/**
 * Server-side Markdown → HTML converter.
 * Converts AI-generated markdown notes into TipTap-compatible rich text HTML.
 * Uses a lightweight regex-based approach to avoid heavy remark/rehype dependencies.
 *
 * Handles: headings, bold, italic, strikethrough, code blocks, inline code,
 * links, images, lists (ul/ol), blockquotes, horizontal rules, tables, paragraphs.
 */

export function markdownToHtml(markdown: string): string {
  if (!markdown || !markdown.trim()) return ''

  let html = markdown

  // Escape HTML entities (but preserve markdown)
  html = html.replace(/&/g, '&')
  html = html.replace(/</g, '&lt;')
  html = html.replace(/>/g, '&gt;')

  // Code blocks (``` ... ```) — protect from further processing
  const codeBlocks: string[] = []
  html = html.replace(/```(\w*)\n([\s\S]*?)```/g, (_match, lang, code) => {
    const idx = codeBlocks.length
    codeBlocks.push(`<pre><code class="language-${lang || 'plaintext'}">${code.trim()}</code></pre>`)
    return `%%CODEBLOCK_${idx}%%`
  })

  // Inline code (`...`)
  html = html.replace(/`([^`]+)`/g, '<code>$1</code>')

  // Images (![alt](url))
  html = html.replace(/!\[([^\]]*)\]\(([^)]+)\)/g, '<img src="$2" alt="$1" />')

  // Links ([text](url))
  html = html.replace(/\[([^\]]+)\]\(([^)]+)\)/g, '<a href="$2" target="_blank" rel="noopener noreferrer">$1</a>')

  // Headings (h1-h6)
  html = html.replace(/^######\s+(.+)$/gm, '<h6>$1</h6>')
  html = html.replace(/^#####\s+(.+)$/gm, '<h5>$1</h5>')
  html = html.replace(/^####\s+(.+)$/gm, '<h4>$1</h4>')
  html = html.replace(/^###\s+(.+)$/gm, '<h3>$1</h3>')
  html = html.replace(/^##\s+(.+)$/gm, '<h2>$1</h2>')
  html = html.replace(/^#\s+(.+)$/gm, '<h1>$1</h1>')

  // Bold (**text** or __text__)
  html = html.replace(/\*\*([^*]+)\*\*/g, '<strong>$1</strong>')
  html = html.replace(/__([^_]+)__/g, '<strong>$1</strong>')

  // Italic (*text* or _text_)
  html = html.replace(/(?<!\*)\*([^*]+)\*(?!\*)/g, '<em>$1</em>')
  html = html.replace(/(?<!_)_([^_]+)_(?!_)/g, '<em>$1</em>')

  // Strikethrough (~~text~~)
  html = html.replace(/~~([^~]+)~~/g, '<s>$1</s>')

  // Horizontal rules (---, ***, ___)
  html = html.replace(/^(-{3,}|\*{3,}|_{3,})$/gm, '<hr />')

  // Tables
  html = convertTables(html)

  // Blockquotes (> text)
  html = html.replace(/^&gt;\s+(.+)$/gm, '<blockquote><p>$1</p></blockquote>')
  // Merge consecutive blockquotes
  html = html.replace(/<\/blockquote>\n<blockquote>/g, '\n')

  // Unordered lists (- item or * item)
  html = convertUnorderedLists(html)

  // Ordered lists (1. item)
  html = convertOrderedLists(html)

  // Restore code blocks
  codeBlocks.forEach((block, idx) => {
    html = html.replace(`%%CODEBLOCK_${idx}%%`, block)
  })

  // Paragraphs — wrap remaining loose text in <p> tags
  html = wrapParagraphs(html)

  // Clean up empty paragraphs
  html = html.replace(/<p>\s*<\/p>/g, '')

  return html.trim()
}

function convertTables(html: string): string {
  // Simple table conversion: | header | header |\n| --- | --- |\n| cell | cell |
  const tableRegex = /(?:^|\n)((?:\|[^\n]+\|\n)+)/g

  return html.replace(tableRegex, (match) => {
    const rows = match.trim().split('\n').filter(r => r.trim())
    if (rows.length < 2) return match

    // Check if second row is separator
    const separator = rows[1].trim()
    if (!/^[\s|:-]+$/.test(separator)) return match

    let table = '<table>'

    // Header row
    const headers = parseTableRow(rows[0])
    if (headers.length > 0) {
      table += '<thead><tr>'
      headers.forEach(h => { table += `<th>${h}</th>` })
      table += '</tr></thead>'
    }

    // Body rows (skip separator)
    const bodyRows = rows.slice(2)
    if (bodyRows.length > 0) {
      table += '<tbody>'
      bodyRows.forEach(row => {
        const cells = parseTableRow(row)
        table += '<tr>'
        cells.forEach(c => { table += `<td>${c}</td>` })
        table += '</tr>'
      })
      table += '</tbody>'
    }

    table += '</table>'
    return '\n' + table + '\n'
  })
}

function parseTableRow(row: string): string[] {
  return row.split('|')
    .map(cell => cell.trim())
    .filter((_, i, arr) => i > 0 && i < arr.length) // Skip first and last empty from leading/trailing |
}

function convertUnorderedLists(html: string): string {
  const lines = html.split('\n')
  const result: string[] = []
  let inList = false

  for (const line of lines) {
    const listMatch = line.match(/^(\s*)[-*]\s+(.+)$/)
    if (listMatch) {
      if (!inList) {
        result.push('<ul>')
        inList = true
      }
      result.push(`<li>${listMatch[2]}</li>`)
    } else {
      if (inList) {
        result.push('</ul>')
        inList = false
      }
      result.push(line)
    }
  }
  if (inList) result.push('</ul>')

  return result.join('\n')
}

function convertOrderedLists(html: string): string {
  const lines = html.split('\n')
  const result: string[] = []
  let inList = false

  for (const line of lines) {
    const listMatch = line.match(/^(\s*)\d+\.\s+(.+)$/)
    if (listMatch) {
      if (!inList) {
        result.push('<ol>')
        inList = true
      }
      result.push(`<li>${listMatch[2]}</li>`)
    } else {
      if (inList) {
        result.push('</ol>')
        inList = false
      }
      result.push(line)
    }
  }
  if (inList) result.push('</ol>')

  return result.join('\n')
}

function wrapParagraphs(html: string): string {
  const blockTags = new Set(['h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'ul', 'ol', 'li', 'blockquote', 'pre', 'table', 'thead', 'tbody', 'tr', 'th', 'td', 'hr', 'p', 'div', 'img'])

  const lines = html.split('\n')
  const result: string[] = []
  let buffer: string[] = []

  const flushBuffer = () => {
    const text = buffer.join('\n').trim()
    if (text) {
      // Don't double-wrap if already starts with a block tag
      const firstTag = text.match(/^<(\w+)/)
      if (firstTag && blockTags.has(firstTag[1].toLowerCase())) {
        result.push(text)
      } else {
        result.push(`<p>${text}</p>`)
      }
    }
    buffer = []
  }

  for (const line of lines) {
    const trimmed = line.trim()

    // Check if this line is a block-level element
    const isBlockLine = trimmed.startsWith('<') && (() => {
      const tag = trimmed.match(/^<(\w+)/)
      return tag ? blockTags.has(tag[1].toLowerCase()) : false
    })()

    if (isBlockLine || trimmed === '') {
      flushBuffer()
      if (isBlockLine) result.push(trimmed)
    } else {
      buffer.push(trimmed)
    }
  }
  flushBuffer()

  return result.join('\n')
}