Plugins: Common String Helpers (Splash)

Adds ArrayOfString helpers for libSplash write calls.
See
  ComputationalRadiationPhysics/libSplash#220
for the description of the problem.

If a user wants to write an array of strings they need to be padded
to the longest one. This helper does that.

Use it like this:
```C++

// [...]
helper::GetSplashArrayOfString getSplashArrayOfString;
helper::GetSplashArrayOfString::Result myArrOfStr;

std::list<std::string> myListOfStr;
myListOfStr.push_back("short");
myListOfStr.push_back("middle");
myListOfStr.push_back("loooong");
myArrOfStr = getSplashArrayOfString( myListOfStr );
```

Also adds a `helper::` namespace to avoid polluting the `PIConGPU`
namespace.

src/picongpu/include/plugins/common/stringHelpers.hpp

@@ -23,9 +23,17 @@
 #include <string>
 #include <sstream>
 #include <ctime>
+// libSplash
+#if(ENABLE_HDF5 == 1)
+#  include <splash/splash.h>
+#  include <list>
+#  include <algorithm>
+#endif
 
 
 namespace picongpu
+{
+namespace helper
 {
     /** Return the current date as string
      *
@@ -50,4 +58,83 @@ namespace picongpu
         return dateString.str();
     }
 
+#if(ENABLE_HDF5 == 1)
+    /** Create c-strings suitable for libSplash
+     *
+     * Convert a std::list of strings to a format that is suitable to
+     * be written into libSplash attributes (list of constant c-strings).
+     * Strings will be padded to longest string.
+     *
+     * Independent of the padding you chose, the final strings will be '\0'
+     * terminated.
+     */
+    class GetSplashArrayOfString
+    {
+    private:
+        // compare two std::string by their size
+        struct CompStrBySize
+        {
+            bool operator()( std::string i, std::string j)
+            {
+                return i.size() < j.size();
+            }
+        };
+
+    public:
+        // resulting type containing all attributes for a libSplash write call
+        struct Result
+        {
+            splash::ColTypeString colType;
+            std::vector<char*> listIndex;
+            std::vector<char> buffers; // all of same length
+        };
+
+        Result operator()(
+            std::list<std::string> listOfStrings,
+            char padding = '\0'
+        )
+        {
+            // find length of longest string in list
+            CompStrBySize compStrBySize;
+            std::string longestString = *std::max_element(
+                    listOfStrings.begin(),
+                    listOfStrings.end(),
+                    compStrBySize
+            );
+            size_t lenMax = longestString.size();
+
+            // create libSplash data type suitable for longest string
+            Result result;
+            result.colType = splash::ColTypeString(lenMax);
+
+            // prepare buffer with padding
+            //   size per buffer must include terminator \0 !
+            const size_t bytesPerEntry = lenMax + 1;
+            const size_t lenAllBuffers = listOfStrings.size() * bytesPerEntry;
+            result.buffers.assign( lenAllBuffers, padding );
+
+            // copy buffers
+            std::list<std::string>::iterator listIt = listOfStrings.begin();
+            for(
+                size_t i = 0;
+                i < listOfStrings.size();
+                ++i, ++listIt
+            )
+            {
+                // index points to each part of the buffer individually
+                const size_t startIdx = i * bytesPerEntry;
+                char* start = &result.buffers.at( startIdx );
+                result.listIndex.push_back( start );
+
+                // copy byte wise
+                std::copy( listIt->c_str(), listIt->c_str() + bytesPerEntry, start );
+            }
+
+            // return
+            return result;
+        }
+    };
+#endif
+
+} // namespace helper
 } // namespace picongpu