root/media/base/container_names.cc

DEFINITIONS

1. Read16
2. Read24
3. Read32
4. Read32LE
5. StartsWith
6. StartsWith
7. ReadBits
8. CheckAac
9. CheckAc3
10. CheckEac3
11. CheckBink
12. CheckCaf
13. CheckDts
14. CheckDV
15. CheckGsm
16. AdvanceToStartCode
17. CheckH261
18. CheckH263
19. CheckH264
20. CheckHls
21. CheckMJpeg
22. CheckMpeg2ProgramStream
23. CheckMpeg2TransportStream
24. CheckMpeg4BitStream
25. CheckMov
26. ValidMpegAudioFrameHeader
27. GetMp3HeaderSize
28. CheckMp3
29. VerifyNumber
30. VerifyCharacters
31. CheckSrt
32. GetElementId
33. GetVint
34. CheckWebm
35. CheckVC1
36. LookupContainerByFirst4
37. DetermineContainer

// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "media/base/container_names.h"

#include <cctype>
#include <limits>

#include "base/basictypes.h"
#include "base/logging.h"
#include "media/base/bit_reader.h"

namespace media {

namespace container_names {

#define TAG(a, b, c, d) \
    ((static_cast<uint8>(a) << 24) | (static_cast<uint8>(b) << 16) | \
     (static_cast<uint8>(c) << 8) | (static_cast<uint8>(d)))

#define RCHECK(x)     \
    do {              \
      if (!(x))       \
        return false; \
    } while (0)

#define UTF8_BYTE_ORDER_MARK "\xef\xbb\xbf"

// Helper function to read 2 bytes (16 bits, big endian) from a buffer.
static int Read16(const uint8* p) {
  return p[0] << 8 | p[1];
}

// Helper function to read 3 bytes (24 bits, big endian) from a buffer.
static uint32 Read24(const uint8* p) {
  return p[0] << 16 | p[1] << 8 | p[2];
}

// Helper function to read 4 bytes (32 bits, big endian) from a buffer.
static uint32 Read32(const uint8* p) {
  return p[0] << 24 | p[1] << 16 | p[2] << 8 | p[3];
}

// Helper function to read 4 bytes (32 bits, little endian) from a buffer.
static uint32 Read32LE(const uint8* p) {
  return p[3] << 24 | p[2] << 16 | p[1] << 8 | p[0];
}

// Helper function to do buffer comparisons with a string without going off the
// end of the buffer.
static bool StartsWith(const uint8* buffer,
                       size_t buffer_size,
                       const char* prefix) {
  size_t prefix_size = strlen(prefix);
  return (prefix_size <= buffer_size &&
          memcmp(buffer, prefix, prefix_size) == 0);
}

// Helper function to do buffer comparisons with another buffer (to allow for
// embedded \0 in the comparison) without going off the end of the buffer.
static bool StartsWith(const uint8* buffer,
                       size_t buffer_size,
                       const uint8* prefix,
                       size_t prefix_size) {
  return (prefix_size <= buffer_size &&
          memcmp(buffer, prefix, prefix_size) == 0);
}

// Helper function to read up to 64 bits from a bit stream.
static uint64 ReadBits(BitReader* reader, int num_bits) {
  DCHECK_GE(reader->bits_available(), num_bits);
  DCHECK((num_bits > 0) && (num_bits <= 64));
  uint64 value;
  reader->ReadBits(num_bits, &value);
  return value;
}

const int kAc3FrameSizeTable[38][3] = {
  { 128, 138, 192 }, { 128, 140, 192 }, { 160, 174, 240 }, { 160, 176, 240 },
  { 192, 208, 288 }, { 192, 210, 288 }, { 224, 242, 336 }, { 224, 244, 336 },
  { 256, 278, 384 }, { 256, 280, 384 }, { 320, 348, 480 }, { 320, 350, 480 },
  { 384, 416, 576 }, { 384, 418, 576 }, { 448, 486, 672 }, { 448, 488, 672 },
  { 512, 556, 768 }, { 512, 558, 768 }, { 640, 696, 960 }, { 640, 698, 960 },
  { 768, 834, 1152 }, { 768, 836, 1152 }, { 896, 974, 1344 },
  { 896, 976, 1344 }, { 1024, 1114, 1536 }, { 1024, 1116, 1536 },
  { 1280, 1392, 1920 }, { 1280, 1394, 1920 }, { 1536, 1670, 2304 },
  { 1536, 1672, 2304 }, { 1792, 1950, 2688 }, { 1792, 1952, 2688 },
  { 2048, 2228, 3072 }, { 2048, 2230, 3072 }, { 2304, 2506, 3456 },
  { 2304, 2508, 3456 }, { 2560, 2768, 3840 }, { 2560, 2770, 3840 }
};

// Checks for an ADTS AAC container.
static bool CheckAac(const uint8* buffer, int buffer_size) {
  // Audio Data Transport Stream (ADTS) header is 7 or 9 bytes
  // (from http://wiki.multimedia.cx/index.php?title=ADTS)
  RCHECK(buffer_size > 6);

  int offset = 0;
  while (offset + 6 < buffer_size) {
    BitReader reader(buffer + offset, 6);

    // Syncword must be 0xfff.
    RCHECK(ReadBits(&reader, 12) == 0xfff);

    // Skip MPEG version.
    reader.SkipBits(1);

    // Layer is always 0.
    RCHECK(ReadBits(&reader, 2) == 0);

    // Skip protection + profile.
    reader.SkipBits(1 + 2);

    // Check sampling frequency index.
    RCHECK(ReadBits(&reader, 4) != 15);  // Forbidden.

    // Skip private stream, channel configuration, originality, home,
    // copyrighted stream, and copyright_start.
    reader.SkipBits(1 + 3 + 1 + 1 + 1 + 1);

    // Get frame length (includes header).
    int size = ReadBits(&reader, 13);
    RCHECK(size > 0);
    offset += size;
  }
  return true;
}

const uint16 kAc3SyncWord = 0x0b77;

// Checks for an AC3 container.
static bool CheckAc3(const uint8* buffer, int buffer_size) {
  // Reference: ATSC Standard: Digital Audio Compression (AC-3, E-AC-3)
  //            Doc. A/52:2012
  // (http://www.atsc.org/cms/standards/A52-2012(12-17).pdf)

  // AC3 container looks like syncinfo | bsi | audblk * 6 | aux | check.
  RCHECK(buffer_size > 6);

  int offset = 0;
  while (offset + 6 < buffer_size) {
    BitReader reader(buffer + offset, 6);

    // Check syncinfo.
    RCHECK(ReadBits(&reader, 16) == kAc3SyncWord);

    // Skip crc1.
    reader.SkipBits(16);

    // Verify fscod.
    int sample_rate_code = ReadBits(&reader, 2);
    RCHECK(sample_rate_code != 3);  // Reserved.

    // Verify frmsizecod.
    int frame_size_code = ReadBits(&reader, 6);
    RCHECK(frame_size_code < 38);  // Undefined.

