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POOff By One errors and Mutation Testing
 

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    <p>In the process of writing an "Off By One" mutation tester for my favourite mutation testing framework (<a href="http://ninjaturtles.codeplex.com" rel="nofollow noreferrer">NinjaTurtles</a>), I wrote the following code to provide an opportunity to check the correctness of my implementation:</p> <pre><code>public int SumTo(int max) { int sum = 0; for (var i = 1; i &lt;= max; i++) { sum += i; } return sum; } </code></pre> <p>now this seems simple enough, and it didn't strike me that there would be a problem trying to mutate all the literal integer constants in the IL. After all, there are only 3 (the <code>0</code>, the <code>1</code>, and the <code>++</code>). </p> <p><strong>WRONG!</strong></p> <p>It became very obvious on the first run that it was never going to work in this particular instance. Why? Because changing the code to </p> <pre><code>public int SumTo(int max) { int sum = 0; for (var i = 0; i &lt;= max; i++) { sum += i; } return sum; } </code></pre> <p>only adds 0 (zero) to the sum, and this obviously has no effect. Different story if it was the multiple set, but in this instance it was not.</p> <p>Now there's a fairly easy algorithm for working out the sum of integers</p> <pre><code>sum = max * (max + 1) / 2; </code></pre> <p>which I could have fail the mutations easily, since adding or subtracting 1 from either of the constants there will result in an error. (given that <code>max &gt;= 0</code>)</p> <p>So, problem solved for this particular case. Although it did not do what I wanted for the test of the mutation, which was to check what would happen when I lost the <code>++</code> - effectively an infinite loop. But that's another problem.</p> <p><strong>So - My Question:</strong> Are there any trivial or non-trivial cases where a loop starting from 0 or 1 may result in a "mutation off by one" test failure that cannot be refactored (code under test or test) in a similar way? (examples please)</p> <p><strong><em>Note</strong>: Mutation tests fail when the test suite passes after a mutation has been applied.</em></p> <p><strong>Update:</strong> an example of something less trivial, but something that could still have the test refactored so that it failed would be the following</p> <pre><code>public int SumArray(int[] array) { int sum = 0; for (var i = 0; i &lt; array.Length; i++) { sum += array[i]; } return sum; } </code></pre> <p>Mutation testing against this code would fail when changing the <code>var i=0</code> to <code>var i=1</code> if the test input you gave it was <code>new[] {0,1,2,3,4,5,6,7,8,9}</code>. However change the test input to <code>new[] {9,8,7,6,5,4,3,2,1,0}</code>, and the mutation testing will fail. So a successful refactor proves the testing.</p>
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