UnrealEngineUWP/Engine/Shaders/Private/HashTable.ush

// Copyright Epic Games, Inc. All Rights Reserved.

uint MurmurAdd( uint Hash, uint Element )
{
	Element *= 0xcc9e2d51;
	Element = ( Element << 15 ) | ( Element >> (32 - 15) );
	Element *= 0x1b873593;
    
	Hash ^= Element;
	Hash = ( Hash << 13 ) | ( Hash >> (32 - 13) );
	Hash = Hash * 5 + 0xe6546b64;
	return Hash;
}

uint MurmurMix( uint Hash )
{
	Hash ^= Hash >> 16;
	Hash *= 0x85ebca6b;
	Hash ^= Hash >> 13;
	Hash *= 0xc2b2ae35;
	Hash ^= Hash >> 16;
	return Hash;
}

// Linear probing hash table
Buffer< uint >		HashTable;
RWBuffer< uint >	RWHashTable;
uint			HashTableSize;

// Returns true if key is added for the first time.
// Index output is the hash table bucket this key is stored in.
bool HashTableAdd( uint Key, out uint Index )
{
	// Zero is reserved as invalid
	Key++;

	uint NumLoops = 0;
	LOOP
	[allow_uav_condition]
	for(Index = MurmurMix(Key); ; Index++ )
	{
		//Belt and braces safety code to prevent inf loops if tables are malformed.
		if(++NumLoops > HashTableSize)
		{
			break;
		}

		//Index &= HashTableSize - 1;
		Index = Index % HashTableSize;

		uint StoredKey = RWHashTable[ Index ];
		if( StoredKey != Key )
		{
			if( StoredKey != 0 )
				continue;

			uint PrevKey;
			InterlockedCompareExchange( RWHashTable[ Index ], 0, Key, PrevKey );
			
			if( PrevKey == 0 )
				return true;
			else if( PrevKey != Key )
				continue;
		}

		break;
	}

	return false;
}

// Returns true if key is found.
// Index output is the hash table bucket this key is stored in if found.
bool HashTableFind( uint Key, out uint Index )
{
	// Zero is reserved as invalid
	Key++;

	uint NumLoops = 0;
	LOOP
	[allow_uav_condition]
	for( Index = MurmurMix( Key );; Index++ )
	{
		//Belt and braces safety code to prevent inf loops if tables are malformed.
		if (++NumLoops > HashTableSize)
		{
			break;
		}

		//Index &= HashTableSize - 1;
		Index = Index % HashTableSize;

		uint StoredKey = HashTable[ Index ];
		if( StoredKey != Key )
		{
			if( StoredKey != 0 )
				continue;
		}
		else
		{
			return true;
		}

		break;
	}

	return false;
}