Diffs (Extra Credit)

This is the second of two possible extra-credit features. Again, we will only count one of them.

Usage: java gitlet.Diff [ branch-name [ branch-name ] ]

Description: The git diff command compares the contents of a commit with a working directory or compares two commits, and presents any differences as a unified diff. Suppose that the latest commit on the current branch contains the following three files:

    f.txt              g.txt              h.txt
    ---------------------------------------------------------
    Line 1.            This is a wug.     This is not a wug.
    Line 2.            
    Line 3.            
    Line 4.            
    Line 5.            
    Line 6.            
    Line 7.            
    Line 8.            
    Line 9.            
    Line 10.           
    Line 11.           
    Line 12.           
    Line 13.           
    Line 14.           
    Line 15.           
    Line 16.           
    Line 17.           

and suppose that the working directory contains the following two files:

    f.txt              g.txt       
    -------------------------------
    Line 0.            This is a wug.
    Line 0.1.  
    Line 1.    
    Line 3.    
    Line 4.    
    Line 7.    
    Line 8.    
    Line 9.    
    Line 9.1.  
    Line 9.2.  
    Line 10.   
    Line 11.   
    Line 11.1. 
    Line 12.   
    Line 13.1  
    Line 14.   
    Line 15.   
    Line 16.1  
    Line 17.1  
    Line 18.   

Performing the command java gitlet.Main diff should produce the following output:

    diff --git a/f.txt b/f.txt
    --- a/f.txt
    +++ b/f.txt
    @@ -0,0 +1,2 @@
    +Line 0.
    +Line 0.1.
    @@ -2 +3,0 @@
    -Line 2.
    @@ -5,2 +5,0 @@
    -Line 5.
    -Line 6.
    @@ -9,0 +9,2 @@
    +Line 9.1.
    +Line 9.2.
    @@ -11,0 +13 @@
    +Line 11.1.
    @@ -13 +15 @@
    -Line 13.
    +Line 13.1
    @@ -16,2 +18,3 @@
    -Line 16.
    -Line 17.
    +Line 16.1
    +Line 17.1
    +Line 18.
    diff --git a/h.txt /dev/null
    --- a/h.txt
    +++ /dev/null
    @@ -1 +0,0 @@
    -This is not a wug.

The two lines starting diff --git indicate the start of the differences for one of the files in the two versions (the head of the current branch and the current (uncommitted) contents of the working directory. These diff lines are followed by a --- and +++ line, also giving the file name. Next come a sequence of edits, each starting with a line beginning and ending with @@. The entry

     @@ -L1,N1 +L2,N2

says that this entry indicates that to get the second version of the file from the first, one removes N1 lines starting at line L1 of the first version, and inserts N2 lines from the second version starting with line L2 of the second version. When N1 is 0, L1 is the number of lines before the lines to be deleted. Likewise, when N2 is 0, L2 is the number of lines in the second version that correspond to the lines preceding line L1 in the first version. When N1 or N2 is 1, it (and the comma in front of it) is not printed (e.g., @@ -13 +15 @@ in the example.) When one of the versions of the file is missing (as for h.txt here), its name is printed as /dev/null (the standard Unix empty file), and it is in fact treated as an empty file when comparing. If both versions of a file are identical, no diff is given for that file.

With no arguments, we compare the commit at the head of the current branch with the files in the working directory. Any files in the working directory and not in the current branch are ignored.

With one argument, as in git diff branch1, the contents of the commit at the head of the branch named branch1 are compared to the versions in the working directory.

With two arguments, as in git diff branch1 branch2, the contents of the commit at the head of the branch named branch1 are compared with those in branch2. It is only in this case, by the way, that the first (a/) file can be /dev/null.

Failure cases: If the branch in the one-argument case does not exist, print the message A branch with that name does not exist. If one or both of the branches in the two-argument case does not exist, print the message At least one branch does not exist.

Dangerous?: No.