using System.Text.Json;
using System.Text.RegularExpressions;

if (args.Length != 1)
{
    Console.WriteLine("Usage: Program <DiscourseTopicUrl>");
    return;
}

string discourseTopic = args[0];
int pageNumber = 0;
HttpClient client = new();
client.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/5.0 (compatible; Chrome)");

Console.WriteLine();

while (true)
{
    pageNumber++;
    string currentPage = $"{discourseTopic}.json?page={pageNumber}";

    try
    {
        string response = await client.GetStringAsync(currentPage);
        using JsonDocument document = JsonDocument.Parse(response);
        JsonElement allPosts = document.RootElement.GetProperty("post_stream").GetProperty("posts");

        foreach (var post in allPosts.EnumerateArray())
        {
            if (!post.TryGetProperty("link_counts", out var allLinks) ||
                allLinks.ValueKind != JsonValueKind.Array) continue;

            foreach (var link in allLinks.EnumerateArray())
            {
                ProcessLink(link, post);
            }
        }
    }
    catch (HttpRequestException)
    {
        break;
    }
}

return;

static void ProcessLink(JsonElement link, JsonElement post)
{
    if (!link.TryGetProperty("internal", out var internalLink) ||
        internalLink.ValueKind != JsonValueKind.False) return;

    int postNumber = post.GetProperty("post_number").GetInt32();

    string? url = link.GetProperty("url").GetString();
    if (url == null)
        throw new NullReferenceException($"link: {link} url property from the parsed json post: {post} is null!");

    string? username = post.GetProperty("username").GetString();
    if (username == null)
        throw new NullReferenceException($"link: {link} username property from the parsed json post: {post} is null!");

    string? postBody = post.GetProperty("cooked").GetString();
    if (postBody == null)
        throw new NullReferenceException($"link: {link} cooked property from the parsed json post: {post} is null!");

    string escapedLink = Regex.Escape(url);
    string pattern = $"""<a\s+[^>]*href=\"{escapedLink}\"[^>]*>(.*?)</a>""";

    Console.WriteLine($"External link found in post number: {postNumber} by {username}");
    Console.WriteLine(url);

    Match match = Regex.Match(postBody, pattern);
    if (match.Success)
    {
        string linkText = match.Groups[1].Value;
        Console.WriteLine($"Link Text: {linkText}\n");
    }
    else
    {
        Console.WriteLine("No link text found\n");
    }
}