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Author: Lelenadamtew

Rabin-Karp.py

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+def rabin_karp(text, pattern, base=256, prime=101):
+    """
+    Rabin-Karp Algorithm for substring search.
+    
+    Parameters:
+    text (str): The main text where the pattern is to be searched.
+    pattern (str): The pattern to search for.
+    base (int): The base used for hashing (default: 256).
+    prime (int): A prime number used to reduce hash values (default: 101).
+    
+    Returns:
+    list: Starting indices of all occurrences of the pattern in the text.
+    """
+    n, m = len(text), len(pattern)
+    if m > n:
+        return []
+
+    pattern_hash = 0  # Hash value for pattern
+    text_hash = 0     # Hash value for current window in text
+    h = 1             # The value of base^(m-1)
+
+    # Compute h = (base^(m-1)) % prime
+    for _ in range(m - 1):
+        h = (h * base) % prime
+
+    # Compute initial hash values for pattern and first window of text
+    for i in range(m):
+        pattern_hash = (base * pattern_hash + ord(pattern[i])) % prime
+        text_hash = (base * text_hash + ord(text[i])) % prime
+
+    # List to store starting indices of pattern matches
+    result = []
+
+    # Slide the pattern over text one character at a time
+    for i in range(n - m + 1):
+        # Check if the hash values match
+        if pattern_hash == text_hash:
+            # If hash matches, check characters one by one for a real match
+            if text[i:i + m] == pattern:
+                result.append(i)
+
+        # Compute hash for next window of text
+        if i < n - m:
+            text_hash = (base * (text_hash - ord(text[i]) * h) + ord(text[i + m])) % prime
+
+            # Make sure text_hash is positive
+            if text_hash < 0:
+                text_hash += prime
+
+    return result
+
+
+# Example Usage
+if __name__== "__main__":
+    text = "abracadabra"
+    pattern = "abra"
+    matches = rabin_karp(text, pattern)
+    print(f"Pattern '{pattern}' found at indices: {matches}")