Repetitive hashing collision

gistfile1.py

import hashlib

iteration_length = 10
hash_length = 5
set_length = pow(16, hash_length)
fs = set()

for i in range(0, set_length):
    x = format(i, 'x').zfill(hash_length)
    fs.add(x)

print len(fs)
 
for i in range(0, iteration_length): 
    nfs = set()

    for i in fs:
        h = hashlib.md5()
        h.update(i)
        nfs.add(h.hexdigest()[:hash_length])
     
    print len(nfs)

    fs = nfs

#############################
Hash length: 1
16
8
5
5
5
5

Hash length: 2
256
161
116
93
78
67

Hash length: 3
4096
2608
1940
1559
1316
1142

Hash length: 4
65536
41486
30774
24579
20537
17705

Hash length: 5
1048576
662422
490920
391934
327017
280856