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.
      *

src/test/java/ch/fhnw/kry/SPNTest.java

@@ -73,4 +73,16 @@ class SPNTest {
         assertEquals(y, spn.encryptBlock(key, x));
         assertEquals(x, spn.decryptBlock(key, y));
     }
+
+    @Test
+    void strToArray() {
+        var spn = new SPN();
+
+        String input = "00000100110100100000101110111000000000101000111110001110011111110110000001010001010000111010000000010011011001110010101110110000";
+        int[] data = spn.strToArray(input);
+
+        assertEquals(8, data.length);
+        assertEquals(0x04D2, data[0]);
+        assertEquals(0x2BB0, data[data.length - 1]);
+    }
 }