Kata.java

package kata.colouredtriangles;

public class Kata {
    private static final int R = 0;
    private static final int G = 1;
    private static final int B = 2;
    public static char triangle(final String firstRow) {
        StringBuilder row = new StringBuilder(firstRow);
        while (row.length() != 1) {
            row = triangleOneTime(row);
        }
        return row.charAt(0);
    }

    private static StringBuilder triangleOneTime(StringBuilder firstRow) {
        StringBuilder row = new StringBuilder();
        for(int i = 1; i < firstRow.length(); ++i) {
             row.append(getChild(getValue(firstRow.charAt(i -1)), getValue(firstRow.charAt(i))));
        }
        return row;
    }

    private static int getValue(char rgb) {
        if (rgb == 'R') return R;
        if (rgb == 'G') return G;
        if (rgb == 'B') return B;
        throw new IllegalArgumentException();
    }

    private static char getChild(int parentBefore, int parentActual) {
        switch (parentBefore + parentActual) {
            case 0:
            case 3: return 'R';
            case 2: return 'G';
            case 4:
            case 1: return 'B';
            default: throw  new IllegalArgumentException();
        }
    }
}