Bitap Algorithm for Exact String Matching

docs/source/pydatastructs/strings/algorithms.rst

@@ -1,4 +1,5 @@
 Algorithms
 ==========
 
-.. autofunction:: pydatastructs.find
+.. autofunction:: pydatastructs.find
+.. autofunction:: pydatastructs.bitap_search

pydatastructs/strings/__init__.py

@@ -12,7 +12,8 @@
 __all__.extend(trie.__all__)
 
 from .algorithms import (
-    find
+    find,
+    bitap_search
 )
 
 __all__.extend(algorithms.__all__)

pydatastructs/strings/algorithms.py

@@ -4,7 +4,8 @@
     Backend, raise_if_backend_is_not_python)
 
 __all__ = [
-    'find'
+    'find',
+    'bitap_search'
 ]
 
 PRIME_NUMBER, MOD = 257, 1000000007
@@ -83,6 +84,25 @@ def find(text, query, algorithm, **kwargs):
         %(algorithm))
     return getattr(algorithms, func)(text, query)
 
+def bitap_search(text, pattern):
+    """
+    Bitap Algorithm (Shift-Or Algorithm) for exact string matching.
+    Returns the starting index of the pattern in the text, or -1 if not found.
+    """
+    m = len(pattern)
+    R = ~1
+    pattern_mask = {}
+
+    for i in range(m):
+        pattern_mask[pattern[i]] = pattern_mask.get(pattern[i], ~0) & ~(1 << i)
+
+    for i in range(len(text)):
+        R |= pattern_mask.get(text[i], ~0)
+        R <<= 1
+        if (R & (1 << m)) == 0:
+            return i - m + 1
+
+    return -1
 
 def _knuth_morris_pratt(text, query):
     if len(text) == 0 or len(query) == 0:

pydatastructs/strings/tests/test_algorithms.py

@@ -1,4 +1,4 @@
-from pydatastructs.strings import find
+from pydatastructs.strings import find, bitap_search
 
 import random, string
 
@@ -14,6 +14,15 @@ def test_bm():
 def test_zf():
     _test_common_string_matching('z_function')
 
+def test_bitap_search():
+    assert bitap_search("hello world", "world") == 6
+    assert bitap_search("abcdef", "def") == 3
+    assert bitap_search("abcdef", "gh") == -1
+    assert bitap_search("aaaaa", "aa") == 0
+    assert bitap_search("abababab", "bab") == 1
+    assert bitap_search("", "a") == -1
+    print("All tests passed.")
+
 def _test_common_string_matching(algorithm):
     true_text_pattern_dictionary = {
         "Knuth-Morris-Pratt": "-Morris-",