#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <sys/socket.h>
#include <unistd.h>

#define PORT 6008
#define BACKLOG 16
#define RECV_BUF 256

int main()
{
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    int opt = 1;
    setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    struct sockaddr_in addr = {0};
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = INADDR_ANY;
    addr.sin_port = htons(PORT);

    bind(server_fd, (struct sockaddr *)&addr, sizeof(addr));
    listen(server_fd, BACKLOG);

    printf("listening port %d\nbacklog %d\nrecv buffer %d\n", PORT, BACKLOG, RECV_BUF);

    char buf[RECV_BUF];
    const char *hdr = "X-Forwarded-For:";
    const size_t hdrlen = strlen(hdr);

    while (1)
    {
        int client = accept(server_fd, NULL, NULL);
        if (client < 0)
            continue;
        ssize_t r = recv(client, buf, sizeof(buf) - 1, 0);
        if (r <= 0)
        {
            close(client);
            continue;
        }
        buf[r] = 0;
        // detect optional JSON response request on the first line
        bool wants_json = false;
        const char *line_end = strstr(buf, "\r\n");
        size_t line_len = line_end ? (size_t)(line_end - buf) : (size_t)r;
        const char *space1 = memchr(buf, ' ', line_len);
        if (space1 && space1 < buf + line_len)
        {
            const char *path = space1 + 1;
            const char *space2 = memchr(path, ' ', line_len - (size_t)(path - buf));
            if (space2 && space2 > path)
            {
                const char *query = memchr(path, '?', (size_t)(space2 - path));
                if (query && ++query < space2)
                {
                    size_t remaining = (size_t)(space2 - query);
                    const char *token = query;
                    while (remaining > 0)
                    {
                        size_t token_len = 0;
                        while (token_len < remaining && token[token_len] != '&')
                            token_len++;
                        if (token_len == 11 && strncasecmp(token, "format=json", 11) == 0)
                        {
                            wants_json = true;
                            break;
                        }
                        if (token_len >= 7 && strncasecmp(token, "format=", 7) == 0)
                        {
                            size_t value_len = token_len - 7;
                            if (value_len == 4 && strncasecmp(token + 7, "json", 4) == 0)
                            {
                                wants_json = true;
                                break;
                            }
                        }
                        if (token_len == remaining)
                            break;
                        token += token_len + 1;
                        remaining -= token_len + 1;
                    }
                }
            }
        }

        // find header
        char *ip = NULL;
        for (ssize_t i = 0; i < r - (ssize_t)hdrlen; ++i)
        {
            if (strncasecmp(buf + i, hdr, hdrlen) == 0)
            {
                ip = buf + i + hdrlen;
                while (*ip == ' ' || *ip == '\t')
                    ip++;
                break;
            }
        }
        // find end
        char *end = ip;
        if (ip)
        {
            while (*end && *end != '\r' && *end != '\n' && *end != ',')
                end++;
            while (end > ip && (end[-1] == ' ' || end[-1] == '\t'))
                end--;
            *end = 0;
        }

        // response
        char head[128];
        char jsonbuf[RECV_BUF];
        const char *body = ip ? ip : "";
        const char *content_type = "text/plain";
        int body_len = ip ? (int)strlen(ip) : 0;

        if (wants_json)
        {
            content_type = "application/json";
            if (ip && *ip)
            {
                body_len = snprintf(jsonbuf, sizeof(jsonbuf), "{\"ip\":\"%s\"}", ip);
                if (body_len < 0 || body_len >= (int)sizeof(jsonbuf))
                    body_len = snprintf(jsonbuf, sizeof(jsonbuf), "{\"ip\":\"%s\"}", "");
            }
            else
            {
                body_len = snprintf(jsonbuf, sizeof(jsonbuf), "{\"ip\":null}");
            }
            body = jsonbuf;
        }

        int head_len = snprintf(head, sizeof(head),
                                "HTTP/1.1 200 OK\r\nContent-Type: %s\r\nContent-Length: %d\r\n\r\n",
                                content_type, body_len);
        if (head_len > 0)
            send(client, head, (size_t)head_len, 0);
        if (body_len > 0)
            send(client, body, (size_t)body_len, 0);
        close(client);
    }
}