import 'package:flutter/material.dart';

/// Renders a markdown string as formatted widgets.
///
/// Supports the markdown subset that the reports actually use:
/// headings (#..######), unordered and ordered lists, block quotes,
/// fenced code blocks, horizontal rules and the inline styles
/// bold, italic, strikethrough, inline code and links.
class MarkdownText extends StatelessWidget{
  const MarkdownText({super.key, required this.data});

  final String data;

  @override
  Widget build(BuildContext context) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      spacing: 8,
      children: _buildBlocks(context),
    );
  }

  List<Widget> _buildBlocks(BuildContext context) {

    final List<Widget> blocks = <Widget>[];
    final List<String> lines = data.replaceAll('\r\n', '\n').split('\n');

    final List<String> paragraph = <String>[];

    void flushParagraph() {
      if (paragraph.isEmpty) {
        return;
      }

      blocks.add(_paragraph(context, paragraph.join(' ')));
      paragraph.clear();
    }

    for (int i = 0; i < lines.length; i++) {

      final String line = lines[i];
      final String trimmed = line.trim();

      // Fenced code block
      if (trimmed.startsWith('```')) {
        flushParagraph();

        final List<String> code = <String>[];
        i++;

        while (i < lines.length && !lines[i].trim().startsWith('```')) {
          code.add(lines[i]);
          i++;
        }

        blocks.add(_codeBlock(context, code.join('\n')));
        continue;
      }

      // Empty line ends the current paragraph
      if (trimmed.isEmpty) {
        flushParagraph();
        continue;
      }

      // Horizontal rule
      if (RegExp(r'^(\*\s*){3,}$|^(-\s*){3,}$|^(_\s*){3,}$').hasMatch(trimmed)) {
        flushParagraph();
        blocks.add(const Divider());
        continue;
      }

      // Heading
      final RegExpMatch? heading = RegExp(r'^(#{1,6})\s+(.*)$').firstMatch(trimmed);

      if (heading != null) {
        flushParagraph();
        blocks.add(_heading(context, heading.group(1)!.length, heading.group(2)!));
        continue;
      }

      // Block quote
      if (trimmed.startsWith('>')) {
        flushParagraph();
        blocks.add(_quote(context, trimmed.substring(1).trim()));
        continue;
      }

      // Unordered list item
      final RegExpMatch? bullet = RegExp(r'^[-*+]\s+(.*)$').firstMatch(trimmed);

      if (bullet != null) {
        flushParagraph();
        blocks.add(_listItem(context, '\u2022', bullet.group(1)!, _indentOf(line)));
        continue;
      }

      // Ordered list item
      final RegExpMatch? numbered = RegExp(r'^(\d+)[.)]\s+(.*)$').firstMatch(trimmed);

      if (numbered != null) {
        flushParagraph();
        blocks.add(_listItem(context, '${numbered.group(1)}.', numbered.group(2)!, _indentOf(line)));
        continue;
      }

      paragraph.add(trimmed);
    }

    flushParagraph();

    return blocks;
  }

  /// Number of leading spaces of a line, a tab counts as two spaces.
  int _indentOf(String line) {
    int spaces = 0;

    for (int i = 0; i < line.length; i++) {
      if (line[i] == ' ') {
        spaces++;
      } else if (line[i] == '\t') {
        spaces += 2;
      } else {
        break;
      }
    }

    return spaces;
  }

  Widget _paragraph(BuildContext context, String text) {
    return SelectableText.rich(
      _inline(context, text, Theme.of(context).textTheme.bodyMedium),
    );
  }

  Widget _heading(BuildContext context, int level, String text) {

    final TextTheme theme = Theme.of(context).textTheme;

    TextStyle? style;

    switch (level) {
      case 1:
        style = theme.headlineMedium;
      case 2:
        style = theme.headlineSmall;
      case 3:
        style = theme.titleLarge;
      case 4:
        style = theme.titleMedium;
      default:
        style = theme.titleSmall;
    }

    return Padding(
      padding: EdgeInsetsGeometry.only(top: 8),
      child: SelectableText.rich(_inline(context, text, style)),
    );
  }