    // Verify bsid.
    RCHECK(ReadBits(&reader, 5) < 10);  // Normally 8 or 6, 16 used by EAC3.

    offset += kAc3FrameSizeTable[frame_size_code][sample_rate_code];
  }
  return true;
}

// Checks for an EAC3 container (very similar to AC3)
static bool CheckEac3(const uint8* buffer, int buffer_size) {
  // Reference: ATSC Standard: Digital Audio Compression (AC-3, E-AC-3)
  //            Doc. A/52:2012
  // (http://www.atsc.org/cms/standards/A52-2012(12-17).pdf)

  // EAC3 container looks like syncinfo | bsi | audfrm | audblk* | aux | check.
  RCHECK(buffer_size > 6);

  int offset = 0;
  while (offset + 6 < buffer_size) {
    BitReader reader(buffer + offset, 6);

    // Check syncinfo.
    RCHECK(ReadBits(&reader, 16) == kAc3SyncWord);

    // Verify strmtyp.
    RCHECK(ReadBits(&reader, 2) != 3);

    // Skip substreamid.
    reader.SkipBits(3);

    // Get frmsize. Include syncinfo size and convert to bytes.
    int frame_size = (ReadBits(&reader, 11) + 1) * 2;
    RCHECK(frame_size >= 7);

    // Skip fscod, fscod2, acmod, and lfeon.
    reader.SkipBits(2 + 2 + 3 + 1);

    // Verify bsid.
    int bit_stream_id = ReadBits(&reader, 5);
    RCHECK(bit_stream_id >= 11 && bit_stream_id <= 16);

    offset += frame_size;
  }
  return true;
}

// Additional checks for a BINK container.
static bool CheckBink(const uint8* buffer, int buffer_size) {
  // Reference: http://wiki.multimedia.cx/index.php?title=Bink_Container
  RCHECK(buffer_size >= 44);

  // Verify number of frames specified.
  RCHECK(Read32LE(buffer + 8) > 0);

  // Verify width in range.
  int width = Read32LE(buffer + 20);
  RCHECK(width > 0 && width <= 32767);

  // Verify height in range.
  int height = Read32LE(buffer + 24);
  RCHECK(height > 0 && height <= 32767);

  // Verify frames per second specified.
  RCHECK(Read32LE(buffer + 28) > 0);

  // Verify video frames per second specified.
  RCHECK(Read32LE(buffer + 32) > 0);

  // Number of audio tracks must be 256 or less.
  return (Read32LE(buffer + 40) <= 256);
}

// Additional checks for a CAF container.
static bool CheckCaf(const uint8* buffer, int buffer_size) {
  // Reference: Apple Core Audio Format Specification 1.0
  // (https://developer.apple.com/library/mac/#documentation/MusicAudio/Reference/CAFSpec/CAF_spec/CAF_spec.html)
  RCHECK(buffer_size >= 52);
  BitReader reader(buffer, buffer_size);

  // mFileType should be "caff".
  RCHECK(ReadBits(&reader, 32) == TAG('c', 'a', 'f', 'f'));

  // mFileVersion should be 1.
  RCHECK(ReadBits(&reader, 16) == 1);

  // Skip mFileFlags.
  reader.SkipBits(16);

  // First chunk should be Audio Description chunk, size 32l.
  RCHECK(ReadBits(&reader, 32) == TAG('d', 'e', 's', 'c'));
  RCHECK(ReadBits(&reader, 64) == 32);

  // CAFAudioFormat.mSampleRate(float64) not 0
  RCHECK(ReadBits(&reader, 64) != 0);

  // CAFAudioFormat.mFormatID not 0
  RCHECK(ReadBits(&reader, 32) != 0);

  // Skip CAFAudioFormat.mBytesPerPacket and mFramesPerPacket.
  reader.SkipBits(32 + 32);

  // CAFAudioFormat.mChannelsPerFrame not 0
  RCHECK(ReadBits(&reader, 32) != 0);
  return true;
}

static bool kSamplingFrequencyValid[16] = { false, true, true, true, false,
                                            false, true, true, true, false,
                                            false, true, true, true, false,
                                            false };
static bool kExtAudioIdValid[8] = { true, false, true, false, false, false,
                                    true, false };

// Additional checks for a DTS container.
static bool CheckDts(const uint8* buffer, int buffer_size) {
  // Reference: ETSI TS 102 114 V1.3.1 (2011-08)
  // (http://www.etsi.org/deliver/etsi_ts/102100_102199/102114/01.03.01_60/ts_102114v010301p.pdf)
  RCHECK(buffer_size > 11);

  int offset = 0;
  while (offset + 11 < buffer_size) {
    BitReader reader(buffer + offset, 11);

    // Verify sync word.
    RCHECK(ReadBits(&reader, 32) == 0x7ffe8001);

    // Skip frame type and deficit sample count.
    reader.SkipBits(1 + 5);

    // Verify CRC present flag.
    RCHECK(ReadBits(&reader, 1) == 0);  // CPF must be 0.

    // Verify number of PCM sample blocks.
    RCHECK(ReadBits(&reader, 7) >= 5);

    // Verify primary frame byte size.
    int frame_size = ReadBits(&reader, 14);
    RCHECK(frame_size >= 95);

    // Skip audio channel arrangement.
    reader.SkipBits(6);

    // Verify core audio sampling frequency is an allowed value.
    RCHECK(kSamplingFrequencyValid[ReadBits(&reader, 4)]);

    // Verify transmission bit rate is valid.
    RCHECK(ReadBits(&reader, 5) <= 25);

    // Verify reserved field is 0.
    RCHECK(ReadBits(&reader, 1) == 0);

    // Skip dynamic range flag, time stamp flag, auxiliary data flag, and HDCD.
    reader.SkipBits(1 + 1 + 1 + 1);

    // Verify extension audio descriptor flag is an allowed value.
    RCHECK(kExtAudioIdValid[ReadBits(&reader, 3)]);

    // Skip extended coding flag and audio sync word insertion flag.
    reader.SkipBits(1 + 1);

    // Verify low frequency effects flag is an allowed value.
    RCHECK(ReadBits(&reader, 2) != 3);

    offset += frame_size + 1;
  }
  return true;
}

// Checks for a DV container.
static bool CheckDV(const uint8* buffer, int buffer_size) {
  // Reference: SMPTE 314M (Annex A has differences with IEC 61834).
  // (http://standards.smpte.org/content/978-1-61482-454-1/st-314-2005/SEC1.body.pdf)
  RCHECK(buffer_size > 11);

  int offset = 0;
  int current_sequence_number = -1;
  int last_block_number[6];
  while (offset + 11 < buffer_size) {
    BitReader reader(buffer + offset, 11);

    // Decode ID data. Sections 5, 6, and 7 are reserved.
    int section = ReadBits(&reader, 3);
    RCHECK(section < 5);

    // Next bit must be 1.
    RCHECK(ReadBits(&reader, 1) == 1);

    // Skip arbitrary bits.
    reader.SkipBits(4);

    int sequence_number = ReadBits(&reader, 4);

    // Skip FSC.
    reader.SkipBits(1);

    // Next 3 bits must be 1.
    RCHECK(ReadBits(&reader, 3) == 7);

    int block_number = ReadBits(&reader, 8);

    if (section == 0) {  // Header.
      // Validate the reserved bits in the next 8 bytes.
      reader.SkipBits(1);
      RCHECK(ReadBits(&reader, 1) == 0);
      RCHECK(ReadBits(&reader, 11) == 0x7ff);
      reader.SkipBits(4);
      RCHECK(ReadBits(&reader, 4) == 0xf);
      reader.SkipBits(4);
      RCHECK(ReadBits(&reader, 4) == 0xf);
      reader.SkipBits(4);
      RCHECK(ReadBits(&reader, 4) == 0xf);
      reader.SkipBits(3);
      RCHECK(ReadBits(&reader, 24) == 0xffffff);
      current_sequence_number = sequence_number;
      for (size_t i = 0; i < arraysize(last_block_number); ++i)
        last_block_number[i] = -1;
    } else {
      // Sequence number must match (this will also fail if no header seen).
      RCHECK(sequence_number == current_sequence_number);
      // Block number should be increasing.
      RCHECK(block_number > last_block_number[section]);
      last_block_number[section] = block_number;
    }

