refactor(strings): Improve KMP implementation and tests

Author: msaadsbr

strings/knuth_morris_pratt.py

@@ -1,101 +1,122 @@
+"""
+Implementation of the Knuth-Morris-Pratt (KMP) string searching algorithm.
+The KMP algorithm searches for all occurrences of a "pattern" within a main "text"
+by employing the observation that when a mismatch occurs, the pattern itself
+embodies sufficient information to determine where the next match could begin,
+thus bypassing re-examination of previously matched characters.
+
+This results in an optimal time complexity of O(n + m), where n is the length
+of the text and m is the length of the pattern.
+
+Source: https://en.wikipedia.org/wiki/Knuth–Morris–Pratt_algorithm
+"""
 from __future__ import annotations
 
 
-def knuth_morris_pratt(text: str, pattern: str) -> int:
+def _compute_lps_array(pattern: str) -> list[int]:
     """
-    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.
+    Computes the Longest Proper Prefix Suffix (LPS) array for the KMP algorithm.
+    The LPS array for a pattern of length m is an array lps of size m where lps[i]
+    is the length of the longest proper prefix of pattern[0...i] that is also a
+    suffix of pattern[0...i].
+
+    A "proper prefix" is a prefix of the string, but not the whole string.
+    A "proper suffix" is a suffix of the string, but not the whole string.
+
+    :param pattern: The pattern string to compute the LPS array for.
+    :return: The LPS array, which is used to guide the search.
+
+    >>> _compute_lps_array("aabaabaaa")
+    [0, 1, 0, 1, 2, 3, 4, 5, 2]
+    >>> _compute_lps_array("ababaca")
+    [0, 0, 1, 2, 3, 0, 1]
+    >>> _compute_lps_array("AAAA")
+    [0, 1, 2, 3]
+    >>> _compute_lps_array("abcde")
+    [0, 0, 0, 0, 0]
+    """
+    m = len(pattern)
+    lps = [0] * m
+    length = 0  # Length of the previous longest prefix suffix
+    i = 1
+
+    while i < m:
+        if pattern[i] == pattern[length]:
+            length += 1
+            lps[i] = length
+            i += 1
+        else:
+            if length != 0:
+                length = lps[length - 1]
+            else:
+                lps[i] = 0
+                i += 1
+    return lps
 
-    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
 
-    >>> kmp = "knuth_morris_pratt"
-    >>> all(
-    ...    knuth_morris_pratt(kmp, s) == kmp.find(s)
-    ...    for s in ("kn", "h_m", "rr", "tt", "not there")
-    ... )
-    True
+def knuth_morris_pratt_search(text: str, pattern: str) -> list[int]:
+    """
+    Finds all occurrences of a pattern in a text using the KMP algorithm.
+
+    :param text: The text to search in.
+    :param pattern: The pattern to search for.
+    :return: A list of starting indices of all occurrences of the pattern.
+             Returns an empty list if the pattern is not found or is empty.
+
+    >>> # Test cases from the original file
+    >>> knuth_morris_pratt_search("alskfjaldsabc1abc1abc12k23adsfabcabc", "abc1abc12")
+    [10]
+    >>> knuth_morris_pratt_search("alskfjaldsk23adsfabcabc", "abc1abc12")
+    []
+    >>> knuth_morris_pratt_search("ABABZABABYABABX", "ABABX")
+    [10]
+    >>> knuth_morris_pratt_search("ABAAAAAB", "AAAB")
+    [4]
+    >>> knuth_morris_pratt_search("abcxabcdabxabcdabcdabcy", "abcdabcy")
+    [15]
+    >>> # More comprehensive test cases
+    >>> knuth_morris_pratt_search("AABAACAADAABAABA", "AABA")
+    [0, 9, 12]
+    >>> knuth_morris_pratt_search("knuth_morris_pratt", "kn")
+    [0]
+    >>> knuth_morris_pratt_search("knuth_morris_pratt", "h_m")
+    [4]
+    >>> knuth_morris_pratt_search("knuth_morris_pratt", "rr")
+    [12]
+    >>> knuth_morris_pratt_search("knuth_morris_pratt", "tt")
+    [16]
+    >>> knuth_morris_pratt_search("knuth_morris_pratt", "not there")
+    []
+    >>> knuth_morris_pratt_search("test", "")
+    []
     """
+    n = len(text)
+    m = len(pattern)
+    if m == 0:
+        return []
 
-    # 1) Construct the failure array
-    failure = get_failure_array(pattern)
+    lps = _compute_lps_array(pattern)
+    found_indices = []
+    i = 0  # index for text
+    j = 0  # index for pattern
 
-    # 2) Step through text searching for pattern
-    i, j = 0, 0  # index into text, pattern
-    while i < len(text):
+    while i < n:
         if pattern[j] == text[i]:
-            if j == (len(pattern) - 1):
-                return i - j
+            i += 1
             j += 1
 
-        # if this is a prefix in our pattern
-        # just go back far enough to continue
-        elif j > 0:
-            j = failure[j - 1]
-            continue
-        i += 1
-    return -1
-
-
-def get_failure_array(pattern: str) -> list[int]:
-    """
-    Calculates the new index we should go to if we fail a comparison
-    :param pattern:
-    :return:
-    """
-    failure = [0]
-    i = 0
-    j = 1
-    while j < len(pattern):
-        if pattern[i] == pattern[j]:
-            i += 1
-        elif i > 0:
-            i = failure[i - 1]
-            continue
-        j += 1
-        failure.append(i)
-    return failure
+        if j == m:
+            found_indices.append(i - j)
+            j = lps[j - 1]
+        elif i < n and pattern[j] != text[i]:
+            if j != 0:
+                j = lps[j - 1]
+            else:
+                i += 1
+    return found_indices
 
 
 if __name__ == "__main__":
     import doctest
 
-    doctest.testmod()
-
-    # Test 1)
-    pattern = "abc1abc12"
-    text1 = "alskfjaldsabc1abc1abc12k23adsfabcabc"
-    text2 = "alskfjaldsk23adsfabcabc"
-    assert knuth_morris_pratt(text1, pattern)
-    assert knuth_morris_pratt(text2, pattern)
-
-    # Test 2)
-    pattern = "ABABX"
-    text = "ABABZABABYABABX"
-    assert knuth_morris_pratt(text, pattern)
-
-    # Test 3)
-    pattern = "AAAB"
-    text = "ABAAAAAB"
-    assert knuth_morris_pratt(text, pattern)
-
-    # Test 4)
-    pattern = "abcdabcy"
-    text = "abcxabcdabxabcdabcdabcy"
-    assert knuth_morris_pratt(text, pattern)
-
-    # Test 5) -> Doctests
-    kmp = "knuth_morris_pratt"
-    assert all(
-        knuth_morris_pratt(kmp, s) == kmp.find(s)
-        for s in ("kn", "h_m", "rr", "tt", "not there")
-    )
-
-    # Test 6)
-    pattern = "aabaabaaa"
-    assert get_failure_array(pattern) == [0, 1, 0, 1, 2, 3, 4, 5, 2]
+    doctest.testmod()