BitSource.java

/*
 * Copyright 2007 ZXing authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.zxing.common;

/**
 * <p>
 * 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.
 * </p>
 * 
 * <p>
 * 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.
 * </p>
 * 
 * @author Sean Owen
 */
public final class BitSource {

	private final byte[] bytes;
	private int byteOffset;
	private int bitOffset;

	/**
	 * @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 BitSource(byte[] 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 int getBitOffset() {
		return bitOffset;
	}

	/**
	 * @return index of next byte in input byte array which would be read by the
	 *         next call to {@link #readBits(int)}.
	 */
	public int getByteOffset() {
		return 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 IllegalArgumentException
	 *             if numBits isn't in [1,32] or more than is available
	 */
	public int readBits(int numBits) {
		if (numBits < 1 || numBits > 32 || numBits > available()) {
			throw new IllegalArgumentException(String.valueOf(numBits));
		}

		int result = 0;

		// First, read remainder from current byte
		if (bitOffset > 0) {
			int bitsLeft = 8 - bitOffset;
			int toRead = numBits < bitsLeft ? numBits : bitsLeft;
			int bitsToNotRead = bitsLeft - toRead;
			int mask = (0xFF >> (8 - toRead)) << bitsToNotRead;
			result = (bytes[byteOffset] & mask) >> bitsToNotRead;
			numBits -= toRead;
			bitOffset += toRead;
			if (bitOffset == 8) {
				bitOffset = 0;
				byteOffset++;
			}
		}

		// Next read whole bytes
		if (numBits > 0) {
			while (numBits >= 8) {
				result = (result << 8) | (bytes[byteOffset] & 0xFF);
				byteOffset++;
				numBits -= 8;
			}

			// Finally read a partial byte
			if (numBits > 0) {
				int bitsToNotRead = 8 - numBits;
				int mask = (0xFF >> bitsToNotRead) << bitsToNotRead;
				result = (result << numBits)
						| ((bytes[byteOffset] & mask) >> bitsToNotRead);
				bitOffset += numBits;
			}
		}

		return result;
	}

	/**
	 * @return number of bits that can be read successfully
	 */
	public int available() {
		return 8 * (bytes.length - byteOffset) - bitOffset;
	}

}