checkout

Checkout is a general command that can do a few different things depending on what its arguments are. There are 3 possible use cases. In each section below, you'll see 3 bullet points. Each corresponds to the respective usage of checkout.

• Usages:

  1. java gitlet.Main checkout -- [file name]
  2. java gitlet.Main checkout [commit id] -- [file name]
  3. java gitlet.Main checkout [branch name]

• Descriptions:

  1. Takes the version of the file as it exists in the head commit, the front of the current branch, and puts it in the working directory, overwriting the version of the file that's already there if there is one. The new version of the file is not staged.
  2. Takes the version of the file as it exists in the commit with the given id, and puts it in the working directory, overwriting the version of the file that's already there if there is one. The new version of the file is not staged.
  3. Takes all files in the commit at the head of the given branch, and puts them in the working directory, overwriting the versions of the files that are already there if they exist. Also, at the end of this command, the given branch will now be considered the current branch (HEAD). Any files that are tracked in the current branch but are not present in the checked-out branch are deleted. The staging area is cleared, unless the checked-out branch is the current branch (see Failure cases below).

• Run times:

  1. Should be linear relative to the size of the file being checked out.
  2. Should be linear with respect to the total size of the files in the commit's snapshot. Should be constant with respect to any measure involving number of commits. Should be constant with respect to the number of branches.

• Failure cases:

  1. If the file does not exist in the previous commit, abort, printing the error message File does not exist in that commit.
  2. If no commit with the given id exists, print No commit with that id exists. Otherwise, if the file does not exist in the given commit, print the same message as for failure case 1.
  3. If no branch with that name exists, print No such branch exists. If that branch is the current branch, print No need to checkout the current branch. If a working file is untracked in the current branch and would be overwritten by the checkout, print There is an untracked file in the way; delete it, or add and commit it first. and exit; perform this check before doing anything else.
• Differences from real git: Real git does not clear the staging area and stages the file that is checked out. Also, it won't do a checkout that would overwrite or undo changes (additions or removals) that you have staged.

A [commit id] is, as described earlier, a hexadecimal numeral. A convenient feature of real Git is that one can abbreviate commits with a unique prefix. For example, one might abbreviate

a0da1ea5a15ab613bf9961fd86f010cf74c7ee48

as

a0da1e

in the (likely) event that no other object exists with a SHA-1 identifier that starts with the same six digits. You should arrange for the same thing to happen for commit ids that contain fewer than 40 characters. Unfortunately, using shortened ids might slow down the finding of objects if implemented naively (making the time to find a file linear in the number of objects), so we won't worry about timing for commands that use shortened ids. We suggest, however, that you poke around in a .git directory (specifically, .git/objects) and see how it manages to speed up its search. You will perhaps recognize a familiar data structure implemented with the file system rather than pointers.

Only version 3 (checkout of a full branch) modifies the staging area: otherwise files scheduled for addition or removal remain so.

• Dangerous?: Yes!

• Our line counts:

  • ~15
  • ~5
  • ~15