Custom hash algorithms, detect hash collisions

[blakeembrey]

• Pass customised hash algorithm around internal class instantiations.
• Implement method to get full identifiers (E.g. CSS style) to detect hash collisions

Closes #21.

Edit: If you're implementing your own hash, you'll need to remember to escape (https://github.com/mathiasbynens/cssesc). Don't really feel the need to take on a 150 line dependency for this feature.

[coveralls]

Coverage decreased (-0.3%) to 99.31% when pulling a2ad111087de1f820d264b549698128c26327595 on configure-hash into 9563a50cd71e40b0918d66d7d75a83f74a20afbf on master.

[dmitrage]

Unfortunately, this change doesn't cover all possible collisions :( Styles with nesting and keyframes are not always catched, because of collision in accumulated hash string.

Gist with failing test cases for the default hash function: https://gist.github.com/dmitrage/1daa3bc5306b176d0df300096441abfb

Keyframes: Identifier for rules is computed from accumulated hash strings, in my case "fromcolor:#0008datocolor:red" and "fromcolor:#000f8ctocolor:red". Default hash function produces same hash value for both: "uybv9". So rules are constructed with rule = "@keyframes huybv9" and styles = "". getIdentifier returns same value, collision not detected.

[blakeembrey]

Great catch, I'm not handling any accumulated hash strings in the detection. Not sure of the best way to do that right now, but I'll think on it.

[dmitrage]

The simplest thing I've found is to add hashString property to Selector and Rule and use it in getIdentifier. Not sure if it is 100% correct but at least all tests are passing. I've forked this branch and created a commit on top to show the idea.

[blakeembrey]

Thanks for the idea, I was hoping not to have to keep around the extra data but it makes sense that I may have to (though the identifier diff is probably going to be a lot more confusing). I'll review and try to get that into the PR for the next release 👍

[coveralls]

Coverage decreased (-0.3%) to 99.324% when pulling b09fd3079e9cc9c89a7ba30e2456226381c4099b on configure-hash into fb53fdfc6f8a8f3781ad6fb1c58c746998a48fd3 on master.

[coveralls]

Coverage decreased (-0.3%) to 99.324% when pulling 1daf4df5265c1d6b0de64ae362e259ba2f84c0bd on configure-hash into fb53fdfc6f8a8f3781ad6fb1c58c746998a48fd3 on master.

[coveralls]

Coverage decreased (-0.3%) to 99.324% when pulling bdc9dc67ccfc11b1baf4c67d81ea83f9d00c81c4 on configure-hash into fb53fdfc6f8a8f3781ad6fb1c58c746998a48fd3 on master.