Implemented Str.Count() method

[SudhanshuJoshi09]

1. Implemented Str.Count() method
2. Included tests for validating str.count.

[Thirumalai-Shaktivel]

This could have been implemented in IntrinsicFunction.
But this is fine for now, I think.

Looks good, thank you!

[anutosh491]

looks good !

[certik]

Thanks!

[OculusMode]

Small nitpicking on this but why to use an algorithm having O(n * m), we do have algos like boyer-moore, or even rabin karp if we do not wish to eat space.

some ref for this: https://en.wikipedia.org/wiki/String-searching_algorithm

or if you want to check out more of what cpython does, this is a nice place:

/* fast search/count implementation, based on a mix between boyer-
moore and horspool, with a few more bells and whistles on the top.
for some more background, see:
https://web.archive.org/web/20201107074620/http://effbot.org/zone/stringlib.htm */

via: https://github.com/python/cpython/blob/main/Objects/stringlib/fastsearch.h

[certik]

@OculusMode awesome, thanks! Can you post your comment as a new issue?

Then we can send PRs to implement it. If you are interested in this, that would be great.

[OculusMode]

Sure, can do that.