sc4/lib/core/crc.js

A command line utility for automating SimCity 4 modding tasks & modifying savegames

// # crc.ts
import table from './crc-table.js';
const IV = 0xffffffff;
const MAX = 250000;
// # crc(buffer, offset = 0)
export default function crc(buffer, offset = 0) {
    // It's possible that an offset was given for the CRC so we can discard 
    // the SIZE CRC part easily.
    let input = offset > 0 ? buffer.subarray(offset) : buffer;
    // See #4. Apparently the game doesn't calculate crcs for buffers larger 
    // than 250 000 bytes. It cuts them off, so we'll clamp the crc to it.
    const size = Math.min(MAX, input.length);
    // Now perform the *actual* checksum algorithm on the capped buffer.
    return crc32(input, size);
}
// Given that JS performs bitwise operators on *signed* integers, we need a 
// way to interpret signed integers as unsigned ones, which is what this 
// function does.
const intArray = new Int32Array(1);
const uintArray = new Uint32Array(intArray.buffer);
function uint(x) {
    intArray[0] = x;
    return uintArray[0];
}
// # crc32(buffer, buffer.length, iv = 0xffffffff)
// The JS implementation of the crc checksum algorithm. The key difference 
// here is that JS uses *signed* integers for bitwise operations. This means 
// that we have to do two things:
//   1. Cast the index to look for in the table to an unsigned integer because 
//      negative indices are obviously not possible.
//   2. Cast the result to an unsigned integer.
function crc32(buffer, length = buffer.length, iv = IV) {
    let crc = iv;
    for (let i = 0; i < length; i++) {
        let index = uint(((crc >> 24) ^ buffer[i]) & 0xff);
        crc = (crc << 8) ^ table[index];
    }
    return uint(crc);
}