Mr. Dickinson thank you for doing this.  I do not know how to help with 
a patch.  If it helps, here is the code I use in python:

def factorial(n, _known=[1]):
    assert isinstance(n, int), "Need an integer. This isn't a gamma"
    assert n >= 0, "Sorry, can't factorilize a negative"
    assert n < 1000, "No way! That's too large"
    try:
        return _known[n]
    except IndexError:
        pass
    for i in range(len(_known), n+1):
        _known.append(_known[-1] * i)
    return _known[n]

When the assertions are turned-off, this runs pretty fast.