In this case, there are two versions of the section, and the version which is in the common ancestor "C" is discarded, while the version that differs is preserved in the output. The three-way merge looks for sections which are the same in only two of the three files. Three way merge can be done on raw text (sequence of lines) or on structured trees. It is a rough merging method, but widely applicable since it only requires one common ancestor to reconstruct the changes that are to be merged.
In these cases, revision control systems resort to the user to specify the intended merge result.Ĭ is the origin, A and B are derivatives of C, and D is the new output versionĪ three-way merge is performed after an automated difference analysis between a file "A" and a file "B" while also considering the origin, or common ancestor, of both files "C".
Manual merging is also required when automatic merging runs into a change conflict for instance, very few automatic merge tools can merge two changes to the same line of code (say, one that changes a function name, and another that adds a comment). For instance, if two systems have slightly differing versions of a configuration file and a user wants to have the good stuff in both, this can usually be achieved by merging the configuration files by hand, picking the wanted changes from both sources (this is also called two-way merging). Manual merging is what people have to resort to (possibly assisted by merging tools) when they have to reconcile files that differ. For instance, Wikipedia allows two people to edit the same article at the same time when the latter contributor saves, their changes are merged into the article instead of overwriting the previous set of changes.
Also, other pieces of software deploy automatic merging if they allow for editing the same content simultaneously. Workflow Īutomatic merging is what version control software does when it reconciles changes that have happened simultaneously (in a logical sense). To overcome this problem, it is required to do some high-fidelity pretty-printing, which is essentially language-specific. Pretty-printing back from AST to source code may result in a completely different formatting style on the merged files. Yet, it also creates a new problem: as the AST abstracts away formatting, the This allows for a fine-grained merge that avoid spurious conflicts. Structured merge tools, or AST merge, turn the source code into aįully resolved AST.
This is limited, as a line of text does not represent the structure of source code. This is what Unix tools (diff/patch) and CVS tools (SVN, Git) use. Unstructured merge operates on raw text, typically using lines of text as atomic There are two types of merges: automatic and manual.