YouTube-Manager/Manager.YouTube/Parsers/HtmlParser.cs

using DotBased.Monads;
using HtmlAgilityPack;

namespace Manager.YouTube.Parsers;

public static class HtmlParser
{
    public static Result<(string, string)> GetStateJson(string html)
    {
        if (string.IsNullOrWhiteSpace(html))
        {
            return ResultError.Fail("html cannot be empty!");
        }

        var htmlDocument = new HtmlDocument();
        htmlDocument.LoadHtml(html);

        const string setFunction = "ytcfg.set({";
        var scriptNode = htmlDocument.DocumentNode.SelectSingleNode($"//script[contains(., '{setFunction}')]");
        if (string.IsNullOrWhiteSpace(scriptNode.InnerText))
            return ResultError.Fail($"Could not find {setFunction} in html script nodes!");
        
        var json = ExtractJson(scriptNode.InnerText, "ytcfg.set(");
        var jsonText = ExtractJson(scriptNode.InnerText, "setMessage(");

        if (string.IsNullOrWhiteSpace(json) || string.IsNullOrWhiteSpace(jsonText))
        {
            return ResultError.Fail($"Could not find {setFunction} in html script nodes!");
        }

        return (json, jsonText);
    }
    
    static string? ExtractJson(string input, string marker)
    {
        var start = input.IndexOf(marker, StringComparison.Ordinal);
        if (start < 0) return null;

        start += marker.Length;

        // Skip until first '{'
        while (start < input.Length && input[start] != '{')
            start++;

        if (start >= input.Length) return null;

        var depth = 0;
        var i = start;

        for (; i < input.Length; i++)
        {
            if (input[i] == '{') depth++;
            else if (input[i] == '}')
            {
                depth--;
                if (depth != 0) continue;
                i++;
                break;
            }
        }

        return input[start..i];
    }
}