Merge pull request #208 from damelLP/add_string_algos

Added a python implementation of knuth-morris-pratt string search algo

Author: harshildarji

Diff for: strings/knuth-morris-pratt.py

@@ -0,0 +1,57 @@
+def kmp(pattern, text, len_p=None, len_t=None):
+    """
+    The Knuth-Morris-Pratt Algorithm for finding a pattern within a piece of text
+    with complexity O(n + m)
+
+    1) Preprocess pattern to identify any suffixes that are identical to prefixes
+
+        This tells us where to continue from if we get a mismatch between a character in our pattern
+        and the text.
+
+    2) Step through the text one character at a time and compare it to a character in the pattern
+        updating our location within the pattern if necessary
+
+    """
+
+    # 1) Construct the failure array
+    failure = [0]
+    i = 0
+    for index, char in enumerate(pattern[1:]):
+        if pattern[i] == char:
+            i += 1
+        else:
+            i = 0
+        failure.append(i)
+
+    # 2) Step through text searching for pattern
+    i, j = 0, 0  # index into text, pattern
+    while i < len(text):
+        if pattern[j] == text[i]:
+            if j == (len(pattern) - 1):
+                return True
+            i += 1
+            j += 1
+
+        # if this is a prefix in our pattern
+        # just go back far enough to continue
+        elif failure[j] > 0:
+            j = failure[j] - 1
+        else:
+            i += 1
+    return False
+
+
+if __name__ == '__main__':
+
+    # Test 1)
+    pattern = "abc1abc12"
+    text1 = "alskfjaldsabc1abc1abc12k23adsfabcabc"
+    text2 = "alskfjaldsk23adsfabcabc"
+    assert kmp(pattern, text1) and not kmp(pattern, text2)
+
+    # Test 2)
+    pattern = "ABABX"
+    text = "ABABZABABYABABX"
+    assert kmp(pattern, text)
+
+