package picalculator;

import java.math.BigDecimal;
import java.math.MathContext;
import java.util.ArrayList;

public class Vars {

    public static final BigDecimal ZERO = new BigDecimal("0" );
    public static final BigDecimal ONE = new BigDecimal("1");
    public static final BigDecimal TWO = new BigDecimal("2");
    public static final BigDecimal FOUR = new BigDecimal("4");

    public static BigDecimal LastPI = ZERO.plus();
    public static BigDecimal PI = new BigDecimal( "0.0", MathContext.DECIMAL128 );
    public static BigDecimal Count = new BigDecimal("0");

    public static ArrayList<BigDecimal> PIAvgs = new ArrayList<>( 1000000 );

    public static boolean shouldStop = false;

    public static final Object piLock = new Object();

}

package picalculator;

import java.math.BigDecimal;
import java.math.MathContext;
import static picalculator.Vars.*;

public class BDFuncs {

    // The magic happens here
    // This function calculates the next PI in the series, as well as the actual child series while creating other news (see updateAverage()).
    public static void nextCicle() {
        synchronized ( piLock ) {
            LastPI = PI.plus();
            if ( shouldSubstract() ) PI = PI.subtract( getCicleTerm() );
            else PI = PI.add( getCicleTerm() );
            updateAverage();
            Count = Count.add( ONE );
        }
    }

    private static BigDecimal getCicleTerm() {
        return ONE.divide( Count.multiply( TWO ).add( ONE ), MathContext.DECIMAL128 );
    }

    private static boolean shouldSubstract() {
        if ( Count.remainder( TWO ).compareTo( ONE ) == 0 ) return true;
        else return false;
    }

    // This function works by calculating ONLY the averages at the last side of the tree.
    // It first calcles the last average of the first child series, and then using that
    // average and the last average of the same child series(if any), calculates its average
    // and sets it as the second child series' last child, repeating so until it reaches the
    // top of the tree, or in this case, while the index is more or equal than 0.
    private static void updateAverage() {
        PIAvgs.add( average( LastPI, PI ) );
        for ( int i = PIAvgs.size() - 2; i >= 0; i-- ) {
            PIAvgs.set( i, average( PIAvgs.get( i ) , PIAvgs.get( i + 1 ) ) );
        }
    }

    private static BigDecimal average( BigDecimal bd1, BigDecimal bd2 ) {
        return bd1.add( bd2 ).divide( TWO );
    }

    public static BigDecimal[] getData() {
        BigDecimal[] tempArr = new BigDecimal[3];
        synchronized ( piLock ) {
            tempArr[0] = PI.plus();
            tempArr[1] = ( PIAvgs.isEmpty() ? ZERO : PIAvgs.get(0) );
            tempArr[2] = Count.plus();
        }
        return tempArr;
    }

}