ppm_decode.py

#!/usr/bin/env python3    
                        
import sys              
                        
ZERO_COUNT = 4          
ONE_COUNT = 8           
                        
def ppm_scan(s):        
    count = 0           
    for bit in s:       
        if bit == "1":    
            if count > 0:    
                yield(count)    
            count = 0    
        elif bit =="0":    
            count = count + 1    
        else:           
            raise ValueError("got %s expects 0 or 1" % bit)    
                        
    if count > 0:       
        yield(count)    
                        
def count_to_bits(counts):    
    for count in counts:    
        if count == ZERO_COUNT: yield(0)    
        elif count == ONE_COUNT: yield(1)    
        else: raise ValueError("got %s expects %0 or %0" % (count, ZERO_COUNT, ONE_COUNT))    
                        
for bit in count_to_bits(ppm_scan(sys.argv[1])):    
    sys.stdout.write(str(bit))