meeting-assistant/MeetingAssistant/Workflow/WorkflowRulesEditorMarkdown.cs

namespace MeetingAssistant.Workflow;

internal enum WorkflowRulesEditorMarkdownInlineStyle
{
    Normal,
    Italic,
    Bold,
    Code
}

internal abstract record WorkflowRulesEditorMarkdownBlock;

internal sealed record WorkflowRulesEditorMarkdownParagraph(
    IReadOnlyList<WorkflowRulesEditorMarkdownInline> Inlines) : WorkflowRulesEditorMarkdownBlock;

internal sealed record WorkflowRulesEditorMarkdownCodeBlock(string Text) : WorkflowRulesEditorMarkdownBlock;

internal sealed record WorkflowRulesEditorMarkdownTable(
    IReadOnlyList<string> Header,
    IReadOnlyList<IReadOnlyList<string>> Rows) : WorkflowRulesEditorMarkdownBlock;

internal sealed record WorkflowRulesEditorMarkdownInline(
    string Text,
    WorkflowRulesEditorMarkdownInlineStyle Style);

internal static class WorkflowRulesEditorMarkdown
{
    public static IReadOnlyList<WorkflowRulesEditorMarkdownBlock> Parse(string text)
    {
        if (string.IsNullOrEmpty(text))
        {
            return [];
        }

        var blocks = new List<WorkflowRulesEditorMarkdownBlock>();
        var paragraph = new List<string>();
        var code = new List<string>();
        var inCodeBlock = false;
        var lines = ReadLines(text).ToArray();

        for (var index = 0; index < lines.Length; index++)
        {
            var line = lines[index];
            if (line.TrimStart().StartsWith("```", StringComparison.Ordinal))
            {
                if (inCodeBlock)
                {
                    blocks.Add(new WorkflowRulesEditorMarkdownCodeBlock(string.Join(Environment.NewLine, code)));
                    code.Clear();
                    inCodeBlock = false;
                }
                else
                {
                    FlushParagraph(blocks, paragraph);
                    inCodeBlock = true;
                }

                continue;
            }

            if (inCodeBlock)
            {
                code.Add(line);
                continue;
            }

            if (IsTableStart(lines, index))
            {
                FlushParagraph(blocks, paragraph);
                var header = ParseTableRow(lines[index]);
                index += 2;
                var rows = new List<IReadOnlyList<string>>();
                while (index < lines.Length && IsTableRow(lines[index]))
                {
                    rows.Add(ParseTableRow(lines[index]));
                    index++;
                }

                blocks.Add(new WorkflowRulesEditorMarkdownTable(header, rows));
                index--;
                continue;
            }

            if (string.IsNullOrWhiteSpace(line))
            {
                FlushParagraph(blocks, paragraph);
            }
            else
            {
                paragraph.Add(line);
            }
        }

        if (inCodeBlock)
        {
            blocks.Add(new WorkflowRulesEditorMarkdownCodeBlock(string.Join(Environment.NewLine, code)));
        }

        FlushParagraph(blocks, paragraph);
        return blocks;
    }

    public static IReadOnlyList<WorkflowRulesEditorMarkdownInline> ParseInline(string text)
    {
        var result = new List<WorkflowRulesEditorMarkdownInline>();
        var index = 0;
        while (index < text.Length)
        {
            var next = FindNextMarker(text, index);
            if (next < 0)
            {
                AddInline(result, text[index..], WorkflowRulesEditorMarkdownInlineStyle.Normal);
                break;
            }

            if (next > index)
            {
                AddInline(result, text[index..next], WorkflowRulesEditorMarkdownInlineStyle.Normal);
            }

            if (text[next] == '`')
            {
                var end = text.IndexOf('`', next + 1);
                if (end > next)
                {
                    AddInline(result, text[(next + 1)..end], WorkflowRulesEditorMarkdownInlineStyle.Code);
                    index = end + 1;
                    continue;
                }
            }
            else if (text.AsSpan(next).StartsWith("**", StringComparison.Ordinal))
            {
                var end = text.IndexOf("**", next + 2, StringComparison.Ordinal);
                if (end > next)
                {
                    AddInline(result, text[(next + 2)..end], WorkflowRulesEditorMarkdownInlineStyle.Bold);
                    index = end + 2;
                    continue;
                }
            }
            else if (text[next] == '*')
            {
                var end = text.IndexOf('*', next + 1);
                if (end > next)
                {
                    AddInline(result, text[(next + 1)..end], WorkflowRulesEditorMarkdownInlineStyle.Italic);
                    index = end + 1;
                    continue;
                }
            }

            AddInline(result, text[next].ToString(), WorkflowRulesEditorMarkdownInlineStyle.Normal);
            index = next + 1;
        }

        return result;
    }

    private static void FlushParagraph(
        List<WorkflowRulesEditorMarkdownBlock> blocks,
        List<string> paragraph)
    {
        if (paragraph.Count == 0)
        {
            return;
        }

        blocks.Add(new WorkflowRulesEditorMarkdownParagraph(ParseInline(string.Join('\n', paragraph))));
        paragraph.Clear();
    }

    private static bool IsTableStart(IReadOnlyList<string> lines, int index)
    {
        return index + 1 < lines.Count &&
            IsTableRow(lines[index]) &&
            IsTableSeparator(lines[index + 1]);
    }

    private static bool IsTableRow(string line)
    {
        return !string.IsNullOrWhiteSpace(line) && line.Contains('|', StringComparison.Ordinal);
    }

    private static bool IsTableSeparator(string line)
    {
        var cells = ParseTableRow(line);
        return cells.Count > 0 &&
            cells.All(cell =>
            {
                var trimmed = cell.Trim();
                return trimmed.Contains('-', StringComparison.Ordinal) &&
                    trimmed.All(character => character is '-' or ':' or ' ');
            });
    }

    private static IReadOnlyList<string> ParseTableRow(string line)
    {
        var trimmed = line.Trim();
        if (trimmed.StartsWith('|'))
        {
            trimmed = trimmed[1..];
        }

        if (trimmed.EndsWith('|'))
        {
            trimmed = trimmed[..^1];
        }

        return trimmed
            .Split('|')
            .Select(cell => cell.Trim())
            .ToArray();
    }

    private static void AddInline(
        List<WorkflowRulesEditorMarkdownInline> result,
        string text,
        WorkflowRulesEditorMarkdownInlineStyle style)
    {
        if (text.Length == 0)
        {
            return;
        }

        if (result.Count > 0 && result[^1].Style == style)
        {
            result[^1] = result[^1] with { Text = result[^1].Text + text };
            return;
        }

        result.Add(new WorkflowRulesEditorMarkdownInline(text, style));
    }

    private static int FindNextMarker(string text, int start)
    {
        var code = text.IndexOf('`', start);
        var bold = text.IndexOf("**", start, StringComparison.Ordinal);
        var italic = text.IndexOf('*', start);
        if (italic >= 0 && italic + 1 < text.Length && text[italic + 1] == '*')
        {
            italic = text.IndexOf('*', italic + 2);
        }

        return new[] { code, bold, italic }
            .Where(index => index >= 0)
            .DefaultIfEmpty(-1)
            .Min();
    }

    private static IEnumerable<string> ReadLines(string text)
    {
        using var reader = new StringReader(text);
        while (reader.ReadLine() is { } line)
        {
            yield return line;
        }
    }
}