Improving performance

[hukkin]

Describe the problem/need and solution

Problem / Idea
According to updated benchmark results (#196) mistletoe beat us. This is embarrassing. Haha, no, hats off to mistletoe authors!

Solution
The profiler (#197) reveals that we spend a significant portion of execution time, around one third, converting string characters to ints here.

As we've researched before, this is a performance hack in upstream JavaScript, but for us it hurts performance in a major way. It also makes the code slightly less readable. We've already resorted to caching these int sequences (diverging from JS upstream), which is basically a performance hack on top of a failed performance hack, where as a result performance still suffers. I'd be interested to move to using the str type only.

The naive way to implement this will break basically all parser extensions I believe (mdit-py-plugins). What we could do is a deprecation period for srcCharCode, where:

• the core library moves to using src for increased performance
• accessing srcCharCode emits DeprecationWarnings
• srcCharCode is generated lazily, only when accessed. This means that the performance loss only occurs when using deprecated extension, not when using core markdown-it

Benefit
Increased performance.

Guide for implementation

No response

Tasks and updates

No response

[hukkin]

What do you think @chrisjsewell ? If you're worried about the breaking change, we could make the deprecation period very long or infinite.

[chrisjsewell]

I think it sounds reasonable 👍

[hukkin]

Great, I can work on this.

Do you agree with this comment that we should just scrap #190 ? Or do you want an out-of-range safe wrapper for src, something like

def srcAt(self, idx: int) -> Optional[str]:
    try:
        return self._src[idx]
    except IndexError:
        return None

Personally I wouldn't do this for reasons explained in the comment I linked.

[hukkin]

Another way to increase performance would be to fill in whatever type annotations that are missing (enforced by the disallow_untyped_defs = true mypy setting) and publishing binary wheels built with mypyc.

What do you think @chrisjsewell ?