AIS 2nd derivative

//+------------------------------------------------------------------+
//|                                           AIS 2nd derivative.mq5 |
//|                                                      Aleksej1966 |
//|                        https://www.mql5.com/ru/users/aleksej1966 |
//+------------------------------------------------------------------+
#property copyright "Aleksej1966"
#property link      "https://www.mql5.com/ru/users/aleksej1966"
#property version   "1.00"
#property indicator_chart_window
#property indicator_buffers 1
#property indicator_plots   1

#property indicator_type1  DRAW_LINE
#property indicator_label1 "2nd derivative"
#property indicator_color1 clrBlue
#property indicator_width1 1
#property indicator_style1 STYLE_SOLID

input ushort iPeriod=14;
double buffer[];
//+------------------------------------------------------------------+
//| Custom indicator initialization function                         |
//+------------------------------------------------------------------+
int OnInit()
  {
//--- indicator buffers mapping
   SetIndexBuffer(0,buffer,INDICATOR_DATA);
   ArraySetAsSeries(buffer,true);
   PlotIndexSetDouble(0,PLOT_EMPTY_VALUE,EMPTY_VALUE);
   PlotIndexSetInteger(0,PLOT_SHIFT,1);
//---
   return(INIT_SUCCEEDED);
  }
//+------------------------------------------------------------------+
//| Custom indicator iteration function                              |
//+------------------------------------------------------------------+
int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],
                const double &open[],
                const double &high[],
                const double &low[],
                const double &close[],
                const long &tick_volume[],
                const long &volume[],
                const int &spread[])
  {
//---
   if(rates_total>prev_calculated)
     {
      ArraySetAsSeries(open,true);
      int bars=prev_calculated>0? rates_total-prev_calculated-1:rates_total-2*iPeriod-2;
      for(int i=bars; i>=0; i--)
        {
         double sum=0;
         for(int j=0; j<iPeriod; j++)
            sum=sum+2*open[i+j]-open[i+2*j+1];

         buffer[i]=sum/iPeriod;
        }
     }
//--- return value of prev_calculated for next call
   return(rates_total);
  }
//+------------------------------------------------------------------+