[json] Introduce TBufferJSON::zipJSON method

It idea to provide string which can be embed in any JavaScript
code and then unzipped whith JSROOT.unzipJSON function.

Primary aim is to improve json handling in jupyter

Author: linev

io/io/inc/TBufferJSON.h

@@ -77,6 +77,8 @@ class TBufferJSON final : public TBufferText {
       return ConvertToJSON(obj, TClass::GetClass<T>(), compact, member_name);
    }
 
+   static TString zipJSON(const char *json);
+
    template <class T>
    static Bool_t FromJSON(T *&obj, const char *json)
    {

io/io/src/TBufferJSON.cxx

@@ -541,6 +541,28 @@ TString TBufferJSON::ConvertToJSON(const TObject *obj, Int_t compact, const char
    return ConvertToJSON(ptr, clActual, compact, member_name);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// zip JSON string and convert into base64 string
+/// to be used with JSROOT unzipJSON() function
+/// Main application - embed large JSON code into jupyter notebooks
+
+TString TBufferJSON::zipJSON(const char *json)
+{
+   std::string buf;
+
+   int srcsize = (int) strlen(json);
+
+   buf.resize(srcsize + 500);
+
+   int tgtsize = buf.length();
+
+   int nout = 0;
+
+   R__zipMultipleAlgorithm(ROOT::RCompressionSetting::ELevel::kDefaultZLIB, &srcsize, (char *)json, &tgtsize, (char *) buf.data(), &nout, ROOT::RCompressionSetting::EAlgorithm::kZLIB);
+
+   return TBase64::Encode(buf.data(), nout);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Set level of space/newline/array compression
 /// Lower digit of compact parameter define formatting rules