etebase/dist/lib/Chunker.js

Etebase TypeScript API for the web and node

/*
 * The rolling sum implementation is based on Rollsum from bup which is in turn based on rollsum from librsync:
 * https://github.com/bup/bup/blob/master/lib/bup/bupsplit.c
 * https://github.com/librsync/librsync/blob/master/src/rollsum.h
 *
 * We tried a few alternatives (see experiments/chunker/ for details) though this was by far the best one.
 *
 * The problem with using such a chunker is that it leaks information about the sizes of different chunks which
 * in turn leaks information about the original file (because the chunking is deterministic).
 * Therefore one needs to make sure to pad the chunks in a way that doesn't leak this information.
 */
const windowSize = 64;
const charOffset = 31;
export class Rollsum {
    constructor() {
        this.window = new Uint8Array(windowSize);
        this.s1 = windowSize * charOffset;
        this.s2 = windowSize * (windowSize - 1) * charOffset;
        this.wofs = 0;
    }
    update(ch) {
        this.rollsumAdd(this.window[this.wofs], ch);
        this.window[this.wofs] = (ch);
        this.wofs = (this.wofs + 1) % windowSize;
    }
    rollsumAdd(drop, add) {
        this.s1 = (this.s1 + add - drop) >>> 0;
        this.s2 = (this.s2 + this.s1 - (windowSize * (drop + charOffset))) >>> 0;
    }
    /**
     * Returns true if splitting is needed, that is when the current digest
     * reaches the given number of the same consecutive low bits.
     */
    split(mask) {
        return ((this.s2 & (mask)) === mask);
    }
}
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