This program has a bug. When given an already encoded input, it encodes it again (replacing % with ‘%25’). For example, the input
https://example.com/t%20c?x=1results in the output
https://example.com/t%2520c?x=1, whereas in fact the output should be the same as the input in this case.
Let’s put our probabilistic thinking caps on and try and to debug the program. We ‘know’ that the
urlprinted on line 19 is wrong, so we can assign low probability (0.05) to this value being correct. Likewise we ‘know’ that the input
urlon line 1 is correct, so we can assign high probability (0.95). (In probabilistic inference, it is standard not to use 0.0 or 1.0, but values close to them instead). Initially we’ll set the probability of every other program variable being set to 0.5, since we don’t know any better yet. If we can find a point in the program where the inputs are correct with relatively high probability, and the outputs are incorrect with relatively high probability, then that’s an interesting place!
urlon line 19 has a low probability of being correct, this suggests that
urlon line 18, and
purl_strat line 12 are also likely to be faulty. PI Debugger actually assigns these probabilities of being correct 0.0441 and 0.0832 respectively. Line 18 is a simple assignment statement, so if the chances of a bug here are fairly low. Now we trace the data flow. If
purl_strat line 12 is likely to be faulty then
sat line 16 is also likely to be faulty (probability 0.1176).
I’m interested to see someone create a practical implementation someday.