生成特征词的的hash值

/**
 * 生成特征词的的hash值
 * @return
 */
private BigInteger hash(String keywords) {
    if (keywords == null || keywords.length() == 0) {
        return new BigInteger("0");
    } else {
        char[] sourceArray = keywords.toCharArray();
        BigInteger x = BigInteger.valueOf(((long) sourceArray[0]) << 7);
        BigInteger m = new BigInteger("1000003");
        BigInteger mask = new BigInteger("2").pow(this.hashbits).subtract(
                new BigInteger("1"));
        for (char item : sourceArray) {
            BigInteger temp = BigInteger.valueOf((long) item);
            x = x.multiply(m).xor(temp).and(mask);
        }
        x = x.xor(new BigInteger(String.valueOf(keywords.length())));
        if (x.equals(new BigInteger("-1"))) {
            x = new BigInteger("-2");
        }
        return x;
    }
}