UDF using RapidJSON to flatten a JSON document to key/value pairs

flatten_udf.cpp

/*
* Compile with: 
* g++ -std=c++11 -shared -I `mysql_config --include` `mysql_config --cxxflags` flatten_udf.cpp -o flatten_udf.so 
*/

#pragma GCC diagnostic ignored "-Wwrite-strings"
#include "presto.h"
#include <iostream>
#include <string>
#include "rapidjson/reader.h"
#include <mysql.h>
#include <sstream>	// std::ostringstream
#include <plugin.h>
#include <unistd.h>


/* to remind me of the signatures */
extern "C" { 
	my_bool rapid_flatten_json_init(UDF_INIT *initid, UDF_ARGS *args, char* message);
	char* rapid_flatten_json(UDF_INIT *initid, UDF_ARGS *args,char *result, long long *length, char *is_null, char *error);
	void rapid_flatten_json_deinit(UDF_INIT *initid);
	
}

#endif

using namespace rapidjson;

/* This function hooks rewrite related PHP output to the general log*/ 
void php_log_output(const char *str){
	std::cerr << str;
}

void flatten_json_deinit(UDF_INIT *initid) {
  delete[] initid->ptr;
}

my_bool rapid_flatten_json_init(UDF_INIT *initid, UDF_ARGS *args, char* message) {
	if(args->arg_count != 1 || args->arg_type[0] != STRING_RESULT) {
		strcpy(message,"Takes one string arg: json_doc");
		return 1;
	}

	return 0;
}

char* rapid_flatten_json(UDF_INIT *initid, UDF_ARGS *args,char *result, long long *length, char *is_null, char *error) {
  if(args->args[0] == NULL) {
    *is_null = 1;
    return 0;
  }

  std::string json(args->args[0], args->lengths[0]);
  struct Handler {
    std::ostringstream oss;
    std::string key;
    bool Null()             { this->oss << this->key << "=" << "NULL" << "\n";  return true; }
    bool Bool(bool b)       { this->oss << this->key << "=" << std::boolalpha << b << "\n" ; return true; }
    bool Int(int i)         { this->oss << this->key << "=" << i << "\n"; return true; }
    bool Uint(unsigned u)   { this->oss << this->key << "=" << u << "\n"; return true; }
    bool Int64(int64_t i)   { this->oss << this->key << "=" << i << "\n"; return true; }
    bool Uint64(uint64_t u) { this->oss << this->key << "=" << u << "\n"; return true; }
    bool Double(double d)   { this->oss << this->key << "=" << d << "\n"; return true; }
    bool String(const char* str, SizeType length, bool copy) { 
        this->oss << this->key << "=" << str << "\n";
        return true;
    }
    bool Key(const char* str, SizeType length, bool copy) { 
        this->key.assign(str, length);
        return true;
    }
    /* these do nothing */
    bool StartObject() { return true; }
    bool EndObject(SizeType memberCount) { return true; }
    bool StartArray() { return true; }
    bool EndArray(SizeType elementCount) { return true; }
  };

  Handler handler;
  Reader reader;
  StringStream ss(json.c_str());
  reader.Parse<0>(ss, handler);

  initid->ptr = (char*)malloc(handler.oss.str().length()+1);
  initid->ptr = strcpy(initid->ptr, handler.oss.str().c_str());
  *length = handler.oss.str().length();
  return initid->ptr;
}

void rapid_flatten_json_deinit(UDF_INIT *initid) {
  delete[] initid->ptr;
}