Add UMA stats for audio/video containers.

media/base/container_names.cc

+// 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
+  // (http://goo.gl/Vgb9r)
+  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://goo.gl/FhHrk)
+  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://goo.gl/kMn6p)
+  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://goo.gl/h2VDS)
+  // 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://goo.gl/OWH0Q)
+  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 <= 0)
+      break;  // Indicates the last atom or length too big.
+    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);
+    }
+    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://goo.gl/fLvaE)
+  // 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