const VALID_LANGUAGE_RE = /^[a-z0-9][a-z0-9+#-]*$/i
const NON_CODE_FENCE_LANGUAGES = new Set(['', 'text', 'plain', 'plaintext', 'md', 'markdown'])
const COMMON_CODE_LANGUAGES = new Set([
  'bash',
  'c',
  'cpp',
  'css',
  'diff',
  'go',
  'html',
  'java',
  'javascript',
  'js',
  'json',
  'jsx',
  'markdown',
  'md',
  'php',
  'python',
  'py',
  'ruby',
  'rust',
  'rs',
  'sh',
  'sql',
  'swift',
  'tsx',
  'ts',
  'typescript',
  'xml',
  'yaml',
  'yml'
])

interface CodeSignals {
  bulletLines: number
  codeSignals: number
  hasMarkdown: boolean
  proseLines: number
  trimmed: string
}

export function sanitizeLanguageTag(tag: string): string {
  const trimmed = tag.trim()
  const first = trimmed.split(/\s/, 1)[0] || ''

  return VALID_LANGUAGE_RE.test(first) && first.length <= 16 ? first.toLowerCase() : ''
}

function proseLineCount(body: string): number {
  return body
    .split('\n')
    .filter(line => {
      const trimmed = line.trim()

      return Boolean(trimmed) && /^[A-Za-z0-9"'`*-]/.test(trimmed)
    })
    .length
}

const CODE_SIGNAL_RE = [
  /(^|\s)(const|let|var|function|class|import|export|return|if|for|while|switch)\b/gim,
  /=>|==|===|!=|!==|\{|\}|;|<\/?[a-z][^>]*>/gi,
  /^\s*(#include|SELECT|INSERT|UPDATE|DELETE|CREATE|DROP)\b/gim
]

function codeSignalCount(body: string): number {
  return CODE_SIGNAL_RE.reduce((total, pattern) => total + (body.match(pattern)?.length ?? 0), 0)
}

function codeSignals(body: string): CodeSignals {
  const trimmed = body.trim()
  const markdownSignals = (trimmed.match(/\*\*[^*]+\*\*/g) || []).length + (trimmed.match(/`[^`\n]+`/g) || []).length

  return {
    bulletLines: (trimmed.match(/^\s*[-*]\s+\S+/gm) || []).length,
    codeSignals: codeSignalCount(trimmed),
    hasMarkdown: markdownSignals > 0,
    proseLines: proseLineCount(trimmed),
    trimmed
  }
}

export function isLikelyProseFence(info: string, body: string): boolean {
  const trimmedInfo = info.trim()
  const rawInfo = trimmedInfo.toLowerCase()
  const language = sanitizeLanguageTag(info)
  const infoToken = trimmedInfo.split(/\s+/, 1)[0] || ''
  const hasInfoTail = Boolean(trimmedInfo) && trimmedInfo !== infoToken

  if (/^[-*+]\s/.test(rawInfo) || /^https?:\/\//.test(rawInfo)) {
    return true
  }

  const signals = codeSignals(body)

  if (!signals.trimmed) {
    return false
  }

  if (hasInfoTail && signals.codeSignals <= 2 && (signals.proseLines >= 2 || signals.bulletLines >= 1)) {
    return true
  }

  if (!NON_CODE_FENCE_LANGUAGES.has(language)) {
    return false
  }

  return (
    (signals.bulletLines >= 2 && signals.hasMarkdown && signals.codeSignals <= 2) ||
    (signals.proseLines >= 3 && signals.codeSignals === 0)
  )
}

export function isLikelyProseCodeBlock(language: string | undefined, code: string | undefined): boolean {
  const cleanLanguage = sanitizeLanguageTag(language || '')
  const signals = codeSignals(code || '')

  if (!signals.trimmed || signals.codeSignals >= 3) {
    return false
  }

  if (signals.bulletLines >= 1 && (signals.hasMarkdown || signals.proseLines >= 2)) {
    return true
  }

  if (NON_CODE_FENCE_LANGUAGES.has(cleanLanguage)) {
    return signals.proseLines >= 3 && signals.codeSignals === 0
  }

  return !COMMON_CODE_LANGUAGES.has(cleanLanguage) && signals.proseLines >= 2 && signals.codeSignals <= 1
}