New hash scheme

[robinheghan]

This PR makes a big change to how classNames are generated.

Previously the flow worked something like this:

• When user creates a node (div, span, whatever)
• • Compile CSS styles to CSS without class selector
• • Hash compiled css to get class name
• When converting to "unstyled" html
• • Compile CSS styles to CSS with class selector retrieved from hash

There are some potential performance improvements to this flow:

1. CSS styles are potentially compiled twice (once without and once with a class name)
2. All styled nodes are hashed, meaning if you have a list of 100 equally styled elements, we hash their styles 100 times

This PR tries to improve performance by:

• When a user creates a node, we compile the CSS styles to CSS with a template selector
• We use this compiled CSS as the key in a dictionary of Styles -> Hash. The style is only hashed if it isn't already in the dictionary.
• Instead of re-compiling the CSS, we perform a String.replace on the compiled css, replacing the template selector with the hashed class name.
• To reduce the overhead of dictionary lookups and hashing, the pretty printer uses a much more compact representation than before. I guess one could argue that it doesn't really pretty print anymore.
• Also reduce the number of allocations required when compiling CSS.

Somewhat unrelated, this PR also replaces a bunch of String.join calls with direct string concatination (++) where possible, as String.join shows up as a hot spot when profiling.

Depending on the browser, this change represents a 18-30% improvement compared to the current master branch.

[robinheghan]

Some questions:

1. There are more spaces that could be removed from the pretty printed output. Less output means faster dict lookup and faster hashing. I kept some of them in, just so that the tests wouldn't be as cryptic. Do you think I should remove the few spaces that remain?
2. Do you have a suggestion for a better classnameStandin value? It should preferably be short, but it also has to be unique so that we don't replace something important.

[brian-carroll]

Cool!
For the stand-in class name, how about a single unprintable control character like the bell character "\a"? It's as short as it gets and it's probably invalid CSS, or at least unlikely to be used by a sane person.

[robinheghan]

New commits produce even less whitespace in generated css for faster hashing and dict lookups.
Also took Brian's suggestion to use '\a' as the standin classname.

[rtfeldman]

@robinheghan Looks like there are some merge conflicts since I merged your other PR. 😅

[robinheghan]

I’ll fix it :)

[robinheghan]

All tests pass, ready to merge :)