Trouble porting a function in Python to C using the Python C API

Question

I have a checksum function in Python:

def checksum(data):
    a = b = 0
    l = len(data)
    for i in range(l):
        a += ord(data[i])
        b += (l - i)*ord(data[i])

    return (b << 16) | a, a, b

that I am trying to port to a C module for speed. Here's the C function:

static PyObject *
checksum(PyObject *self, PyObject *args)
{
    int i, length;
    unsigned long long a = 0, b = 0;
    unsigned long long checksum = 0;
    char *data;

    if (!PyArg_ParseTuple(args, "s#", &data, &length)) {
        return NULL;
    }

    for (i = 0; i < length; i++) {
        a += (int)data[i];
        b += (length - i) * (int)data[i];
    }

    checksum = (b << 16) | a;
    return Py_BuildValue("(Kii)", checksum, (int)a, (int)b);
}

I use it by opening a file and feeding it a 4096 block of data. They both return the same values for small strings, but when I feed it binary data straight from a file, the C version returns wildly different values. Any help would be appreciated.

Answer 1

I would guess that you have some kind of overflow in your local variables. Probably b gets to large. Just dump the values for debugging purposes and you should see if it's the problem. As you mention, that you are porting the method for performance reasons. Have you checked psyco? Might be fast enough and much easier. There are more other tools which compile parts of python code on the fly to C, but I don't have the names in my head.