Relay/RelayClient/Helpers/MarkdownHelper.cs

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

namespace RelayClient.Helpers;

public static class MarkdownHelper
{
    private static readonly Regex FencedCode =
        new(@"```([A-Za-z0-9_+#-]*)\r?\n?(.*?)```", RegexOptions.Singleline | RegexOptions.Compiled);

    private static readonly Color MentionText = Color.FromArgb("#9EA8FF");
    private static readonly Color MentionBg   = Color.FromArgb("#2D2F5C");
    private static readonly Color SpoilerBg   = Color.FromArgb("#1F1F23");

    public static View Render(string markdown, double fontSize = 14)
    {
        if (string.IsNullOrEmpty(markdown))
            return new Label { Text = string.Empty, FontSize = fontSize };

        var stack = new VerticalStackLayout { Spacing = 2 };

        var matches = FencedCode.Matches(markdown);
        int cursor = 0;

        foreach (Match m in matches)
        {
            if (m.Index > cursor)
                AppendTextSegment(stack, markdown[cursor..m.Index], fontSize);

            stack.Children.Add(CreateCodeBlock(m.Groups[1].Value.Trim(), m.Groups[2].Value.TrimEnd()));
            cursor = m.Index + m.Length;
        }

        if (cursor < markdown.Length)
            AppendTextSegment(stack, markdown[cursor..], fontSize);

        return stack.Children.Count == 1 ? (View)stack.Children[0] : stack;
    }

    private static void AppendTextSegment(VerticalStackLayout stack, string segment, double fontSize)
    {
        var paragraphBuffer = new StringBuilder();

        void FlushParagraph()
        {
            if (paragraphBuffer.Length == 0) return;
            stack.Children.Add(CreateInlineLabel(paragraphBuffer.ToString(), fontSize));
            paragraphBuffer.Clear();
        }

        foreach (var rawLine in segment.Split('\n'))
        {
            var line = rawLine.TrimEnd('\r');

            if (string.IsNullOrWhiteSpace(line))
            {
                FlushParagraph();
                continue;
            }

            if (line.StartsWith("### "))
            {
                FlushParagraph();
                stack.Children.Add(CreateHeaderLabel(line[4..], fontSize + 3));
                continue;
            }

            if (line.StartsWith("## "))
            {
                FlushParagraph();
                stack.Children.Add(CreateHeaderLabel(line[3..], fontSize + 6));
                continue;
            }

            if (line.StartsWith("# "))
            {
                FlushParagraph();
                stack.Children.Add(CreateHeaderLabel(line[2..], fontSize + 10));
                continue;
            }

            if (line.StartsWith("-# "))
            {
                FlushParagraph();
                stack.Children.Add(CreateSubtextLabel(line[3..], fontSize - 3));
                continue;
            }

            if (paragraphBuffer.Length > 0)
                paragraphBuffer.Append('\n');
            paragraphBuffer.Append(line);
        }

        FlushParagraph();
    }

    private static View CreateCodeBlock(string language, string code)
    {
        var label = new Label
        {
            FontFamily    = "AnonymousProRegular",
            FontSize      = 12,
            TextColor     = Color.FromArgb("#D4D4D4"),
            LineBreakMode = LineBreakMode.WordWrap
        };

        var spans = SyntaxHighlighter.Highlight(code, language, 12);
        if (spans.Count > 0)
        {
            var fs = new FormattedString();
            foreach (var s in spans) fs.Spans.Add(s);
            label.FormattedText = fs;
        }
        else
        {
            label.Text = code;
        }

        var stack = new VerticalStackLayout { Spacing = 4 };

        if (!string.IsNullOrWhiteSpace(language))
        {
            stack.Children.Add(new Label
            {
                Text           = language.ToLowerInvariant(),
                FontFamily     = "AnonymousProRegular",
                FontSize       = 10,
                TextColor      = Color.FromArgb("#6A9955"),
                FontAttributes = FontAttributes.Bold
            });
        }

        stack.Children.Add(label);

        return new Border
        {
            BackgroundColor = Color.FromArgb("#1E1E1E"),
            StrokeThickness = 0,
            Padding         = new Thickness(10, 6),
            Content         = stack
        };
    }

    private static Label CreateHeaderLabel(string text, double size)
    {
        var label = new Label
        {
            FontSize       = size,
            FontAttributes = FontAttributes.Bold,
            LineBreakMode  = LineBreakMode.WordWrap,
            Margin         = new Thickness(0, 4, 0, 2)
        };

        var fs = new FormattedString();
        var spoilerSpans = new List<Span>();
        ParseInline(text, fs.Spans, size, spoilerSpans);

        if (fs.Spans.Count > 0) label.FormattedText = fs;
        else label.Text = text;

        WireSpoilerTap(label, spoilerSpans);
        return label;
    }

    private static Label CreateSubtextLabel(string text, double size)
    {
        var label = new Label
        {
            FontSize       = size,
            TextColor      = Color.FromArgb("#8E8E93"),
            LineBreakMode  = LineBreakMode.WordWrap
        };

        var fs = new FormattedString();
        var spoilerSpans = new List<Span>();
        ParseInline(text, fs.Spans, size, spoilerSpans);

        if (fs.Spans.Count > 0)
        {
            foreach (var s in fs.Spans)
                s.TextColor ??= Color.FromArgb("#8E8E93");
            label.FormattedText = fs;
        }
        else
        {
            label.Text = text;
        }