    // Move to next block.
    offset += 80;
  }
  return true;
}


// Checks for a GSM container.
static bool CheckGsm(const uint8* buffer, int buffer_size) {
  // Reference: ETSI EN 300 961 V8.1.1
  // (http://www.etsi.org/deliver/etsi_en/300900_300999/300961/08.01.01_60/en_300961v080101p.pdf)
  // also http://tools.ietf.org/html/rfc3551#page-24
  // GSM files have a 33 byte block, only first 4 bits are fixed.
  RCHECK(buffer_size >= 1024);  // Need enough data to do a decent check.

  int offset = 0;
  while (offset < buffer_size) {
    // First 4 bits of each block are xD.
    RCHECK((buffer[offset] & 0xf0) == 0xd0);
    offset += 33;
  }
  return true;
}

// Advance to the first set of |num_bits| bits that match |start_code|. |offset|
// is the current location in the buffer, and is updated. |bytes_needed| is the
// number of bytes that must remain in the buffer when |start_code| is found.
// Returns true if start_code found (and enough space in the buffer after it),
// false otherwise.
static bool AdvanceToStartCode(const uint8* buffer,
                               int buffer_size,
                               int* offset,
                               int bytes_needed,
                               int num_bits,
                               uint32 start_code) {
  DCHECK_GE(bytes_needed, 3);
  DCHECK_LE(num_bits, 24);  // Only supports up to 24 bits.

  // Create a mask to isolate |num_bits| bits, once shifted over.
  uint32 bits_to_shift = 24 - num_bits;
  uint32 mask = (1 << num_bits) - 1;
  while (*offset + bytes_needed < buffer_size) {
    uint32 next = Read24(buffer + *offset);
    if (((next >> bits_to_shift) & mask) == start_code)
      return true;
    ++(*offset);
  }
  return false;
}

// Checks for an H.261 container.
static bool CheckH261(const uint8* buffer, int buffer_size) {
  // Reference: ITU-T Recommendation H.261 (03/1993)
  // (http://www.itu.int/rec/T-REC-H.261-199303-I/en)
  RCHECK(buffer_size > 16);

  int offset = 0;
  bool seen_start_code = false;
  while (true) {
    // Advance to picture_start_code, if there is one.
    if (!AdvanceToStartCode(buffer, buffer_size, &offset, 4, 20, 0x10)) {
      // No start code found (or off end of buffer), so success if
      // there was at least one valid header.
      return seen_start_code;
    }

    // Now verify the block. AdvanceToStartCode() made sure that there are
    // at least 4 bytes remaining in the buffer.
    BitReader reader(buffer + offset, buffer_size - offset);
    RCHECK(ReadBits(&reader, 20) == 0x10);

    // Skip the temporal reference and PTYPE.
    reader.SkipBits(5 + 6);

    // Skip any extra insertion information. Since this is open-ended, if we run
    // out of bits assume that the buffer is correctly formatted.
    int extra = ReadBits(&reader, 1);
    while (extra == 1) {
      if (!reader.SkipBits(8))
        return seen_start_code;
      if (!reader.ReadBits(1, &extra))
        return seen_start_code;
    }

    // Next should be a Group of Blocks start code. Again, if we run out of
    // bits, then assume that the buffer up to here is correct, and the buffer
    // just happened to end in the middle of a header.
    int next;
    if (!reader.ReadBits(16, &next))
      return seen_start_code;
    RCHECK(next == 1);

    // Move to the next block.
    seen_start_code = true;
    offset += 4;
  }
}

// Checks for an H.263 container.
static bool CheckH263(const uint8* buffer, int buffer_size) {
  // Reference: ITU-T Recommendation H.263 (01/2005)
  // (http://www.itu.int/rec/T-REC-H.263-200501-I/en)
  // header is PSC(22b) + TR(8b) + PTYPE(8+b).
  RCHECK(buffer_size > 16);

  int offset = 0;
  bool seen_start_code = false;
  while (true) {
    // Advance to picture_start_code, if there is one.
    if (!AdvanceToStartCode(buffer, buffer_size, &offset, 9, 22, 0x20)) {
      // No start code found (or off end of buffer), so success if
      // there was at least one valid header.
      return seen_start_code;
    }

    // Now verify the block. AdvanceToStartCode() made sure that there are
    // at least 9 bytes remaining in the buffer.
    BitReader reader(buffer + offset, 9);
    RCHECK(ReadBits(&reader, 22) == 0x20);

    // Skip the temporal reference.
    reader.SkipBits(8);

    // Verify that the first 2 bits of PTYPE are 10b.
    RCHECK(ReadBits(&reader, 2) == 2);

    // Skip the split screen indicator, document camera indicator, and full
    // picture freeze release.
    reader.SkipBits(1 + 1 + 1);

    // Verify Source Format.
    int format = ReadBits(&reader, 3);
    RCHECK(format != 0 && format != 6);  // Forbidden or reserved.

    if (format == 7) {
      // Verify full extended PTYPE.
      int ufep = ReadBits(&reader, 3);
      if (ufep == 1) {
        // Verify the optional part of PLUSPTYPE.
        format = ReadBits(&reader, 3);
        RCHECK(format != 0 && format != 7);  // Reserved.
        reader.SkipBits(11);
        // Next 4 bits should be b1000.
        RCHECK(ReadBits(&reader, 4) == 8);  // Not allowed.
      } else {
        RCHECK(ufep == 0);  // Only 0 and 1 allowed.
      }

      // Verify picture type code is not a reserved value.
      int picture_type_code = ReadBits(&reader, 3);
      RCHECK(picture_type_code != 6 && picture_type_code != 7);  // Reserved.

      // Skip picture resampling mode, reduced resolution mode,
      // and rounding type.
      reader.SkipBits(1 + 1 + 1);

      // Next 3 bits should be b001.
      RCHECK(ReadBits(&reader, 3) == 1);  // Not allowed.
    }

    // Move to the next block.
    seen_start_code = true;
    offset += 9;
  }
}

// Checks for an H.264 container.
static bool CheckH264(const uint8* buffer, int buffer_size) {
  // Reference: ITU-T Recommendation H.264 (01/2012)
  // (http://www.itu.int/rec/T-REC-H.264)
  // Section B.1: Byte stream NAL unit syntax and semantics.
  RCHECK(buffer_size > 4);

  int offset = 0;
  int parameter_count = 0;
  while (true) {
    // Advance to picture_start_code, if there is one.
    if (!AdvanceToStartCode(buffer, buffer_size, &offset, 4, 24, 1)) {
      // No start code found (or off end of buffer), so success if
      // there was at least one valid header.
      return parameter_count > 0;
    }

    // Now verify the block. AdvanceToStartCode() made sure that there are
    // at least 4 bytes remaining in the buffer.
    BitReader reader(buffer + offset, 4);
    RCHECK(ReadBits(&reader, 24) == 1);

    // Verify forbidden_zero_bit.
    RCHECK(ReadBits(&reader, 1) == 0);

    // Extract nal_ref_idc and nal_unit_type.
    int nal_ref_idc = ReadBits(&reader, 2);
    int nal_unit_type = ReadBits(&reader, 5);

    switch (nal_unit_type) {
      case 5:  // Coded slice of an IDR picture.
        RCHECK(nal_ref_idc != 0);
        break;
      case 6:   // Supplemental enhancement information (SEI).
      case 9:   // Access unit delimiter.
      case 10:  // End of sequence.
      case 11:  // End of stream.
      case 12:  // Filler data.
        RCHECK(nal_ref_idc == 0);
        break;
      case 7:  // Sequence parameter set.
      case 8:  // Picture parameter set.
        ++parameter_count;
        break;
    }

