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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool"></a><a class="link" href="BoyerMooreHorspool.html" title="Boyer-Moore-Horspool Search">Boyer-Moore-Horspool
  28       Search</a>
  29 </h3></div></div></div>
  30 <h5>
  31 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h0"></a>
  32         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.overview"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.overview">Overview</a>
  33       </h5>
  34 <p>
  35         The header file 'boyer_moore_horspool.hpp' contains an implementation of
  36         the Boyer-Moore-Horspool algorithm for searching sequences of values.
  37       </p>
  38 <p>
  39         The Boyer-Moore-Horspool search algorithm was published by Nigel Horspool
  40         in 1980. It is a refinement of the Boyer-Moore algorithm that trades space
  41         for time. It uses less space for internal tables than Boyer-Moore, and has
  42         poorer worst-case performance.
  43       </p>
  44 <p>
  45         The Boyer-Moore-Horspool algorithm cannot be used with comparison predicates
  46         like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">search</span></code>.
  47       </p>
  48 <h5>
  49 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h1"></a>
  50         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.interface"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.interface">Interface</a>
  51       </h5>
  52 <p>
  53         Nomenclature: I refer to the sequence being searched for as the "pattern",
  54         and the sequence being searched in as the "corpus".
  55       </p>
  56 <p>
  57         For flexibility, the Boyer-Moore-Horspool algorithm has two interfaces; an
  58         object-based interface and a procedural one. The object-based interface builds
  59         the tables in the constructor, and uses operator () to perform the search.
  60         The procedural interface builds the table and does the search all in one
  61         step. If you are going to be searching for the same pattern in multiple corpora,
  62         then you should use the object interface, and only build the tables once.
  63       </p>
  64 <p>
  65         Here is the object interface:
  66 </p>
  67 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">patIter</span><span class="special">&gt;</span>
  68 <span class="keyword">class</span> <span class="identifier">boyer_moore_horspool</span> <span class="special">{</span>
  69 <span class="keyword">public</span><span class="special">:</span>
  70     <span class="identifier">boyer_moore_horspool</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>
  71     <span class="special">~</span><span class="identifier">boyer_moore_horspool</span> <span class="special">();</span>
  72
  73     <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">corpusIter</span><span class="special">&gt;</span>
  74     <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>
  75     <span class="special">};</span>
  76 </pre>
  77 <p>
  78       </p>
  79 <p>
  80         and here is the corresponding procedural interface:
  81       </p>
  82 <p>
  83 </p>
  84 <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</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">&gt;</span>
  85 <span class="identifier">corpusIter</span> <span class="identifier">boyer_moore_horspool_search</span> <span class="special">(</span>
  86         <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>
  87         <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>
  88 </pre>
  89 <p>
  90       </p>
  91 <p>
  92         Each of the functions is passed two pairs of iterators. The first two define
  93         the corpus and the second two define the pattern. Note that the two pairs
  94         need not be of the same type, but they do need to "point" at the
  95         same type. In other words, <code class="computeroutput"><span class="identifier">patIter</span><span class="special">::</span><span class="identifier">value_type</span></code>
  96         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.
  97       </p>
  98 <p>
  99         The return value of the function is an iterator pointing to the start of
 100         the pattern in the corpus. If the pattern is not found, it returns the end
 101         of the corpus (<code class="computeroutput"><span class="identifier">corpus_last</span></code>).
 102       </p>
 103 <h5>
 104 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h2"></a>
 105         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.performance"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.performance">Performance</a>
 106       </h5>
 107 <p>
 108         The execution time of the Boyer-Moore-Horspool algorithm is linear in the
 109         size of the string being searched; it can have a significantly lower constant
 110         factor than many other search algorithms: it doesn't need to check every
 111         character of the string to be searched, but rather skips over some of them.
 112         Generally the algorithm gets faster as the pattern being searched for becomes
 113         longer. Its efficiency derives from the fact that with each unsuccessful
 114         attempt to find a match between the search string and the text it is searching,
 115         it uses the information gained from that attempt to rule out as many positions
 116         of the text as possible where the string cannot match.
 117       </p>
 118 <h5>
 119 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h3"></a>
 120         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.memory_use"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.memory_use">Memory
 121         Use</a>
 122       </h5>
 123 <p>
 124         The algorithm an internal table that has one entry for each member of the
 125         "alphabet" in the pattern. For (8-bit) character types, this table
 126         contains 256 entries.
 127       </p>
 128 <h5>
 129 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h4"></a>
 130         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.complexity"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.complexity">Complexity</a>
 131       </h5>
 132 <p>
 133         The worst-case performance is <span class="emphasis"><em>O(m x n)</em></span>, where <span class="emphasis"><em>m</em></span>
 134         is the length of the pattern and <span class="emphasis"><em>n</em></span> is the length of
 135         the corpus. The average time is <span class="emphasis"><em>O(n)</em></span>. The best case
 136         performance is sub-linear, and is, in fact, identical to Boyer-Moore, but
 137         the initialization is quicker and the internal loop is simpler than Boyer-Moore.
 138       </p>
 139 <h5>
 140 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h5"></a>
 141         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.exception_safety"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.exception_safety">Exception
 142         Safety</a>
 143       </h5>
 144 <p>
 145         Both the object-oriented and procedural versions of the Boyer-Moore-Horspool
 146         algorithm take their parameters by value and do not use any information other
 147         than what is passed in. Therefore, both interfaces provide the strong exception
 148         guarantee.
 149       </p>
 150 <h5>
 151 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h6"></a>
 152         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.notes"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.notes">Notes</a>
 153       </h5>
 154 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 155 <li class="listitem">
 156             When using the object-based interface, the pattern must remain unchanged
 157             for during the searches; i.e, from the time the object is constructed
 158             until the final call to operator () returns.
 159           </li>
 160 <li class="listitem">
 161             The Boyer-Moore-Horspool algorithm requires random-access iterators for
 162             both the pattern and the corpus.
 163           </li>
 164 </ul></div>
 165 <h5>
 166 <a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h7"></a>
 167         <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.customization_points"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.customization_points">Customization
 168         points</a>
 169       </h5>
 170 <p>
 171         The Boyer-Moore-Horspool object takes a traits template parameter which enables
 172         the caller to customize how the precomputed table is stored. This table,
 173         called the skip table, contains (logically) one entry for every possible
 174         value that the pattern can contain. When searching 8-bit character data,
 175         this table contains 256 elements. The traits class defines the table to be
 176         used.
 177       </p>
 178 <p>
 179         The default traits class uses a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code>
 180         for small 'alphabets' and a <code class="computeroutput"><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_map</span></code>
 181         for larger ones. The array-based skip table gives excellent performance,
 182         but could be prohibitively large when the 'alphabet' of elements to be searched
 183         grows. The unordered_map based version only grows as the number of unique
 184         elements in the pattern, but makes many more heap allocations, and gives
 185         slower lookup performance.
 186       </p>
 187 <p>
 188         To use a different skip table, you should define your own skip table object
 189         and your own traits class, and use them to instantiate the Boyer-Moore-Horspool
 190         object. The interface to these objects is described TBD.
 191       </p>
 192 </div>
 193 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 194 <td align="left"></td>
 195 <td align="right"><div class="copyright-footer">Copyright &#169; 2010-2012 Marshall Clow<p>
 196         Distributed under the Boost Software License, Version 1.0. (See accompanying
 197         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>)
 198       </p>
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