From 15d44eddaeb59460ed3c3395e1ad3095fe4ca6dd Mon Sep 17 00:00:00 2001
From: kartiknad <56294396+kartiknad@users.noreply.github.com>
Date: Mon, 7 Mar 2022 07:20:41 +0530
Subject: [PATCH] chore(Python): add Rabin Karp algorithm (#699)

Co-authored-by: Arsenic <54987647+Arsenic-ATG@users.noreply.github.com>
---
 algorithms/Python/README.md                   |  1 +
 .../Python/strings/rabin-karp-algorithm.py    | 98 +++++++++++++++++++
 2 files changed, 99 insertions(+)
 create mode 100644 algorithms/Python/strings/rabin-karp-algorithm.py

diff --git a/algorithms/Python/README.md b/algorithms/Python/README.md
index da6ca875..048c61ee 100644
--- a/algorithms/Python/README.md
+++ b/algorithms/Python/README.md
@@ -53,6 +53,7 @@
 - [Longest Common Subsequence](strings/longest_common_subsequence.py)
 - [Unique Character](strings/unique_character.py)
 - [Add String](strings/add_string.py)
+- [Rabin Karp algorithm](strings/rabin-karp-algorithm.py)
 
 ## Dynamic Programming
 - [Print Fibonacci Series Up To N-th Term](dynamic_programming/fibonacci_series.py)
diff --git a/algorithms/Python/strings/rabin-karp-algorithm.py b/algorithms/Python/strings/rabin-karp-algorithm.py
new file mode 100644
index 00000000..11eee76e
--- /dev/null
+++ b/algorithms/Python/strings/rabin-karp-algorithm.py
@@ -0,0 +1,98 @@
+'''
+String pattern matching algorithm which performs efficiently for large text and patterns
+
+Algorithm: 
+Rabin Karp works on the concept of hashing. If the substring of the given text 
+is same as the pattern then the corresponding hash value should also be same. Exploiting this idea 
+and designing a hash function which can be computed in O(m) time for both pattern and initial window 
+of text. The subsequent  window each will require only O(1) time. And we slide the window n-m times 
+after the initial window. Therefore the overall complexity of calculating hash function for text is O(n-m+1)
+Once the hash value matches, the underlying string is again checked with pattern for matching
+
+
+Complexity:
+    Best case:  O(n-m+1)
+    Worst case: O(nm)
+
+
+'''
+
+def rabin_karp(T: str, P: str, q: int ,d: int = 256) -> None :
+    '''
+    Parameters:
+    
+            T: string
+               The string where the pattern needs to be searched
+
+            P: string 
+               The pattern to be searched
+
+            q: int
+               An appropriately chosen prime number based on length of input strings
+               The higher the prime number, the lower the collisions and spurious hits
+               
+            d: int, default value 256
+               Denotes the no of unique character that is used for encoding
+    
+    
+
+    
+    
+    
+    Example:
+    
+    >>> pos = rabin_karp("AAEXCRTDDEAAFT","AA",101)
+    Pattern found at pos: 0
+    Pattern found at pos: 10
+    
+    '''
+    
+    n = len(T)   # length of text
+    m = len(P)   # length of pattern
+    p=0          # Hash value of pattern
+    t=0          # Hash value of text
+    
+    #Computing h: (h=d^m-1 mod q)
+    h=1
+    for i in range(1,m):
+        h = (h*d)%q
+    
+    #Computing hash value of pattern and initial window (of size m) of text
+    for j in range(m):
+        p = (d*p + ord(P[j])) % q
+        t = (d*t + ord(T[j])) % q
+    
+    
+    found = False
+    pos=[] # To store positions
+    
+    #Sliding window and matching
+    for s in range(n-m+1):
+        if p==t: # if hash value matches
+            if P == T[s:s+m]: # check for string match
+                pos.append(s)
+                if not found:
+                    found = True
+    
+        if s<n-m:
+            t = (d*(t-ord(T[s])*h) + ord(T[s+m])) % q # updating hash value of t for next window
+            if t<0:
+                t = t+q # To make sure t is positive integer
+    
+    if not found: # If pattern not found in text
+        pos.append(-1)
+        
+    #Printing results
+    if pos[0]==-1:
+        print("Pattern not found")
+    else:
+        for i in pos:
+            print(f"Pattern found at pos: {i}")
+
+
+
+if __name__ == "__main__":
+    T = "AAEXCRTDDEAAFT"
+    P = "AA"
+
+    rabin_karp(T,P,101)