java - Find indexOf a byte array within another byte array

Question

Given a byte array, how can I find within it, the position of a (smaller) byte array?

This documentation looked promising, using ArrayUtils, but if I'm correct it would only let me find an individual byte within the array to be searched.

(I can't see it mattering, but just in case: sometimes the search byte array will be regular ASCII characters, other times it will be control characters or extended ASCII characters. So using String operations would not always be appropriate)

The large array could be between 10 and about 10000 bytes, and the smaller array around 10. In some cases I will have several smaller arrays that I want found within the larger array in a single search. And I will at times want to find the last index of an instance rather than the first.

Answer 1

The simpelst way would be to compare each element:

public int indexOf(byte[] outerArray, byte[] smallerArray) {
    for(int i = 0; i < outerArray.length - smallerArray.length+1; ++i) {
        boolean found = true;
        for(int j = 0; j < smallerArray.length; ++j) {
           if (outerArray[i+j] != smallerArray[j]) {
               found = false;
               break;
           }
        }
        if (found) return i;
     }
   return -1;  
}  

Some tests:

@Test
public void testIndexOf() {
  byte[] outer = {1, 2, 3, 4};
  assertEquals(0, indexOf(outer, new byte[]{1, 2}));
  assertEquals(1, indexOf(outer, new byte[]{2, 3}));
  assertEquals(2, indexOf(outer, new byte[]{3, 4}));
  assertEquals(-1, indexOf(outer, new byte[]{4, 4}));
  assertEquals(-1, indexOf(outer, new byte[]{4, 5}));
  assertEquals(-1, indexOf(outer, new byte[]{4, 5, 6, 7, 8}));
}

As you updated your question: Java Strings are UTF-16 Strings, they do not care about the extended ASCII set, so you could use string.indexOf()