import { mimeTypes } from '../generated/mime-types.ts'
import { customMimeTypeByExtension } from './define-mime-type.ts'

/**
 * Detects the MIME type for a given file extension or filename.
 *
 * Custom MIME types registered via {@link import('./define-mime-type.ts').defineMimeType}
 * take precedence over built-in types.
 *
 * @param extension The file extension (e.g. "txt", ".txt") or filename (e.g. "file.txt")
 * @returns The MIME type string, or undefined if not found
 *
 * @example
 * detectMimeType('txt')           // 'text/plain'
 * detectMimeType('.txt')          // 'text/plain'
 * detectMimeType('file.txt')      // 'text/plain'
 * detectMimeType('unknown')       // undefined
 */
export function detectMimeType(extension: string): string | undefined {
  let ext = extension.trim().toLowerCase()

  let customMimeType = detectCustomMimeType(ext)
  if (customMimeType !== undefined) {
    return customMimeType
  }

  let idx = ext.lastIndexOf('.')
  // If no dot found (~idx === -1, so !~idx === true), use ext as-is.
  // Otherwise, skip past the dot (++idx) and extract the extension.
  // Credit to mrmime for this technique.
  ext = !~idx ? ext : ext.substring(++idx)
  return mimeTypes[ext]
}

function detectCustomMimeType(extension: string): string | undefined {
  let customMimeTypes = customMimeTypeByExtension
  if (!customMimeTypes) return undefined

  let ext = extension.startsWith('.') ? extension.slice(1) : extension
  let mimeType = customMimeTypes.get(ext)
  if (mimeType !== undefined) {
    return mimeType
  }

  let slashIdx = Math.max(extension.lastIndexOf('/'), extension.lastIndexOf('\\'))
  let filename = ~slashIdx ? extension.slice(slashIdx + 1) : extension
  if (filename.includes('.')) {
    ext = filename.startsWith('.') ? filename.slice(1) : filename
    mimeType = customMimeTypes.get(ext)
    if (mimeType !== undefined) {
      return mimeType
    }
  }

  let idx = filename.indexOf('.')
  while (~idx) {
    mimeType = customMimeTypes.get(filename.substring(idx + 1))
    if (mimeType !== undefined) {
      return mimeType
    }

    idx = filename.indexOf('.', idx + 1)
  }
}