Software testing example with TDD problem: Commission

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

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# Execute the testing
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1. make
2. ./gtest_commission --gtest_filter=CommissionSpecTest.*

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# Generate coverage report
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1. ./gtest_commission --gtest_filter=CommissionCodeTest.CodeCoverage/(C0 | C1 | C2 | MCDC)/
2. gcov -b -c gtest_commission.c
3. refer to the commission.c.gcov for the detail report