Revision b49dda04-7efd-415c-9c0d-aab9dfe39e5d - Code Golf Stack Exchange
# [C (clang)], <s>82</s> 78 bytes
<!-- language-all: lang-c -->
d;f(*s,*r,n){for(*r=n>3?strcmp(s,++s):0;*s;*r*=!d|d<1^*r>0)d=strcmp(s-1,s++);}
[Try it online!][TIO-m7de0a9k]
[C (clang)]: http://clang.llvm.org/
[TIO-m7de0a9k]: https://tio.run/##dVJdT8IwFH3nV1QSpCtdgvHNOkwdBRrHSmCRGD8Ssg@yZEyyzvCg/HVnWQtOo/ehO7n3nHN7exfaYbbK11UVkQQiiVGBc@s9eS0gKpx8cHkjyyLcbKHEvZ60rvoESYIK5JxFH9H1xQsqBn0rco4k@wLLXs8i@yrNS7BZpTm0wHsLqFAJBMpYlo/PwDG5Q3htOmSzoI0VumVj7mvEPU8DsVgYIBaa5U74dFajO18sFWh42cdo40Z2MRP@mNUSBYVuEcyF6bEU3ugH/2@XJQ3cSS1gdB5o5HPf2I2E0Pekc21K701H5jE34Ib2wPQ4vmYP2Yj7/FRVLZiRc1/LXTHUgFI6UVFjXp/TqS7curRx0X6N9qT@qEUCeNhF6vSJfv70mYBULamxgwOhiOVbVhLwK06cBO5CGb1toTGxMDjXGgxUJYvzU8Ui384JgMjkgeOArt21Wk3/baGaJ7DdyeRTrsY5enxbxJmM/5OoP6kTNXXYzGH0@9a@@gyTbLWWlb37Ag "C (clang) – Try It Online"
-4 bytes thanks to @ceilingcat!
Returns:<br>
- < 0 for Increasing Wavy
- > 0 for Decreasing Wavy
- 0 for not Wavy