Mutation testing

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Mutation testing (sometimes also called mutation analysis) is a method of software testing, which involves modifying program's source code in small ways.^[1] These, so-called mutations, are based on well-defined mutation operators that either mimic typical user mistakes (such as using the wrong operator or variable name) or force the creation of valuable tests (such as driving each expression to zero). The purpose is to help the tester develop effective tests or locate weaknesses in the test data used for the program or in sections of the code that are seldom or never accessed during execution.

Pioneered in the 1970s, mutation testing was originally intended to locate and expose weaknesses in test suites. The theory was that if a mutation was introduced without the behavior (generally output) of the program being affected, this indicated either that the code that had been mutated was never executed (redundant code) or that the testing suite was unable to locate the fault. In order for this to function at any scale, a large number of mutations had to be introduced into a large program, leading to the compilation and execution of an extremely large number of copies of the program. This problem of the expense of mutation testing, has reduced its practical use as a method of software testing.

Mutation testing was originally proposed by Richard Lipton as a student in 1971,^[2] and first developed and published by DeMillo, Lipton and Sayward. The first implementation of a mutation testing tool was by Timothy Budd as part of his PhD work (titled Mutation Analysis) in 1980 from Yale University.

Recently, with the availability of massive computing power, there has been a resurgence of mutation analysis within computer science community, and work has been done to define methods of applying mutation testing to object oriented programming languages and non-procedural languages such as XML, SMV, and finite state machines.

In 2007 a company called Certess Inc. extended many of the principles into the hardware verification domain. Whereas mutation analysis only expects to detect a difference in the output produced, Certess extends this by verifying that a checker in the testbench will actually detect the difference. This extension means that all three stages of verification, namely: activation, propagation and detection are evaluated. They have called this functional qualification.

[edit] Mutation testing overview

Mutation testing is done by selecting a set of mutation operators and then applying them to the source program one at a time for each applicable piece of the source code. The result of applying of one mutation operator to the program is called a mutant. If the test suite is able to detect the change (i.e. one of tests fails), then mutant is said to be killed.

For example, let's consider the following C++ code fragment:

if (a && b)
    c = 1;
else
    c = 0;

The condition mutation operator would replace '&&' with '||' and produce the following mutant:

if (a || b)
    c = 1;
else
    c = 0;

Now, for the test to kill this mutant, the following condition should be met:

• Test input data should cause different program states for mutant and original program. For example, a test with a=1 and b=0 would do this.
• The value of 'c' should be propagated to the program's output and checked by the test.

Weak mutation testing (or weak mutation coverage) requires that only the first condition is satisfied. Strong mutation testing requires that both conditions are satisfied. Strong mutation is more poweful, since it ensures that the test suite can really catch the problems. Weak mutation is closely related to code coverage methods. It requires much less computing power to ensure that the test suite satisfies weak mutation testing than strong mutation testing.

[edit] Equivalent mutants

Many mutation operators can produce equivalent mutants. For example, let's consider the following code fragment:

int index=0;
while (...)
{
    . . .; 
    index++;
    if (index==10)
        break;
}

Boolean relation mutation operator will replace "==" with ">=" and produce the following mutant:

int index=0;
while (...)
{
    . . .; 
    index++;
    if (index>=10)
        break;
}

However, it is not possible to find a test case, which could kill this mutant. The resulting program is equaivalent to the original one. Such mutants are called equivalent mutants.

[edit] Mutation operators

A variety of mutation operators were explored by researchers. Here are some examples of mutation operators for imperative languages:

• Statement deletion.
• Replace each boolean subexpression with true and false.
• Replace each arithmetic operation with another one, e.g. + with *, - and /.
• Replace each boolean relation with another one, e.g. > with >=, == and <=.
• Replace each variable with another variable declared in the same scope (variable types should be the same).

Beside this, there are mutation operators for object-oriented languages^[3] , for concurrent constructions^[4], complex objects like containers^[5] etc.

[edit] See also

• Fault injection

[edit] References

[edit] External links

• Mutation testing online an open community which brings together the Hardware and Software research communities studying mutation testing.
• Mutation testing list of tools and publications by Jeff Offutt.
• Certitude Software from Certess Inc.