ulong (UInt64) compression based on https://github.com/wolfgarbe/EliasFanoCompression

UInt64Compressor.cs

byte[] EliasFanoCompress(ulong[] UlongsArray)
{	
	long maxCompressedSize = (long)((2 + Math.Ceiling(Math.Log((double)UlongsArray[UlongsArray.Length - 1] / (double)UlongsArray.Length, 2))) * UlongsArray.Length / 8) + 6;
	byte[] compressedBuffer = new byte[maxCompressedSize];
	ulong compressedBufferPointer2 = 0;
	ulong compressedBufferPointer1 = 0;
	
	UInt64 lastID = 0;
	UInt64 buffer1 = 0;
	UInt64 bufferLength1 = 0;
	UInt64 buffer2 = 0;
	UInt64 bufferLength2 = 0;

	ulong largestblockID = (ulong)UlongsArray[UlongsArray.Length - 1];
	double averageDelta = (double)largestblockID / (double)UlongsArray.Length;
	double averageDeltaLog = Math.Log(averageDelta, 2);
	ulong lowBitsLength = (ulong)Math.Floor(averageDeltaLog); 
	if (lowBitsLength < 0) lowBitsLength = 0;
	ulong lowBitsMask = (((ulong)1 << (int)lowBitsLength) - 1);	

	compressedBufferPointer2 = lowBitsLength * (ulong)UlongsArray.Length / 8 + 6;
	compressedBuffer[compressedBufferPointer1++] = (byte)(UlongsArray.Length & 255);
	compressedBuffer[compressedBufferPointer1++] = (byte)((UlongsArray.Length >> 8) & 255);
	compressedBuffer[compressedBufferPointer1++] = (byte)((UlongsArray.Length >> 16) & 255);
	compressedBuffer[compressedBufferPointer1++] = (byte)((UlongsArray.Length >> 24) & 255);

	compressedBuffer[compressedBufferPointer1++] = (byte)lowBitsLength;

	foreach (ulong docID in UlongsArray)
	{
		ulong docIdDelta = (docID - lastID - 1);
		buffer1 <<= (int)lowBitsLength;
		buffer1 |= (docIdDelta & lowBitsMask);
		bufferLength1 += lowBitsLength;
		while (bufferLength1 > 7)
		{
			bufferLength1 -= 8;
			compressedBuffer[compressedBufferPointer1++] = (byte)(buffer1 >> (int)bufferLength1);
		}
		ulong unaryCodeLength = (ulong)(docIdDelta >> (int)lowBitsLength) + 1;
		buffer2 <<= (int)unaryCodeLength;
		buffer2 |= 1;
		bufferLength2 += unaryCodeLength;
		while (bufferLength2 > 7)
		{
			bufferLength2 -= 8;
			compressedBuffer[compressedBufferPointer2++] = (byte)(buffer2 >> (int)bufferLength2);
		}
		lastID = docID;
	}

	if (bufferLength1 > 0) compressedBuffer[compressedBufferPointer1++] = (byte)(buffer1 << (int)(8 - bufferLength1));
	if (bufferLength2 > 0) compressedBuffer[compressedBufferPointer2++] = (byte)(buffer2 << (int)(8 - bufferLength2));	
	
	Array.Resize(ref compressedBuffer, (int)compressedBufferPointer2);
	return compressedBuffer;
}

public static UInt64[,] decodingTableHighBits = new UInt64[256, 8];
public static byte[] decodingTableNumber = new byte[256];
public static byte[] decodingTableHighBitsCarryover = new byte[256];

UInt64[] EliasFanoDecompress(byte[] compressedBuffer)
{
	if (decodingTableHighBitsCarryover[0] == 0)
	{
		for (int i = 0; i < 256; i++)
		{
			byte zeroCount = 0;
			for (int j = 7; j >= 0; j--)
			{
				if ((i & (1 << j)) > 0)
				{
					decodingTableHighBits[i, decodingTableNumber[i]] = zeroCount;
					decodingTableNumber[i]++;
					zeroCount = 0;
				}
				else zeroCount++;
			}
			decodingTableHighBitsCarryover[i] = zeroCount;
		}
	}
	
	UInt64 compressedBufferPointer = (ulong)compressedBuffer.LongLength;	
	UInt64 resultPointer = 0;

	ulong lowBitsPointer = 0;
	ulong lastDocID = 0;
	ulong docID = 0;

	UInt64 postingListCount = compressedBuffer[lowBitsPointer++];
	postingListCount |= (UInt64)compressedBuffer[lowBitsPointer++] << 8;
	postingListCount |= (UInt64)compressedBuffer[lowBitsPointer++] << 16;
	postingListCount |= (UInt64)compressedBuffer[lowBitsPointer++] << 24;
	ulong[] decompressedUlongs = new ulong[postingListCount];

	byte lowBitsLength = compressedBuffer[lowBitsPointer++];
	byte lowBitsCount = 0;
	byte lowBits = 0;

	byte cb = 1;
	for (UInt64 highBitsPointer = lowBitsLength * postingListCount / 8 + 6; highBitsPointer < compressedBufferPointer; highBitsPointer++)
	{
		docID += decodingTableHighBitsCarryover[cb];
		cb = compressedBuffer[highBitsPointer];
		UInt64 docIdNumber = decodingTableNumber[cb];
		for (UInt64 i = 0; i < docIdNumber; i++)
		{
			docID <<= lowBitsCount;
			docID |= lowBits & ((1u << lowBitsCount) - 1u);
			while (lowBitsCount < lowBitsLength)
			{
				docID <<= 8;
				lowBits = compressedBuffer[lowBitsPointer++];
				docID |= lowBits;
				lowBitsCount += 8;
			}
			lowBitsCount -= lowBitsLength;
			docID >>= lowBitsCount;
			docID += (decodingTableHighBits[cb, i] << lowBitsLength) + lastDocID + 1u;
			decompressedUlongs[resultPointer++] = docID;
			lastDocID = docID;
			docID = 0;
		}
	}
	
	return decompressedUlongs;
}