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) | \

[acolwell GONE FROM CHROMIUM, 2013/05/18 01:22:09]

nit: Is it always safe to shift a uint8 more than 8 bits? Any reason you can't
cast to uint32 instead?

[jrummell, 2013/05/22 18:27:39]

The problem with casting to uint32 is that each value gets sign-extended (and
thus mess up the result). The alternative would be to cast to uint32 and then
AND each value with 0xff.

+     (static_cast<uint8>(c) << 8) | (static_cast<uint8>(d)))
+
+// Helper function to read 2 bytes (16 bits, big endian) from a buffer.
+static uint32 Read16(const uint8* p) {

[acolwell GONE FROM CHROMIUM, 2013/05/18 01:22:09]

I think these ReadXXX methods should just be replaced with BitStream reader
calls. I think that would reduce a bunch of the offset calculations in the
methods that use them.

[jrummell, 2013/05/22 18:27:39]

The problem with that is that most formats have a frame size in the header (that
includes all the optional bits). So I would need to keep track of how many bits
have been read in order to advance to the next header start, which I think
complicates things too much.

+  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];

[acolwell GONE FROM CHROMIUM, 2013/05/18 01:22:09]

It would be nice to see this method rewritten in terms of BitReader as well.

+}
+
+// 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* search) {
+  size_t search_size = strlen(search);
+  return (search_size <= buffer_size &&
+          memcmp(buffer, search, search_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* search,
+                       size_t search_size) {
+  return (search_size <= buffer_size &&
+          memcmp(buffer, search, search_size) == 0);

[xhwang, 2013/05/20 23:24:24]

nit: how about s/search/prefix ?

[jrummell, 2013/05/22 18:27:39]

Done.

+}
+
+// Helper function to read bits from a bit stream.
+static int ReadBits(BitReader* reader, int num_bits) {

[xhwang, 2013/05/20 23:24:24]

I wonder whether it's always more convenient to wrap BitReader::ReadBits like
this (so that we DCHECK the size and return the read value instead of return
boolean). If so, we probably want to change BitReader::ReadBits to do so.

[jrummell, 2013/05/22 18:27:39]

This is used in the MP4 code. They want to fail if the bits aren't in the
stream.

+  DCHECK_GE(reader->bits_available(), num_bits);
+  int value;
+  reader->ReadBits(num_bits, &value);
+  return value;
+}
+
+// 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[].
+#define BYTE_ORDER_MARK      "\xef\xbb\xbf"

[xhwang, 2013/05/20 23:24:24]

drop extra white spaces?

[jrummell, 2013/05/22 18:27:39]

Done.

+
+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[] = BYTE_ORDER_MARK "[Script Info]";
+static const char kHlsSignature[] = "#EXTM3U";

[xhwang, 2013/05/20 23:24:24]

Define consts right before where where it's used. For example, define
kHlsSignature around line 660.

[jrummell, 2013/05/22 18:27:39]

Done.

+static const uint8 kWtvSignature[] = { 0xb7, 0xd8, 0x00, 0x20, 0x37, 0x49, 0xda,
+                                       0x11, 0xa6, 0x4e, 0x00, 0x07, 0xe9, 0x5e,
+                                       0xad, 0x8d };
+
+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 },

[xhwang, 2013/05/20 23:24:24]

nit: drop the ',' at EOL?

[jrummell, 2013/05/22 18:27:39]

Done.

+};
+
+// 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)
+  if (buffer_size < 7)
+    return false;
+
+  int offset = 0;
+  while (offset + 6 < buffer_size) {

[xhwang, 2013/05/20 23:24:24]

It's a bit confusing that we use 6 here but the header is 7 or 9 bytes. I read
the spec and understand why, but can use use 7 here for less confusion? Or add a
comment?

[jrummell, 2013/05/22 18:27:39]

Done.

+    BitReader reader(buffer + offset, 6);
+
+    // Syncword must be 0xfff.
+    if (ReadBits(&reader, 12) != 0xfff)
+      return false;
+
+    // Skip MPEG version.
+    reader.SkipBits(1);
+
+    // Layer is always 0.
+    if (ReadBits(&reader, 2) != 0)
+      return false;
+
+    // Skip protection + profile.
+    reader.SkipBits(1 + 2);
+
+    // Check sampling frequency index.
+    if (ReadBits(&reader, 4) == 15)  // Forbidden.
+      return false;
+
+    // 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);
+    if (size == 0)
+      return false;
+
+    offset += size;
+  }
+  return true;
+}
+
+#define AC3_SYNCWORD 0x0b77

[xhwang, 2013/05/20 23:24:24]

const int instead of macro?

[jrummell, 2013/05/22 18:27:39]

Done.

+
+// 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.
+  if (buffer_size < 6)
+    return false;
+
+  int offset = 0;
+  while (offset + 6 < buffer_size) {
+    BitReader reader(buffer + offset, 6);
+
+    // Check syncinfo.
+    if (ReadBits(&reader, 16) != AC3_SYNCWORD)
+      return false;
+
+    // Skip crc1.
+    reader.SkipBits(16);
+
+    // Verify fscod.
+    int sample_rate_code = ReadBits(&reader, 2);
+    if (sample_rate_code == 3)  // Reserved.

[xhwang, 2013/05/20 23:24:24]

merge the above two lines?

[jrummell, 2013/05/22 18:27:39]

sample_rate_code is used below to determine the frame size.

+      return false;
+
+    // Verify frmsizecod.
+    int frame_size_code = ReadBits(&reader, 6);
+    if (frame_size_code >= 38)  // Undefined.

[xhwang, 2013/05/20 23:24:24]

ditto?

[jrummell, 2013/05/22 18:27:39]

frame_size_code is used below to determine the frame size.