    // Skip the current start_code_prefix and move to the next.
    offset += 4;
  }
}

static const char kHlsSignature[] = "#EXTM3U";
static const char kHls1[] = "#EXT-X-STREAM-INF:";
static const char kHls2[] = "#EXT-X-TARGETDURATION:";
static const char kHls3[] = "#EXT-X-MEDIA-SEQUENCE:";

// Additional checks for a HLS container.
static bool CheckHls(const uint8* buffer, int buffer_size) {
  // HLS is simply a play list used for Apple HTTP Live Streaming.
  // Reference: Apple HTTP Live Streaming Overview
  // (http://goo.gl/MIwxj)

  if (StartsWith(buffer, buffer_size, kHlsSignature)) {
    // Need to find "#EXT-X-STREAM-INF:", "#EXT-X-TARGETDURATION:", or
    // "#EXT-X-MEDIA-SEQUENCE:" somewhere in the buffer. Other playlists (like
    // WinAmp) only have additional lines with #EXTINF
    // (http://en.wikipedia.org/wiki/M3U).
    int offset = strlen(kHlsSignature);
    while (offset < buffer_size) {
      if (buffer[offset] == '#') {
        if (StartsWith(buffer + offset, buffer_size - offset, kHls1) ||
            StartsWith(buffer + offset, buffer_size - offset, kHls2) ||
            StartsWith(buffer + offset, buffer_size - offset, kHls3)) {
          return true;
        }
      }
      ++offset;
    }
  }
  return false;
}

// Checks for a MJPEG stream.
static bool CheckMJpeg(const uint8* buffer, int buffer_size) {
  // Reference: ISO/IEC 10918-1 : 1993(E), Annex B
  // (http://www.w3.org/Graphics/JPEG/itu-t81.pdf)
  RCHECK(buffer_size >= 16);

  int offset = 0;
  int last_restart = -1;
  int num_codes = 0;
  while (offset + 5 < buffer_size) {
    // Marker codes are always a two byte code with the first byte xFF.
    RCHECK(buffer[offset] == 0xff);
    uint8 code = buffer[offset + 1];
    RCHECK(code >= 0xc0 || code == 1);

    // Skip sequences of xFF.
    if (code == 0xff) {
      ++offset;
      continue;
    }

    // Success if the next marker code is EOI (end of image)
    if (code == 0xd9)
      return true;

    // Check remaining codes.
    if (code == 0xd8 || code == 1) {
      // SOI (start of image) / TEM (private use). No other data with header.
      offset += 2;
    } else if (code >= 0xd0 && code <= 0xd7) {
      // RST (restart) codes must be in sequence. No other data with header.
      int restart = code & 0x07;
      if (last_restart >= 0)
        RCHECK(restart == (last_restart + 1) % 8);
      last_restart = restart;
      offset += 2;
    } else {
      // All remaining marker codes are followed by a length of the header.
      int length = Read16(buffer + offset + 2) + 2;

      // Special handling of SOS (start of scan) marker since the entropy
      // coded data follows the SOS. Any xFF byte in the data block must be
      // followed by x00 in the data.
      if (code == 0xda) {
        int number_components = buffer[offset + 4];
        RCHECK(length == 8 + 2 * number_components);

        // Advance to the next marker.
        offset += length;
        while (offset + 2 < buffer_size) {
          if (buffer[offset] == 0xff && buffer[offset + 1] != 0)
            break;
          ++offset;
        }
      } else {
        // Skip over the marker data for the other marker codes.
        offset += length;
      }
    }
    ++num_codes;
  }
  return (num_codes > 1);
}

enum Mpeg2StartCodes {
  PROGRAM_END_CODE = 0xb9,
  PACK_START_CODE = 0xba
};

// Checks for a MPEG2 Program Stream.
static bool CheckMpeg2ProgramStream(const uint8* buffer, int buffer_size) {
  // Reference: ISO/IEC 13818-1 : 2000 (E) / ITU-T Rec. H.222.0 (2000 E).
  RCHECK(buffer_size > 14);

  int offset = 0;
  while (offset + 14 < buffer_size) {
    BitReader reader(buffer + offset, 14);

    // Must start with pack_start_code.
    RCHECK(ReadBits(&reader, 24) == 1);
    RCHECK(ReadBits(&reader, 8) == PACK_START_CODE);

    // Determine MPEG version (MPEG1 has b0010, while MPEG2 has b01).
    int mpeg_version = ReadBits(&reader, 2);
    if (mpeg_version == 0) {
      // MPEG1, 10 byte header
      // Validate rest of version code
      RCHECK(ReadBits(&reader, 2) == 2);
    } else {
      RCHECK(mpeg_version == 1);
    }

    // Skip system_clock_reference_base [32..30].
    reader.SkipBits(3);

    // Verify marker bit.
    RCHECK(ReadBits(&reader, 1) == 1);

    // Skip system_clock_reference_base [29..15].
    reader.SkipBits(15);

    // Verify next marker bit.
    RCHECK(ReadBits(&reader, 1) == 1);

    // Skip system_clock_reference_base [14..0].
    reader.SkipBits(15);

    // Verify next marker bit.
    RCHECK(ReadBits(&reader, 1) == 1);

    if (mpeg_version == 0) {
      // Verify second marker bit.
      RCHECK(ReadBits(&reader, 1) == 1);

      // Skip mux_rate.
      reader.SkipBits(22);

      // Verify next marker bit.
      RCHECK(ReadBits(&reader, 1) == 1);

      // Update offset to be after this header.
      offset += 12;
    } else {
      // Must be MPEG2.
      // Skip program_mux_rate.
      reader.SkipBits(22);

      // Verify pair of marker bits.
      RCHECK(ReadBits(&reader, 2) == 3);

      // Skip reserved.
      reader.SkipBits(5);

      // Update offset to be after this header.
      int pack_stuffing_length = ReadBits(&reader, 3);
      offset += 14 + pack_stuffing_length;
    }

    // Check for system headers and PES_packets.
    while (offset + 6 < buffer_size && Read24(buffer + offset) == 1) {
      // Next 8 bits determine stream type.
      int stream_id = buffer[offset + 3];

      // Some stream types are reserved and shouldn't occur.
      if (mpeg_version == 0)
        RCHECK(stream_id != 0xbc && stream_id < 0xf0);
      else
        RCHECK(stream_id != 0xfc && stream_id != 0xfd && stream_id != 0xfe);

      // Some stream types are used for pack headers.
      if (stream_id == PACK_START_CODE)  // back to outer loop.
        break;
      if (stream_id == PROGRAM_END_CODE)  // end of stream.
        return true;

      int pes_length = Read16(buffer + offset + 4);
      RCHECK(pes_length > 0);
      offset = offset + 6 + pes_length;
    }
  }
  // Success as we are off the end of the buffer and liked everything
  // in the buffer.
  return true;
}

const uint8 kMpeg2SyncWord = 0x47;

// Checks for a MPEG2 Transport Stream.
static bool CheckMpeg2TransportStream(const uint8* buffer, int buffer_size) {
  // Spec: ISO/IEC 13818-1 : 2000 (E) / ITU-T Rec. H.222.0 (2000 E).
  // Normal packet size is 188 bytes. However, some systems add various error
  // correction data at the end, resulting in packet of length 192/204/208
  // (https://en.wikipedia.org/wiki/MPEG_transport_stream). Determine the
  // length with the first packet.
  RCHECK(buffer_size >= 250);  // Want more than 1 packet to check.

  int offset = 0;
  int packet_length = -1;
  while (buffer[offset] != kMpeg2SyncWord && offset < 20) {
    // Skip over any header in the first 20 bytes.
    ++offset;
  }

  while (offset + 6 < buffer_size) {
    BitReader reader(buffer + offset, 6);

