export const MIME_CLASSES = ["image", "video", "audio", "document", "archive", "other"] as const;
export type MimeClass = (typeof MIME_CLASSES)[number];

export interface SniffedMime {
  mime: string;
  mimeClass: MimeClass;
  extension: string | null;
}

/** Bytes needed to recognise every supported signature. */
export const SNIFF_BYTES = 16;

const UNKNOWN: SniffedMime = { mime: "application/octet-stream", mimeClass: "other", extension: null };

function startsWith(head: Uint8Array, signature: readonly number[], offset = 0): boolean {
  if (head.length < offset + signature.length) return false;
  return signature.every((byte, i) => head[offset + i] === byte);
}

function ascii(text: string): number[] {
  return [...text].map((c) => c.charCodeAt(0));
}

/**
 * MIME is decided by magic bytes only; the sender's declared `info.mimetype`
 * is never trusted. Unrecognised payloads are stored as octet-stream/other.
 */
export function sniffMime(head: Uint8Array): SniffedMime {
  if (startsWith(head, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])) {
    return { mime: "image/png", mimeClass: "image", extension: "png" };
  }
  if (startsWith(head, [0xff, 0xd8, 0xff])) return { mime: "image/jpeg", mimeClass: "image", extension: "jpg" };
  if (startsWith(head, ascii("GIF87a")) || startsWith(head, ascii("GIF89a"))) {
    return { mime: "image/gif", mimeClass: "image", extension: "gif" };
  }
  if (startsWith(head, ascii("%PDF-"))) return { mime: "application/pdf", mimeClass: "document", extension: "pdf" };
  if (startsWith(head, [0x50, 0x4b, 0x03, 0x04])) {
    return { mime: "application/zip", mimeClass: "archive", extension: "zip" };
  }
  if (startsWith(head, ascii("ftyp"), 4)) return { mime: "video/mp4", mimeClass: "video", extension: "mp4" };
  if (startsWith(head, [0x1a, 0x45, 0xdf, 0xa3])) return { mime: "video/webm", mimeClass: "video", extension: "webm" };
  if (startsWith(head, ascii("OggS"))) return { mime: "audio/ogg", mimeClass: "audio", extension: "ogg" };
  return UNKNOWN;
}