A neat little numerical puzzle

gistfile1.txt

This puzzle is (allegedly) solved by elementary school children in 5 to 10 minutes,
but adults usually struggle with it much longer.

8809=6
7111=0
2172=0
6666=4
1111=0
3213=0
7662=2
9313=1
0000=4
2222=0
3333=0
5555=0
8193=3
8096=5
7777=0
9999=4
7756=1
6855=3
9881=5
5531=0

2581=?

(Bonus: Write a program that computes this function)

(Hint #1: It's language-agnostic. Actual numbers matter, not their names.)
(Hint #2: The program would take like 3-4 lines tops in most languages.)
(Hint #3: The function does not have well-defined value for certain class of inputs.)