    // Must start with sync byte.
    RCHECK(ReadBits(&reader, 8) == kMpeg2SyncWord);

    // Skip transport_error_indicator, payload_unit_start_indicator, and
    // transport_priority.
    reader.SkipBits(1 + 1 + 1);

    // Verify the pid is not a reserved value.
    int pid = ReadBits(&reader, 13);
    RCHECK(pid < 3 || pid > 15);

    // Skip transport_scrambling_control.
    reader.SkipBits(2);

    // Adaptation_field_control can not be 0.
    int adaptation_field_control = ReadBits(&reader, 2);
    RCHECK(adaptation_field_control != 0);

    // If there is an adaptation_field, verify it.
    if (adaptation_field_control >= 2) {
      // Skip continuity_counter.
      reader.SkipBits(4);

      // Get adaptation_field_length and verify it.
      int adaptation_field_length = ReadBits(&reader, 8);
      if (adaptation_field_control == 2)
        RCHECK(adaptation_field_length == 183);
      else
        RCHECK(adaptation_field_length <= 182);
    }

    // Attempt to determine the packet length on the first packet.
    if (packet_length < 0) {
      if (buffer[offset + 188] == kMpeg2SyncWord)
        packet_length = 188;
      else if (buffer[offset + 192] == kMpeg2SyncWord)
        packet_length = 192;
      else if (buffer[offset + 204] == kMpeg2SyncWord)
        packet_length = 204;
      else
        packet_length = 208;
    }
    offset += packet_length;
  }
  return true;
}

enum Mpeg4StartCodes {
  VISUAL_OBJECT_SEQUENCE_START_CODE = 0xb0,
  VISUAL_OBJECT_SEQUENCE_END_CODE = 0xb1,
  VISUAL_OBJECT_START_CODE = 0xb5,
  VOP_START_CODE = 0xb6
};

// Checks for a raw MPEG4 bitstream container.
static bool CheckMpeg4BitStream(const uint8* buffer, int buffer_size) {
  // Defined in ISO/IEC 14496-2:2001.
  // However, no length ... simply scan for start code values.
  // Note tags are very similar to H.264.
  RCHECK(buffer_size > 4);

  int offset = 0;
  int sequence_start_count = 0;
  int sequence_end_count = 0;
  int visual_object_count = 0;
  int vop_count = 0;
  while (true) {
    // Advance to start_code, if there is one.
    if (!AdvanceToStartCode(buffer, buffer_size, &offset, 6, 24, 1)) {
      // Not a complete sequence in memory, so return true if we've seen a
      // visual_object_sequence_start_code and a visual_object_start_code.
      return (sequence_start_count > 0 && visual_object_count > 0);
    }

    // Now verify the block. AdvanceToStartCode() made sure that there are
    // at least 6 bytes remaining in the buffer.
    BitReader reader(buffer + offset, 6);
    RCHECK(ReadBits(&reader, 24) == 1);

    int start_code = ReadBits(&reader, 8);
    RCHECK(start_code < 0x30 || start_code > 0xaf);  // 30..AF and
    RCHECK(start_code < 0xb7 || start_code > 0xb9);  // B7..B9 reserved

    switch (start_code) {
      case VISUAL_OBJECT_SEQUENCE_START_CODE: {
        ++sequence_start_count;
        // Verify profile in not one of many reserved values.
        int profile = ReadBits(&reader, 8);
        RCHECK(profile > 0);
        RCHECK(profile < 0x04 || profile > 0x10);
        RCHECK(profile < 0x13 || profile > 0x20);
        RCHECK(profile < 0x23 || profile > 0x31);
        RCHECK(profile < 0x35 || profile > 0x41);
        RCHECK(profile < 0x43 || profile > 0x60);
        RCHECK(profile < 0x65 || profile > 0x70);
        RCHECK(profile < 0x73 || profile > 0x80);
        RCHECK(profile < 0x83 || profile > 0x90);
        RCHECK(profile < 0x95 || profile > 0xa0);
        RCHECK(profile < 0xa4 || profile > 0xb0);
        RCHECK(profile < 0xb5 || profile > 0xc0);
        RCHECK(profile < 0xc3 || profile > 0xd0);
        RCHECK(profile < 0xe4);
        break;
      }

      case VISUAL_OBJECT_SEQUENCE_END_CODE:
        RCHECK(++sequence_end_count == sequence_start_count);
        break;

      case VISUAL_OBJECT_START_CODE: {
        ++visual_object_count;
        if (ReadBits(&reader, 1) == 1) {
          int visual_object_verid = ReadBits(&reader, 4);
          RCHECK(visual_object_verid > 0 && visual_object_verid < 3);
          RCHECK(ReadBits(&reader, 3) != 0);
        }
        int visual_object_type = ReadBits(&reader, 4);
        RCHECK(visual_object_type > 0 && visual_object_type < 6);
        break;
      }

      case VOP_START_CODE:
        RCHECK(++vop_count <= visual_object_count);
        break;
    }
    // Skip this block.
    offset += 6;
  }
}

// Additional checks for a MOV/QuickTime/MPEG4 container.
static bool CheckMov(const uint8* buffer, int buffer_size) {
  // Reference: ISO/IEC 14496-12:2005(E).
  // (http://standards.iso.org/ittf/PubliclyAvailableStandards/c061988_ISO_IEC_14496-12_2012.zip)
  RCHECK(buffer_size > 8);

  int offset = 0;
  while (offset + 8 < buffer_size) {
    int atomsize = Read32(buffer + offset);
    uint32 atomtype = Read32(buffer + offset + 4);
    // Only need to check for ones that are valid at the top level.
    switch (atomtype) {
      case TAG('f','t','y','p'):
      case TAG('p','d','i','n'):
      case TAG('m','o','o','v'):
      case TAG('m','o','o','f'):
      case TAG('m','f','r','a'):
      case TAG('m','d','a','t'):
      case TAG('f','r','e','e'):
      case TAG('s','k','i','p'):
      case TAG('m','e','t','a'):
      case TAG('m','e','c','o'):
      case TAG('s','t','y','p'):
      case TAG('s','i','d','x'):
      case TAG('s','s','i','x'):
      case TAG('p','r','f','t'):
      case TAG('b','l','o','c'):
        break;
      default:
        return false;
    }
    if (atomsize == 1) {
      // Indicates that the length is the next 64bits.
      if (offset + 16 > buffer_size)
        break;
      if (Read32(buffer + offset + 8) != 0)
        break;  // Offset is way past buffer size.
      atomsize = Read32(buffer + offset + 12);
    }
    if (atomsize <= 0)
      break;  // Indicates the last atom or length too big.
    offset += atomsize;
  }
  return true;
}

enum MPEGVersion {
  VERSION_25 = 0,
  VERSION_RESERVED,
  VERSION_2,
  VERSION_1
};
enum MPEGLayer {
  L_RESERVED = 0,
  LAYER_3,
  LAYER_2,
  LAYER_1
};

static int kSampleRateTable[4][4] = { { 11025, 12000, 8000, 0 },   // v2.5
                                      { 0, 0, 0, 0 },              // not used
                                      { 22050, 24000, 16000, 0 },  // v2
                                      { 44100, 48000, 32000, 0 }   // v1
};

static int kBitRateTableV1L1[16] = { 0, 32, 64, 96, 128, 160, 192, 224, 256,
                                     288, 320, 352, 384, 416, 448, 0 };
static int kBitRateTableV1L2[16] = { 0, 32, 48, 56, 64, 80, 96, 112, 128, 160,
                                     192, 224, 256, 320, 384, 0 };
static int kBitRateTableV1L3[16] = { 0, 32, 40, 48, 56, 64, 80, 96, 112, 128,
                                     160, 192, 224, 256, 320, 0 };
static int kBitRateTableV2L1[16] = { 0, 32, 48, 56, 64, 80, 96, 112, 128, 144,
                                     160, 176, 192, 224, 256, 0 };
static int kBitRateTableV2L23[16] = { 0, 8, 16, 24, 32, 40, 48, 56, 64, 80, 96,
                                      112, 128, 144, 160, 0 };

static bool ValidMpegAudioFrameHeader(const uint8* header,
                                      int header_size,
                                      int* framesize) {
  // Reference: http://mpgedit.org/mpgedit/mpeg_format/mpeghdr.htm.
  DCHECK_GE(header_size, 4);
  *framesize = 0;
  BitReader reader(header, 4);  // Header can only be 4 bytes long.

