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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="algorithm.Searching"></a><a class="link" href="Searching.html" title="Searching Algorithms">Searching Algorithms</a> |
| </h2></div></div></div> |
| <div class="toc"><dl class="toc"> |
| <dt><span class="section"><a href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore">Boyer-Moore |
| Search</a></span></dt> |
| <dt><span class="section"><a href="../the_boost_algorithm_library/Searching/BoyerMooreHorspool.html">Boyer-Moore-Horspool |
| Search</a></span></dt> |
| <dt><span class="section"><a href="../the_boost_algorithm_library/Searching/KnuthMorrisPratt.html">Knuth-Morris-Pratt |
| Search</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore"></a><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore" title="Boyer-Moore Search">Boyer-Moore |
| Search</a> |
| </h3></div></div></div> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h0"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.overview"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.overview">Overview</a> |
| </h5> |
| <p> |
| The header file 'boyer_moore.hpp' contains an implementation of the Boyer-Moore |
| algorithm for searching sequences of values. |
| </p> |
| <p> |
| The Boyer–Moore string search algorithm is a particularly efficient string |
| searching algorithm, and it has been the standard benchmark for the practical |
| string search literature. The Boyer-Moore algorithm was invented by Bob Boyer |
| and J. Strother Moore, and published in the October 1977 issue of the Communications |
| of the ACM , and a copy of that article is available at <a href="http://www.cs.utexas.edu/~moore/publications/fstrpos.pdf" target="_top">http://www.cs.utexas.edu/~moore/publications/fstrpos.pdf</a>. |
| </p> |
| <p> |
| The Boyer-Moore algorithm uses two precomputed tables to give better performance |
| than a naive search. These tables depend on the pattern being searched for, |
| and give the Boyer-Moore algorithm larger a memory footprint and startup |
| costs than a simpler algorithm, but these costs are recovered quickly during |
| the searching process, especially if the pattern is longer than a few elements. |
| </p> |
| <p> |
| However, the Boyer-Moore algorithm cannot be used with comparison predicates |
| like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">search</span></code>. |
| </p> |
| <p> |
| Nomenclature: I refer to the sequence being searched for as the "pattern", |
| and the sequence being searched in as the "corpus". |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h1"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.interface"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.interface">Interface</a> |
| </h5> |
| <p> |
| For flexibility, the Boyer-Moore algorithm has two interfaces; an object-based |
| interface and a procedural one. The object-based interface builds the tables |
| in the constructor, and uses operator () to perform the search. The procedural |
| interface builds the table and does the search all in one step. If you are |
| going to be searching for the same pattern in multiple corpora, then you |
| should use the object interface, and only build the tables once. |
| </p> |
| <p> |
| Here is the object interface: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">patIter</span><span class="special">></span> |
| <span class="keyword">class</span> <span class="identifier">boyer_moore</span> <span class="special">{</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">boyer_moore</span> <span class="special">(</span> <span class="identifier">patIter</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">patIter</span> <span class="identifier">last</span> <span class="special">);</span> |
| <span class="special">~</span><span class="identifier">boyer_moore</span> <span class="special">();</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">corpusIter</span><span class="special">></span> |
| <span class="identifier">corpusIter</span> <span class="keyword">operator</span> <span class="special">()</span> <span class="special">(</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_first</span><span class="special">,</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_last</span> <span class="special">);</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| and here is the corresponding procedural interface: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">patIter</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">corpusIter</span><span class="special">></span> |
| <span class="identifier">corpusIter</span> <span class="identifier">boyer_moore_search</span> <span class="special">(</span> |
| <span class="identifier">corpusIter</span> <span class="identifier">corpus_first</span><span class="special">,</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_last</span><span class="special">,</span> |
| <span class="identifier">patIter</span> <span class="identifier">pat_first</span><span class="special">,</span> <span class="identifier">patIter</span> <span class="identifier">pat_last</span> <span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Each of the functions is passed two pairs of iterators. The first two define |
| the corpus and the second two define the pattern. Note that the two pairs |
| need not be of the same type, but they do need to "point" at the |
