Awesome

yadi.vim

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.

Commands

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 algorithm

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.

Integration with statuslines

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"}'
      \ },
      \ }

Why yet another clone of DetectIndent?

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

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.