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POBorland string::find bug
 

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    <p>I'm supporting a C++ application written using Borland C++ Builder 5.02 (from 1997). The find() method on the Borland string class does not behave how I would expect:</p> <pre><code>#include &lt;cstring&gt; #include &lt;iostream&gt; int main (int argc, char *argv[]) { string needle = "length == eighteen"; string haystack = "&lt;" + needle + "&gt;"; if (haystack.find(needle) != NPOS) cout &lt;&lt; "Found it!" &lt;&lt; endl; else cout &lt;&lt; "Not found" &lt;&lt; endl; return 0; } </code></pre> <p>This program outputs <code>Not found</code>. If I change the needle to something shorter it outputs <code>Found it!</code>. If I exchange the angle brackets for some other characters it finds it. Spaces work, but parentheses also don't.</p> <p>Note that I am using the Borland string library here: if I <code>#include &lt;string&gt;</code> and use <code>std::string</code> instead then it works exactly how I would expect. Sadly changing the whole application to use STL strings is not a feasible answer!</p> <p>From the documentation it seems that Borland uses a hash-based algorithm for string search. I can't find any more details about this, and I've stepped through the disassembly but am not much the wiser.</p> <p>I find it very hard to believe that this is really a bug in the string library, particularly since if it were then I would expect to be able to find an article or something about it. I can't find any such information.</p> <p>However, I've run out of ideas! Is this a known bug? Is there a fix?</p> <p>EDIT: Having looked again at the disassembly, I think it's trying to do something like the Rabin-Karp algorithm, where the hash function is calculated mod 33554393 (the largest prime &lt; 2^25). It could well be the polynomial hash function with a base of 32 (i.e. a_0 + 32 a_1 + 32^2 a_2 + .. + 32^n a_n) but that's just a hunch. Sounds like a possible overflow as Daniel Fischer suggested.</p>
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    1. USJohan
    1. USDave Turner
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      1. USdten
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    1. CO"hash-based algorithm", "If I change the needle to something shorter" <- that smells of integer overflow.
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      1. USDaniel Fischer
    2. COThis is what happens when you rely on 15 year old compilers. Time to move on.
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      1. USDavid Heffernan
    3. CO@DavidHeffernan It can be difficult to move on when there's no budget.
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      1. USPeter Wood
 

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