This provides an easy abstraction to read bits at a time from a sequence of bytes, where the * number of bits read is not often a multiple of 8.

* *

This class is thread-safe but not reentrant -- unless the caller modifies the bytes array * it passed in, in which case all bets are off.

* * @author Sean Owen */ final class BitSource { private $bytes; private $byteOffset = 0; private $bitOffset = 0; /** * @param bytes bytes from which this will read bits. Bits will be read from the first byte first. * Bits are read within a byte from most-significant to least-significant bit. */ public function __construct($bytes) { $this->bytes = $bytes; } /** * @return index of next bit in current byte which would be read by the next call to {@link #readBits(int)}. */ public function getBitOffset() { return $this->bitOffset; } /** * @return index of next byte in input byte array which would be read by the next call to {@link #readBits(int)}. */ public function getByteOffset() { return $this->byteOffset; } /** * @param numBits number of bits to read * * @return int representing the bits read. The bits will appear as the least-significant * bits of the int * @throws InvalidArgumentException if numBits isn't in [1,32] or more than is available */ public function readBits($numBits) { if ($numBits < 1 || $numBits > 32 || $numBits > $this->available()) { throw new \InvalidArgumentException(strval($numBits)); } $result = 0; // First, read remainder from current byte if ($this->bitOffset > 0) { $bitsLeft = 8 - $this->bitOffset; $toRead = $numBits < $bitsLeft ? $numBits : $bitsLeft; $bitsToNotRead = $bitsLeft - $toRead; $mask = (0xFF >> (8 - $toRead)) << $bitsToNotRead; $result = ($this->bytes[$this->byteOffset] & $mask) >> $bitsToNotRead; $numBits -= $toRead; $this->bitOffset += $toRead; if ($this->bitOffset == 8) { $this->bitOffset = 0; $this->byteOffset++; } } // Next read whole bytes if ($numBits > 0) { while ($numBits >= 8) { $result = ($result << 8) | ($this->bytes[$this->byteOffset] & 0xFF); $this->byteOffset++; $numBits -= 8; } // Finally read a partial byte if ($numBits > 0) { $bitsToNotRead = 8 - $numBits; $mask = (0xFF >> $bitsToNotRead) << $bitsToNotRead; $result = ($result << $numBits) | (($this->bytes[$this->byteOffset] & $mask) >> $bitsToNotRead); $this->bitOffset += $numBits; } } return $result; } /** * @return number of bits that can be read successfully */ public function available() { return 8 * (count($this->bytes) - $this->byteOffset) - $this->bitOffset; } }