How to generate unique string of length 28 from two unique strings of length 28?

Question

I want to generate unique alphanumeric string of length 28 from two unique alphanumeric strings. Is it mathematically possible to have collision free string from two unique strings?

here is what I did,

ASCII_NUMBER_RANGE_START = 48;
ASCII_ALPHABET_RANGE_START =55;

 for (int i = 0; i < firstArray.length; i++) {
        int tempASCIIValue = (Character.getNumericValue(firstArray[i]) + Character.getNumericValue(secondArray[i])) % 35;
        if (tempASCIIValue <= 9) {
            FINAL_ASCII_VALUE = tempASCIIValue + ASCII_NUMBER_RANGE_START;
        } else {
            FINAL_ASCII_VALUE = tempASCIIValue + ASCII_ALPHABET_RANGE_START;
        }
        combinedArray[i] = (char) FINAL_ASCII_VALUE;
    }
    return new String(combinedArray);
}

In above code, i am not sure whether the resultant string is as uniquely strong as its parent strings.

Note: the generated string to have same length as the parent string

Any help is appreciated. Thanks.

Answer 1

Given that collisions are unavoidable. We can look at ideas like hash code generation. In a hashtable, you want to generate a hash code for each object. Ideally you want a Perfect hash function, but thats quite tricky to implement.

You might be able to get away with a hash function see for example Best implementation for hashCode method. A simple one with two integer variables is

int generateHashCode(int a,int b) {
    // Start with a non-zero constant. Prime is preferred
    int result = 17;
    // For each field multiply the previous result by a prime and add
    result = 31 * result + a;         
    result = 31 * result + b;         
    return result;
}

For your implementation you could change this to work with character. If you are happy to lose one character giving 26 + 26 + 9 = 64 possibilities for each character. This means you can use 6 bits for each character and 168 bits for the whole input, that can be fit into 6 integer. Then just run the generateHashCode() method on each pair of integers.