+      return false;
+
+    // Verify bsid.
+    if (ReadBits(&reader, 5) >= 10)  // Normally 8 or 6, 16 used by EAC3.
+      return false;
+
+    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.
+  if (buffer_size < 6)
+    return false;
+
+  int offset = 0;
+  while (offset + 6 < buffer_size) {
+    BitReader reader(buffer + offset, 6);
+
+    // Check syncinfo.
+    if (ReadBits(&reader, 16) != AC3_SYNCWORD)
+      return false;
+
+    // Verify strmtyp.
+    if (ReadBits(&reader, 2) == 3)
+      return false;
+
+    // Skip substreamid.
+    reader.SkipBits(3);
+
+    // Get frmsize. Include syncinfo size and convert to bytes.
+    int frame_size = (ReadBits(&reader, 11) + 1) * 2;
+    if (frame_size < 7)
+      return false;
+
+    // Skip fscod, fscod2, acmod, and lfeon.
+    reader.SkipBits(2 + 2 + 3 + 1);
+
+    // Verify bsid.
+    int bit_stream_id = ReadBits(&reader, 5);
+    if (bit_stream_id < 11 || bit_stream_id > 16)
+      return false;
+
+    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
+  if (buffer_size < 44)
+    return false;
+
+  int frames = Read32LE(buffer + 8);
+  int width = Read32LE(buffer + 20);
+  int height = Read32LE(buffer + 24);
+  int fps = Read32LE(buffer + 28);
+  int den = Read32LE(buffer + 32);
+  int audio_tracks = Read32LE(buffer + 40);
+  return (frames > 0 && fps > 0 && den > 0 && audio_tracks <= 256 &&
+          (width > 0 && width <= 32767) &&
+          (height > 0 && height <= 32767));
+}
+
+// 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)
+  if (buffer_size < 52)
+    return false;
+
+  // mFileVersion should be 1.
+  if (Read16(buffer + 4) != 1)
+    return false;
+  // First chunk should be Audio Description chunk, size 32.
+  if (Read32(buffer + 8) != TAG('d','e','s','c'))
+    return false;
+  if (Read32(buffer + 12) != 0 || Read32(buffer + 16) != 32)
+    return false;
+  // CAFAudioFormat.mSampleRate not 0
+  if (Read32(buffer + 20) == 0 && Read32(buffer + 24) == 0)
+    return false;
+  // CAFAudioFormat.mFormatID not 0
+  if (Read32(buffer + 28) == 0)
+    return false;
+  // CAFAudioFormat.mChannelsPerFrame not 0
+  if (Read32(buffer + 44) == 0)
+    return false;
+  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)

[xhwang, 2013/05/20 23:24:24]

here and elsewhere, URL is not subject to 80-char limitation. 
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Line_L...

[jrummell, 2013/05/22 18:27:39]

Guide says "that line may be longer than 80 characters". However, I will change
it (and see if presubmit allows it).

+  if (buffer_size < 12)
+    return false;
+
+  int offset = 0;
+  while (offset + 11 < buffer_size) {
+    BitReader reader(buffer + offset, 11);

[xhwang, 2013/05/20 23:24:24]

Again, requiring 12 bytes but only parsing 11 bytes is a bit confusing.

[jrummell, 2013/05/22 18:27:39]

Done.

+
+    // Verify sync word.
+    if (ReadBits(&reader, 32) != 0x7ffe8001)
+      return false;
+
+    // Skip frame type and deficit sample count.
+    reader.SkipBits(1 + 5);
+
+    // Verify CRC present flag.
+    if (ReadBits(&reader, 1) != 0)  // CPF must be 0.
+      return false;
+
+    // Verify number of PCM sample blocks.
+    if (ReadBits(&reader, 7) < 5)
+      return false;
+
+    // Verify primary frame byte size.
+    int frame_size = ReadBits(&reader, 14);
+    if (frame_size < 95)
+      return false;
+
+    // Skip audio channel arrangement.
+    reader.SkipBits(6);
+
+    // Verify core audio sampling frequency is an allowed value.
+    int sfreq = ReadBits(&reader, 4);
+    if (!kSamplingFrequencyValid[sfreq])
+      return false;
+
+    // Verify transmission bit rate is valid.
+    if (ReadBits(&reader, 5) > 25)
+      return false;
+
+    // Verify reserved field is 0.
+    if (ReadBits(&reader, 1) != 0)
+      return false;
+
+    // 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.
+    int ext_audio_id = ReadBits(&reader, 3);
+    if (!kExtAudioIdValid[ext_audio_id])
+      return false;
+
+    // Skip extended coding flag and audio sync word insertion flag.
+    reader.SkipBits(1 + 1);
+
+    // Verify low frequency effects flag is an allowed value.
+    if (ReadBits(&reader, 2) == 3)
+      return false;
+
+    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)
+  if (buffer_size < 12)
+    return false;
+
+  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);
+    if (section >= 5)
+      return false;
+
+    // Next bit must be 1.
+    if (ReadBits(&reader, 1) != 1)
+      return false;
+
+    // Skip arbitrary bits.
+    reader.SkipBits(4);
+
+    int sequence_number = ReadBits(&reader, 4);
+
+    // Skip FSC.
+    reader.SkipBits(1);
+
+    // Next 3 bits must be 1.
+    if (ReadBits(&reader, 3) != 7)
+      return false;
+
+    int block_number = ReadBits(&reader, 8);
+
+    if (section == 0) {  // Header.
+      // Validate the reserved bits in the next 8 bytes.
+      reader.SkipBits(1);
+      if (ReadBits(&reader, 1) != 0)
+        return false;
+      if (ReadBits(&reader, 11) != 0x7ff)
+        return false;
+      reader.SkipBits(4);
+      if (ReadBits(&reader, 4) != 0xf)
+        return false;
+      reader.SkipBits(4);
+      if (ReadBits(&reader, 4) != 0xf)
+        return false;
+      reader.SkipBits(4);
+      if (ReadBits(&reader, 4) != 0xf)
+        return false;
+      reader.SkipBits(3);
+      if (ReadBits(&reader, 24) != 0xffffff)
+        return false;
+      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).
+      if (sequence_number != current_sequence_number)
+        return false;
+      // Block number should be increasing.
+      if (block_number <= last_block_number[section])
+        return false;
+      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.
+  if (buffer_size < 1024)  // Need enough data to do a decent check.
+    return false;
+
+  int offset = 0;
+  while (offset < buffer_size) {
+    // First 4 bits of each block are xD.
+    if ((buffer[offset] & 0xf0) != 0xd0)
+      return false;
+    offset = offset + 33;

[xhwang, 2013/05/20 23:24:24]

nit: offset += 33;

[jrummell, 2013/05/22 18:27:39]

Done.

+  }
+  return true;
+}
+
+// Check that value is a picture_start_code for H.261.
+static inline bool IsH261PictureStartCode(uint32 value) {
+  return (value & 0xfffff000) == 0x10000;
+}
+
+// Checks for a H.261 container.

[xhwang, 2013/05/20 23:24:24]

nit: s/a/an

[jrummell, 2013/05/22 18:27:39]

Done.

