/**
 * Lightweight MIME sniffing helpers for agent image inputs.
 *
 * The checks here avoid trusting file extensions and reject unsupported image
 * variants before provider upload paths try to process them.
 */
import { open } from "node:fs/promises";

const IMAGE_TYPE_SNIFF_BYTES = 4100;
const PNG_SIGNATURE = [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a];

/** Detects supported image MIME types from leading file bytes. */
function detectSupportedImageMimeType(buffer: Uint8Array): string | null {
  if (startsWith(buffer, [0xff, 0xd8, 0xff])) {
    return buffer[3] === 0xf7 ? null : "image/jpeg";
  }
  if (startsWith(buffer, PNG_SIGNATURE)) {
    return isPng(buffer) && !isAnimatedPng(buffer) ? "image/png" : null;
  }
  if (startsWithAscii(buffer, 0, "GIF")) {
    return "image/gif";
  }
  if (startsWithAscii(buffer, 0, "RIFF") && startsWithAscii(buffer, 8, "WEBP")) {
    return "image/webp";
  }
  return null;
}

/** Reads a bounded prefix from disk and detects its supported image MIME type. */
export async function detectSupportedImageMimeTypeFromFile(
  filePath: string,
): Promise<string | null> {
  const fileHandle = await open(filePath, "r");
  try {
    const buffer = Buffer.alloc(IMAGE_TYPE_SNIFF_BYTES);
    const { bytesRead } = await fileHandle.read(buffer, 0, IMAGE_TYPE_SNIFF_BYTES, 0);
    return detectSupportedImageMimeType(buffer.subarray(0, bytesRead));
  } finally {
    await fileHandle.close();
  }
}

function isPng(buffer: Uint8Array): boolean {
  return (
    buffer.length >= 16 &&
    readUint32BE(buffer, PNG_SIGNATURE.length) === 13 &&
    startsWithAscii(buffer, 12, "IHDR")
  );
}

function isAnimatedPng(buffer: Uint8Array): boolean {
  let offset = PNG_SIGNATURE.length;
  while (offset + 8 <= buffer.length) {
    const chunkLength = readUint32BE(buffer, offset);
    const chunkTypeOffset = offset + 4;
    if (startsWithAscii(buffer, chunkTypeOffset, "acTL")) {
      return true;
    }
    if (startsWithAscii(buffer, chunkTypeOffset, "IDAT")) {
      return false;
    }

    // PNG chunk length is untrusted input; bail if advancing would wrap or exceed the sniffed bytes.
    const nextOffset = offset + 8 + chunkLength + 4;
    if (nextOffset <= offset || nextOffset > buffer.length) {
      return false;
    }
    offset = nextOffset;
  }
  return false;
}

function readUint32BE(buffer: Uint8Array, offset: number): number {
  return (
    (buffer[offset] ?? 0) * 0x1000000 +
    ((buffer[offset + 1] ?? 0) << 16) +
    ((buffer[offset + 2] ?? 0) << 8) +
    (buffer[offset + 3] ?? 0)
  );
}

function startsWith(buffer: Uint8Array, bytes: number[]): boolean {
  if (buffer.length < bytes.length) {
    return false;
  }
  return bytes.every((byte, index) => buffer[index] === byte);
}

function startsWithAscii(buffer: Uint8Array, offset: number, text: string): boolean {
  if (buffer.length < offset + text.length) {
    return false;
  }
  for (let index = 0; index < text.length; index++) {
    if (buffer[offset + index] !== text.charCodeAt(index)) {
      return false;
    }
  }
  return true;
}
