But this time it's faster:

    /**
     * @param s non-empty string that contains only digits
     * @return true only if the given digits pass the luhn check
     */
    boolean isLuhn(String s) {
        char[] ca = s.toCharArray();
        int sum = ca[ca.length-1] - '0';
        
        for (int i=ca.length-2; i >= 0; i-=2) {
            int x2 = (ca[i] - '0') << 1;
            sum += x2 < 10 ? x2 : 1 + (x2 - 10);
        }
        for (int i=ca.length-3; i >= 0; i-=2)
            sum += ca[i] - '0';
        return sum % 10 == 0;
    }

    /**
     * @param s non-empty string that contains only digits
     * @return the generated Luhn digit
     */
    int calculateLuhnDigit(String s) {
        char[] ca = s.toCharArray();
        int sum = 0;
        
        for (int i=ca.length-1; i >= 0; i-=2) {
            int x2 = (ca[i] - '0') << 1;
            sum += x2 < 10 ? x2 : 1 + (x2 - 10);
        }
        for (int i=ca.length-2; i >= 0; i-=2)
            sum += ca[i] - '0';
        int mod = sum % 10;
        return mod == 0 ? 0 : 10 - mod;
    }

# posted by rot13(Unafba Pune) @ 4:15 PM