/*
 *  call-seq:
 *     str.sum(n=16)   => integer
 *  
 *  Returns a basic <em>n</em>-bit checksum of the characters in <i>str</i>,
 *  where <em>n</em> is the optional <code>Fixnum</code> parameter, defaulting
 *  to 16. The result is simply the sum of the binary value of each character in
 *  <i>str</i> modulo <code>2n - 1</code>. This is not a particularly good
 *  checksum.
 */

static VALUE
rb_str_sum(argc, argv, str)
    int argc;
    VALUE *argv;
    VALUE str;
{
    VALUE vbits;
    int bits;
    char *ptr, *p, *pend;
    long len;

    if (rb_scan_args(argc, argv, "01", &vbits) == 0) {
        bits = 16;
    }
    else bits = NUM2INT(vbits);

    ptr = p = RSTRING(str)->ptr;
    len = RSTRING(str)->len;
    pend = p + len;
    if (bits >= sizeof(long)*CHAR_BIT) {
        VALUE sum = INT2FIX(0);

        while (p < pend) {
            str_mod_check(str, ptr, len);
            sum = rb_funcall(sum, '+', 1, INT2FIX((unsigned char)*p));
            p++;
        }
        if (bits != 0) {
            VALUE mod;

            mod = rb_funcall(INT2FIX(1), rb_intern("<<"), 1, INT2FIX(bits));
            mod = rb_funcall(mod, '-', 1, INT2FIX(1));
            sum = rb_funcall(sum, '&', 1, mod);
        }
        return sum;
    }
    else {
       unsigned long sum = 0;

        while (p < pend) {
            str_mod_check(str, ptr, len);
            sum += (unsigned char)*p;
            p++;
        }
        if (bits != 0) {
           sum &= (((unsigned long)1)<<bits)-1;
        }
        return rb_int2inum(sum);
    }
}