private byte[] GetOnlyRelevantSourceBytes(byte[] sourceBits, int sourceArrayBitIndex, int numBits,
int firstByteIndexInSource, int lastByteIndexInSource, int lastSourceBitIndex,
int numBitPositionsToShiftSourceBy)
{
if (numBits == 0)
{
return(new byte[0]);
}
// A copy of the source array allows us to mask-out bits before sourceArrayBitIndex and after the last
// bit (sourceArrayBitIndex + numBits) so we don't accidently "borrow" bits we're not supposed to --
// without modifying the array passed in as a reference.
var nominalNumberOfBytesForCopy = GetByteIndexFromBitIndex(sourceArrayBitIndex % 8 + numBits - 1) + 1;
var numBitsAvailableInLastSourceByte =
duoBitArrayUtilities.GetNumUnusedBitsInByteArray(sourceArrayBitIndex + numBits);
var extraByteNeededForBorrowBits = (numBitPositionsToShiftSourceBy > numBitsAvailableInLastSourceByte);
var arrayCopySize = nominalNumberOfBytesForCopy + (extraByteNeededForBorrowBits ? 1 : 0);
var sourceBitsCopy = new byte[arrayCopySize];
for (var i = 1; i < sourceBitsCopy.Length - 1; i++)
{
sourceBitsCopy[i] = sourceBits[firstByteIndexInSource + i];
}
sourceBitsCopy[0] = GetLsbMaskedSourceByte(sourceBits[firstByteIndexInSource]);
if (lastByteIndexInSource > 0)
{
sourceBitsCopy[sourceBitsCopy.Length - 1] =
GetMsbMaskedSourceByte(sourceBits[lastByteIndexInSource]);
}
return(sourceBitsCopy);
// These are C# 7 "local functions"; if porting to another language, make them private methods.
byte GetMsbMaskedSourceByte(byte candidateDestinationByte)
{
var numBitsInLastSourceByteToKeep =
duoBitArrayUtilities.GetNumBitsInUseInLastByte(lastSourceBitIndex + 1);
var sourceByteMask = duoBitArrayUtilities.GetByteMsbMask(numBitsInLastSourceByteToKeep);
var maskedSourceByte = (byte)(candidateDestinationByte & sourceByteMask);
return(maskedSourceByte);
}
byte GetLsbMaskedSourceByte(byte candidateDestinationByte)
{
var numBitsInFirstSourceByteToKeep = 8 - sourceArrayBitIndex;
var sourceByteMask = duoBitArrayUtilities.GetByteLsbMask(numBitsInFirstSourceByteToKeep);
var maskedSourceByte = (byte)(candidateDestinationByte & sourceByteMask);
return(maskedSourceByte);
}
}
private byte[] GetMaskForConsecutiveLeftBits(int numBits)
{
var numBytes = duoBitArrayUtilities.GetMinNumBytesToStoreBits(numBits);
var generatedMask = new byte[numBytes];
generatedMask[numBytes - 1] =
duoBitArrayUtilities.GetByteMsbMask(duoBitArrayUtilities.GetNumBitsInUseInLastByte(numBits));
for (var i = 0; i < numBytes - 2; i++)
{
generatedMask[i] = 0xFF;
}
return(generatedMask);
}