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  1. #include <iostream>
  2. #include <cstdint>
  3. #include <cassert>
  4.  
  5. //----------[ generischer Kram ]----------
  6.  
  7. // extracts a sequence of bits from an unsigned char array
  8. // at arbitrary bit positions (most significant bits first)
  9. std::uint32_t get_bits_msbf(
  10.     const unsigned char data[],
  11.     unsigned bit_offset, unsigned nbits)
  12. {
  13.     // TODO: Diese Implementierung kann man vielleicht noch vereinfachen.
  14.     assert(nbits <= 32);
  15.     if (nbits == 0) return 0u;
  16.     data += bit_offset / 8;
  17.     bit_offset &= 7;
  18.     unsigned tlen = 8 - bit_offset;
  19.     std::uint32_t temp = *data & (0xFFu >> bit_offset);
  20.     if (tlen >= nbits)
  21.         return temp >> (tlen-nbits);
  22.     while (tlen+8 <= nbits) {
  23.         ++data;
  24.         temp = (temp << 8) | (*data & 0xFFu);
  25.         tlen += 8;
  26.     }
  27.     unsigned missing = nbits - tlen;
  28.     if (missing) {
  29.         ++data;
  30.         temp = (temp << missing) | (*data >> (8-missing));
  31.     }
  32.     return temp;
  33. }
  34.  
  35. template<unsigned BitOffset, unsigned BitLen, class MessageTag=void>
  36. struct field
  37. {
  38.     typedef MessageTag message_tag;
  39.     static const unsigned bit_offset = BitOffset;
  40.     static const unsigned bit_len = BitLen;
  41. };
  42.  
  43. template<class MessageTag=void>
  44. struct message_ptr
  45. {
  46.     const unsigned char* data;
  47.     unsigned length;
  48.  
  49.     explicit message_ptr(const unsigned char *ptr, unsigned len)
  50.         : data(ptr), length(len)
  51.     {}
  52.  
  53.     template<unsigned BitOffset, unsigned BitLen>
  54.     std::uint32_t get(field<BitOffset,BitLen,MessageTag>) const
  55.     {
  56.         assert(BitOffset+BitLen <= 8*length);
  57.         return get_bits_msbf(data,BitOffset,BitLen);
  58.     }
  59. };
  60.  
  61. //----------[ Anwendungsfall ]----------
  62.  
  63. namespace mp3_stuff {
  64.  
  65. struct frame_header_tag {};
  66. const field<0,11,frame_header_tag> sync = {};
  67. const field<11,2,frame_header_tag> mpegid = {};
  68. const field<13,2,frame_header_tag> layer = {};
  69. const field<15,1,frame_header_tag> protection_bit = {};
  70. const field<16,4,frame_header_tag> bitrate_index = {};
  71. const field<20,2,frame_header_tag> sampling_rate_index = {};
  72. const field<22,1,frame_header_tag> padding_bit = {};
  73. const field<23,1,frame_header_tag> private_bit = {};
  74. const field<24,2,frame_header_tag> channel_mode = {};
  75. const field<26,2,frame_header_tag> cm_extension = {};
  76. const field<28,1,frame_header_tag> copyrighted_bit = {};
  77. const field<29,1,frame_header_tag> original_bit = {};
  78. const field<30,2,frame_header_tag> emphasis_mode = {};
  79.  
  80. static const char* const mpegid2str[] = {"MPEG 2.5", "reserved", "MPEG 2", "MPEG 1"};
  81. static const char* const chanmodes[] = {"Stereo", "Joint Stereo", "Dual Channel", "Mono"};
  82. static const long srate[] = {44100,48000,32000};
  83.  
  84. } // namespace mp3_stuff
  85.  
  86. void test()
  87. {
  88.     using namespace std;
  89.     using namespace mp3_stuff;
  90.     unsigned char data[] = {0xFF, 0xFB, 0xA0, 0x64};
  91.     message_ptr<frame_header_tag> mptr (data,4);
  92.     if (mptr.get(sync) == 0x7FF) {
  93.         cout << "MPEG          : " << mpegid2str[mptr.get(mpegid)] << endl;
  94.         cout << "Layer         : " << 4-mptr.get(layer) << endl;
  95.         cout << "Bitrate index : " << mptr.get(bitrate_index) << endl;
  96.         const int srdivisor = 4-mptr.get(mpegid);
  97.         cout << "Sampling rate : " << srate[mptr.get(sampling_rate_index)]/srdivisor << endl;
  98.         cout << "Channel mode  : " << chanmodes[mptr.get(channel_mode)] << endl;
  99.     } else {
  100.         cout << "Sync bits invalid\n";
  101.     }
  102. }
  103.  
  104. int main()
  105. {
  106.     test();
  107. }
Success #stdin #stdout 0s 2856KB
comments (?)
stdin 
Standard input is empty
stdout 
MPEG          : MPEG 1
Layer         : 3
Bitrate index : 10
Sampling rate : 44100
Channel mode  : Joint Stereo
https://ideone.com/5Hon0s
language:
C++14 (gcc 8.3)
created:
13 years ago
visibility:
secret

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