  Widget _quote(BuildContext context, String text) {

    final Color color = Theme.of(context).colorScheme.primary;

    return Container(
      padding: EdgeInsetsGeometry.fromLTRB(12, 4, 0, 4),
      decoration: BoxDecoration(
        border: Border(left: BorderSide(color: color, width: 4)),
      ),
      child: SelectableText.rich(
        _inline(
          context,
          text,
          Theme.of(context).textTheme.bodyMedium?.copyWith(fontStyle: FontStyle.italic),
        ),
      ),
    );
  }

  Widget _listItem(BuildContext context, String marker, String text, int indent) {
    return Padding(
      padding: EdgeInsetsGeometry.only(left: 8.0 + indent * 8.0),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        spacing: 8,
        children: [
          Text(marker, style: Theme.of(context).textTheme.bodyMedium),
          Expanded(
            child: SelectableText.rich(
              _inline(context, text, Theme.of(context).textTheme.bodyMedium),
            ),
          ),
        ],
      ),
    );
  }

  Widget _codeBlock(BuildContext context, String code) {
    return Container(
      width: double.infinity,
      padding: EdgeInsetsGeometry.all(12),
      decoration: BoxDecoration(
        color: Colors.black45,
        borderRadius: BorderRadius.circular(8),
      ),
      child: SingleChildScrollView(
        scrollDirection: Axis.horizontal,
        child: SelectableText(
          code,
          style: Theme.of(context).textTheme.bodySmall?.copyWith(
            fontFamily: 'monospace',
          ),
        ),
      ),
    );
  }

  /// Parses the inline markdown syntax of a single block of text.
  TextSpan _inline(BuildContext context, String text, TextStyle? baseStyle) {

    final List<InlineSpan> spans = <InlineSpan>[];
    final StringBuffer buffer = StringBuffer();

    void flush() {
      if (buffer.isEmpty) {
        return;
      }

      spans.add(TextSpan(text: buffer.toString()));
      buffer.clear();
    }

    final RegExp pattern = RegExp(
      r'(\*\*\*|___)(.+?)\1'          // bold + italic
      r'|(\*\*|__)(.+?)\3'            // bold
      r'|(\*|_)(.+?)\5'               // italic
      r'|~~(.+?)~~'                   // strike through
      r'|`([^`]+)`'                   // inline code
      r'|!?\[([^\]]*)\]\(([^)]*)\)',  // link / image
    );

    int index = 0;

    for (final RegExpMatch match in pattern.allMatches(text)) {

      if (match.start > index) {
        buffer.write(text.substring(index, match.start));
      }

      flush();

      if (match.group(2) != null) {
        spans.add(TextSpan(
          text: match.group(2),
          style: const TextStyle(fontWeight: FontWeight.bold, fontStyle: FontStyle.italic),
        ));
      } else if (match.group(4) != null) {
        spans.add(TextSpan(
          text: match.group(4),
          style: const TextStyle(fontWeight: FontWeight.bold),
        ));
      } else if (match.group(6) != null) {
        spans.add(TextSpan(
          text: match.group(6),
          style: const TextStyle(fontStyle: FontStyle.italic),
        ));
      } else if (match.group(7) != null) {
        spans.add(TextSpan(
          text: match.group(7),
          style: const TextStyle(decoration: TextDecoration.lineThrough),
        ));
      } else if (match.group(8) != null) {
        spans.add(TextSpan(
          text: match.group(8),
          style: TextStyle(
            fontFamily: 'monospace',
            backgroundColor: Colors.black45,
          ),
        ));
      } else {
        final String label = match.group(9) ?? '';
        final String target = match.group(10) ?? '';

        spans.add(TextSpan(
          text: label.isEmpty ? target : label,
          style: TextStyle(
            color: Theme.of(context).colorScheme.primary,
            decoration: TextDecoration.underline,
          ),
        ));
      }

      index = match.end;
    }

    if (index < text.length) {
      buffer.write(text.substring(index));
    }

    flush();

    return TextSpan(style: baseStyle, children: spans);
  }
}