export interface DetectedType {
  contentType: string
  extension: string
}

const startsWith = (bytes: Uint8Array, signature: number[], offset = 0) => signature.every((byte, i) => bytes[offset + i] === byte)
const ascii = (bytes: Uint8Array, from: number, to: number) => String.fromCharCode(...bytes.subarray(from, to))

/**
 * Identifies photos and PDFs from their first bytes, ignoring the client's claimed type and file name.
 * Returns null for anything else, so only these formats are ever stored and served.
 */
export function detectFileType(bytes: Uint8Array): DetectedType | null {
  if (startsWith(bytes, [0xff, 0xd8, 0xff])) return { contentType: 'image/jpeg', extension: 'jpg' }
  if (startsWith(bytes, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])) return { contentType: 'image/png', extension: 'png' }
  if (ascii(bytes, 0, 6) === 'GIF87a' || ascii(bytes, 0, 6) === 'GIF89a') return { contentType: 'image/gif', extension: 'gif' }
  if (ascii(bytes, 0, 4) === 'RIFF' && ascii(bytes, 8, 12) === 'WEBP') return { contentType: 'image/webp', extension: 'webp' }
  if (ascii(bytes, 4, 8) === 'ftyp' && ['heic', 'heix', 'mif1', 'msf1'].includes(ascii(bytes, 8, 12))) return { contentType: 'image/heic', extension: 'heic' }
  if (ascii(bytes, 0, 5) === '%PDF-') return { contentType: 'application/pdf', extension: 'pdf' }
  return null
}