        WireSpoilerTap(label, spoilerSpans);
        return label;
    }

    private static Label CreateInlineLabel(string text, double fontSize)
    {
        var label = new Label { FontSize = fontSize, LineBreakMode = LineBreakMode.WordWrap };
        var fs = new FormattedString();
        var spoilerSpans = new List<Span>();
        ParseInline(text, fs.Spans, fontSize, spoilerSpans);

        if (fs.Spans.Count > 0) label.FormattedText = fs;
        else label.Text = text;

        WireSpoilerTap(label, spoilerSpans);
        return label;
    }

    private static void WireSpoilerTap(Label label, List<Span> spoilerSpans)
    {
        if (spoilerSpans.Count == 0) return;

        var tap = new TapGestureRecognizer();
        tap.Tapped += (_, _) =>
        {
            foreach (var s in spoilerSpans)
            {
                s.BackgroundColor = Colors.Transparent;
                s.TextColor       = null; // fall back to default label color
            }
        };
        label.GestureRecognizers.Add(tap);
    }

    private static void ParseInline(string text, IList<Span> spans, double fontSize, List<Span> spoilerSpans)
    {
        var plain = new StringBuilder();
        int i = 0;

        void Flush()
        {
            if (plain.Length == 0) return;
            spans.Add(new Span { Text = plain.ToString(), FontSize = fontSize });
            plain.Clear();
        }

        while (i < text.Length)
        {
            char c = text[i];

            if (c == '|' && Peek(text, i + 1) == '|')
            {
                int end = text.IndexOf("||", i + 2, StringComparison.Ordinal);
                if (end > i + 2)
                {
                    Flush();
                    var span = new Span
                    {
                        Text            = text[(i + 2)..end],
                        FontSize        = fontSize,
                        BackgroundColor = SpoilerBg,
                        TextColor       = SpoilerBg  // text invisible until revealed
                    };
                    spans.Add(span);
                    spoilerSpans.Add(span);
                    i = end + 2;
                    continue;
                }
            }

            if (c == '@' && i + 1 < text.Length &&
                (char.IsLetter(text[i + 1]) || text[i + 1] == '_'))
            {
                int end = i + 1;
                while (end < text.Length && (char.IsLetterOrDigit(text[end]) || text[end] == '_'))
                    end++;

                Flush();
                spans.Add(new Span
                {
                    Text            = text[i..end],
                    TextColor       = MentionText,
                    BackgroundColor = MentionBg,
                    FontAttributes  = FontAttributes.Bold,
                    FontSize        = fontSize
                });
                i = end;
                continue;
            }

            if (c == '~' && Peek(text, i + 1) == '~')
            {
                int end = text.IndexOf("~~", i + 2, StringComparison.Ordinal);
                if (end > i + 2)
                {
                    Flush();
                    spans.Add(new Span
                    {
                        Text            = text[(i + 2)..end],
                        FontSize        = fontSize,
                        TextDecorations = TextDecorations.Strikethrough
                    });
                    i = end + 2; continue;
                }
            }

            if (c == '_' && Peek(text, i + 1) == '_')
            {
                int end = text.IndexOf("__", i + 2, StringComparison.Ordinal);
                if (end > i + 2)
                {
                    Flush();
                    spans.Add(new Span
                    {
                        Text            = text[(i + 2)..end],
                        FontSize        = fontSize,
                        TextDecorations = TextDecorations.Underline
                    });
                    i = end + 2; continue;
                }
            }

            if (c == '*' && Peek(text, i + 1) == '*')
            {
                int end = text.IndexOf("**", i + 2, StringComparison.Ordinal);
                if (end > i + 2)
                {
                    Flush();
                    spans.Add(new Span
                    {
                        Text           = text[(i + 2)..end],
                        FontSize       = fontSize,
                        FontAttributes = FontAttributes.Bold
                    });
                    i = end + 2; continue;
                }
            }

            if (c == '*' && Peek(text, i + 1) != '*')
            {
                int end = FindClosingSingle(text, '*', i + 1);
                if (end > i + 1)
                {
                    Flush();
                    spans.Add(new Span
                    {
                        Text           = text[(i + 1)..end],
                        FontSize       = fontSize,
                        FontAttributes = FontAttributes.Italic
                    });
                    i = end + 1; continue;
                }
            }

            if (c == '`')
            {
                int end = text.IndexOf('`', i + 1);
                if (end > i + 1)
                {
                    Flush();
                    spans.Add(new Span
                    {
                        Text            = text[(i + 1)..end],
                        FontFamily      = "AnonymousProRegular",
                        FontSize        = fontSize - 1,
                        BackgroundColor = Color.FromArgb("#2D2D2D"),
                        TextColor       = Color.FromArgb("#CE9178")
                    });
                    i = end + 1; continue;
                }
            }

            plain.Append(c);
            i++;
        }

        Flush();
    }

    private static char Peek(string text, int index) => index < text.Length ? text[index] : '\0';

    private static int FindClosingSingle(string text, char marker, int start)
    {
        for (int i = start; i < text.Length; i++)
            if (text[i] == marker && Peek(text, i + 1) != marker)
                return i;
        return -1;
    }
}