1 /* 2 * Copyright (C) 2017 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package android.location.cts.asn1.base; 18 19 /** 20 * Reads a stream of bits. 21 * 22 * <p>This class is not thread-safe. 23 * 24 */ 25 public class BitStreamReader { 26 private static final int BITS_IN_BYTE = 8; 27 28 private final byte[] buffer; 29 private int position = 0; 30 private int bitsRead = 0; 31 BitStreamReader(byte[] bytes)32 public BitStreamReader(byte[] bytes) { 33 buffer = bytes; 34 } 35 36 /** 37 * Returns true if the next bit in the stream is set. 38 * @throws IndexOutOfBoundsException if there is no more data. 39 */ readBit()40 public boolean readBit() { 41 bitsRead++; 42 if (bitsRead > BITS_IN_BYTE) { 43 position++; 44 bitsRead = 1; 45 } 46 return ((buffer[position] >> (BITS_IN_BYTE - bitsRead)) & 1) == 1; 47 } 48 49 /** 50 * Returns true if there is another readable bit in the stream. 51 */ hasBit()52 public boolean hasBit() { 53 return position + 1 < buffer.length 54 || (bitsRead < BITS_IN_BYTE && position < buffer.length); 55 } 56 spoolToByteBoundary()57 public void spoolToByteBoundary() { 58 if (bitsRead == 0) { 59 return; 60 } 61 bitsRead = 0; 62 position++; 63 } 64 65 /** 66 * Returns next byte's worth of data (8 bits) from the stream. 67 * @throws IndexOutOfBoundsException if there is no more data. 68 */ readByte()69 public byte readByte() { 70 int mask = (1 << (8 - bitsRead)) - 1; 71 byte result = (byte) ((buffer[position] & mask) << bitsRead); 72 position++; 73 if (bitsRead > 0) { 74 result = (byte) (result | (buffer[position] & 0xFF) >>> (8 - bitsRead)); 75 } 76 return result; 77 } 78 79 /** 80 * Returns next {@code howMany} bits as the low bits in the returned byte. 81 * @throws IndexOutOfBoundsException if there is no more data. 82 */ readLowBits(int howMany)83 public int readLowBits(int howMany) { 84 int result = 0; 85 for (int i = 0; i < howMany; i++) { 86 result <<= 1; 87 result |= (readBit() ? 1 : 0); 88 } 89 return result; 90 } 91 } 92