/**
 * @module utils/crc32c
 * @description CRC-32C (Castagnoli, polynomial 0x1EDC6F41, reflected 0x82F63B78)
 * — the storage-industry frame checksum (ext4, iSCSI, SCTP, LSM segment files).
 * Used to frame generation-fact segments: every appended record carries the
 * CRC-32C of its payload, so a torn tail (crash mid-append) or bit rot is
 * DETECTED at scan time and never silently read as data.
 *
 * Table-driven, dependency-free reference implementation. Native providers may
 * substitute a hardware-accelerated (SSE4.2 / ARMv8 CRC) implementation — the
 * polynomial is the contract, byte-identical results required.
 */

/** The 256-entry lookup table for the reflected CRC-32C polynomial. */
const TABLE: Uint32Array = (() => {
  const table = new Uint32Array(256)
  for (let n = 0; n < 256; n++) {
    let c = n
    for (let k = 0; k < 8; k++) {
      c = c & 1 ? 0x82f63b78 ^ (c >>> 1) : c >>> 1
    }
    table[n] = c >>> 0
  }
  return table
})()

/**
 * Compute the CRC-32C checksum of a byte buffer.
 *
 * Known-answer vectors (RFC 3720 appendix / the standard test suite):
 * - ASCII "123456789" → 0xE3069283
 * - 32 zero bytes → 0x8A9136AA
 *
 * @param bytes - The payload to checksum.
 * @returns The CRC-32C as an unsigned 32-bit integer.
 */
export function crc32c(bytes: Uint8Array): number {
  let crc = 0xffffffff
  for (let i = 0; i < bytes.length; i++) {
    crc = TABLE[(crc ^ bytes[i]) & 0xff] ^ (crc >>> 8)
  }
  return (crc ^ 0xffffffff) >>> 0
}
