package bigpi;

import java.math.BigDecimal;

import java.io.* ;

public class Bigpi {

 public static void main(String args[]) throws IOException{

   String I1, I2 ,If;

   BigDecimal one, sixteen, four, oneby5, oneby239, atn15, atn239, xsq, powercum, bigpi;

 PrintWriter dos = new PrintWriter(System.out, true) ;    

 System.out.println("         ------------------------------------    ");

 System.out.println("          Calculation of Pi by Machin method    ");

 System.out.println("         ------------------------------------   ");

 System.out.println("                      by M. Gallant 10/16/97   ");

 System.out.println("\n ");

     try (BufferedReader disys = new BufferedReader(new InputStreamReader(System.in))) {

         System.out.println("Enter digits of precision to calculate PI:") ;

         I1=disys.readLine() ;

         

         int tscale = Integer.parseInt(I1) +2;

         

         one = new BigDecimal("1") ;

         four = new BigDecimal("4") ;

         sixteen = new BigDecimal("16") ;

         oneby5 = new BigDecimal("0.2") ;

         oneby239 = one.divide(new BigDecimal("239"), tscale, BigDecimal.ROUND_DOWN) ;

         

         //calculate arctan(1/5) to tscale digits of precision:

         // Based on log10(1/5) need about 1.5*tscale maximum exponent.

         

         atn15 = oneby5 ;      // initialize to first term of arctan series.

         powercum = oneby5;   // start with first power.

         xsq = oneby5.multiply(oneby5) ;

         

         System.out.println("Starting Calculation with "+tscale+" terms ....\n") ;

         for (int i=3; i<=(3*tscale/2); i+=4) {

             powercum = powercum.multiply(xsq) ;

             atn15 = atn15.subtract(powercum.divide(new BigDecimal(String.valueOf(i)), tscale, BigDecimal.ROUND_DOWN));

             powercum = powercum.multiply(xsq) ;

             atn15 = atn15.add(powercum.divide(new BigDecimal(String.valueOf(i+2)), tscale, BigDecimal.ROUND_DOWN));

             atn15 = atn15.setScale(tscale, BigDecimal.ROUND_DOWN) ;

         }

         

         //calculate arctan(1/239) to tscale digits of precision:

         // Based on log10(1/239) need about 0.5*tscale maximum exponent.

         atn239 = oneby239 ;      // initialize to first term of arctan series.

         powercum = oneby239;   // start with first power.

         xsq = oneby239.multiply(oneby239) ;

         

         for (int i=3; i<=tscale/2; i+=4) {

             powercum = (powercum.multiply(xsq)).setScale(tscale, BigDecimal.ROUND_DOWN) ;

             atn239 = atn239.subtract(powercum.divide(new BigDecimal(String.valueOf(i)), BigDecimal.ROUND_DOWN));

             powercum = (powercum.multiply(xsq)).setScale(tscale, BigDecimal.ROUND_DOWN) ;

             atn239 = atn239.add(powercum.divide(new BigDecimal(String.valueOf(i+2)),  BigDecimal.ROUND_DOWN));

             atn239 = atn239.setScale(tscale, BigDecimal.ROUND_DOWN) ;

         }

         

         

         bigpi = sixteen.multiply(atn15) ;

         bigpi = bigpi.subtract(four.multiply(atn239)) ;

         

         System.out.println("BigPi (terms summed= "+tscale+")\n" + bigpi) ;

         // System.out.println("\n\nPi: "   + Math.PI) ;

         

         

           }

 }

}

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