How to Efficiently Extract Variable Length Bit Sequences from a Byte Array?

How can I efficiently extract bit sequences of arbitrary lengths from a byte[] array?

// Example of extracting bits from a byte array
public static int extractBits(byte[] byteArray, int startBit, int length) {
    int startByte = startBit / 8; // Get the beginning byte index
    int startPos = startBit % 8; // Get the position within the byte
    int result = 0;
    int totalBits = 0;

    for (int i = startByte; totalBits < length && i < byteArray.length; i++) {
        int bitsToRead = Math.min(8 - startPos, length - totalBits);
        int mask = (1 << bitsToRead) - 1;
        int bits = ((byteArray[i] >> startPos) & mask);
        result |= (bits << totalBits);

        totalBits += bitsToRead;
        startPos = 0;
    }
    return result;
}

Extracting bit sequences of arbitrary lengths from a byte array is a common task in areas such as network programming, cryptography, and data compression. This can be achieved by careful manipulation of byte-level data.

// Using the provided code snippet to extract bits
int extractedBits = extractBits(myByteArray, 18, 13); // Extract 13 bits starting from bit 18

Causes

• Understanding byte allocation and bit manipulation is key.
• Calculating start positions and lengths accurately avoids data loss.

Solutions

• Calculate which byte and bit positions correspond to your target sequence.
• Implement a loop to extract bits from the byte array as needed.

Mistake: Not accounting for byte boundaries, leading to incorrect bit reading.

Solution: Always calculate `startByte` and `startPos` correctly.

Mistake: Accessing out-of-bounds indices in the byte array.

Solution: Check the length of the byte array before extraction.