Similar code detector

I've used MOSS in the past: http://theory.stanford.edu/~aiken/moss/ to detect plagiarized code. Since it works on a semantic level, it will detect the situations you presented above. The tool is language-aware, so comments are not considered in the analysis, and it goes a long way in detecting code that has been modified through simple search-and-replace of variable and/or function names.

Note: I used the tool a few years ago when I taught computer science in grad school, and it worked wonderfully in detecting code that had been yanked from the internet. Here is a well-documented account of similar application: http://fie2012.org/sites/fie2012.org/history/fie99/papers/1110.pdf

If you google "measure software similarity", you should find a few more useful hits: http://www.ics.heacademy.ac.uk/resources/assessment/plagiarism/detectiontools_sourcecode.html

Your problem in Computer Science Terminology maybe stated as Source Code Plagiarism Detection. A good start would be to read this article on Dr Dobbs: Detecting Source-Code Plagiarism. It lists the Algorithms for detecting Plagiarism in the source code.

Note: What you have asked for is indeed a tough computing problem :)

May be Copy-paste-detector from PMD?