add bit string to int array conversion

src/main/java/ch/fhnw/kry/SPN.java

@@ -67,6 +67,29 @@ public class SPN {
         return null;
     }
 
+    /**
+     * Convert a bit string into an integer array.
+     *
+     * Because later we only ever look at the lower 16 bits,
+     * we stuff every block of 16 bits into the lower half of an int
+     * (meaning the igh 16 bits of the ints in the array are always 0).
+     *
+     * @param bits Bit string. Its length must be a multiple of 16.
+     * @return int array, with every int's lower 16 bits set to 16 bits of the input string.
+     */
+    public int[] strToArray(String bits) {
+        int[] data = new int[bits.length() / 16];
+
+        for (int i = 0; i < data.length; i++) {
+            int startIdx = i * 16;
+            String wordBits = bits.substring(startIdx, startIdx + 16);
+            int word = Integer.parseInt(wordBits, 2);
+            data[i] = word;
+        }
+
+        return data;
+    }
+
     /**
      * Encrypt a block with the defined SPN.
      *