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      1 /*
      2  * Copyright (C) 2010 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 #include "ABitReader.h"
     18 
     19 #include <media/stagefright/foundation/ADebug.h>
     20 
     21 namespace android {
     22 
     23 ABitReader::ABitReader(const uint8_t *data, size_t size)
     24     : mData(data),
     25       mSize(size),
     26       mReservoir(0),
     27       mNumBitsLeft(0),
     28       mOverRead(false) {
     29 }
     30 
     31 ABitReader::~ABitReader() {
     32 }
     33 
     34 bool ABitReader::fillReservoir() {
     35     if (mSize == 0) {
     36         mOverRead = true;
     37         return false;
     38     }
     39 
     40     mReservoir = 0;
     41     size_t i;
     42     for (i = 0; mSize > 0 && i < 4; ++i) {
     43         mReservoir = (mReservoir << 8) | *mData;
     44 
     45         ++mData;
     46         --mSize;
     47     }
     48 
     49     mNumBitsLeft = 8 * i;
     50     mReservoir <<= 32 - mNumBitsLeft;
     51     return true;
     52 }
     53 
     54 uint32_t ABitReader::getBits(size_t n) {
     55     uint32_t ret;
     56     CHECK(getBitsGraceful(n, &ret));
     57     return ret;
     58 }
     59 
     60 uint32_t ABitReader::getBitsWithFallback(size_t n, uint32_t fallback) {
     61     uint32_t ret = fallback;
     62     (void)getBitsGraceful(n, &ret);
     63     return ret;
     64 }
     65 
     66 bool ABitReader::getBitsGraceful(size_t n, uint32_t *out) {
     67     if (n > 32) {
     68         return false;
     69     }
     70 
     71     uint32_t result = 0;
     72     while (n > 0) {
     73         if (mNumBitsLeft == 0) {
     74             if (!fillReservoir()) {
     75                 return false;
     76             }
     77         }
     78 
     79         size_t m = n;
     80         if (m > mNumBitsLeft) {
     81             m = mNumBitsLeft;
     82         }
     83 
     84         result = (result << m) | (mReservoir >> (32 - m));
     85         mReservoir <<= m;
     86         mNumBitsLeft -= m;
     87 
     88         n -= m;
     89     }
     90 
     91     *out = result;
     92     return true;
     93 }
     94 
     95 bool ABitReader::skipBits(size_t n) {
     96     uint32_t dummy;
     97     while (n > 32) {
     98         if (!getBitsGraceful(32, &dummy)) {
     99             return false;
    100         }
    101         n -= 32;
    102     }
    103 
    104     if (n > 0) {
    105         return getBitsGraceful(n, &dummy);
    106     }
    107     return true;
    108 }
    109 
    110 void ABitReader::putBits(uint32_t x, size_t n) {
    111     if (mOverRead) {
    112         return;
    113     }
    114 
    115     CHECK_LE(n, 32u);
    116 
    117     while (mNumBitsLeft + n > 32) {
    118         mNumBitsLeft -= 8;
    119         --mData;
    120         ++mSize;
    121     }
    122 
    123     mReservoir = (mReservoir >> n) | (x << (32 - n));
    124     mNumBitsLeft += n;
    125 }
    126 
    127 size_t ABitReader::numBitsLeft() const {
    128     return mSize * 8 + mNumBitsLeft;
    129 }
    130 
    131 const uint8_t *ABitReader::data() const {
    132     return mData - (mNumBitsLeft + 7) / 8;
    133 }
    134 
    135 NALBitReader::NALBitReader(const uint8_t *data, size_t size)
    136     : ABitReader(data, size),
    137       mNumZeros(0) {
    138 }
    139 
    140 bool NALBitReader::atLeastNumBitsLeft(size_t n) const {
    141     // check against raw size and reservoir bits first
    142     size_t numBits = numBitsLeft();
    143     if (n > numBits) {
    144         return false;
    145     }
    146 
    147     ssize_t numBitsRemaining = (ssize_t)n - (ssize_t)mNumBitsLeft;
    148 
    149     size_t size = mSize;
    150     const uint8_t *data = mData;
    151     int32_t numZeros = mNumZeros;
    152     while (size > 0 && numBitsRemaining > 0) {
    153         bool isEmulationPreventionByte = (numZeros >= 2 && *data == 3);
    154 
    155         if (*data == 0) {
    156             ++numZeros;
    157         } else {
    158             numZeros = 0;
    159         }
    160 
    161         if (!isEmulationPreventionByte) {
    162             numBitsRemaining -= 8;
    163         }
    164 
    165         ++data;
    166         --size;
    167     }
    168 
    169     return (numBitsRemaining <= 0);
    170 }
    171 
    172 bool NALBitReader::fillReservoir() {
    173     if (mSize == 0) {
    174         mOverRead = true;
    175         return false;
    176     }
    177 
    178     mReservoir = 0;
    179     size_t i = 0;
    180     while (mSize > 0 && i < 4) {
    181         bool isEmulationPreventionByte = (mNumZeros >= 2 && *mData == 3);
    182 
    183         if (*mData == 0) {
    184             ++mNumZeros;
    185         } else {
    186             mNumZeros = 0;
    187         }
    188 
    189         // skip emulation_prevention_three_byte
    190         if (!isEmulationPreventionByte) {
    191             mReservoir = (mReservoir << 8) | *mData;
    192             ++i;
    193         }
    194 
    195         ++mData;
    196         --mSize;
    197     }
    198 
    199     mNumBitsLeft = 8 * i;
    200     mReservoir <<= 32 - mNumBitsLeft;
    201     return true;
    202 }
    203 
    204 }  // namespace android
    205