TagReaderTagParser: Fix reading and writing rating

ext/libstrawberry-tagreader/tagreaderbase.cpp

@@ -25,3 +25,27 @@ const std::string TagReaderBase::kEmbeddedCover = "(embedded)";
 
 TagReaderBase::TagReaderBase() = default;
 TagReaderBase::~TagReaderBase() = default;
+
+float TagReaderBase::ConvertPOPMRating(const int POPM_rating) {
+
+  if (POPM_rating < 0x01) return 0.0F;
+  else if (POPM_rating < 0x40) return 0.20F;
+  else if (POPM_rating < 0x80) return 0.40F;
+  else if (POPM_rating < 0xC0) return 0.60F;
+  else if (POPM_rating < 0xFC) return 0.80F;
+
+  return 1.0F;
+
+}
+
+int TagReaderBase::ConvertToPOPMRating(const float rating) {
+
+  if (rating < 0.20) return 0x00;
+  else if (rating < 0.40) return 0x01;
+  else if (rating < 0.60) return 0x40;
+  else if (rating < 0.80) return 0x80;
+  else if (rating < 1.0)  return 0xC0;
+
+  return 0xFF;
+
+}