## Fuzzy Testing

I’ve been thinking about various aspects of the upcoming project to create a testing framework for LCFG components. One thing we really need is a replacement for what I am going to term “fuzzy matching”. With the current framework there is support for embedding tags [% and %] to mark sections which are going to change with every run (timestamps, version numbers, etc.)

This is a really nice idea and is very useful but there are a couple of downsides to the current implementation. In particular, the tags need to appear in the expected output and also the generated output (and consequently the code which is being tested). However, we do not want these tags to be appear in the generated output on a “live” system so we end up having to rebuild the packages in a special way to get the tags inserted into the code producing the output. This leads to a situation where the code we are testing is not identical to that on the live system, at best the difference is a few strings but in some cases completely different code paths are followed.

Having thought about this for a while I reckoned it should be possible to do away completely with the need to add tags to the generated output and just markup the expected output. Effectively the expected output becomes like a template. The inspiration for this approach came from Template::Extract and initially I thought I could just build directly on top of that module but I didn’t have too much success.

I’ve now come up with Test::FuzzyMatch, this is most definitely alpha-quality software but it is already quite useful. I think this demonstrates what I want to be able to do:

Here is part of a logfile from the boot component:

06/10/08 03:02:01: >> run
07/10/08 03:02:02: >> run
08/10/08 03:02:01: >> run
09/10/08 03:02:01: >> run
10/10/08 03:02:01: >> run
11/10/08 03:02:01: >> run
12/10/08 03:02:02: >> run


Here is the template it needs to match:

[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run
[% \d{2} %]/[% \d{2} %]/[% \d{2} %] [% \d{2} %]:[% \d{2} %]:[% \d{2} %]: >> run


Any regular expression can be embedded inside the tags, anything which is not inside the tags is a simple text-string. For each line a regular expression is assembled from the parts (with the text-strings being first passed through the quotemeta function. Each line in the input file is then compared with the regular expression generated from the same line in the template.

It can be used something like:

use Test::More tests => 1;
use Test::FuzzyMatch;

is_fuzzy_equal_files( 't/boot.tmpl', 't/boot.log' ,
'checking that the log file is correctly formatted');


This could be achieved by just writing every line in the expected output as a regular expression. I think this is clearer in terms of both reading and writing. It also means that we could optimise for lines where no fuzzy-matching is required.

I’d like to take this idea a bit further and add support for simple loops to handle repetition. The boot logfile example above shows how it would be nice to say something like “the next 7 lines must match this”.

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### 2 Comments on “Fuzzy Testing”

1. dcspaul Says:

I like the idea of tightening up the “fuzzy templating” mechanism for testing. I’m less sure about the idea of including the markup *only* in the expected output. The problem here is that you have to maintain the “expected output” file as well as the code – if you made a significant change to (say) the layout of the output, then you would pretty much have to throw away your “expected output” file and hand-markup a new one, based on the new output format. If you retain the markup generation in the code, then it makes more sense – the code ought “know” what is significant for testing and what is not …

2. squinney Says:

What I particularly want to avoid is having to do a special build of a component just to do the testing. We really do want to test exactly the same code as we run in the live environment.

Yes, this does lead to the need to completely update the “expected output” for a large code change. If that is not happening very often then I think that’s acceptable, certainly this is basically what everyone else does. Often I find I end up doing this at the moment anyway and that is with the “generated output” containing markup.

I think it comes down to the author deciding what they care about when they are testing and producing output which is testable. I gave the example of timestamps in log files because it would be good to check that the output is correctly formatted without worrying about the actual date and time. My proposed scheme would allow the author quite a lot of flexibility. A match could ignore most of a line with something like [% .*? %] and then just check one small, important part as a string.