+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)
+  if (buffer_size < 16)
+    return false;
+
+  // Advance to picture_start_code, if there is one.
+  int offset = 0;
+  uint32 next = 0;
+  while (offset + 4 < buffer_size) {
+    next = Read32(buffer + offset);
+    if (IsH261PictureStartCode(next))
+      break;
+    ++offset;
+  }

[xhwang, 2013/05/20 23:24:24]

Can we move this block into the bit while loop on line 470? It seems duplicate
to have this start code searching code in two places.

[jrummell, 2013/05/22 18:27:39]

Done.

+  // The picture_start_code must exist.
+  if (!IsH261PictureStartCode(next))
+    return false;
+
+  while (offset + 4 < buffer_size) {
+    BitReader reader(buffer + offset, buffer_size - offset);
+
+    // Skip the picture_start_code, temporal reference, and PTYPE.
+    reader.SkipBits(20 + 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 true;
+      if (!reader.ReadBits(1, &extra))
+        return true;
+    }
+
+    // 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.
+    if (!reader.ReadBits(16, &next))
+      return true;
+    if (next != 1)
+      return false;
+
+    // Move to the next picture_start_code.
+    offset += 4;
+    while (offset + 4 < buffer_size) {
+      next = Read32(buffer + offset);
+      if (IsH261PictureStartCode(next))
+        break;
+      ++offset;
+    }
+  }
+  return true;
+}
+
+// Check that value is a picture_start_code for H.263.
+static inline bool IsH263PictureStartCode(uint32 value) {
+  return (value & 0xfffffc00) == 0x8000;
+}
+
+// Checks for a 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).
+  if (buffer_size < 16)
+    return false;
+
+  // Advance to picture_start_code, if there is one.
+  int offset = 0;
+  uint32 next = 0;
+  while (offset + 6 < buffer_size) {
+    next = Read32(buffer + offset);
+    if (IsH263PictureStartCode(next))
+      break;
+    ++offset;
+  }

[xhwang, 2013/05/20 23:24:24]

ditto about duplicate start code searching code.

[jrummell, 2013/05/22 18:27:39]

Done.

+  // The picture_start_code must exist.
+  if (!IsH263PictureStartCode(next))
+    return false;
+
+  while (offset + 9 < buffer_size) {
+    BitReader reader(buffer + offset, 9);
+
+    // Skip the picture start code and temporal reference.
+    reader.SkipBits(22 + 8);
+
+    // Verify that the first 2 bits of PTYPE are 10b.
+    if (ReadBits(&reader, 2) != 2)
+      return false;
+
+    // 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);
+    if (format == 0 || format == 6)  // Forbidden or reserved.
+      return false;
+
+    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);
+        if (format == 0 || format == 7)  // Reserved.
+          return false;
+        reader.SkipBits(11);
+        // Next 4 bits should be b1000.
+        if (ReadBits(&reader, 4) != 8)  // Not allowed.
+          return false;
+      } else if (ufep != 0)  // Only 0 and 1 allowed.
+        return false;
+
+      // Verify picture type code is not a reserved value.
+      int picture_type_code = ReadBits(&reader, 3);
+      if (picture_type_code == 6 || picture_type_code == 7)  // Reserved.
+        return false;
+
+      // Skip picture resampling mode, reduced resolution mode,
+      // and rounding type.
+      reader.SkipBits(1 + 1 + 1);
+
+      // Next 3 bits should be b001.
+      if (ReadBits(&reader, 3) != 1)  // Not allowed.
+        return false;
+    }
+
+    // Move to the next picture_start_code.
+    offset += 4;
+    while (offset + 4 < buffer_size) {
+      next = Read32(buffer + offset);
+      if (IsH263PictureStartCode(next))
+        break;
+      ++offset;
+    }
+  }
+  return true;
+}
+
+// Checks for a 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.
+  if (buffer_size < 4)
+    return false;
+
+  int offset = 0;
+  int parameter_count = 0;
+
+  // Advance to start_code_prefix (x000001), if there is one.
+  while (offset + 4 < buffer_size) {
+    if (Read24(buffer + offset) == 1)
+      break;
+    ++offset;
+  }

[xhwang, 2013/05/20 23:24:24]

ditto. Also can we make all startcode searching code consistent?

[jrummell, 2013/05/22 18:27:39]

Done.

