Differentiate missing keys and previously seen keys in dicom tag cache

This change allows the tag cache to record when keys have been
previously accessed but were either missing from the instance or
had empty string values.  In either case, they fileValue method
should return an empty string; with this change the file does not
get re-parsed when the missing/empty value is requested subsequently.

Libs/DICOM/Core/ctkDICOMDatabase.cpp

@@ -62,6 +62,10 @@
 static ctkLogger logger("org.commontk.dicom.DICOMDatabase" );
 //------------------------------------------------------------------------------
 
+// Flag for tag cache to avoid repeated serarches for
+// tags that do no exist.
+static QString TagNotInInstance("__TAG_NOT_IN_INSTANCE__");
+
 //------------------------------------------------------------------------------
 class ctkDICOMDatabasePrivate
 {
@@ -676,6 +680,10 @@ QString ctkDICOMDatabase::headerValue (QString key)
 QString ctkDICOMDatabase::instanceValue(QString sopInstanceUID, QString tag)
 {
   QString value = this->cachedTag(sopInstanceUID, tag);
+  if (value == TagNotInInstance)
+    {
+    return "";
+    }
   if (value != "")
     {
     return value;
@@ -690,6 +698,10 @@ QString ctkDICOMDatabase::instanceValue(const QString sopInstanceUID, const unsi
 {
   QString tag = this->groupElementToTag(group,element);
   QString value = this->cachedTag(sopInstanceUID, tag);
+  if (value == TagNotInInstance)
+    {
+    return "";
+    }
   if (value != "")
     {
     return value;
@@ -714,6 +726,10 @@ QString ctkDICOMDatabase::fileValue(const QString fileName, QString tag)
   this->tagToGroupElement(tag, group, element);
   QString sopInstanceUID = this->instanceForFile(fileName);
   QString value = this->cachedTag(sopInstanceUID, tag);
+  if (value == TagNotInInstance)
+    {
+    return "";
+    }
   if (value != "")
     {
     return value;
@@ -740,6 +756,10 @@ QString ctkDICOMDatabase::fileValue(const QString fileName, const unsigned short
   QString tag = this->groupElementToTag(group, element);
   QString sopInstanceUID = this->instanceForFile(fileName);
   QString value = this->cachedTag(sopInstanceUID, tag);
+  if (value == TagNotInInstance)
+    {
+    return "";
+    }
   if (value != "")
     {
     return value;
@@ -1502,10 +1522,15 @@ bool ctkDICOMDatabase::cacheTag(const QString sopInstanceUID, const QString tag,
       return false;
       }
     }
+  QString valueToInsert(value);
+  if (valueToInsert == "")
+    {
+    valueToInsert = TagNotInInstance;
+    }
   QSqlQuery insertTag( d->TagCacheDatabase );
   insertTag.prepare( "INSERT OR REPLACE INTO TagCache VALUES(:sopInstanceUID, :tag, :value)" );
   insertTag.bindValue(":sopInstanceUID",sopInstanceUID);
   insertTag.bindValue(":tag",tag);
-  insertTag.bindValue(":value",value);
+  insertTag.bindValue(":value",valueToInsert);
   return d->loggedExec(insertTag);
 }