Add Boyer-Moore-Horspool string-search algorithm

DIRECTORY.md

@@ -1327,6 +1327,7 @@
   * [Autocomplete Using Trie](strings/autocomplete_using_trie.py)
   * [Barcode Validator](strings/barcode_validator.py)
   * [Bitap String Match](strings/bitap_string_match.py)
+  * [Boyer Moore Horspool](strings/boyer_moore_horspool.py)
   * [Boyer Moore Search](strings/boyer_moore_search.py)
   * [Camel Case To Snake Case](strings/camel_case_to_snake_case.py)
   * [Can String Be Rearranged As Palindrome](strings/can_string_be_rearranged_as_palindrome.py)

strings/boyer_moore_horspool.py

@@ -0,0 +1,131 @@
+"""
+Boyer-Moore-Horspool string-search algorithm.
+
+A simplification of the Boyer-Moore algorithm that keeps only the
+bad-character shift table (Horspool's variant).  It still runs in
+sub-linear time on average (roughly O(n / m) for random text) while
+worst case is O(n * m).  Memory is O(sigma) where sigma is the size
+of the alphabet that appears in the pattern.
+
+Reference: https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore%E2%80%93Horspool_algorithm
+"""
+
+from __future__ import annotations
+
+
+def _build_shift_table(pattern: str) -> dict[str, int]:
+    """
+    Build the bad-character shift table for ``pattern``.
+
+    For every character in the pattern except the last one, the table
+    stores the distance from that character to the end of the pattern.
+    Characters that do not appear in the pattern fall back to ``len(pattern)``
+    at lookup time.
+
+    >>> _build_shift_table("abcab")
+    {'a': 1, 'b': 3, 'c': 2}
+    >>> _build_shift_table("a")
+    {}
+    >>> _build_shift_table("")
+    {}
+    >>> _build_shift_table("aaaa")
+    {'a': 1}
+    """
+    pattern_length = len(pattern)
+    table: dict[str, int] = {}
+    for index in range(pattern_length - 1):
+        table[pattern[index]] = pattern_length - 1 - index
+    return table
+
+
+def boyer_moore_horspool_search(text: str, pattern: str) -> int:
+    """
+    Return the index of the first occurrence of ``pattern`` in ``text``
+    or ``-1`` if the pattern does not appear.
+
+    An empty pattern matches at position ``0`` (the same convention used by
+    :py:meth:`str.find`).
+
+    >>> boyer_moore_horspool_search("ABAAABCD", "ABC")
+    4
+    >>> boyer_moore_horspool_search("hello world", "world")
+    6
+    >>> boyer_moore_horspool_search("hello world", "Python")
+    -1
+    >>> boyer_moore_horspool_search("aaaaa", "aa")
+    0
+    >>> boyer_moore_horspool_search("anything", "")
+    0
+    >>> boyer_moore_horspool_search("", "x")
+    -1
+    >>> sample = "the quick brown fox jumps over the lazy dog"
+    >>> boyer_moore_horspool_search(sample, "fox") == sample.find("fox")
+    True
+    >>> boyer_moore_horspool_search(sample, "cat") == sample.find("cat")
+    True
+    """
+    pattern_length = len(pattern)
+    text_length = len(text)
+    if pattern_length == 0:
+        return 0
+    if pattern_length > text_length:
+        return -1
+
+    shift_table = _build_shift_table(pattern)
+    skip = 0
+    while text_length - skip >= pattern_length:
+        index = pattern_length - 1
+        while index >= 0 and pattern[index] == text[skip + index]:
+            index -= 1
+        if index < 0:
+            return skip
+        skip += shift_table.get(text[skip + pattern_length - 1], pattern_length)
+    return -1
+
+
+def boyer_moore_horspool_search_all(text: str, pattern: str) -> list[int]:
+    """
+    Return every starting index where ``pattern`` occurs in ``text``.
+
+    Overlapping matches are reported (e.g. ``"aaa"`` contains ``"aa"``
+    at indices ``0`` and ``1``).  An empty pattern matches at every
+    position from ``0`` to ``len(text)`` inclusive, mirroring
+    :py:meth:`str.find` and :py:func:`re.finditer` conventions.
+
+    >>> boyer_moore_horspool_search_all("ababcabab", "ab")
+    [0, 2, 5, 7]
+    >>> boyer_moore_horspool_search_all("aaaa", "aa")
+    [0, 1, 2]
+    >>> boyer_moore_horspool_search_all("abcdef", "gh")
+    []
+    >>> boyer_moore_horspool_search_all("abc", "")
+    [0, 1, 2, 3]
+    >>> boyer_moore_horspool_search_all("", "abc")
+    []
+    """
+    pattern_length = len(pattern)
+    text_length = len(text)
+    if pattern_length == 0:
+        return list(range(text_length + 1))
+    if pattern_length > text_length:
+        return []
+
+    shift_table = _build_shift_table(pattern)
+    matches: list[int] = []
+    skip = 0
+    while text_length - skip >= pattern_length:
+        index = pattern_length - 1
+        while index >= 0 and pattern[index] == text[skip + index]:
+            index -= 1
+        if index < 0:
+            matches.append(skip)
+            skip += 1
+        else:
+            skip += shift_table.get(text[skip + pattern_length - 1], pattern_length)
+    return matches
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()