+
+  // The start_code_prefix must exist.
+  if (Read24(buffer + offset) != 1)
+    return false;
+
+  while (offset + 4 < buffer_size) {
+    BitReader reader(buffer + offset, 4);
+
+    // Verify start_code_prefix.
+    if (ReadBits(&reader, 24) != 1)
+      return false;
+
+    // Verify forbidden_zero_bit.
+    if (ReadBits(&reader, 1) != 0)
+      return false;
+
+    // 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.
+        if (nal_ref_idc == 0)
+          return false;
+        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.
+        if (nal_ref_idc != 0)
+          return false;
+        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 += 3;
+    while (offset + 4 < buffer_size && Read24(buffer + offset) != 1)
+      ++offset;
+  }
+
+  return (parameter_count > 0);
+}
+
+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://developer.apple.com/library/ios/#documentation/NetworkingInternet/
+  //         Conceptual/StreamingMediaGuide/Introduction/Introduction.html)
+
+  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)
+  if (buffer_size < 20)
+    return false;
+
+  int offset = 0;
+  int last_restart = -1;
+  while (offset + 5 < buffer_size) {
+    // Marker codes are always a two byte code with the first byte xFF.
+    if (buffer[offset] != 0xff)
+      return false;
+    uint8 code = buffer[offset + 1];
+
+    // Skip sequences of xFF.
+    if (code == 0xff) {
+      ++offset;
+      continue;
+    }
+
+    // Fail if marker code is a reserved value.
+    if (code < 0xc0 && code != 1)
+      return false;
+
+    // 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) {
+        if (restart != (last_restart + 1) % 8)
+          return false;
+      }
+      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 entrophy
+      // 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];
+        if (length != 8 + 2 * number_components)
+          return false;
+
+        // 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;
+      }
+    }
+  }
+  return true;
+}
+
+// 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).
+  if (buffer_size < 14)
+    return false;
+
+  int offset = 0;
+  while (offset + 14 < buffer_size) {
+    BitReader reader(buffer + offset, 14);
+
+    // Must start with pack_start_code.
+    if (ReadBits(&reader, 32) != 0x000001ba)
+      return false;
+
+    // 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
+      if (ReadBits(&reader, 2) != 2)
+        return false;
+    } else if (mpeg_version != 1) {
+      // Not recognized.
+      return false;
+    }
+
+    // Skip system_clock_reference_base [32..30].
+    reader.SkipBits(3);
+
+    // Verify marker bit.
+    if (ReadBits(&reader, 1) != 1)
+      return false;
+
+    // Skip system_clock_reference_base [29..15].
+    reader.SkipBits(15);
+
+    // Verify next marker bit.
+    if (ReadBits(&reader, 1) != 1)
+      return false;
+
+    // Skip system_clock_reference_base [14..0].
+    reader.SkipBits(15);
+
+    // Verify next marker bit.
+    if (ReadBits(&reader, 1) != 1)
+      return false;
+
+    if (mpeg_version == 0) {
+      // Verify second marker bit.
+      if (ReadBits(&reader, 1) != 1)
+        return false;
+
+      // Skip mux_rate.
+      reader.SkipBits(22);
+
+      // Verify next marker bit.
+      if (ReadBits(&reader, 1) != 1)
+        return false;
+
+      // 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.
+      if (ReadBits(&reader, 2) != 3)
+        return false;
+
+      // Skip reserved.
+      reader.SkipBits(5);
+
+      // Update offset to be after this header.
+      int pack_stuffing_length = ReadBits(&reader, 3);
+      offset += 14 + pack_stuffing_length;
+    }
+
+    if (offset + 6 < buffer_size && Read32(buffer + offset) == 0x000001bb) {
+      // If there is a System header, skip it.
+      offset = offset + 6 + Read16(buffer + offset + 4);
+    }
+    // Check for 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) {
+        if (stream_id == 0xbc || stream_id >= 0xf0)
+          return false;
+      } else {
+        if (stream_id == 0xfc || stream_id == 0xfd || stream_id == 0xfe)
+          return false;
+      }
+
+      // Some stream types are used for pack headers.
+      if (stream_id == 0xba || stream_id == 0xbb || stream_id == 0xb9)
+        break;
+
+      int pes_length = Read16(buffer + offset + 4);
+      if (pes_length == 0)
+        return false;
+      offset = offset + 6 + pes_length;
+    }
+  }
+  // Success if we are off the end of the buffer or the stream ends
+  // with the program_end_code.
+  return (offset + 4 > buffer_size || Read32(buffer + offset) == 0x000001b9);
+}
+
+// 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.
+  if (buffer_size < 250)  // Want more than 1 packet to check.
+    return false;
+
+  int offset = 0;
+  int packet_length = -1;
+  while (buffer[offset] != 0x47 && 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.
+    if (ReadBits(&reader, 8) != 0x47)
+      return false;
+
+    // Skip transport_error_indicator, payload_unit_start_indicator, and
+    // transport_priority.A
+    reader.SkipBits(1 + 1 + 1);
+
+    // Verify the pid is not a reserved value.
+    int pid = ReadBits(&reader, 13);
+    if (pid >= 3 && pid <= 15)
+      return false;
+
+    // Skip transport_scrambling_control.
+    reader.SkipBits(2);
+
+    // Adaptation_field_control can not be 0.
+    int adaptation_field_control = ReadBits(&reader, 2);
+    if (adaptation_field_control == 0)
+      return false;
+
+    // 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) {
+        if (adaptation_field_length != 183)
+          return false;
+      } else if (adaptation_field_length > 182)
+        return false;
+    }
+
+    // Attempt to determine the packet length on the first packet.
+    if (packet_length < 0) {
+      if (buffer[offset + 188] == 0x47)
+        packet_length = 188;
+      else if (buffer[offset + 192] == 0x47)
+        packet_length = 192;
+      else if (buffer[offset + 204] == 0x47)
+        packet_length = 204;
+      else
+        packet_length = 208;
+    }
+    offset += packet_length;
+  }
+  return true;
+}
+
+#define VISUAL_OBJECT_SEQUENCE_START_CODE 0xb0
+#define VISUAL_OBJECT_SEQUENCE_END_CODE 0xb1
+#define VISUAL_OBJECT_START_CODE 0xb5
+#define VOP_START_CODE 0xb6

[xhwang, 2013/05/20 23:24:24]

static const int?

[jrummell, 2013/05/22 18:27:39]

Converted to an enum.

+
+// 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.
+  // Expect to see SEQ | VO1 | VOL* | VO2 ...

[xhwang, 2013/05/20 23:24:24]

Are those "..." something to finish?

[jrummell, 2013/05/22 18:27:39]

Removed.

