Testing Approaches: Functional Testing versus Structural Testing and Mutation testing

In functional testing, functionality of the module is tested and structure is not considered. Test cases based on specifications and internals of modules are not considered. This type of testing is also known as black box testing. The objective here is to ensure that `correct results'' are obtained when `good inputs'' are applied to the various parts of the design, and when `bad inputs'' are applied the design operates in a predictable manner. Functional testing can therefore be considered as concentrating on checking that the data paths operate correctly.

Structural testing is used to test the implementation of the program. Structural testing, also known as white box or open box testing, is normally applied to sequential HDL code and concentrates on checking that all executable statements within each module. Structural testing involves :
- Statement coverage : This ensures that each and every statement is tested. Software tools called profilers are used to carry this statement coverage.
- Branch coverage : Each and every condition is taken, and inputs are given in such a way that each branch is executed at least once.
- Path Coverage : To test loops, this is required. For example the statement
for(i=o;i<=100;i++) is executed 101 times.

Mutation testing is required to ensure that the software does not fail. It is a good debugging mechanism. The program is modified slightly to obtain mutants of the program. Different mutants are tested with the same test cases. If the mutant fails, and the actual program works correctly, confidence is gained in the program and test cases are considered good. To produce mutants, mutation operators are defined.
- Constant replacement
- Variable replacement
- Arithmetic operator replacement
- Relational operator replacement
- Goto label replacement