libpsdxx [WP]: provide LogIndex to reverse log-scale

src/libs/libpsdxx/helper.h

@@ -43,6 +43,21 @@ namespace psd {
 
   namespace helper {
 
+    /*! Class function to return index on logarithmic scale
+     */
+    class LogIndex 
+    {
+      public:
+        LogIndex(const double& f0,
+                 const double& f1,
+                 const double& f2);
+        double operator()(const double& f);
+      private:
+        double Mn_per_decade;
+        double Mc;
+        double Mdf;
+    }; // class LogIndex
+
   } // namespace helper
 
 } // namespace psd

src/libs/libpsdxx/log_index.cc

@@ -34,11 +34,34 @@
   "PSDXX_LOG_INDEX_CC   V1.0"
 
 #include <psdxx/helper.h>
+#include <psdxx/error.h>
 
 namespace psd {
 
   namespace helper {
 
+    LogIndex::LogIndex(const double& f0,
+                       const double& f1,
+                       const double& f2)
+    {
+      PSDXX_assert(f1>f0, "frequency values must increase");
+      PSDXX_assert(f2>f1, "frequency values must increase");
+      Mdf=f0;
+      double logfac=(f2-f1)/(f1-f0);
+      Mn_per_decade=std::log(10.)/std::log(logfac);
+      Mc=(f1-f0)/(f0*(logfac-1.));
+    } // LogIndex::LogIndex(const double& f0,
+      //                    const double& f1,
+      //                    const double& f2)
+
+
+  /* ---------------------------------------------------------------------- */
+   
+    double LogIndex::operator()(const double& f)
+    {
+     double l =Mn_per_decade*std::log(1.+((f/Mdf)-1.)/Mc)/std::log(10.);
+    } // double LogIndex::operator()(const double& f)
+
   } // namespace helper
 
 } // namespace psd