+  // Note tags are very similar to H.264.
+
+  if (buffer_size < 4)
+    return false;
+
+  int offset = 0;
+  int sequence_start_count = 0;
+  int sequence_end_count = 0;
+  int visual_object_count = 0;
+  int vop_count = 0;
+  while (offset + 4 < buffer_size) {
+    int start_code = Read24(buffer + offset);
+    if (start_code == 1) {
+      // Fail if it is a reserved value.
+      offset += 3;
+      if (buffer[offset] >= 0x30 && buffer[offset] <= 0xaf)
+        return false;
+      if (buffer[offset] >= 0xb7 && buffer[offset] <= 0xb9)
+        return false;
+
+      switch (buffer[offset]) {
+        case VISUAL_OBJECT_SEQUENCE_START_CODE:
+          ++sequence_start_count;
+          break;
+        case VISUAL_OBJECT_SEQUENCE_END_CODE:
+          if (++sequence_end_count > sequence_start_count)
+            return false;
+          break;
+        case VISUAL_OBJECT_START_CODE:
+          ++visual_object_count;
+          break;
+        case VOP_START_CODE:
+          if (++vop_count > visual_object_count)
+            return false;
+          break;
+      }
+    }
+    // Start codes can start on any byte boundary.
+    ++offset;
+  }
+  // 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);
+}
+
+// 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)
+  if (buffer_size < 8)
+    return false;
+
+  int offset = 0;
+  while (offset + 8 < buffer_size) {
+    int atomsize = Read32(buffer + offset);
+    uint32 atomtype = Read32(buffer + offset + 4);
+    // Valid atoms from http://www.mp4ra.org/atoms.html.
+    switch (atomtype) {
+      case TAG('a','i','n','f'):
+      case TAG('a','v','c','n'):
+      case TAG('b','l','o','c'):
+      case TAG('b','p','c','c'):
+      case TAG('b','u','f','f'):
+      case TAG('b','x','m','l'):
+      case TAG('c','c','i','d'):
+      case TAG('c','d','e','f'):
+      case TAG('c','m','a','p'):
+      case TAG('c','o','6','4'):
+      case TAG('c','o','l','r'):
+      case TAG('c','r','h','d'):
+      case TAG('c','s','l','g'):
+      case TAG('c','t','t','s'):
+      case TAG('c','v','r','u'):
+      case TAG('d','i','n','f'):
+      case TAG('d','r','e','f'):
+      case TAG('d','s','g','d'):
+      case TAG('d','s','t','g'):
+      case TAG('e','d','t','s'):
+      case TAG('e','l','s','t'):
+      case TAG('f','e','c','i'):
+      case TAG('f','e','c','r'):
+      case TAG('f','i','i','n'):
+      case TAG('f','i','r','e'):
+      case TAG('f','p','a','r'):
+      case TAG('f','r','e','e'):
+      case TAG('f','r','m','a'):
+      case TAG('f','t','y','p'):
+      case TAG('g','i','t','n'):
+      case TAG('g','r','p','i'):
+      case TAG('h','d','l','r'):
+      case TAG('h','m','h','d'):
+      case TAG('h','p','i','x'):
+      case TAG('i','c','n','u'):
+      case TAG('I','D','3','2'):
+      case TAG('i','d','a','t'):
+      case TAG('i','h','d','r'):
+      case TAG('i','i','n','f'):
+      case TAG('i','l','o','c'):
+      case TAG('i','m','i','f'):
+      case TAG('i','n','f','u'):
+      case TAG('i','o','d','s'):
+      case TAG('i','p','h','d'):
+      case TAG('i','p','m','c'):
+      case TAG('i','p','r','o'):
+      case TAG('i','r','e','f'):
+      case TAG('j','P',' ',' '):
+      case TAG('j','p','2','c'):
+      case TAG('j','p','2','h'):
+      case TAG('j','p','2','i'):
+      case TAG('l','r','c','u'):
+      case TAG('m','7','h','d'):
+      case TAG('m','d','a','t'):
+      case TAG('m','d','h','d'):
+      case TAG('m','d','i','a'):
+      case TAG('m','d','r','i'):
+      case TAG('m','e','c','o'):
+      case TAG('m','e','h','d'):
+      case TAG('m','e','r','e'):
+      case TAG('m','e','t','a'):
+      case TAG('m','f','h','d'):
+      case TAG('m','f','r','a'):
+      case TAG('m','f','r','o'):
+      case TAG('m','i','n','f'):
+      case TAG('m','j','h','d'):
+      case TAG('m','o','o','f'):
+      case TAG('m','o','o','v'):
+      case TAG('m','v','c','g'):
+      case TAG('m','v','c','i'):
+      case TAG('m','v','e','x'):
+      case TAG('m','v','h','d'):
+      case TAG('m','v','r','a'):
+      case TAG('n','m','h','d'):
+      case TAG('o','c','h','d'):
+      case TAG('o','d','a','f'):
+      case TAG('o','d','d','a'):
+      case TAG('o','d','h','d'):
+      case TAG('o','d','h','e'):
+      case TAG('o','d','r','b'):
+      case TAG('o','d','r','m'):
+      case TAG('o','d','t','t'):
+      case TAG('o','h','d','r'):
+      case TAG('p','a','d','b'):
+      case TAG('p','a','e','n'):
+      case TAG('p','c','l','r'):
+      case TAG('p','d','i','n'):
+      case TAG('p','i','t','m'):
+      case TAG('r','e','s',' '):
+      case TAG('r','e','s','c'):
+      case TAG('r','e','s','d'):
+      case TAG('s','b','g','p'):
+      case TAG('s','c','h','i'):
+      case TAG('s','c','h','m'):
+      case TAG('s','d','e','p'):
+      case TAG('s','d','h','d'):
+      case TAG('s','d','t','p'):
+      case TAG('s','d','v','p'):
+      case TAG('s','e','g','r'):
+      case TAG('s','e','n','c'):
+      case TAG('s','g','p','d'):
+      case TAG('s','i','d','x'):
+      case TAG('s','i','n','f'):
+      case TAG('s','k','i','p'):
+      case TAG('s','m','h','d'):
+      case TAG('s','r','m','b'):
+      case TAG('s','r','m','c'):
+      case TAG('s','r','p','p'):
+      case TAG('s','t','b','l'):
+      case TAG('s','t','c','o'):
+      case TAG('s','t','d','p'):
+      case TAG('s','t','h','d'):
+      case TAG('s','t','s','c'):
+      case TAG('s','t','s','d'):
+      case TAG('s','t','s','h'):
+      case TAG('s','t','s','s'):
+      case TAG('s','t','s','z'):
+      case TAG('s','t','t','s'):
+      case TAG('s','t','y','p'):
+      case TAG('s','t','z','2'):
+      case TAG('s','u','b','s'):
+      case TAG('s','w','t','c'):
+      case TAG('t','f','a','d'):
+      case TAG('t','f','h','d'):
+      case TAG('t','f','m','a'):
+      case TAG('t','f','r','a'):
+      case TAG('t','i','b','r'):
+      case TAG('t','i','r','i'):
+      case TAG('t','k','h','d'):
+      case TAG('t','r','a','f'):
+      case TAG('t','r','a','k'):
+      case TAG('t','r','e','f'):
+      case TAG('t','r','e','x'):
+      case TAG('t','r','g','r'):
+      case TAG('t','r','i','k'):
+      case TAG('t','r','u','n'):
+      case TAG('u','d','t','a'):
+      case TAG('u','i','n','f'):
+      case TAG('U','I','T','S'):
+      case TAG('u','l','s','t'):
+      case TAG('u','r','l',' '):
+      case TAG('u','u','i','d'):
+      case TAG('v','m','h','d'):
+      case TAG('v','w','d','i'):
+      case TAG('x','m','l',' '):

[xhwang, 2013/05/20 23:24:24]

Up to this point all tags are ordered. But the tags after this point aren't. Any
reason for that?

[jrummell, 2013/05/22 18:27:39]

This is the order on www.mp4ra.org. However, list adjusted to just have
top-level ids in it.

