Class Hash

Utilities for implementing GetHashCode().

Inheritance
Object
Hash
Inherited Members
Namespace: Microsoft.ML.Probabilistic.Utilities
Assembly: Microsoft.ML.Probabilistic.dll
Syntax
public static class Hash
Remarks

To hash an object with two fields x and y: 

return Hash.Combine(x.GetHashCode(), y.GetHashCode());

To hash an array: 

int hash = Hash.Start;
for(int i = 0; i < Count; i++)
    hash = Hash.Combine(hash, this[i].GetHashCode());
return hash;

Algorithm: FNV hash from http://www.isthe.com/chongo/tech/comp/fnv/

Note: You should not use xor to combine hash codes, even though it is recommended by MSDN. xor is invariant to permutation, which means "abc" and "bac" and "cba" will hash to the same value (bad). Also xoring a hash value with itself produces 0, so "aab" and "b" will hash to the same value (bad).

Fields

Start

The recommended start value for a combined hash value

Declaration
public const int Start = -2128831035
Field Value
Type Description
Int32

Methods

Combine(Int32, Double)

Incorporates the hash key of a double into an existing hash key

Declaration
public static int Combine(int hash, double number)
Parameters
Type Name Description
Int32 hash

Exisiting hash key
Double number

Floating point number to incorporate
Returns
Type Description
Int32

The new, combined hash key

Combine(Int32, Int32)

Combines two int32 hash keys

Declaration
public static int Combine(int hash, int key)
Parameters
Type Name Description
Int32 hash

First hash key
Int32 key

Second hash key
Returns
Type Description
Int32

Incorporates the second hash key into the first hash key and returns the new, combined hash key

GetHashCodeAsSequence(IEnumerable)

Declaration
public static int GetHashCodeAsSequence(IEnumerable seq)
Parameters
Type Name Description
IEnumerable seq
Returns
Type Description
Int32

GetHashCodeAsSequence<T>(IEnumerable<T>)

Declaration
public static int GetHashCodeAsSequence<T>(IEnumerable<T> seq)
Parameters
Type Name Description
IEnumerable<T> seq
Returns
Type Description
Int32
Type Parameters
Name Description
T

GetHashCodeAsSet<T>(IEnumerable<T>)

Declaration
public static int GetHashCodeAsSet<T>(IEnumerable<T> set)
Parameters
Type Name Description
IEnumerable<T> set
Returns
Type Description
Int32
Type Parameters
Name Description
T

GetHashCodeAsSet<T>(IEnumerable<T>, IEqualityComparer<T>)

Declaration
public static int GetHashCodeAsSet<T>(IEnumerable<T> set, IEqualityComparer<T> comparer)
Parameters
Type Name Description
IEnumerable<T> set
IEqualityComparer<T> comparer
Returns
Type Description
Int32
Type Parameters
Name Description
T