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New issue from Hewill: "boyer_moore_searcher and boyer_moore_horspool_searcher should be constexpr-friendly"
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‎xml/issue4365.xml

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<?xml version='1.0' encoding='utf-8' standalone='no'?>
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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
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<issue num="4365" status="New">
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<title>`boyer_moore_searcher` and `boyer_moore_horspool_searcher` should be `constexpr`-friendly</title>
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<section>
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<sref ref="[func.search.bm]"/><sref ref="[func.search.bmh]"/>
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</section>
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<submitter>Hewill Kang</submitter>
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<date>03 Sep 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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Currently, `boyer_moore_searcher` and `boyer_moore_horspool_searcher` are not
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`constexpr`-friendly because their underlying implementation needs to precompute
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the shift table, which usually requires a `vector` or `unordered_map` to store.
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<p/>
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Thanks to <paper num="P3372R3"/>, unordered containers are now `constexpr`-friendly.
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Although `std::hash` still lacks `constexpr` support, users can provide their own
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hash functions to use unordered containers at compile time.
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<p/>
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Given that both `boyer_moore_searcher` and `boyer_moore_horspool_searcher` can
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take a custom hash, it makes perfect sense that they could be `constexpr`-friendly.
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<p/>
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Not to mention that library implementations usually simply use arrays instead of
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hash tables for the common string case because characters only have 256 values,
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so `unordered_map` is not actually used.
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</p>
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</discussion>
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<resolution>
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<p>
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This wording is relative to <paper num="N5014"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[func.search.bm]"/> as indicated:</p>
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<blockquote>
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<blockquote>
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<pre>
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namespace std {
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template&lt;class RandomAccessIterator1,
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class Hash = hash&lt;typename iterator_traits&lt;RandomAccessIterator1&gt;::value_type&gt;,
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class BinaryPredicate = equal_to&lt;&gt;&gt;
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class boyer_moore_searcher {
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public:
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<ins>constexpr</ins> boyer_moore_searcher(RandomAccessIterator1 pat_first,
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RandomAccessIterator1 pat_last,
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Hash hf = Hash(),
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BinaryPredicate pred = BinaryPredicate());
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template&lt;class RandomAccessIterator2&gt;
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<ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
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operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
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private:
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RandomAccessIterator1 pat_first_; // <i>exposition only</i>
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RandomAccessIterator1 pat_last_; // <i>exposition only</i>
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Hash hash_; // <i>exposition only</i>
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BinaryPredicate pred_; // <i>exposition only</i>
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};
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}
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</pre>
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</blockquote>
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<pre>
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<ins>constexpr</ins> boyer_moore_searcher(RandomAccessIterator1 pat_first,
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RandomAccessIterator1 pat_last,
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Hash hf = Hash(),
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BinaryPredicate pred = BinaryPredicate());
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</pre>
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<blockquote>
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<p>
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-1- <i>Preconditions</i>: The value type of <code>RandomAccessIterator1</code> meets the
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<i>Cpp17DefaultConstructible</i>, <i>Cpp17CopyConstructible</i>, and <i>Cpp17CopyAssignable</i>
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requirements.
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</p>
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</blockquote>
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<blockquote>
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[&hellip;]
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</blockquote>
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<pre>
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template&lt;class RandomAccessIterator2&gt;
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<ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
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operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
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</pre>
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<blockquote>
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<p>
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-5- <i>Mandates</i>: <code>RandomAccessIterator1</code> and <code>RandomAccessIterator2</code>
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have the same value type.
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[func.search.bmh]"/> as indicated:</p>
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<blockquote>
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<blockquote>
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<pre>
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namespace std {
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template&lt;class RandomAccessIterator1,
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class Hash = hash&lt;typename iterator_traits&lt;RandomAccessIterator1&gt;::value_type&gt;,
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class BinaryPredicate = equal_to&lt;&gt;&gt;
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class boyer_moore_horspool_searcher {
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public:
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<ins>constexpr</ins> boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
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RandomAccessIterator1 pat_last,
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Hash hf = Hash(),
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BinaryPredicate pred = BinaryPredicate());
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template&lt;class RandomAccessIterator2&gt;
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<ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
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operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
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private:
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RandomAccessIterator1 pat_first_; // <i>exposition only</i>
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RandomAccessIterator1 pat_last_; // <i>exposition only</i>
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Hash hash_; // <i>exposition only</i>
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BinaryPredicate pred_; // <i>exposition only</i>
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};
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}
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</pre>
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</blockquote>
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<pre>
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<ins>constexpr</ins> boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
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RandomAccessIterator1 pat_last,
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Hash hf = Hash(),
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BinaryPredicate pred = BinaryPredicate());
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</pre>
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<blockquote>
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<p>
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-1- <i>Preconditions</i>: The value type of <code>RandomAccessIterator1</code> meets the
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<i>Cpp17DefaultConstructible</i>, <i>Cpp17CopyConstructible</i>, and <i>Cpp17CopyAssignable</i>
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requirements.
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</p>
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</blockquote>
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<blockquote>
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[&hellip;]
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</blockquote>
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<pre>
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template&lt;class RandomAccessIterator2&gt;
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<ins>constexpr</ins> pair&lt;RandomAccessIterator2, RandomAccessIterator2&gt;
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operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;
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</pre>
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<blockquote>
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<p>
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-5- <i>Mandates</i>: <code>RandomAccessIterator1</code> and <code>RandomAccessIterator2</code>
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have the same value type.
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</p>
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</blockquote>
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</blockquote>
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</li>
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</ol></resolution>
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</issue>

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