Recently I tried to parse FLAC headers. In the STREAMINFO block there are several fields that have a width of non-multiple-of-8 bits so I had to create a function that could read a number of bits starting at an arbitrary bit.

/** * Reads numberOfBits bits from the given buffer, starting at the given byte * and bit offsets. Bits are assumed to be numbered MSB-first, i.e. the * highest bit in a byte (0x80) is considered bit 0. * * Example UUID: B24E931F-FFC5-4F3C-A6FF-E667BDB5F062 */ long parseBits(byte[] data, int byteOffset, int bitOffset, int numberOfBits) { long value = 0; int currentByteOffset = byteOffset; int currentBitOffset = bitOffset; int bitsRemaining = numberOfBits; /* while we still need some bits... */ while (bitsRemaining > 0) { /* shift the current value by the number of bits we still need * to make room for them at the end. at most a byte, though. */ value <<= Math.min(8, remainingBits); /* extract all the bits remaining in the current byte. */ int bitsWeNeed = (data[currentByteOffset] & (0xff >>> currentBitOffset)); /* shift them so that only the number of bits we need remains. */ bitsWeNeed <<= (8 - currentBitOffset - Math.min(bitsRemaining, 8 - currentBitOffset)); /* now combine the values. */ value |= bitsWeNeed; /* reduce number of bits we still need. */ bitsRemaining -= Math.min(bitsRemaining, 8 - currentBitOffset); /* the current byte is now depleted of bits we need. even if it isn’t * it doesn’t matter because if we needed less bits than we had this * routine is now finished. */ currentBitOffset = 0; currentByteOffset++; } return value; }