  // Verify frame sync (11 bits) are all set.
  RCHECK(ReadBits(&reader, 11) == 0x7ff);

  // Verify MPEG audio version id.
  int version = ReadBits(&reader, 2);
  RCHECK(version != 1);  // Reserved.

  // Verify layer.
  int layer = ReadBits(&reader, 2);
  RCHECK(layer != 0);

  // Skip protection bit.
  reader.SkipBits(1);

  // Verify bitrate index.
  int bitrate_index = ReadBits(&reader, 4);
  RCHECK(bitrate_index != 0xf);

  // Verify sampling rate frequency index.
  int sampling_index = ReadBits(&reader, 2);
  RCHECK(sampling_index != 3);

  // Get padding bit.
  int padding = ReadBits(&reader, 1);

  // Frame size:
  // For Layer I files = (12 * BitRate / SampleRate + Padding) * 4
  // For others = 144 * BitRate / SampleRate + Padding
  // Unfortunately, BitRate and SampleRate are coded.
  int sampling_rate = kSampleRateTable[version][sampling_index];
  int bitrate;
  if (version == VERSION_1) {
    if (layer == LAYER_1)
      bitrate = kBitRateTableV1L1[bitrate_index];
    else if (layer == LAYER_2)
      bitrate = kBitRateTableV1L2[bitrate_index];
    else
      bitrate = kBitRateTableV1L3[bitrate_index];
  } else {
    if (layer == LAYER_1)
      bitrate = kBitRateTableV2L1[bitrate_index];
    else
      bitrate = kBitRateTableV2L23[bitrate_index];
  }
  if (layer == LAYER_1)
    *framesize = ((12000 * bitrate) / sampling_rate + padding) * 4;
  else
    *framesize = (144000 * bitrate) / sampling_rate + padding;
  return (bitrate > 0 && sampling_rate > 0);
}

// Extract a size encoded the MP3 way.
static int GetMp3HeaderSize(const uint8* buffer, int buffer_size) {
  DCHECK_GE(buffer_size, 9);
  int size = ((buffer[6] & 0x7f) << 21) + ((buffer[7] & 0x7f) << 14) +
             ((buffer[8] & 0x7f) << 7) + (buffer[9] & 0x7f) + 10;
  if (buffer[5] & 0x10)  // Footer added?
    size += 10;
  return size;
}

// Additional checks for a MP3 container.
static bool CheckMp3(const uint8* buffer, int buffer_size, bool seenHeader) {
  RCHECK(buffer_size >= 10);  // Must be enough to read the initial header.

  int framesize;
  int numSeen = 0;
  int offset = 0;
  if (seenHeader) {
    offset = GetMp3HeaderSize(buffer, buffer_size);
  } else {
    // Skip over leading 0's.
    while (offset < buffer_size && buffer[offset] == 0)
      ++offset;
  }

  while (offset + 3 < buffer_size) {
    RCHECK(ValidMpegAudioFrameHeader(
        buffer + offset, buffer_size - offset, &framesize));

    // Have we seen enough valid headers?
    if (++numSeen > 10)
      return true;
    offset += framesize;
  }
  // Off the end of the buffer, return success if a few valid headers seen.
  return numSeen > 2;
}

// Check that the next characters in |buffer| represent a number. The format
// accepted is optional whitespace followed by 1 or more digits. |max_digits|
// specifies the maximum number of digits to process. Returns true if a valid
// number is found, false otherwise.
static bool VerifyNumber(const uint8* buffer,
                         int buffer_size,
                         int* offset,
                         int max_digits) {
  RCHECK(*offset < buffer_size);

  // Skip over any leading space.
  while (isspace(buffer[*offset])) {
    ++(*offset);
    RCHECK(*offset < buffer_size);
  }

  // Need to process up to max_digits digits.
  int numSeen = 0;
  while (--max_digits >= 0 && isdigit(buffer[*offset])) {
    ++numSeen;
    ++(*offset);
    if (*offset >= buffer_size)
      return true;  // Out of space but seen a digit.
  }

  // Success if at least one digit seen.
  return (numSeen > 0);
}

// Check that the next character in |buffer| is one of |c1| or |c2|. |c2| is
// optional. Returns true if there is a match, false if no match or out of
// space.
static inline bool VerifyCharacters(const uint8* buffer,
                                    int buffer_size,
                                    int* offset,
                                    char c1,
                                    char c2) {
  RCHECK(*offset < buffer_size);
  char c = static_cast<char>(buffer[(*offset)++]);
  return (c == c1 || (c == c2 && c2 != 0));
}

// Checks for a SRT container.
static bool CheckSrt(const uint8* buffer, int buffer_size) {
  // Reference: http://en.wikipedia.org/wiki/SubRip
  RCHECK(buffer_size > 20);

  // First line should just be the subtitle sequence number.
  int offset = StartsWith(buffer, buffer_size, UTF8_BYTE_ORDER_MARK) ? 3 : 0;
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 100));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, '\n', '\r'));

  // Skip any additional \n\r.
  while (VerifyCharacters(buffer, buffer_size, &offset, '\n', '\r')) {}
  --offset;  // Since VerifyCharacters() gobbled up the next non-CR/LF.

  // Second line should look like the following:
  //   00:00:10,500 --> 00:00:13,000
  // Units separator can be , or .
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 100));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ':', 0));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 2));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ':', 0));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 2));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ',', '.'));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 3));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ' ', 0));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, '-', 0));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, '-', 0));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, '>', 0));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ' ', 0));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 100));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ':', 0));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 2));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ':', 0));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 2));
  RCHECK(VerifyCharacters(buffer, buffer_size, &offset, ',', '.'));
  RCHECK(VerifyNumber(buffer, buffer_size, &offset, 3));
  return true;
}

// Read a Matroska Element Id.
static int GetElementId(BitReader* reader) {
  // Element ID is coded with the leading zero bits (max 3) determining size.
  // If it is an invalid encoding or the end of the buffer is reached,
  // return -1 as a tag that won't be expected.
  if (reader->bits_available() >= 8) {
    int num_bits_to_read = 0;
    static int prefix[] = { 0x80, 0x4000, 0x200000, 0x10000000 };
    for (int i = 0; i < 4; ++i) {
      num_bits_to_read += 7;
      if (ReadBits(reader, 1) == 1) {
        if (reader->bits_available() < num_bits_to_read)
          break;
        // prefix[] adds back the bits read individually.
        return ReadBits(reader, num_bits_to_read) | prefix[i];
      }
    }
  }
  // Invalid encoding, return something not expected.
  return -1;
}

