Calculate pressure on select row/position in Pyramid

CheckPyramid.java

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {

    public static void main(String... args) {
        int row = 322, position = 156;
        PyramidPressure pyramidPressure = new PyramidPressure();
        BigDecimal value = pyramidPressure.getPressure(row, position);
        System.out.println("Result = " + value);
    }
}

class PyramidPressure {
    private static final BigDecimal TWO = BigDecimal.valueOf(2);

    public BigDecimal getPressure(int row, int position) throws IllegalArgumentException {
        if (position > row) throw new IllegalArgumentException("Position is much more then a row");
        BigDecimal[][] pyramid = calcPyramid(row + 1);

        int half = getHalfLength(row);
        if ((position + 1) > half) {
            int part = row - position + 1;
            part = half - part;
            position = half - part - 1;
        }

        return pyramid[row][position].subtract(BigDecimal.ONE);
    }

    private BigDecimal[][] calcPyramid(int height) {
        final BigDecimal weight = BigDecimal.ONE;

        //todo improve this calculate, keep array to cache
        BigDecimal[][] pyramid = new BigDecimal[height][height];
        for (int i = 0; i < height; i++) {
            if (isFirstElement(i)) {
                pyramid[i][0] = weight;
                continue;
            }

            int half = getHalfLength(i);
            for (int j = 0; j <= i; j++) {
                if (j < half) {
                    BigDecimal weightBlock1;
                    BigDecimal weightBlock2;
                    if (isFirstElement(j)) {
                        weightBlock1 = BigDecimal.ZERO;
                        weightBlock2 = divide(pyramid, i - 1, j);
                    } else if (isLastElement(i, j, half)) {
                        weightBlock1 = divide(pyramid, i - 1, j - 1);
                        weightBlock2 = weightBlock1;
                    } else {
                        weightBlock1 = divide(pyramid, i - 1, j - 1);
                        weightBlock2 = divide(pyramid, i - 1, j);
                    }

                    pyramid[i][j] = weight.add(weightBlock2.add(weightBlock1));
                }
            }
        }
        return pyramid;
    }

    private boolean isFirstElement(int n) {
        return n == 0;
    }

    private boolean isLastElement(int i, int j, int half) {
        return (i % 2) == 0 && j == (half - 1);
    }

    private BigDecimal divide(BigDecimal[][] pyramid, int i, int j) {
        return pyramid[i][j].divide(TWO, 14, RoundingMode.HALF_EVEN).stripTrailingZeros();
    }

    private int getHalfLength(int i) {
        int row = i + 1;
        int half = (row / 2);
        if ((row % 2) != 0) {
            half = half + 1;
        }
        return half;
    }
}