Midora/extension/bloomfilter.js

116 lines
3.7 KiB
JavaScript
Raw Normal View History

2018-09-14 20:14:35 +02:00
(function(exports) {
exports.BloomFilter = BloomFilter;
exports.fnv_1a = fnv_1a;
var typedArrays = typeof ArrayBuffer !== "undefined";
// Creates a new bloom filter. If *m* is an array-like object, with a length
// property, then the bloom filter is loaded with data from the array, where
// each element is a 32-bit integer. Otherwise, *m* should specify the
// number of bits. Note that *m* is rounded up to the nearest multiple of
// 32. *k* specifies the number of hashing functions.
function BloomFilter(m, k) {
var a;
if (typeof m !== "number") a = m, m = a.length * 32;
var n = Math.ceil(m / 32),
i = -1;
this.m = m = n * 32;
this.k = k;
if (typedArrays) {
var kbytes = 1 << Math.ceil(Math.log(Math.ceil(Math.log(m) / Math.LN2 / 8)) / Math.LN2),
array = kbytes === 1 ? Uint8Array : kbytes === 2 ? Uint16Array : Uint32Array,
kbuffer = new ArrayBuffer(kbytes * k),
buckets = this.buckets = new Int32Array(n);
if (a) while (++i < n) buckets[i] = a[i];
this._locations = new array(kbuffer);
} else {
var buckets = this.buckets = [];
if (a) while (++i < n) buckets[i] = a[i];
else while (++i < n) buckets[i] = 0;
this._locations = [];
}
}
// See http://willwhim.wpengine.com/2011/09/03/producing-n-hash-functions-by-hashing-only-once/
BloomFilter.prototype.locations = function(v) {
var k = this.k,
m = this.m,
r = this._locations,
a = fnv_1a(v),
b = fnv_1a(v, 1576284489), // The seed value is chosen randomly
x = a % m;
for (var i = 0; i < k; ++i) {
r[i] = x < 0 ? (x + m) : x;
x = (x + b) % m;
}
return r;
};
BloomFilter.prototype.add = function(v) {
var l = this.locations(v + ""),
k = this.k,
buckets = this.buckets;
for (var i = 0; i < k; ++i) buckets[Math.floor(l[i] / 32)] |= 1 << (l[i] % 32);
};
BloomFilter.prototype.test = function(v) {
var l = this.locations(v + ""),
k = this.k,
buckets = this.buckets;
for (var i = 0; i < k; ++i) {
var b = l[i];
if ((buckets[Math.floor(b / 32)] & (1 << (b % 32))) === 0) {
return false;
}
}
return true;
};
// Estimated cardinality.
BloomFilter.prototype.size = function() {
var buckets = this.buckets,
bits = 0;
for (var i = 0, n = buckets.length; i < n; ++i) bits += popcnt(buckets[i]);
return -this.m * Math.log(1 - bits / this.m) / this.k;
};
// http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
function popcnt(v) {
v -= (v >> 1) & 0x55555555;
v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
return ((v + (v >> 4) & 0xf0f0f0f) * 0x1010101) >> 24;
}
// Fowler/Noll/Vo hashing.
// Nonstandard variation: this function optionally takes a seed value that is incorporated
// into the offset basis. According to http://www.isthe.com/chongo/tech/comp/fnv/index.html
// "almost any offset_basis will serve so long as it is non-zero".
function fnv_1a(v, seed) {
var a = 2166136261 ^ (seed || 0);
for (var i = 0, n = v.length; i < n; ++i) {
var c = v.charCodeAt(i),
d = c & 0xff00;
if (d) a = fnv_multiply(a ^ d >> 8);
a = fnv_multiply(a ^ c & 0xff);
}
return fnv_mix(a);
}
// a * 16777619 mod 2**32
function fnv_multiply(a) {
return a + (a << 1) + (a << 4) + (a << 7) + (a << 8) + (a << 24);
}
// See https://web.archive.org/web/20131019013225/http://home.comcast.net/~bretm/hash/6.html
function fnv_mix(a) {
a += a << 13;
a ^= a >>> 7;
a += a << 3;
a ^= a >>> 17;
a += a << 5;
return a & 0xffffffff;
}
})(typeof exports !== "undefined" ? exports : this);