Add a minimum data threshold to CharDistributionAnalysis::GetConfidence() to reduce false positives. Bug 306272, r=jshin, sr=rbs

extensions/universalchardet/src/base/CharDistribution.cpp

@@ -46,11 +46,15 @@
 #define SURE_YES 0.99f
 #define SURE_NO  0.01f
 
+#define MINIMUM_DATA_THRESHOLD  4
+
 //return confidence base on received data
 float CharDistributionAnalysis::GetConfidence()
 { 
-  //if we didn't receive any character in our consideration range, return negative answer
-  if (mTotalChars <= 0)
+  //if we didn't receive any character in our consideration range, or the
+  // number of frequent characters is below the minimum threshold, return
+  // negative answer
+  if (mTotalChars <= 0 || mFreqChars <= MINIMUM_DATA_THRESHOLD)
     return SURE_NO;
 
   if (mTotalChars != mFreqChars) {