// Read a Matroska Unsigned Integer (VINT).
static uint64 GetVint(BitReader* reader) {
  // Values are coded with the leading zero bits (max 7) determining size.
  // If it is an invalid coding or the end of the buffer is reached,
  // return something that will go off the end of the buffer.
  if (reader->bits_available() >= 8) {
    int num_bits_to_read = 0;
    for (int i = 0; i < 8; ++i) {
      num_bits_to_read += 7;
      if (ReadBits(reader, 1) == 1) {
        if (reader->bits_available() < num_bits_to_read)
          break;
        return ReadBits(reader, num_bits_to_read);
      }
    }
  }
  // Incorrect format (more than 7 leading 0's) or off the end of the buffer.
  // Since the return value is used as a byte size, return a value that will
  // cause a failure when used.
  return (reader->bits_available() / 8) + 2;
}

// Additional checks for a WEBM container.
static bool CheckWebm(const uint8* buffer, int buffer_size) {
  // Reference: http://www.matroska.org/technical/specs/index.html
  RCHECK(buffer_size > 12);

  BitReader reader(buffer, buffer_size);

  // Verify starting Element Id.
  RCHECK(GetElementId(&reader) == 0x1a45dfa3);

  // Get the header size, and ensure there are enough bits to check.
  int header_size = GetVint(&reader);
  RCHECK(reader.bits_available() / 8 >= header_size);

  // Loop through the header.
  while (reader.bits_available() > 0) {
    int tag = GetElementId(&reader);
    int tagsize = GetVint(&reader);
    switch (tag) {
      case 0x4286:  // EBMLVersion
      case 0x42f7:  // EBMLReadVersion
      case 0x42f2:  // EBMLMaxIdLength
      case 0x42f3:  // EBMLMaxSizeLength
      case 0x4287:  // DocTypeVersion
      case 0x4285:  // DocTypeReadVersion
      case 0xec:    // void
      case 0xbf:    // CRC32
        RCHECK(reader.SkipBits(tagsize * 8));
        break;

      case 0x4282:  // EBMLDocType
        // Need to see "webm" or "matroska" next.
        switch (ReadBits(&reader, 32)) {
          case TAG('w', 'e', 'b', 'm') :
            return true;
          case TAG('m', 'a', 't', 'r') :
            return (ReadBits(&reader, 32) == TAG('o', 's', 'k', 'a'));
        }
        return false;

      default:  // Unrecognized tag
        return false;
    }
  }
  return false;
}

enum VC1StartCodes {
  VC1_FRAME_START_CODE = 0x0d,
  VC1_ENTRY_POINT_START_CODE = 0x0e,
  VC1_SEQUENCE_START_CODE = 0x0f
};

// Checks for a VC1 bitstream container.
static bool CheckVC1(const uint8* buffer, int buffer_size) {
  // Reference: SMPTE 421M
  // (http://standards.smpte.org/content/978-1-61482-555-5/st-421-2006/SEC1.body.pdf)
  // However, no length ... simply scan for start code values.
  // Expect to see SEQ | [ [ ENTRY ] PIC* ]*
  // Note tags are very similar to H.264.

  RCHECK(buffer_size >= 24);

  // First check for Bitstream Metadata Serialization (Annex L)
  if (buffer[0] == 0xc5 &&
      Read32(buffer + 4) == 0x04 &&
      Read32(buffer + 20) == 0x0c) {
    // Verify settings in STRUCT_C and STRUCT_A
    BitReader reader(buffer + 8, 12);

    int profile = ReadBits(&reader, 4);
    if (profile == 0 || profile == 4) {  // simple or main
      // Skip FRMRTQ_POSTPROC, BITRTQ_POSTPROC, and LOOPFILTER.
      reader.SkipBits(3 + 5 + 1);

      // Next bit must be 0.
      RCHECK(ReadBits(&reader, 1) == 0);

      // Skip MULTIRES.
      reader.SkipBits(1);

      // Next bit must be 1.
      RCHECK(ReadBits(&reader, 1) == 1);

      // Skip FASTUVMC, EXTENDED_MV, DQUANT, and VSTRANSFORM.
      reader.SkipBits(1 + 1 + 2 + 1);

      // Next bit must be 0.
      RCHECK(ReadBits(&reader, 1) == 0);

      // Skip OVERLAP, SYNCMARKER, RANGERED, MAXBFRAMES, QUANTIZER, and
      // FINTERPFLAG.
      reader.SkipBits(1 + 1 + 1 + 3 + 2 + 1);

      // Next bit must be 1.
      RCHECK(ReadBits(&reader, 1) == 1);

    } else {
      RCHECK(profile == 12);  // Other profile values not allowed.
      RCHECK(ReadBits(&reader, 28) == 0);
    }

    // Now check HORIZ_SIZE and VERT_SIZE, which must be 8192 or less.
    RCHECK(ReadBits(&reader, 32) <= 8192);
    RCHECK(ReadBits(&reader, 32) <= 8192);
    return true;
  }

  // Buffer isn't Bitstream Metadata, so scan for start codes.
  int offset = 0;
  int sequence_start_code = 0;
  int frame_start_code = 0;
  while (true) {
    // Advance to start_code, if there is one.
    if (!AdvanceToStartCode(buffer, buffer_size, &offset, 5, 24, 1)) {
      // Not a complete sequence in memory, so return true if we've seen a
      // sequence start and a frame start (not checking entry points since
      // they only occur in advanced profiles).
      return (sequence_start_code > 0 && frame_start_code > 0);
    }

    // Now verify the block. AdvanceToStartCode() made sure that there are
    // at least 5 bytes remaining in the buffer.
    BitReader reader(buffer + offset, 5);
    RCHECK(ReadBits(&reader, 24) == 1);

    // Keep track of the number of certain types received.
    switch (ReadBits(&reader, 8)) {
      case VC1_SEQUENCE_START_CODE: {
        ++sequence_start_code;
        switch (ReadBits(&reader, 2)) {
          case 0:  // simple
          case 1:  // main
            RCHECK(ReadBits(&reader, 2) == 0);
            break;
          case 2:  // complex
            return false;
          case 3:  // advanced
            RCHECK(ReadBits(&reader, 3) <= 4);  // Verify level = 0..4
            RCHECK(ReadBits(&reader, 2) == 1);  // Verify colordiff_format = 1
            break;
        }
        break;
      }

      case VC1_ENTRY_POINT_START_CODE:
        // No fields in entry data to check. However, it must occur after
        // sequence header.
        RCHECK(sequence_start_code > 0);
        break;

      case VC1_FRAME_START_CODE:
        ++frame_start_code;
        break;
    }
    offset += 5;
  }
}

// For some formats the signature is a bunch of characters. They are defined
// below. Note that the first 4 characters of the string may be used as a TAG
// in LookupContainerByFirst4. For signatures that contain embedded \0, use
// uint8[].
static const char kAmrSignature[] = "#!AMR";
static const uint8 kAsfSignature[] = { 0x30, 0x26, 0xb2, 0x75, 0x8e, 0x66, 0xcf,
                                       0x11, 0xa6, 0xd9, 0x00, 0xaa, 0x00, 0x62,
                                       0xce, 0x6c };
static const char kAssSignature[] = "[Script Info]";
static const char kAssBomSignature[] = UTF8_BYTE_ORDER_MARK "[Script Info]";
static const uint8 kWtvSignature[] = { 0xb7, 0xd8, 0x00, 0x20, 0x37, 0x49, 0xda,
                                       0x11, 0xa6, 0x4e, 0x00, 0x07, 0xe9, 0x5e,
                                       0xad, 0x8d };

