Software testing example with TDD problem: NextDate

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

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# Execute the testing
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1. make
2. ./gtest_nextdate --gtest_filter=NextDateSpecTest.*

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