TrimMarkdownDocument.cs

public const int NewLineWeight = 4;

public static void TrimDocument(MarkdownDocument document, int numberOfCharactersToKeep)
{
    TrimObject(document, ref numberOfCharactersToKeep);
}

private static void TrimObject(MarkdownObject obj, ref int charactersAvailable)
{
    if (obj is ContainerBlock containerBlock)
    {
        int i;
        for (i = 0; i < containerBlock.Count && charactersAvailable > 0; i++)
        {
            TrimObject(containerBlock[i], ref charactersAvailable);
        }

        for (int removeIndex = containerBlock.Count - 1; removeIndex >= i; removeIndex--)
        {
            containerBlock.RemoveAt(removeIndex);
        }
    }
    else if (obj is ContainerInline containerInline)
    {
        var inline = containerInline.FirstChild;

        while (inline != null && charactersAvailable > 0)
        {
            TrimObject(inline, ref charactersAvailable);
            inline = inline.NextSibling;
        }

        if (charactersAvailable <= 0)
        {
            while (inline != null)
            {
                var next = inline.NextSibling;
                inline.Remove();
                inline = next;
            }
        }
    }
    else if (obj is LeafBlock leafBlock)
    {
        if (leafBlock.Inline != null)
        {
            TrimObject(leafBlock.Inline, ref charactersAvailable);
        }

        ref var lines = ref leafBlock.Lines;
        var stringLines = lines.Lines;

        if (stringLines != null)
        {
            int i;
            for (i = 0; i < lines.Count && charactersAvailable > 0; i++)
            {
                TrimStringSlice(ref stringLines[i].Slice, ref charactersAvailable);
                charactersAvailable -= NewLineWeight;
            }

            for (int removeIndex = lines.Count - 1; removeIndex >= i; removeIndex--)
            {
                lines.RemoveAt(removeIndex);
            }
        }

        charactersAvailable -= NewLineWeight;
    }
    else if (obj is LeafInline leafInline)
    {
        if (leafInline is LiteralInline literal)
        {
            TrimStringSlice(ref literal.Content, ref charactersAvailable);
        }
        else if (leafInline is CodeInline code)
        {
            code.Content = TrimString(code.Content, ref charactersAvailable);
        }
        else if (leafInline is AutolinkInline autoLink)
        {
            autoLink.Url = TrimString(autoLink.Url, ref charactersAvailable);
        }
        else if (leafInline is LineBreakInline)
        {
            charactersAvailable -= NewLineWeight;
        }
    }
}
private static void TrimStringSlice(ref StringSlice slice, ref int charactersAvailable)
{
    if (slice.IsEmpty)
    {
        return;
    }
    else if (charactersAvailable <= 0)
    {
        slice = new StringSlice(null);
    }
    else
    {
        var span = slice.Text.AsSpan(slice.Start, slice.Length);
        var trimmed = TrimSpan(span, ref charactersAvailable);
        slice.End = slice.Start + trimmed.Length - 1;
    }
}
private static string TrimString(string text, ref int charactersAvailable)
{
    if (string.IsNullOrEmpty(text))
    {
        return text;
    }
    else if (charactersAvailable <= 0)
    {
        return string.Empty;
    }
    else
    {
        var span = TrimSpan(text.AsSpan(), ref charactersAvailable);
        return span.Length == text.Length ? text : span.ToString();
    }
}
private static ReadOnlySpan<char> TrimSpan(ReadOnlySpan<char> span, ref int charactersAvailable)
{
    int i = 0;
    while (i < span.Length && charactersAvailable > 0)
    {
        int nextLine = span.Slice(i).IndexOf('\n');
        int lineLength = nextLine == -1 ? span.Length - i : nextLine - i;

        if (lineLength <= charactersAvailable)
        {
            i += lineLength;
            charactersAvailable -= lineLength;

            if (charactersAvailable > NewLineWeight && nextLine != -1)
            {
                i++;
                charactersAvailable -= NewLineWeight;
            }
            else
            {
                if (nextLine != -1)
                {
                    charactersAvailable = 0;
                }
                break;
            };
        }
        else
        {
            i += charactersAvailable;
            charactersAvailable = 0;
        }
    }
    return span.Slice(0, i);
}