/**
 * Inline text rendering for guide blocks.
 *
 * A guide stores its text as a RESTRICTED Markdown subset - bold, italic,
 * inline code, links, line breaks - and nothing else. This turns that into
 * HTML.
 *
 * ESCAPE FIRST, THEN FORMAT. The order is the whole security model: every
 * angle bracket and ampersand becomes an entity before any pattern is applied,
 * so no markup an author pasted can survive into the DOM. There is no path here
 * that emits a tag the code below did not write itself.
 *
 * The restriction is not only about safety. Pasted rich text carries fonts and
 * hardcoded colours, which then fight the theme - black text on a dark
 * background, and nobody can say why. Emphasis is semantic; the stylesheet
 * decides how it looks. That is what makes dark mode work without an author
 * thinking about it, and why a guide written next year still matches one
 * written today.
 */

function escapeHtml (text) {
  return String(text == null ? '' : text)
    .replace(/&/g, '&')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#39;')
}

// Only http(s) and mailto. A javascript: or data: URL in a link is the one way
// an author could otherwise smuggle script past the escaping above.
function safeHref (url) {
  const trimmed = String(url || '').trim()
  if (/^(https?:|mailto:)/i.test(trimmed)) return trimmed
  if (trimmed.startsWith('/')) return trimmed   // in-app link
  return null
}

export function renderInline (text) {
  let html = escapeHtml(text)

  // Inline code first, so formatting characters inside `code` stay literal -
  // a path like C:\**temp** should not turn half of itself bold.
  const codeSpans = []
  html = html.replace(/`([^`]+)`/g, (match, code) => {
    codeSpans.push(code)
    return '\u0000CODE' + (codeSpans.length - 1) + '\u0000'
  })

  // [label](url). The URL pattern allows ONE level of balanced parentheses so
  // real links survive - Wikipedia-style /wiki/Foo_(bar) is the common case,
  // and a naive [^)]+ silently truncates it into a broken link.
  html = html.replace(/\[([^\]]+)\]\(((?:[^()\s]|\([^()\s]*\))+)\)/g, (match, label, url) => {
    const href = safeHref(url)
    if (!href) return label
    const external = /^https?:/i.test(href)
    return '<a href="' + escapeHtml(href) + '"' +
      (external ? ' target="_blank" rel="noopener noreferrer"' : '') +
      '>' + label + '</a>'
  })

  // **bold** then *italic*, longest marker first so **x** is not read as
  // *(*x*)*.
  html = html.replace(/\*\*([^*]+)\*\*/g, '<strong>$1</strong>')
  html = html.replace(/(^|[^*])\*([^*]+)\*(?!\*)/g, '$1<em>$2</em>')

  html = html.replace(/\n/g, '<br>')

  html = html.replace(/\u0000CODE(\d+)\u0000/g, (match, index) =>
    '<code>' + codeSpans[Number(index)] + '</code>')

  return html
}

/** Plain text, for titles and anywhere markup would be noise. */
export function stripInline (text) {
  return String(text == null ? '' : text)
    .replace(/`([^`]+)`/g, '$1')
    .replace(/\[([^\]]+)\]\([^)\s]+\)/g, '$1')
    .replace(/\*\*([^*]+)\*\*/g, '$1')
    .replace(/\*([^*]+)\*/g, '$1')
}