public void UniversalHashTestBias()
{
Pseudorandom pseudo = new Pseudorandom();
UniversalHashFunction f = new UniversalHashFunction("Louis Tully as played by Rick Moranis!");
ulong trials = 100000000;
ulong[] bitCounts = new ulong[64];
for (ulong trial = 0; trial < trials; trial++)
{
string randomString = pseudo.GetString(8);
UInt64 supposedlyUnbiasedBits = f.Hash(randomString, UniversalHashFunction.MaximumNumberOfResultBitsAllowing32BiasedBits);
for (int bit=0; bit < bitCounts.Length; bit++)
{
if ((supposedlyUnbiasedBits & (0x8000000000000000ul >> bit)) != 0ul)
bitCounts[bit]++;
}
}
double[] biases = bitCounts.Select(count => ( (0.5d - (((double)count) / ((double)trials)))) / 0.5d ).ToArray();
/// The first 32 bits should be unbiased
for (int bit = 0; bit < 32; bit++)
{
double bias = biases[bit];
double biasAbs = Math.Abs(bias);
Assert.True(biasAbs < 0.0005d);
}
}
/// <summary>
/// Construct a filter array.
/// </summary>
/// <param name="numberOfBitsInArray">The size of the array in bits.</param>
/// <param name="maximumBitIndexesPerElement">The maximum (and default) number of indexes (bits) in the array to associate with elements.</param>
/// <param name="initilizeBitsOfArrayAtRandom">If set to true, the bits of the filter array will be set to 0 or 1 at random (indpendently, each with probability 0.5).</param>
/// <param name="saltForHashFunctions">A salt used to generate the hash functions.
/// Any two filter arrays generated with the same salt will use the same hash functions.
/// The salt should be kept secret from attackerse who might try to manipulate the selection of elements,
/// such as to intentionally cause bit collisions with the array.</param>
public FilterArray(int numberOfBitsInArray, int maximumBitIndexesPerElement, bool initilizeBitsOfArrayAtRandom,
string saltForHashFunctions = "")
{
// Align on byte boundary to guarantee no less than numberOfBitsInArray
int capacityInBytes = (numberOfBitsInArray + 7) / 8;
// Create hash functions to map elements to indexes in the bit array.
HashFunctionsMappingElementsToBitsInTheArray = new UniversalHashFunction[maximumBitIndexesPerElement];
for (int i = 0; i < HashFunctionsMappingElementsToBitsInTheArray.Length; i++)
{
HashFunctionsMappingElementsToBitsInTheArray[i] =
new UniversalHashFunction(i + ":" + saltForHashFunctions, 64);
}
if (initilizeBitsOfArrayAtRandom)
{
// Initialize the bit array setting ~half the bits randomly to zero by using the
// cryptographic random number generator.
byte[] initialBitValues = new byte[capacityInBytes];
StrongRandomNumberGenerator.GetBytes(initialBitValues);
BitArray = new BitArray(initialBitValues);
}
else
{
// Start with all bits of the array set to zero.
BitArray = new BitArray(capacityInBytes * 8);
}
}
/// <summary>
/// Construct a binomial sketch, in which a set of k hash functions (k=NumberOfIndexes) will map any
/// key to k points with an array of n bits (sizeInBits).
/// When one Adds a key to a binomial sketch, a random bit among the subset of k that are currently 0 will be set to 1.
/// To ensure roughly half the bits remain zero, a random index from the subset of all k bits that are currently 1 will be set to 0.
///
/// Over time, popular keys will have almost all of their bits set and unpopular keys will be expected to have roughly half their bits set.
