XOR two lines to see differences

Question

I need to compute a "XOR"-like of two lines to find every char that differs between the two lines. I know the vim-diff feature can easily achieve that, but for comparing two lines I wanted to keep everything in the same buffer.

e.g.

1wYT4SH8f8VjRA25u8AItlrTdAY4Y7n-g8if8VjRA3q25u8AItlrTdwTbv8if8VjRA3q25u8AItlrTdJjMY
1wYT4SH8f8VjRA25u8AItlrTdAY4Y7n-g8if8VjRA3q25u8AIt1rTdwTbv8if8VjRA3q25u8AItlrTdJjMY
                                                  1

How can it be done ?

Answer 1

This solution doesn't show the difference between the two lines but highlight them and allow you to navigate through them with n and N like this:

To do so add this function to your .vimrc:

function! CompareLines(line1, line2)
    let l1 = getline(a:line1)
    let l2 = getline(a:line2)
    let pattern = ""

    for i in range(strlen(l1))
        if strpart(l1, i, 1) != strpart(l2, i, 1)
            if pattern != ""
                let pattern = pattern . "\\|"
            endif
            let pattern = pattern . "\\%" . a:line1 . "l" . "\\%" . ( i+1 ) . "c"
            let pattern = pattern . "\\|" . "\\%" . a:line2 . "l" . "\\%" . ( i+1 ) . "c"
        endif
    endfor

    execute "let @/='" . pattern . "'"
    set hlsearch
    normal n
endfunction

You can call the function with :call CompareLines(1, 2) where the arguments are the numbers of the lines to compare.

To add a bit more details about how the function works:

• The first lines gets the lines to compare in some variables.

• The loop iterates through these strings. Each characters of the strings are compared and when they don't match a pattern of the form \%Xl\%Yc is added to the variable pattern where X and Y are the line and the column of a character.

  (See :h /ordinary-atom and especially the last lines which explains that the pattern \%23l allows to match the 23rd line and \%23c the 23rd column)

• Once the pattern is build it is injected in the search register (let @/= allow to define the content of the register and the pattern has to be quoted).

• Finally set hlsearch is used to highlight all the matches of the pattern and normal n brings the cursor to the first different character.

Note 1 The function will change your search history.

Note 2 The part which uses the pattern to make the search could probably be greatly improved.

And as a bonus Add this line to your .vimrc to create a command which will faster the call of the function:

command! -nargs=* CompLines call CompareLines(<f-args>)

This way you can simple use :CompLines 1 2 to compares two lines.

---

EDIT And here is an improved version of the function and of the command. The difference is that you can call it in 3 different ways:

• :CL will diff the current line and the following one
• :CL 23 will diff the current line and the line 23
• :CL 23 42 will diff the lines 23 and 42

Here is the code:

command! -nargs=* CL call CompareLines(<f-args>)

function! CompareLines(...)

    " Check the number of arguments
    " And get lines numbers
    if len(a:000) == 0
        let l1=line(".")
        let l2=line(".")+1
    elseif len(a:000) == 1
        let l1 =line(".")
        let l2 =a:1
    elseif len(a:000) == 2
        let l1 = a:1
        let l2 = a:2
    else
        echom "bad number of arguments"
        return;
    endif

    " Get the content of the lines
    let line1 = getline(l1)
    let line2 = getline(l2)

    let pattern = ""

    " Compare lines and create pattern of diff
    for i in range(strlen(line1))
        if strpart(line1, i, 1) != strpart(line2, i, 1)
            if pattern != ""
                let pattern = pattern . "\\|"
            endif
            let pattern = pattern . "\\%" . l1 . "l" . "\\%" . ( i+1 ) . "c"
            let pattern = pattern . "\\|" . "\\%" . l2 . "l" . "\\%" . ( i+1 ) . "c"
        endif
    endfor

    " Search and highlight the diff
    execute "let @/='" . pattern . "'"
    set hlsearch
    normal n
endfunction

---

EDIT 2 I made it a plugin. You can install it with a regular plugin manager with (e.g.):

Plug 'statox/vim-compare-lines'