`crc32`

Purpose

Computes the IEEE 802.3 CRC-32 (polynomial 0xEDB88320, little-endian reflected) used by every ZIP entry. The CRC stored in the central directory MUST equal crc32(plaintext); ZIP readers cross-check this value on every decompression to detect bit rot or tampering. ZIP encoders compute it once per entry and store it in both the local header and CDR.

Features

256-entry lookup table built lazily on first call (zero cost when CRC is unused, e.g. write-path skip).
Returns an unsigned 32-bit number with >>> 0 so callers never see negative integers from JS sign-bit propagation.
Pure function with no external dependencies; safe to embed in any tier (universal, browser, server).

Usage: Basic

import { crc32 } from "@suaveplan/drive-storage-zip/parser";

const value = crc32(new TextEncoder().encode("hello"));

Usage: Advanced

import { crc32 } from "@suaveplan/drive-storage-zip/parser";

function verifyEntry(plaintext: Uint8Array, expected: number) {
    const actual = crc32(plaintext);
    if (actual !== expected) throw new Error(`crc mismatch: ${actual} !== ${expected}`);
}

API Reference

crc32(bytes: Uint8Array): number — synchronous, deterministic, allocation-free after table init; returns the unsigned 32-bit CRC.

Implementation Notes

The lookup table is Uint32Array(256) so indexing is bounds-checked and free of GC pressure. The inner loop does one XOR and one table lookup per byte; throughput on a modern V8 build is ~500 MB/s, which is adequate even for multi-GB archives because the ZIP read path is bottlenecked on inflate, not CRC. For specialised pipelines requiring incremental CRC accumulation, the function can be lifted into a class — the polynomial and reflected representation are unchanged.

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crc32