/**
 * CRC32 checksum utilities for data integrity
 */

// CRC32 lookup table
const CRC32_TABLE: number[] = [];

// Initialize the CRC32 table
function initCRC32Table(): void {
  if (CRC32_TABLE.length > 0) return;

  for (let i = 0; i < 256; i++) {
    let crc = i;
    for (let j = 0; j < 8; j++) {
      crc = (crc & 1) ? (0xEDB88320 ^ (crc >>> 1)) : (crc >>> 1);
    }
    CRC32_TABLE[i] = crc >>> 0;
  }
}

/**
 * Calculate CRC32 checksum for a string
 */
export function calculateCRC32(data: string): number {
  initCRC32Table();

  let crc = 0xFFFFFFFF;
  for (let i = 0; i < data.length; i++) {
    const byte = data.charCodeAt(i) & 0xFF;
    crc = CRC32_TABLE[(crc ^ byte) & 0xFF] ^ (crc >>> 8);
  }
  return (~crc) >>> 0;
}

/**
 * Calculate CRC32 checksum for a Buffer
 */
export function calculateCRC32Buffer(data: Buffer): number {
  initCRC32Table();

  let crc = 0xFFFFFFFF;
  for (let i = 0; i < data.length; i++) {
    crc = CRC32_TABLE[(crc ^ data[i]) & 0xFF] ^ (crc >>> 8);
  }
  return (~crc) >>> 0;
}

/**
 * Calculate checksum for a document (serialized as JSON)
 */
export function calculateDocumentChecksum(doc: Record<string, any>): number {
  // Exclude _checksum field from calculation
  const { _checksum, ...docWithoutChecksum } = doc;
  const json = JSON.stringify(docWithoutChecksum);
  return calculateCRC32(json);
}

/**
 * Add checksum to a document
 */
export function addChecksum<T extends Record<string, any>>(doc: T): T & { _checksum: number } {
  const checksum = calculateDocumentChecksum(doc);
  return { ...doc, _checksum: checksum };
}

/**
 * Verify checksum of a document
 * Returns true if checksum is valid or if document has no checksum
 */
export function verifyChecksum(doc: Record<string, any>): boolean {
  if (!('_checksum' in doc)) {
    // No checksum to verify
    return true;
  }

  const storedChecksum = doc._checksum;
  const calculatedChecksum = calculateDocumentChecksum(doc);

  return storedChecksum === calculatedChecksum;
}

/**
 * Remove checksum from a document
 */
export function removeChecksum<T extends Record<string, any>>(doc: T): Omit<T, '_checksum'> {
  const { _checksum, ...docWithoutChecksum } = doc;
  return docWithoutChecksum as Omit<T, '_checksum'>;
}