version control - Is simplified semantics for the 'blame' command a good thing?

Question

I'm working on a new weave-based data structure for storing version control history. This will undoubtedly cause some religious wars about whether it's The Right Way Of Doing Things when it comes out, but that isn't my question right now.

My question has to do with what output blame should give. When a line of code has been added, removed, and merged into itself a number of times, it isn't always clear what revision should get blame for it. Notably this means that when a section of code is deleted, all records of it having been there is gone, and there is no blame for the removal. Everyone I've gone over this issue with has said that trying to do better simply isn't worth it. Sometimes people put in the hack that the line after the section which got deleted has its blame changed from whatever it actually was to the revision when the section got deleted. Presumably if the section is at the end then the last line get its blame changed, and if the file winds up empty then the blame really does disappear into the aether, because there's literally nowhere left to put blame information. For various technical reasons I won't be using this hack, but assume that continuing but with this completely undocumented but de facto standard practice will be uncontroversial (but feel free to flame me and get it out of your system).

Moving on to my actual question. Usually in blame for each line you look at the complete history of where it was added and removed in the history and using three-way merge (or, in the case of criss-cross merges, random bullshit) and based on the relationships between those you determine whether the line should have been there based on its history, and if it shouldn't but is then you mark it as new with the current revision. In the case where a line occurs in multiple ancestors with different blames then it picks which one to inherit arbitrarily. Again, I assume that continuing with this completely undocumented but de facto standard practice will be uncontroversial.

Where my new system diverges is that rather than doing a complicated calculation of whether a given line should be in the current revision based on a complex calculation of the whole history, it simply looks at the immediate ancestors, and if the line is in any of them it picks an arbitrary one to inherit the blame from. I'm making this change for largely technical reasons (and it's entirely possible that other blame implementations do the same thing, for similar technical reasons and a lack of caring) but after thinking about it a bit part of me actually prefers the new behavior as being more intuitive and predictable than the old one. What does everybody think?

question from:https://stackoverflow.com/questions/14077470/is-simplified-semantics-for-the-blame-command-a-good-thing

Answer 1

I actually wrote a one of the blame implementations out there (Subversion's current one I believe, unless someone replaced it in the past year or two). I helped with some others as well.

At least most implementations of blame don't do what you describe:

Usually in blame for each line you look at the complete history of where it was added and removed in the history and using three way merge (or, in the case of criss-cross merges, random bullshit) and based on the relationships between those you determine whether the line should have been there based on its history, and if it shouldn't but is then you mark it as new with the current revision. In the case where a line occurs in multiple ancestors with different blames then it picks which one to inherit arbitrarily. Again, I assume that continuing with this completely undocumented but de facto standard practice will be uncontroversial.

Actually, most blames are significantly less complex than this and don't bother trying to use the relationships at all, but they just walk parents in some arbitrary order, using simple delta structures (usually the same internal structure whatever diff algorithm they have uses before it turns it into textual output) to see if the chunk changed, and if so, blame it, and mark that line as done.

For example, Mercurial just does an iterative depth first search until all lines are blamed. It doesn't try to take into account whether the relationships make it unlikely it blamed the right one.

Git does do something a bit more complicated, but still, not quite like you describe.

Subversion does what Mercurial does, but the history graph is very simple, so it's even easier.

In turn, what you are suggesting is, in fact, what all of them really do:

Pick an arbitrary ancestor and follow that path down the rabbit hole until it's done, and if it doesn't cause you to have blamed all the lines, arbitrarily pick the next ancestor, continue until all blame is assigned.