101 lines
3.0 KiB
JavaScript
101 lines
3.0 KiB
JavaScript
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/**
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* @module rabin
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*
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* Very efficient & versatile fingerprint/hashing algorithm. However, it is not cryptographically
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* secure. Well suited for fingerprinting.
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*/
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import * as buffer from '../buffer.js'
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import * as map from '../map.js'
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export const StandardIrreducible8 = new Uint8Array([1, 221])
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export const StandardIrreducible16 = new Uint8Array([1, 244, 157])
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export const StandardIrreducible32 = new Uint8Array([1, 149, 183, 205, 191])
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export const StandardIrreducible64 = new Uint8Array([1, 133, 250, 114, 193, 250, 28, 193, 231])
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export const StandardIrreducible128 = new Uint8Array([1, 94, 109, 166, 228, 6, 222, 102, 239, 27, 128, 184, 13, 50, 112, 169, 199])
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/**
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* Maps from a modulo to the precomputed values.
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*
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* @type {Map<string,Uint8Array>}
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*/
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const _precomputedFingerprintCache = new Map()
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/**
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* @param {Uint8Array} m
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*/
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const ensureCache = m => map.setIfUndefined(_precomputedFingerprintCache, buffer.toBase64(m), () => {
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const byteLen = m.byteLength
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const cache = new Uint8Array(256 * byteLen)
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// Use dynamic computing to compute the cached results.
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// Starting values: cache(0) = 0; cache(1) = m
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cache.set(m, byteLen)
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for (let bit = 1; bit < 8; bit++) {
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const mBitShifted = buffer.shiftNBitsLeft(m, bit)
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const bitShifted = 1 << bit
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for (let j = 0; j < bitShifted; j++) {
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// apply the shifted result (reducing the degree of the polynomial)
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const msb = bitShifted | j
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const rest = msb ^ mBitShifted[0]
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for (let i = 0; i < byteLen; i++) {
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// rest is already precomputed in the cache
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cache[msb * byteLen + i] = cache[rest * byteLen + i] ^ mBitShifted[i]
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}
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// if (cache[(bitShifted | j) * byteLen] !== (bitShifted | j)) { error.unexpectedCase() }
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}
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}
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return cache
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})
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export class RabinEncoder {
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/**
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* @param {Uint8Array} m assert(m[0] === 1)
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*/
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constructor (m) {
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this.m = m
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this.blen = m.byteLength
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this.bs = new Uint8Array(this.blen)
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this.cache = ensureCache(m)
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/**
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* This describes the position of the most significant byte (starts with 0 and increases with
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* shift)
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*/
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this.bpos = 0
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}
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/**
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* @param {number} byte
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*/
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write (byte) {
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// assert(this.bs[0] === 0)
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// Shift one byte to the left, add b
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this.bs[this.bpos] = byte
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this.bpos = (this.bpos + 1) % this.blen
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const msb = this.bs[this.bpos]
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for (let i = 0; i < this.blen; i++) {
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this.bs[(this.bpos + i) % this.blen] ^= this.cache[msb * this.blen + i]
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}
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// assert(this.bs[this.bpos] === 0)
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}
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getFingerprint () {
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const result = new Uint8Array(this.blen - 1)
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for (let i = 0; i < result.byteLength; i++) {
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result[i] = this.bs[(this.bpos + i + 1) % this.blen]
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}
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return result
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}
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}
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/**
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* @param {Uint8Array} irreducible
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* @param {Uint8Array} data
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*/
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export const fingerprint = (irreducible, data) => {
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const encoder = new RabinEncoder(irreducible)
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for (let i = 0; i < data.length; i++) {
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encoder.write(data[i])
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}
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return encoder.getFingerprint()
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}
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