Toward an Implementation of the “Form Template Method” Refactoring Nicolas Juillerat University of Fribourg Switzerland

27/09/07SCAM'072 1.Problem definition public void rotateAt(Point center, double amount) { translate(mult(center, )); rotate(amount, center); translate(center); normalize(amount); } public void skewAt(Point center, double amount) { translate(mult(center, )); amount = skew(amount); translate(center); normalize(amount); } public void templateMethod( Point center, double amount) { translate(mult(center, d1())); amount = d2(amount, center); translate(center); normalize(amount); }

27/09/07SCAM' Challenges How to compare two code snippets? –Use clone detection techniques? –Yes, but… small differences The matching order is important Renamed variables are more difficult to handle

27/09/07SCAM' Challenges ABCDEABCDE ABDCEABDCE ABCDEABCDE Matching order Clone detection 4 clones

27/09/07SCAM' Challenges ABCDEABCDE ABDCEABDCE ABCDEABCDE ABDCEABDCE ABCDEABCDE ABDCEABDCE Matching order Template method (2 solutions)‏

27/09/07SCAM' Challenges Renamed variables int x = z * 2; print(x); store(x); int y = x - 4; return y; int y = z * 2 print(y); store(y); y++; return y; int x = z * 2; print(x); store(x); int y = x - 4; return y; int y = z * 2 print(y); store(y); y++; return y; x --- x, y --- y, z --- z 2 matches x --- y, y --- x, z --- z 3 matches

27/09/07SCAM' Challenges Renamed variables int x = z * 2; print(x); store(x); int y = x - 4; return y; int y = z * 2 print(y); store(y); y++; return y; int x = z * 2; print(x); store(x); int y = x - 4; return y; int y = z * 2 print(y); store(y); y++; return y; We need to know what variables are matched before we can match statements. But we need to know what statements are matched before we can match variables Reason: unlike with clone detection, variable matching must be the same for multiple matched statements

27/09/07SCAM' Challenges Method extraction –Known problem, but the template method can only be formed if all differences can be extracted AXDFHKLAXDFHKL BXCFMKPBXCFMKP e1 X e2 F e3 K e4 e1 X e2 F H/M K e4 Successfully extracted Failed to extract. No template method can be formed

27/09/07SCAM' Challenges Method extraction –Furthermore, we have to extract pairs of different methods AXDFHKLAXDFHKL BXCFMKPBXCFMKP e1 X e2 F e3 K e4 void D(int, String); float C(int, boolean); float e2(int, String, boolean);  Merging of arguments / results

27/09/07SCAM' Proposed solutions

27/09/07SCAM' Proposed solutions Variable matching (before matched statements are known)‏ –Based on Maximum Matching of Bipartite Graphs (Heuristic, used to supply a proposal)‏ Not in paper (ongoing work)‏ Statement comparison –Post-order AST traversal (  Tokens list)‏ –Longest Common Subsequence (LCM)‏ Or modified LZ77 (data compression algorithm)‏ –Common ancestor (tree)‏ And various other tree algorithms

27/09/07SCAM' Proposed solutions Method Extraction –If impossible, various tricks to make the extraction possible Divide the method to extract into multiple methods Extend a method with matched statements –Some duplications remains Control flow to dataflow conversion Etc –More freedom than with method extraction alone

27/09/07SCAM' Discussion/Conclusion

27/09/07SCAM' Discussion/Conclusion Algorithms: AST Traversal  List Longest Common SubsequenceLongest Common Subsequence Common Ancestor (tree)‏Common Ancestor (tree)‏ Maximum matching of bipartite graphsMaximum matching of bipartite graphs Control flow to data flow conversionControl flow to data flow conversion Data/Control flow analysis LZ77LZ77 … Existing techniques / abstractions: Graph transformations Graph rewriting languages AST Traversals/Rewriting Scripting languages Code queries … Questions: Are the existing techniques suitable for really complex refactorings? Isn’t a general purpose language (+ suitable library) more adequate?