Smallest Missing Number in a Python Iterable

gistfile1.py

def smallest_missing_number(stream, min_val, max_val, bin_exp):
    ###
    # Returns the smallest counting number _not_ contained in a bounded
    #  sequence of counting numbers.
    #
    # Inputs:
    #    stream: iterator of integer values
    #    min_val: lower bound on values in stream
    #    max_val: upper bound on values in stream
    #    bin_exp: size of bin given as exponent of 2 (bin_size = 2^bin_exp)
    # Output:
    #    The smallest counting number not in the stream
    ###
    
    # If max = min, recusion is complete. Return value.
    if max_val == min_val: return max_val
    
    # Determine bin size.
    if bin_exp > 0:
        bin_size = 2 << bin_exp
    else:
        bin_size = 1
        
    # Construct hashset to represent bins
    bins = { i : 0 for i in xrange(0, (max_val - min_val) / bin_size + 1)}
    
    # For each value in stream, increase the counter for its bin
    for s in stream:
        if s < min_val or s > max_val: continue
        bin = (s - min_val) / bin_size
        bins[bin] = bins[bin] + 1
        
    # Iterate over bins. For the first bin that isn't full
    #  call this function where the max and min values 
    #  are the max and mins of the bin. Also decrease the 
    #  bin size
    
    for i in xrange(0, (max_val - min_val) / bin_size + 1):
        if bins[i] != bin_size:
            return smallest_missing_number(
                    l, 
                    min_val + bin_size * i, 
                    min_val + bin_size * (i + 1) - 1,
                    max(bin_exp - 1, 0)
                )