Yet Another Detect Indent. Here is how it works in my .vimrc:

" Try to auto detect and use the indentation of a file when opened. 
autocmd BufRead * DetectIndent

" Otherwise use file type specific indentation. E.g. tabs for Makefiles
" and 4 spaces for Python. This is optional.
filetype plugin indent on

" Set a fallback here in case detection fails and there is no file type
" plugin available. You can also omit this, then Vim defaults to tabs.
set expandtab shiftwidth=4 softtabstop=4

" You stay in control of your tabstop setting.
set tabstop=4

The plugin is built on the following principles:

• Ignore outliers. A few bad whitespace should not stop the plugin from working. But as a user I don't want to be warned of or worse required to fix those inconsistencies. That's not in the scope of this plugin.
• Leave settings unchanged if detection fails. The users .vimrc or file type plugins should then determine the indentation style.
• Only ever touch the 3 settings relevant to indentation: expandtab, shiftwidth and softtabstop
• Be a single, short and easy to comprehend Vimscript file.

Not convinced yet? Take a look at the code, it fits on a single page.

The :DetectIndent command tries to auto detect the indentation style in the current buffer. If it finds tabs it sets noexpandtab shiftwidth=0 softtabstop=0, if it finds n spaces it sets expandtab shiftwidth=n softtabstop=n. If the algorithm can't confidently determine the indentation style no settings are changed. It comes naturally to set this as an autocommand as can be seen above.

The :IndentTabs and :IndentSpaces <n> commands explicitly apply the settings for tabs and spaces respectively. The argument to :IndentSpaces can be omitted in which case your tabstop setting will be used.

The developers of the Firefox dev tools wrote a great article where they compare different indentation detection strategies for Firefox's built-in source editor. yadi.vim uses the "comparing lines" strategy from the article because it performs well and is easy to implement. The article explains it well:

This method compares the indentation of each line with the previous line, and adds the difference to a tally. So if a line is indented by 10 spaces, and the previous by 8, one more vote would be added for 2-space indentation.

Mixed tabs and spaces as well as 1-space indentation are not supported. This is a deliberate choice as these indentation styles are rare and would complicate the algorithm considerably.

Furthermore there is a heuristic in place to prevent misdetection. More than 80% of the file must be either tabs or spaces for the detection to succeed. Additionally in the case of spaces the most common indentation level must make up more than 60% of all indentations.

To display the current indentation settings in your statusline you could use this expression:

let &statusline = '%{&expandtab?shiftwidth()." sp":"tabs"}'

Or with lightline.vim:

let g:lightline = {
      \ 'active': {
      \   'right': [ [ 'lineinfo' ],
      \              [ 'percent' ],
      \              [ 'fileformat', 'fileencoding', 'filetype', 'indentstyle' ] ]
      \ },
      \ 'component': {
      \   'indentstyle': '%{&expandtab?shiftwidth()." sp":"tabs"}'
      \ },
      \ }

There are a bunch of plugins with DetectIndent-like functionality:

• DetectIndent
• sleuth.vim
• Yaifa
• matchindent.vim
• IndentFinder

But none of them let you fall back to file type plugins if detection fails. Most of them force you to set a fallback indentation style. sleuth.vim is more sophisticated and tries to guess the indentation from surrounding files. It's a different approach which comes at the cost of greater complexity and makes the result rather unpredictable.