From: Tsuda Kageyu <tsuda.kageyu@gmail.com>
Date: Thu, 19 Nov 2015 08:29:25 +0000 (+0900)
Subject: Efficient lookup for an ID3v2 tag in a MPEG file.
X-Git-Tag: v1.11beta~124^2
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=e10684312e641f0d4da668a2851e97e174f8abee;p=taglib

Efficient lookup for an ID3v2 tag in a MPEG file.

An ID3v2 tag or MPEG frame is most likely be at the beginning of the file.
---

diff --git a/taglib/mpeg/mpegfile.cpp b/taglib/mpeg/mpegfile.cpp
index c5f9bbf6..8b7af692 100644
--- a/taglib/mpeg/mpegfile.cpp
+++ b/taglib/mpeg/mpegfile.cpp
@@ -489,7 +489,7 @@ void MPEG::File::read(bool readProperties)
 {
   // Look for an ID3v2 tag
 
-  d->ID3v2Location = findID3v2(0);
+  d->ID3v2Location = findID3v2();
 
   if(d->ID3v2Location >= 0) {
 
@@ -532,115 +532,40 @@ void MPEG::File::read(bool readProperties)
   ID3v1Tag(true);
 }
 
-long MPEG::File::findID3v2(long offset)
+long MPEG::File::findID3v2()
 {
-  // This method is based on the contents of TagLib::File::find(), but because
-  // of some subtleties -- specifically the need to look for the bit pattern of
-  // an MPEG sync, it has been modified for use here.
+  if(!isValid())
+    return -1;
 
-  if(isValid() && ID3v2::Header::fileIdentifier().size() <= bufferSize()) {
+  // An ID3v2 tag or MPEG frame is most likely be at the beginning of the file.
 
-    // The position in the file that the current buffer starts at.
+  const ByteVector headerID = ID3v2::Header::fileIdentifier();
 
-    long bufferOffset = 0;
-    ByteVector buffer;
+  seek(0);
 
-    // These variables are used to keep track of a partial match that happens at
-    // the end of a buffer.
+  const ByteVector data = readBlock(headerID.size());
+  if(data.size() < headerID.size())
+    return -1;
 
-    int previousPartialMatch = -1;
-    bool previousPartialSynchMatch = false;
+  if(data == headerID)
+    return 0;
 
-    // Save the location of the current read pointer.  We will restore the
-    // position using seek() before all returns.
+  if(firstSyncByte(data[0]) && secondSynchByte(data[1]))
+    return -1;
 
-    long originalPosition = tell();
+  // Look for the entire file, if neither an MEPG frame or ID3v2 tag was found
+  // at the beginning of the file.
+  // We don't care about the inefficiency of the code, since this is a seldom case.
 
-    // Start the search at the offset.
+  const long tagOffset = find(headerID);
+  if(tagOffset < 0)
+    return -1;
 
-    seek(offset);
+  const long frameOffset = firstFrameOffset();
+  if(frameOffset < tagOffset)
+    return -1;
 
-    // This loop is the crux of the find method.  There are three cases that we
-    // want to account for:
-    // (1) The previously searched buffer contained a partial match of the search
-    // pattern and we want to see if the next one starts with the remainder of
-    // that pattern.
-    //
-    // (2) The search pattern is wholly contained within the current buffer.
-    //
-    // (3) The current buffer ends with a partial match of the pattern.  We will
-    // note this for use in the next iteration, where we will check for the rest
-    // of the pattern.
-
-    for(buffer = readBlock(bufferSize()); buffer.size() > 0; buffer = readBlock(bufferSize())) {
-
-      // (1) previous partial match
-
-      if(previousPartialSynchMatch && secondSynchByte(buffer[0]))
-        return -1;
-
-      if(previousPartialMatch >= 0 && int(bufferSize()) > previousPartialMatch) {
-        const int patternOffset = (bufferSize() - previousPartialMatch);
-        if(buffer.containsAt(ID3v2::Header::fileIdentifier(), 0, patternOffset)) {
-          seek(originalPosition);
-          return offset + bufferOffset - bufferSize() + previousPartialMatch;
-        }
-      }
-
-      // (2) pattern contained in current buffer
-
-      long location = buffer.find(ID3v2::Header::fileIdentifier());
-      if(location >= 0) {
-        seek(originalPosition);
-        return offset + bufferOffset + location;
-      }
-
-      int firstSynchByte = buffer.find(char(uchar(255)));
-
-      // Here we have to loop because there could be several of the first
-      // (11111111) byte, and we want to check all such instances until we find
-      // a full match (11111111 111) or hit the end of the buffer.
-
-      while(firstSynchByte >= 0) {
-
-        // if this *is not* at the end of the buffer
-
-        if(firstSynchByte < int(buffer.size()) - 1) {
-          if(secondSynchByte(buffer[firstSynchByte + 1])) {
-            // We've found the frame synch pattern.
-            seek(originalPosition);
-            return -1;
-          }
-          else {
-
-            // We found 11111111 at the end of the current buffer indicating a
-            // partial match of the synch pattern.  The find() below should
-            // return -1 and break out of the loop.
-
-            previousPartialSynchMatch = true;
-          }
-        }
-
-        // Check in the rest of the buffer.
-
-        firstSynchByte = buffer.find(char(uchar(255)), firstSynchByte + 1);
-      }
-
-      // (3) partial match
-
-      previousPartialMatch = buffer.endsWithPartialMatch(ID3v2::Header::fileIdentifier());
-
-      bufferOffset += bufferSize();
-    }
-
-    // Since we hit the end of the file, reset the status before continuing.
-
-    clear();
-
-    seek(originalPosition);
-  }
-
-  return -1;
+  return tagOffset;
 }
 
 long MPEG::File::findID3v1()
diff --git a/taglib/mpeg/mpegfile.h b/taglib/mpeg/mpegfile.h
index 858a6a5c..8478d093 100644
--- a/taglib/mpeg/mpegfile.h
+++ b/taglib/mpeg/mpegfile.h
@@ -390,7 +390,7 @@ namespace TagLib {
       File &operator=(const File &);
 
       void read(bool readProperties);
-      long findID3v2(long offset);
+      long findID3v2();
       long findID3v1();
       void findAPE();