+      case TAG('C','o','d','e'):
+      case TAG('a','l','b','m'):
+      case TAG('a','n','g','l'):
+      case TAG('a','u','t','h'):
+      case TAG('c','l','f','n'):
+      case TAG('c','l','i','d'):
+      case TAG('c','l','s','f'):
+      case TAG('c','m','i','d'):
+      case TAG('c','m','n','m'):
+      case TAG('c','o','l','l'):
+      case TAG('c','p','r','t'):
+      case TAG('d','a','t','e'):
+      case TAG('d','s','c','p'):
+      case TAG('g','n','r','e'):
+      case TAG('h','n','t','i'):
+      case TAG('k','y','w','d'):
+      case TAG('l','o','c','i'):
+      case TAG('m','a','n','u'):
+      case TAG('m','o','d','l'):
+      case TAG('p','e','r','f'):
+      case TAG('r','e','e','l'):
+      case TAG('r','t','n','g'):
+      case TAG('s','c','e','n'):
+      case TAG('s','h','o','t'):
+      case TAG('s','l','n','o'):
+      case TAG('s','t','r','k'):
+      case TAG('t','h','m','b'):
+      case TAG('t','s','e','l'):
+      case TAG('t','i','t','l'):
+      case TAG('u','r','a','t'):
+      case TAG('y','r','r','c'):
+      case TAG('c','l','i','p'):
+      case TAG('c','r','g','n'):
+      case TAG('c','t','a','b'):
+      case TAG('d','c','f','D'):
+      case TAG('e','l','n','g'):
+      case TAG('i','m','a','p'):
+      case TAG('k','m','a','t'):
+      case TAG('l','o','a','d'):
+      case TAG('m','a','t','t'):
+      case TAG('p','n','o','t'):
+      case TAG('w','i','d','e'):
+        break;
+      default:
+        return false;
+    }
+    if (atomsize <= 0)

[xhwang, 2013/05/20 23:24:24]

Since we read |atomsize| first, can be handle it first before reading
|atomtype|? 

[jrummell, 2013/05/22 18:27:39]

Not really. If the size is extended, it follows the |atomtype| anyway.

+      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 {
+  Version25 = 0,
+  v_reserved,
+  Version2,
+  Version1
+};
+enum MPEGlayer {
+  l_reserved = 0,
+  Layer3,
+  Layer2,
+  Layer1
+};

[xhwang, 2013/05/20 23:24:24]

style nit: We use UPPER_CASE_0 for enums in chromium.

[jrummell, 2013/05/22 18:27:39]

Done.

+
+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* framesize) {

[xhwang, 2013/05/20 23:24:24]

It's a bit scary to have a function reading memory without specifying size. Can
we DCHECK or at least comment about this?

[jrummell, 2013/05/22 18:27:39]

Done.

+  // Reference: http://mpgedit.org/mpgedit/mpeg_format/mpeghdr.htm.
+  *framesize = 0;
+  BitReader reader(header, 4);  // Header can only be 4 bytes long.
+
+  // Verify frame sync (11 bits) are all set.
+  if (ReadBits(&reader, 11) != 0x7ff)
+    return false;
+
+  // Verify MPEG audio version id.
+  int version = ReadBits(&reader, 2);
+  if (version == 1)  // Reserved.
+    return false;
+
+  // Verify layer.
+  int layer = ReadBits(&reader, 2);
+  if (layer == 0)
+    return false;
+
+  // Skip protection bit.
+  reader.SkipBits(1);
+
+  // Verify bitrate index.
+  int bitrate_index = ReadBits(&reader, 4);
+  if (bitrate_index == 0xf)
+    return false;
+
+  // Verify sampling rate frequency index.
+  int sampling_index = ReadBits(&reader, 2);
+  if (sampling_index == 3)
+    return false;
+
+  // 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 == Version1) {
+    if (layer == Layer1)
+      bitrate = kBitRateTableV1L1[bitrate_index];
+    else if (layer == Layer2)
+      bitrate = kBitRateTableV1L2[bitrate_index];
+    else
+      bitrate = kBitRateTableV1L3[bitrate_index];
+  } else {
+    if (layer == Layer1)
+      bitrate = kBitRateTableV2L1[bitrate_index];
+    else
+      bitrate = kBitRateTableV2L23[bitrate_index];
+  }
+  if (layer == Layer1)
+    *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 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) {
+  if (buffer_size < 10)
+    return false;
+  int framesize;
+  int numSeen = 0;
+  int offset = 0;
+  if (seenHeader)
+    offset = GetMp3HeaderSize(buffer);
+  while (offset + 3 < buffer_size &&
+         ValidMpegAudioFrameHeader(buffer + offset, &framesize)) {
+    ++numSeen;
+    offset += framesize;
+  }
+  return (numSeen > 10 || offset >= buffer_size);
+}
+
+// Read a Matroska TAG, updating offset to point past it.

[xhwang, 2013/05/20 23:24:24]

I am not sure what a TAG or VTAG type is. Use spec language/terminology so that
it's easier to follow?

[jrummell, 2013/05/22 18:27:39]

Done.

+static int GetVtag(const uint8* buffer, int buffer_size, int* offset) {
+  // Size of the tag is determined the same way as VINT,
+  // but the bit is not removed. Maximum of 4 bytes.
+  if (*offset >= buffer_size)
+    return -1;
+  int remaining = buffer_size - *offset;
+  int result = buffer[*offset];
+  if ((result & 0x80) != 0) {
+    // It is a one byte tag.
+    ++*offset;
+  }
+  else if ((result & 0x40) != 0 && remaining >= 2) {
+    // It is a 2 byte tag.
+    result = Read16(buffer + *offset);
+    *offset += 2;
+  }
+  else if ((result & 0x20) != 0 && remaining >= 3) {
+    // It is a 3 byte tag.
+    result = Read24(buffer + *offset);
+    *offset += 3;
+  }
+  else if (remaining >= 4) {
+    // It is a 4 byte tag.
+    result = Read32(buffer + *offset);
+    *offset += 4;
+  }
+  return result;
+}
+
+// Read a Matroska VINT, updating offset to point past it.
+static int GetVint(const uint8* buffer, int buffer_size, int* offset) {
+  // Length = 1 + [number_of_leading_zero_bits].
+  if (*offset >= buffer_size) {
+    // Return something big so it goes off the end of the buffer.
+    return buffer_size;
+  }
+  int remaining = buffer_size - *offset;
+  int size = 1;
+  int mask = 0x80;
+  uint8 b = buffer[*offset];
+  while (mask > 0 && (b & mask) == 0) {
+    ++size;
+    mask = mask >> 1;
+  }
+
+  // Now that we know the size, use the remaining bits plus
+  // following bytes to get the value.
+  if (size > remaining)
+    return buffer_size;
+  int result = buffer[(*offset)++] & (mask - 1);
+  while (--size > 0)
+    result = (result << 8) | buffer[(*offset)++];
+  return result;
+}
+
+// Additional checks for a WEBM container.
+static bool CheckWebm(const uint8* buffer, int buffer_size) {
+  // Reference: http://www.matroska.org/technical/specs/index.html
+  if (buffer_size < 12)
+    return false;
+
+  int offset = 4;
+  int header_size = GetVint(buffer, buffer_size, &offset);
+  int lastoffset = offset + header_size;
+  if (lastoffset > buffer_size)
+    return false;
+  while (offset < lastoffset) {
+    int tag = GetVtag(buffer, buffer_size, &offset);
+    int tagsize = GetVint(buffer, buffer_size, &offset);
+    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
+        offset += tagsize;
+        break;
+      case 0x4282:  // EBMLDocType
+        return (
+            (tagsize >= 4 &&
+             offset < buffer_size &&
+             StartsWith(buffer + offset, buffer_size - offset, "webm")) ||
+            (tagsize >= 8 &&
+             offset < buffer_size &&
+             StartsWith(buffer + offset, buffer_size - offset, "matroska")));
+      default:
+        // Unrecognized tag
+        return false;
+    }
+  }
+  return false;
+}
+
+#define VC1_SEQUENCE_START_CODE 0x0f
+#define VC1_ENTRY_POINT_START_CODE 0x0e
+#define VC1_FRAME_START_CODE 0x0d

