Software testing example with TDD problem: Triangle

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# File annotation
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1. gtest_main.cc:        main() entry point of google test
2. triangle.c:           target function to be tested
3. triangle_unittest.cc: test cases with google testing framework

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# Execute the specification-based testing
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1. make
2. ./gtest_triangle --gtest_filter=TriangleSpecTest.*

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# Execute the code-based testing & generate coverage report
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1. ./gtest_triangle --gtest_filter=TriangleCodeTest.CodeCoverage/(C0 | C1 | C2 | MCDC)/
2. gcov -b -c gtest_triangle.c
3. refer to the triangle.c.gcov for the detail report