INADDR_NONE;
            }
        } else if (c == '.') {
            if (digits_counter == 0) {
                fprintf(log_stream, "Invalid address specified: no digits on byte %u\n", byte_counter + 1);
                return INADDR_NONE;
            }
            digits_counter = 0;
            addr |= byte << (8 * (3 - byte_counter));
            byte = 0;
            byte_counter++;
            if (byte_counter > 3) {
                fprintf(log_stream, "Invalid address specified: unexpected dot after 4th byte\n");
                return INADDR_NONE;
            }
        } else {
            fprintf(log_stream, "Invalid address specified: char '%c' (0x%02x) is not allowed\n", c, c);
            return INADDR_NONE;
        }
    }
    if (addr == INADDR_NONE) {
        fprintf(log_stream, "Invalid address specified\n");
    }
    return addr;
}

int main(int const argc, char *const argv[])
{
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <address> <port>\n", argv[0]);
        return 1;
    }
    unsigned const addr = ParseAddress(argv[1], stderr);
    if (addr == INADDR_NONE) {
        return 1;
    }
    int const port = atoi(argv[2]);
    if (port <= 0 || port > UINT16_MAX) {
        fprintf(stderr, "Invalid port specifed\n");
        return 1;
    }
    int const fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket failed");
        return 1;
    }
    struct sockaddr_in const serveraddr = {
        .sin_family = AF_INET,
        .sin_port = htons(port),
        .sin_addr.s_addr = htonl(addr),
    };
    for (int i = 0; i < 4; i++) {
        ssize_t const ret = sendto(
                fd, "hello", 5, 0, (struct sockaddr const *)&serveraddr, sizeof(serveraddr));
        if (ret < 0) {
            perror("sendto failed");
            break;
        }
        printf("message sent\n");
    }
    close(fd);
}