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POFast algorithm for searching for substrings in a string
 

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    <p>I'd like an efficient algorithm (or library) that I can use in Java to search for substrings in a string. </p> <p>What I would like to do is:</p> <p>Given an input string - <strong>INSTR</strong>:</p> <blockquote> <p>"BCDEFGH"</p> </blockquote> <p>And a set of candidate strings - <strong>CAND</strong>:</p> <blockquote> <p>"AB", "CDE", "FG", "H", "IJ"</p> </blockquote> <p><em>Find any <strong>CAND</strong> strings that match as substrings within <strong>INSTR</em></strong></p> <p>In this example I would match "CDE", "FG", and "H" (but not "AB" and "IJ")</p> <p>There could be many thousand candidate strings (in CAND), but more importantly I will be doing this search many millions of times so I need it to be FAST.</p> <p>I'd like to work with char arrays. Also, I'm not intested in architectural solutions, like distributing the search - just the most efficient function/algorithm for doing it locally.</p> <p>Additionally, all the strings in CAND and INSTR will all be relatively small (&lt; 50 chars) - i.e. the target string INSTR is NOT long relative to the candidate strings.</p> <hr> <p><strong>Update</strong> I should have mentioned, the set of CAND strings is invariant across all values of INSTR. </p> <p><strong>Update</strong> I only need to know that there was a match - and i don't need to know what the match was.</p> <p><strong>Final Update</strong> I opted to try AhoCorsick and Rabin-Karp, due to simplicity of implementation. Because I have variable length patterns I used a modified Rabin-Karp that hashes the first n characters of each pattern, where n is the length of the smallest pattern, N was then the length of my rolling substring search window. For the Aho Corsick I used <a href="http://hkn.eecs.berkeley.edu/~dyoo/java/index.html" rel="noreferrer">this</a></p> <p>In my test i searched for 1000 patterns in two documents news paper articles, averaged across 1000 iterations etc... Normalised times to complete were: </p> <p><strong>AhoCorsick</strong>: 1</p> <p><strong>RabinKarp</strong>: 1.8</p> <p><strong>Naive Search</strong> (check each pattern &amp; use string.contains): 50</p> <hr> <p>*Some resources describing the algos mentioned in the answers below:</p> <p><a href="http://www.seas.gwu.edu/~simhaweb/cs151/lectures/module5/module5.html" rel="noreferrer">http://www.seas.gwu.edu/~simhaweb/cs151/lectures/module5/module5.html</a></p> <p><a href="http://www.cs.princeton.edu/courses/archive/spr09/cos226/lectures/18SubstringSearch-2x2.pdf" rel="noreferrer">http://www.cs.princeton.edu/courses/archive/spr09/cos226/lectures/18SubstringSearch-2x2.pdf</a></p> <p><a href="http://www-igm.univ-mlv.fr/~lecroq/string/index.html" rel="noreferrer">http://www-igm.univ-mlv.fr/~lecroq/string/index.html</a>*</p>
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    1. COBTW - this is NOT homework - but a real world problem!
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    2. COHow long are the input strings in relation to the candidate strings?
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    3. COThey are short. Input strings are typically less then 40 characters, as are the candidate strings.
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