implementation of reservoir sample

stream_sample.py

class Sample(object):
    '''
    This class implements Reservoir Sampling to keep a random sample of an infinite stream.
    See http://gregable.com/2007/10/reservoir-sampling.html for one description.
    '''
    def __init__(self, sample_size=2**14, type='f'):
        self.sample = array.array(type)
        self.sample_size = sample_size
        self.num_vals = 0

    def add_val(self, val):
        if self.num_vals < self.sample_size:
            self.sample.append(val)
        else:
            pos = random.randint(0, self.num_vals)
            if pos < self.sample_size:
                self.sample[pos] = val
        self.num_vals += 1