IntersectingTriangles

// original facebook post:
// https://www.facebook.com/groups/javaharikrishna/permalink/1596271237165097/

// JUnit test classes:
// https://pastebin.com/iiUDpLZG
// https://pastebin.com/9CCBV1C9

// Related "Six Sided Star" classes:
// Implementation:
// https://pastebin.com/7g2keFVE
// Tests:
// https://pastebin.com/zmdNaJ7k

import java.util.BitSet;

public class IntersectingTriangles {

    public static void printIntersectingTriangles(final int triangleLines) {
        final int topLines = (triangleLines + 1) / 3;
        printIntersectingTriangles(triangleLines, topLines);
    }

    public static void printIntersectingTriangles(final int triangleLines, final int topLines) {
        if (triangleLines < 0) {
            throw new IllegalArgumentException("Argument value for 'triangleLines' must not be negative.");
        }

        final int totalLineCount = triangleLines + topLines;
        for (int lineIndex = 0; lineIndex < totalLineCount; ++lineIndex) {
            final BitSet starsOnThisLine = new BitSet();
            if (lineIndex < triangleLines) {
                setTriangleBits(starsOnThisLine, triangleLines, lineIndex);
            }
            final int indexFromEnd = totalLineCount - lineIndex - 1;
            if (indexFromEnd < triangleLines) {
                setTriangleBits(starsOnThisLine, triangleLines, indexFromEnd);
            }
            System.out.println(bitsToStars(starsOnThisLine));
        }
    }

    protected static void setTriangleBits(final BitSet starsOnThisLine, final int triangleLines, final int lineIndex) {
        if (lineIndex + 1 < triangleLines) {
            starsOnThisLine.set(triangleLines - lineIndex - 1, true);
            starsOnThisLine.set(triangleLines + lineIndex - 1, true);
        } else {
            for (int bitIndex = 0; bitIndex < triangleLines * 2 - 1; bitIndex += 2) {
                starsOnThisLine.set(bitIndex, true);
            }
        }
    }

    protected static String bitsToStars(final BitSet starsOnThisLine) {
        final StringBuilder result = new StringBuilder();
        for (int bitIndex = 0; bitIndex < starsOnThisLine.length(); ++bitIndex) {
            result.append(starsOnThisLine.get(bitIndex) ? '*' : ' ');
        }
        return result.toString();
    }

    private static String spaces(final int count) {
        return new String(new char[count]).replace('\0', ' ');
    }

}