[xhwang, 2013/05/20 23:24:24]

static const int?

[jrummell, 2013/05/22 18:27:39]

Converted to an enum.

+
+// 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.
+
+  if (buffer_size < 24)
+    return false;
+
+  // 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.
+      if (ReadBits(&reader, 1) != 0)
+        return false;
+
+      // Skip MULTIRES.
+      reader.SkipBits(1);
+
+      // Next bit must be 1.
+      if (ReadBits(&reader, 1) != 1)
+        return false;
+
+      // Skip FASTUVMC, EXTENDED_MV, DQUANT, and VSTRANSFORM.
+      reader.SkipBits(1 + 1 + 2 + 1);
+
+      // Next bit must be 0.
+      if (ReadBits(&reader, 1) != 0)
+        return false;
+
+      // Skip OVERLAP, SYNCMARKER, RANGERED, MAXBFRAMES, QUANTIZER, and
+      // FINTERPFLAG.
+      reader.SkipBits(1 + 1 + 1 + 3 + 2 + 1);
+
+      // Next bit must be 1.
+      if (ReadBits(&reader, 1) != 1)
+        return false;
+
+    } else if (profile == 12) {  // advanced
+      if (ReadBits(&reader, 28) != 0)
+        return false;
+    } else  // Other profile values not allowed.
+      return false;
+
+    // Now check HORIZ_SIZE and VERT_SIZE, which must be 8192 or less.
+    if (ReadBits(&reader, 32) > 8192)
+      return false;
+    if (ReadBits(&reader, 32) > 8192)
+      return false;
+  }
+
+  // Buffer isn't Bitstream Metadata, so scan for start codes.
+  int offset = 0;
+  int sequence_start_code = 0;
+  int frame_start_code = 0;
+  while (offset + 5 < buffer_size) {
+    int start_code = Read24(buffer + offset);
+    if (start_code == 1) {
+      offset += 3;
+
+      // Keep track of the number of certain types received.
+      switch (buffer[offset]) {
+        case VC1_SEQUENCE_START_CODE: {
+          ++sequence_start_code;
+
+          // Verify some of the information in the header.
+          BitReader reader(buffer + offset, 2);
+          int profile = ReadBits(&reader, 2);
+          switch (profile) {
+            case 0:  // simple
+            case 1:  // main
+              if (ReadBits(&reader, 2) != 0)
+                return false;
+              break;
+            case 2:  // complex
+              return false;
+            case 3:  // advanced
+              // Verify level = 0..4
+              if (ReadBits(&reader, 3) > 4)
+                return false;
+
+              // Verify colordiff_format = 1
+              if (ReadBits(&reader, 2) != 1)
+                return false;
+              break;
+          }
+        } break;

[xhwang, 2013/05/20 23:24:24]

nit: Put break on a separate line.
nit: Here and below, empty line after "break"?

[jrummell, 2013/05/22 18:27:39]

Done.

+        case VC1_ENTRY_POINT_START_CODE:
+          // No fields in entry data to check. However, it must occur after
+          // sequence header.
+          if (sequence_start_code == 0)
+            return false;
+          break;
+        case VC1_FRAME_START_CODE:
+          ++frame_start_code;
+          break;
+      }
+    }
+    // Start codes can start on any byte boundary.
+    ++offset;
+  }
+  // Not a complete sequence in memory, so return true if we've seen a
+  // sequence start (since entry points only occur in advanced profiles).
+  return (sequence_start_code > 0 && frame_start_code > 0);
+}
+
+// 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 FFmpegContainerName LookupContainerByFirst4(const uint8* buffer,
+                                                   int buffer_size) {

[xhwang, 2013/05/20 23:24:24]

s/First4/First4Bytes

It looks like we need at least 12 bytes for this function. Why it's called
"First4"?

[jrummell, 2013/05/22 18:27:39]

Uses the first 4 bytes as an index. Since second4 and third4 are rarely used,
they have been removed.

+  // Minimum size that the code expects to exist without checking size.
+  if (buffer_size < 12)
+    return CONTAINER_UNKNOWN;
+
+  uint32 first4 = Read32(buffer);
+  uint32 second4 = Read32(buffer + 4);
+  uint32 third4 = Read32(buffer + 8);
+
+  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 (second4 == TAG('D','H','D','R'))
+        return CONTAINER_DTSHD;
+      break;
+    case TAG('\x64','\xa3','\x01','\x00'):

[xhwang, 2013/05/20 23:24:24]

s/'\x64'/0x64?

[jrummell, 2013/05/22 18:27:39]

Simplified even farther.

