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Task/Ulam-spiral--for-primes-/Java/ulam-spiral--for-primes-.java

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+import java.util.Arrays;
+
+public class Ulam{
+	enum Direction{
+		RIGHT, UP, LEFT, DOWN;
+	}
+	
+	private static String[][] genUlam(int n){
+		return genUlam(n, 1);
+	}
+
+	private static String[][] genUlam(int n, int i){
+		String[][] spiral = new String[n][n];
+		Direction dir = Direction.RIGHT;
+		int j = i;
+		int y = n / 2;
+		int x = (n % 2 == 0) ? y - 1 : y; //shift left for even n's
+		while(j <= ((n * n) - 1 + i)){
+			spiral[y][x] = isPrime(j) ? String.format("%4d", j) : " ---";
+
+			switch(dir){
+			case RIGHT:
+				if(x <= (n - 1) && spiral[y - 1][x] == null && j > i) dir = Direction.UP; break;
+			case UP:
+				if(spiral[y][x - 1] == null) dir = Direction.LEFT; break;
+			case LEFT:
+				if(x == 0 || spiral[y + 1][x] == null) dir = Direction.DOWN; break;
+			case DOWN:
+				if(spiral[y][x + 1] == null) dir = Direction.RIGHT; break;
+			}
+			
+			switch(dir){
+				case RIGHT:	x++; break;
+				case UP: 	y--; break;
+				case LEFT:	x--; break;
+				case DOWN:	y++; break;
+			}
+			j++;
+		}
+		return spiral;
+	}
+	
+	public static boolean isPrime(int a){
+		   if(a == 2) return true;
+		   if(a <= 1 || a % 2 == 0) return false;
+		   long max = (long)Math.sqrt(a);
+		   for(long n = 3; n <= max; n += 2){
+		      if(a % n == 0) return false;
+		   }
+		   return true;
+		}
+	
+	public static void main(String[] args){
+		String[][] ulam = genUlam(9);
+		for(String[] row : ulam){
+			System.out.println(Arrays.toString(row).replaceAll(",", ""));
+		}
+		System.out.println();
+		
+		for(String[] row : ulam){
+			System.out.println(Arrays.toString(row).replaceAll("\\[\\s+\\d+", "[  * ").replaceAll("\\s+\\d+", "   * ").replaceAll(",", ""));
+		}
+	}
+}