Once there are tests in place, the majority of them will fail and work can begin on implementing the function to correct that – the green phase.

For this example, an initial attempt is made to simply leverage the standard math library by including <math.h> (Figure 7).

Code is written to check that the angle is not out of range. If it is, then it returns 0; otherwise it simply invokes the sin function provided by the math library, multiplies the result by the scaling factor, and converts that into a 16 bit integer. The code is built to ensure that it compiles correctly.

Figure 7: First pass attempt at creating suitable code leverages the standard math library

As before, the 36 tests are executed on this new version of the safe_sine function. Once again the four tests that check if the angle is out of range all pass, but every other function fails. However, this time they fail due to timing, meaning that the execution time exceeds the specified timing budget (Figure 8).

Figure 8: The code is functionally correct when the math library is used but it takes too long to execute

Second iteration: Developing the function (green phase)