| same type. In other words, <code class="computeroutput"><span class="identifier">patIter</span><span class="special">::</span><span class="identifier">value_type</span></code> |
| and <code class="computeroutput"><span class="identifier">curpusIter</span><span class="special">::</span><span class="identifier">value_type</span></code> need to be the same type. |
| </p> |
| <p> |
| The return value of the function is an iterator pointing to the start of |
| the pattern in the corpus. If the pattern is not found, it returns the end |
| of the corpus (<code class="computeroutput"><span class="identifier">corpus_last</span></code>). |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h2"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.performance"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.performance">Performance</a> |
| </h5> |
| <p> |
| The execution time of the Boyer-Moore algorithm, while still linear in the |
| size of the string being searched, can have a significantly lower constant |
| factor than many other search algorithms: it doesn't need to check every |
| character of the string to be searched, but rather skips over some of them. |
| Generally the algorithm gets faster as the pattern being searched for becomes |
| longer. Its efficiency derives from the fact that with each unsuccessful |
| attempt to find a match between the search string and the text it is searching, |
| it uses the information gained from that attempt to rule out as many positions |
| of the text as possible where the string cannot match. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h3"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.memory_use"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.memory_use">Memory |
| Use</a> |
| </h5> |
| <p> |
| The algorithm allocates two internal tables. The first one is proportional |
| to the length of the pattern; the second one has one entry for each member |
| of the "alphabet" in the pattern. For (8-bit) character types, |
| this table contains 256 entries. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h4"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.complexity"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.complexity">Complexity</a> |
| </h5> |
| <p> |
| The worst-case performance to find a pattern in the corpus is <span class="emphasis"><em>O(N)</em></span> |
| (linear) time; that is, proportional to the length of the corpus being searched. |
| In general, the search is sub-linear; not every entry in the corpus need |
| be checked. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h5"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.exception_safety"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.exception_safety">Exception |
| Safety</a> |
| </h5> |
| <p> |
| Both the object-oriented and procedural versions of the Boyer-Moore algorithm |
| take their parameters by value and do not use any information other than |
| what is passed in. Therefore, both interfaces provide the strong exception |
| guarantee. |
| </p> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h6"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.notes"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.notes">Notes</a> |
| </h5> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| When using the object-based interface, the pattern must remain unchanged |
| for during the searches; i.e, from the time the object is constructed |
| until the final call to operator () returns. |
| </li> |
| <li class="listitem"> |
| The Boyer-Moore algorithm requires random-access iterators for both the |
| pattern and the corpus. |
| </li> |
| </ul></div> |
| <h5> |
| <a name="the_boost_algorithm_library.Searching.BoyerMoore.h7"></a> |
| <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMoore.customization_points"></a></span><a class="link" href="Searching.html#the_boost_algorithm_library.Searching.BoyerMoore.customization_points">Customization |
| points</a> |
| </h5> |
| <p> |
| The Boyer-Moore object takes a traits template parameter which enables the |
| caller to customize how one of the precomputed tables is stored. This table, |
| called the skip table, contains (logically) one entry for every possible |
| value that the pattern can contain. When searching 8-bit character data, |
| this table contains 256 elements. The traits class defines the table to be |
| used. |
| </p> |
| <p> |
| The default traits class uses a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code> |
| for small 'alphabets' and a <code class="computeroutput"><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_map</span></code> |
| for larger ones. The array-based skip table gives excellent performance, |
| but could be prohibitively large when the 'alphabet' of elements to be searched |
| grows. The unordered_map based version only grows as the number of unique |
| elements in the pattern, but makes many more heap allocations, and gives |
| slower lookup performance. |
| </p> |
| <p> |
| To use a different skip table, you should define your own skip table object |
| and your own traits class, and use them to instantiate the Boyer-Moore object. |
| The interface to these objects is described TBD. |
| </p> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2010-2012 Marshall Clow<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
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