// Attempt to determine the container type from the buffer provided. This is
// a simple pass, that uses the first 4 bytes of the buffer as an index to get
// a rough idea of the container format.
static MediaContainerName LookupContainerByFirst4(const uint8* buffer,
                                                  int buffer_size) {
  // Minimum size that the code expects to exist without checking size.
  if (buffer_size < 12)
    return CONTAINER_UNKNOWN;

  uint32 first4 = Read32(buffer);
  switch (first4) {
    case 0x1a45dfa3:
      if (CheckWebm(buffer, buffer_size))
        return CONTAINER_WEBM;
      break;

    case 0x3026b275:
      if (StartsWith(buffer,
                     buffer_size,
                     kAsfSignature,
                     sizeof(kAsfSignature))) {
        return CONTAINER_ASF;
      }
      break;

    case TAG('#','!','A','M'):
      if (StartsWith(buffer, buffer_size, kAmrSignature))
        return CONTAINER_AMR;
      break;

    case TAG('#','E','X','T'):
      if (CheckHls(buffer, buffer_size))
        return CONTAINER_HLS;
      break;

    case TAG('.','R','M','F'):
      if (buffer[4] == 0 && buffer[5] == 0)
        return CONTAINER_RM;
      break;

    case TAG('.','r','a','\xfd'):
      return CONTAINER_RM;

    case TAG('B','I','K','b'):
    case TAG('B','I','K','d'):
    case TAG('B','I','K','f'):
    case TAG('B','I','K','g'):
    case TAG('B','I','K','h'):
    case TAG('B','I','K','i'):
      if (CheckBink(buffer, buffer_size))
        return CONTAINER_BINK;
      break;

    case TAG('c','a','f','f'):
      if (CheckCaf(buffer, buffer_size))
        return CONTAINER_CAF;
      break;

    case TAG('D','E','X','A'):
      if (buffer_size > 15 &&
          Read16(buffer + 11) <= 2048 &&
          Read16(buffer + 13) <= 2048) {
        return CONTAINER_DXA;
      }
      break;

    case TAG('D','T','S','H'):
      if (Read32(buffer + 4) == TAG('D','H','D','R'))
        return CONTAINER_DTSHD;
      break;

    case 0x64a30100:
    case 0x64a30200:
    case 0x64a30300:
    case 0x64a30400:
    case 0x0001a364:
    case 0x0002a364:
    case 0x0003a364:
      if (Read32(buffer + 4) != 0 && Read32(buffer + 8) != 0)
        return CONTAINER_IRCAM;
      break;

    case TAG('f','L','a','C'):
      return CONTAINER_FLAC;

    case TAG('F','L','V',0):
    case TAG('F','L','V',1):
    case TAG('F','L','V',2):
    case TAG('F','L','V',3):
    case TAG('F','L','V',4):
      if (buffer[5] == 0 && Read32(buffer + 5) > 8)
        return CONTAINER_FLV;
      break;

    case TAG('F','O','R','M'):
      switch (Read32(buffer + 8)) {
        case TAG('A','I','F','F'):
        case TAG('A','I','F','C'):
          return CONTAINER_AIFF;
      }
      break;

    case TAG('M','A','C',' '):
      return CONTAINER_APE;

    case TAG('O','N','2',' '):
      if (Read32(buffer + 8) == TAG('O','N','2','f'))
        return CONTAINER_AVI;
      break;

    case TAG('O','g','g','S'):
      if (buffer[5] <= 7)
        return CONTAINER_OGG;
      break;

    case TAG('R','F','6','4'):
      if (buffer_size > 16 && Read32(buffer + 12) == TAG('d','s','6','4'))
        return CONTAINER_WAV;
      break;

    case TAG('R','I','F','F'):
      switch (Read32(buffer + 8)) {
        case TAG('A','V','I',' '):
        case TAG('A','V','I','X'):
        case TAG('A','V','I','\x19'):
        case TAG('A','M','V',' '):
          return CONTAINER_AVI;
        case TAG('W','A','V','E'):
          return CONTAINER_WAV;
      }
      break;

    case TAG('[','S','c','r'):
      if (StartsWith(buffer, buffer_size, kAssSignature))
        return CONTAINER_ASS;
      break;

    case TAG('\xef','\xbb','\xbf','['):
      if (StartsWith(buffer, buffer_size, kAssBomSignature))
        return CONTAINER_ASS;
      break;

    case 0x7ffe8001:
    case 0xfe7f0180:
    case 0x1fffe800:
    case 0xff1f00e8:
      if (CheckDts(buffer, buffer_size))
        return CONTAINER_DTS;
      break;

    case 0xb7d80020:
      if (StartsWith(buffer,
                     buffer_size,
                     kWtvSignature,
                     sizeof(kWtvSignature))) {
        return CONTAINER_WTV;
      }
      break;
  }

  // Now try a few different ones that look at something other
  // than the first 4 bytes.
  uint32 first3 = first4 & 0xffffff00;
  switch (first3) {
    case TAG('C','W','S',0):
    case TAG('F','W','S',0):
      return CONTAINER_SWF;

    case TAG('I','D','3',0):
      if (CheckMp3(buffer, buffer_size, true))
        return CONTAINER_MP3;
      break;
  }

  // Maybe the first 2 characters are something we can use.
  uint32 first2 = Read16(buffer);
  switch (first2) {
    case kAc3SyncWord:
      if (CheckAc3(buffer, buffer_size))
        return CONTAINER_AC3;
      if (CheckEac3(buffer, buffer_size))
        return CONTAINER_EAC3;
      break;

    case 0xfff0:
    case 0xfff1:
    case 0xfff8:
    case 0xfff9:
      if (CheckAac(buffer, buffer_size))
        return CONTAINER_AAC;
      break;
  }

  // Check if the file is in MP3 format without the header.
  if (CheckMp3(buffer, buffer_size, false))
    return CONTAINER_MP3;

  return CONTAINER_UNKNOWN;
}

// Attempt to determine the container name from the buffer provided.
MediaContainerName DetermineContainer(const uint8* buffer, int buffer_size) {
  DCHECK(buffer);

  // Since MOV/QuickTime/MPEG4 streams are common, check for them first.
  if (CheckMov(buffer, buffer_size))
    return CONTAINER_MOV;

  // Next attempt the simple checks, that typically look at just the
  // first few bytes of the file.
  MediaContainerName result = LookupContainerByFirst4(buffer, buffer_size);
  if (result != CONTAINER_UNKNOWN)
    return result;

  // Additional checks that may scan a portion of the buffer.
  if (CheckMpeg2ProgramStream(buffer, buffer_size))
    return CONTAINER_MPEG2PS;
  if (CheckMpeg2TransportStream(buffer, buffer_size))
    return CONTAINER_MPEG2TS;
  if (CheckMJpeg(buffer, buffer_size))
    return CONTAINER_MJPEG;
  if (CheckDV(buffer, buffer_size))
    return CONTAINER_DV;
  if (CheckH261(buffer, buffer_size))
    return CONTAINER_H261;
  if (CheckH263(buffer, buffer_size))
    return CONTAINER_H263;
  if (CheckH264(buffer, buffer_size))
    return CONTAINER_H264;
  if (CheckMpeg4BitStream(buffer, buffer_size))
    return CONTAINER_MPEG4BS;
  if (CheckVC1(buffer, buffer_size))
    return CONTAINER_VC1;
  if (CheckSrt(buffer, buffer_size))
    return CONTAINER_SRT;
  if (CheckGsm(buffer, buffer_size))
    return CONTAINER_GSM;

  // AC3/EAC3 might not start at the beginning of the stream,
  // so scan for a start code.
  int offset = 1;  // No need to start at byte 0 due to First4 check.
  if (AdvanceToStartCode(buffer, buffer_size, &offset, 4, 16, kAc3SyncWord)) {
    if (CheckAc3(buffer + offset, buffer_size - offset))
      return CONTAINER_AC3;
    if (CheckEac3(buffer + offset, buffer_size - offset))
      return CONTAINER_EAC3;
  }

  return CONTAINER_UNKNOWN;
}

}  // namespace container_names

}  // namespace media