+    case TAG('\x64','\xa3','\x02','\x00'):
+    case TAG('\x64','\xa3','\x03','\x00'):
+    case TAG('\x64','\xa3','\x04','\x00'):
+    case TAG('\x00','\x01','\xa3','\x64'):
+    case TAG('\x00','\x02','\xa3','\x64'):
+    case TAG('\x00','\x03','\xa3','\x64'):
+      if (second4 != 0 && third4 != 0)
+        return CONTAINER_IRCAM;
+      break;
+    case TAG('f','L','a','C'):
+      return CONTAINER_FLAC;
+    case TAG('F','L','V','\x00'):
+    case TAG('F','L','V','\x01'):
+    case TAG('F','L','V','\x02'):
+    case TAG('F','L','V','\x03'):
+    case TAG('F','L','V','\x04'):
+      if (buffer[5] == 0 && Read32(buffer + 5) > 8)
+        return CONTAINER_FLV;
+      break;
+    case TAG('F','O','R','M'):
+      switch (third4) {
+        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 (third4 == TAG('O','N','2','f'))
+        return CONTAINER_AVI;
+    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 (third4) {
+        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;
+    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 AC3_SYNCWORD:
+      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;
+
+  // Skip over starting 0's, and see if it is MP3.
+  if (buffer[0] == 0) {
+    int offset = 1;
+    int remaining = buffer_size - 1;
+    while (remaining > 0 && buffer[offset] == 0) {
+      ++offset;
+      --remaining;
+    }
+    if (remaining > 4) {
+      if (CheckMp3(buffer + offset, remaining, false)) {
+        return CONTAINER_MP3;
+      }
+    }
+  }
+
+  return CONTAINER_UNKNOWN;
+}
+
+// Helper function to do limited scanf functionality without going off
+// the end of the buffer.
+static bool StringFormatCompare(const uint8* buffer,
+                                int buffer_size,
+                                const char* format) {
+  // This function only supports the following items in the format string:
+  //   %<maxlength>d   -- integer (can start with +/-, maxlength is optional)
+  //   %c              -- any single character
+  //   %[<characters>] -- character must be one of the set
+  //                   -- everything else is a literal
+  // This code assumes that format is correct.
+  int offset = 0;
+  while (offset < buffer_size) {
+    // Determine next format item.
+    if (*format == '\0') {
+      // End of format string, success.
+      return true;
+    } else if (*format != '%') {
+      // Not a specifier, so it must match exactly.
+      if (buffer[offset] != *format)
+        return false;
+      ++offset;
+    } else {
+      ++format;
+      int maxLength = 0;
+      int numSeen = 0;
+      while (isdigit(*format)) {
+        maxLength = maxLength * 10 + (*format - '0');
+        ++format;
+      }
+      if (maxLength < 1)  // If not specified set it to 100 to simplify below.
+        maxLength = 100;
+      switch (*format) {
+        case 'c':
+          ++offset;  // Don't care what the character is.
+          break;
+        case 'd':
+          while (offset < buffer_size && isspace(buffer[offset]))
+            ++offset;
+          if (offset >= buffer_size)
+            return false;
+          if (buffer[offset] == '+' || buffer[offset] == '-')
+            ++offset;
+          // Need to process up to maxLength digits.
+          while (offset < buffer_size &&
+                 --maxLength >= 0 &&
+                 isdigit(buffer[offset])) {
+            ++numSeen;
+            ++offset;
+          }
+          if (numSeen == 0)  // No digits, so it is not a match for %d.
+            return false;
+          break;
+        case '[':
+          ++format;  // Skip [.
+          while (*format != ']') {
+            if (buffer[offset] == *format) {
+              ++numSeen;
+            }
+            ++format;
+          }
+          if (numSeen == 0)  // No character match.
+            return false;
+          ++offset;  // Skip the character matched.
+          break;
+        default:
+          NOTREACHED();
+      }
+    }
+    // Move to the next format specification.
+    ++format;
+  }
+  // Out of buffer, so it doesn't match.
+  return false;
+}
+
+// Attempt to determine the container type by scanning for a set of strings,
+// line by line (delimityed by \n or \r).
+static FFmpegContainerName LookupContainerByStringLine(const uint8* buffer,

[xhwang, 2013/05/20 23:24:24]

Can we rename this to something like CheckSrt to be consistent with other
CheckXxx functions?

[jrummell, 2013/05/22 18:27:39]

Done.

+                                                       int buffer_size) {
+  int offset = StartsWith(buffer, buffer_size, BYTE_ORDER_MARK) ? 3 : 0;
+  int lines = 0;
+
+  while (offset < buffer_size && lines < 3) {
+    const uint8* ptr = buffer + offset;
+    int remaining = buffer_size - offset;;
+
+    if (isdigit(*ptr)) {
+      if (StringFormatCompare(
+              ptr, remaining, "%d:%2d:%2d%[,.]%3d --> %d:%2d:%2d%[,.]%3d")) {

[xhwang, 2013/05/20 23:24:24]

The function StringFormatCompare() looks cool! But I wonder, since we only use
it here for SRT parsing, can we hard-code the parsing instead of using a generic
function to make the parsing simpler?

[jrummell, 2013/05/22 18:27:39]

Done.

+        return CONTAINER_SRT;
+      }
+    }
+
+    // Find the end of the line.
+    while (offset < buffer_size &&
+           buffer[offset] != '\n' && buffer[offset] != '\r') {
+      ++offset;
+    }
+
+    // Skip the \n\r.
+    while (offset < buffer_size &&
+           (buffer[offset] == '\n' || buffer[offset] == '\r')) {
+      ++offset;
+    }
+    ++lines;
+  }
+
+  // Didn't find a string match for any of the formats.
+  return CONTAINER_UNKNOWN;
+}
+
+// Attempt to determine the container name from the buffer provided.
+FFmpegContainerName 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.
+  FFmpegContainerName 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 (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 (CheckMJpeg(buffer, buffer_size))
+    return CONTAINER_MJPEG;
+  if (CheckVC1(buffer, buffer_size))
+    return CONTAINER_VC1;
+
+  // 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.
+  while (offset + 2 < buffer_size && Read16(buffer + offset) != AC3_SYNCWORD)
+    ++offset;
+  if (Read16(buffer + offset) == AC3_SYNCWORD) {
+    if (CheckAc3(buffer + offset, buffer_size - offset))
+      return CONTAINER_AC3;
+    if (CheckEac3(buffer + offset, buffer_size - offset))
+      return CONTAINER_EAC3;
+  }
+
+  // Scan for strings in the buffer.
+  result = LookupContainerByStringLine(buffer, buffer_size);
+  if (result != CONTAINER_UNKNOWN)
+    return result;
+
+  // GSM files have very limited fixed data, so check for them last
+  if (CheckGsm(buffer, buffer_size))
+    return CONTAINER_GSM;
+
+  return CONTAINER_UNKNOWN;
+}
+
+}  // namespace container_names
+
+}  // namespace media