/// </summary>
/// <param name="sizeInBits">The total number of bits to maintain in the table. In theoretical discussions of bloom filters and sketches
/// in general, this is usually referrted to by the letter n.</param>
/// <param name="numberOfIndexes">The number of indexes to map each key to, each of which is assigned a unique pseudorandom
/// hash function.</param>
/// <param name="keyToPreventAlgorithmicComplexityAttacks">A pseudorandom seed that allows the same sketch to be created
/// twice, but (if kept secret) prevents an attacker from knowing the distribution of hashes and thus counters
/// algorithmic complexity attacks.</param>
public BinomialSketch(int sizeInBits, int numberOfIndexes, string keyToPreventAlgorithmicComplexityAttacks)
{
NumberOfIndexes = numberOfIndexes;
string keyToPreventAlgorithmicComplexityAttacks1 = keyToPreventAlgorithmicComplexityAttacks ?? "";
SizeInBits = sizeInBits;
_maxNumberOfObservationsAccountingForAging = (ulong) SizeInBits/(ulong) (NumberOfIndexes*2);
// Align on next byte boundary
if ((SizeInBits & 7) != 0)
SizeInBits = (sizeInBits + 8) ^ 0x7;
int capacityInBytes = SizeInBits / 8;
_universalHashFunctions = new UniversalHashFunction[numberOfIndexes];
for (int i = 0; i < _universalHashFunctions.Length; i++)
{
_universalHashFunctions[i] =
new UniversalHashFunction(i.ToString() + keyToPreventAlgorithmicComplexityAttacks1, 64);
}
// Initialize the sketch setting ~half the bits randomly to zero by using the
// cryptographic random number generator.
byte[] initialSketchValues = new byte[capacityInBytes];
StrongRandomNumberGenerator.GetBytes(initialSketchValues);
_sketch = new BitArray(initialSketchValues);
// binomialProbability[i] = (n choose k) * (p)^k * (1-p)^(n-k)
// since p=.5, this is (n choose k) 0.5^(n)
double[] binomialProbability = new double[numberOfIndexes + 1];
double probabilityOfAnyGivenValue = Math.Pow(0.5d, numberOfIndexes);
double nChooseK = 1d;
for (int k = 0; k <= numberOfIndexes/2; k++)
{
binomialProbability[k] = binomialProbability[numberOfIndexes-k] =
nChooseK * probabilityOfAnyGivenValue;
nChooseK *= (numberOfIndexes - k)/(1d + k);
}
_cumulativeProbabilitySetByChance = new double[numberOfIndexes + 1];
_cumulativeProbabilitySetByChance[numberOfIndexes] = binomialProbability[numberOfIndexes];
for (int k = numberOfIndexes; k > 0; k--)
_cumulativeProbabilitySetByChance[k-1] =
_cumulativeProbabilitySetByChance[k] + binomialProbability[k-1];
}
/// <summary>
/// Create a client for a distributed binomial ladder filter
/// </summary>
/// <param name="numberOfShards">The number of shards that the bit array of the binomial ladder filter will be divided into.
/// The greater the number of shards, the more evently it can be distributed. However, the number of shards should still
/// be a few orders of magnitude smaller than the ladder height.</param>
/// <param name="defaultHeightOfLadder">The default ladder height for elements on the ladder.</param>
/// <param name="shardToHostMapping">An object that maps each shard number to the host responsible for that shard.</param>
/// <param name="configurationKey">A key used to protect the hashing from algorithmic complexity attacks.
/// This key should not be unique to the application using the filter and should not be known to any untrusted
/// systems that might control which elements get sent to the filter. If an attacker could submit elements to the filter
/// and knew this key, the attacker could arrange for all elements to go to the same shard and in so doing overload that shard.</param>
/// <param name="mininmumCacheFreshnessRequired">The maximum time that an element should be kept in the cache of elements at the top of their ladder.
/// In other words, how long to bound the possible time that an element may still appear to be at the top of its ladder in the cache
/// when it is no longer at the top of the ladder based on the filter array. Defaults to one minute.</param>
public DistributedBinomialLadderFilterClient(int numberOfShards, int defaultHeightOfLadder, IDistributedResponsibilitySet<RemoteHost> shardToHostMapping, string configurationKey, TimeSpan? mininmumCacheFreshnessRequired = null)
{
NumberOfShards = numberOfShards;
MaxLadderHeight = defaultHeightOfLadder;
MinimumCacheFreshnessRequired = mininmumCacheFreshnessRequired ?? new TimeSpan(0,0,1);
CacheOfElementsAtTopOfLadder = new FixedSizeLruCache<string, DateTime>(2*NumberOfShards);
ShardHashFunction = new UniversalHashFunction(configurationKey);
ShardToHostMapping = shardToHostMapping;
}
