Added Rabin–Karp algorithm

[pratikgl]

Added Rabin–Karp algorithm to find API

References to other Issues or PRs or Relevant literature

Fixes #402

Brief description of what is fixed or changed

In the find API implemented in the string algorithm. I have added Rabin–Karp algorithm which finds the occurrence of a query string in a text using string hashing.

Other comments

Reference: https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm

[codecov]

Codecov Report

Merging #413 (823fc7b) into master (5cf53a2) will increase coverage by 0.013%.
The diff coverage is 100.000%.

❗ Current head 823fc7b differs from pull request most recent head 98209e4. Consider uploading reports for the commit 98209e4 to get more accurate results

@@              Coverage Diff              @@
##            master      #413       +/-   ##
=============================================
+ Coverage   98.622%   98.635%   +0.013%     
=============================================
  Files           26        26               
  Lines         3411      3444       +33     
=============================================
+ Hits          3364      3397       +33     
  Misses          47        47               

Impacted Files	Coverage Δ
pydatastructs/strings/algorithms.py	100.000% <100.000%> (ø)

[czgdp1807]

Suggested change

	if q == 0:
	for i in range(t):
	positions.append(i)
	return positions
	if q == 0 or t == 0:
	return positions

[czgdp1807]

Suggested change

	def hash_str(string, p=137, m=1e9 + 7):
	def _hash_str(string, p=137, m=1e9 + 7):

[czgdp1807]

Is this implementation of hash function same as in https://www.cs.cmu.edu/afs/cs/academic/class/15451-f14/www/lectures/lec6/karp-rabin-09-15-14.pdf

[pratikgl]

In the implementation, I've used a similar hash function but not the exact same. Instead of doing T[i] * p^(j-i) I've done T[i] * p^(i). So instead of iterating from len(T) to 0, it is iterating from 0 to len(T)
Here is the reference https://cp-algorithms.com/string/string-hashing.html

[czgdp1807]

Instead of repeating 1e9 + 7, let's define it as a constant (PRIME_NUMBER = 10000000007) somewhere at the starting of this file and then use it else where. We also need a Note in documentation of find to explain the details of hash function like what value of p and M we are using and why.
Rest it's fine.

[czgdp1807]

What error?

[pratikgl]

If it is kmp_table = OneDimensionalArray(int, len(query) + 1) then for the case where len(query) is 0 the line kmp_table[pos] = cnd after the while statement will pop out an error index out of range as pos is intialised with 1. I've increased the array size so it won't go out of range. kmp_table = OneDimensionalArray(int, len(query) + 2) will also work though ;).
Rest the increased array size will not affect anything else.

[czgdp1807]

We should do this at the entry point of _kmp.

[pratikgl]

Changes added:

1. Added _p_pow function to decrease redundancy in the code
2. Defined global variables PRIME_NUMBER, MOD

[czgdp1807]

Thanks @pratikgl