branch

• Usage: java gitlet.Main branch [branch name]
• Description: Creates a new branch with the given name, and points it at the current head node. A branch is nothing more than a name for a reference (a SHA-1 identifier) to a commit node. This command does NOT immediately switch to the newly created branch (just as in real Git). Before you ever call branch, your code should be running with a default branch called "master".
• Runtime: Should be constant relative to any significant measure.
• Failure cases: If a branch with the given name already exists, print the error message A branch with that name already exists.
• Dangerous?: No
• Our line count: ~10

All right, let's see what branch does in detail. Suppose our state looks like this:

Now we call java gitlet.Main branch cool-beans. Then we get this:

Hmm... nothing much happened. Let's switch to the branch with java gitlet.Main checkout cool-beans:

Nothing much happened again?! Okay, say we make a commit now. Modify some files, then java gitlet.Main add... then java gitlet.Main commit...

I was told there would be branching. But all I see is a straight line. What's going on? Maybe I should go back to my other branch with java gitlet.Main checkout master:

Now I make a commit...

Phew! So that's the whole idea of branching. Did you catch what's going on? All that creating a branch does is to give us a new pointer. At any given time, one of these pointers is considered the currently active pointer, also called the HEAD pointer (indicated by *). We can switch the currently active head pointer with checkout [branch name]. Whenever we commit, it means we add a child commit to the currently active HEAD commit, even if a child commit is already there. This naturally creates branching behavior, since one parent commit can have multiple children commits.

Make sure that the behavior of your branch, checkout, and commit match what we've described above. This is pretty core functionality of Gitlet that many other commands will depend upon. If any of this core functionality is broken, very many of our autograder tests won't work!