YouCompleteMe: a code-completion engine for Vim

YouCompleteMe is a fast, as-you-type, fuzzy-search code completion engine for Vim. It has two completion engines: an identifier-based engine that works with every programming language and a semantic, Clang-based engine that provides semantic code completion for C/C++/Objective-C/Objective-C++ (from now on referred to as "the C-family languages").

Here's an explanation of what happens in the short GIF demo above.

First, realize that no keyboard shortcuts had to be pressed to get the list of completion candidates at any point in the demo. The user just types and the suggestions pop up by themselves. If the user doesn't find the completion suggestions relevant and/or just wants to type, he can do so; the completion engine will not interfere.

When the user sees a useful completion string being offered, he presses the TAB key to accept it. This inserts the completion string. Repeated presses of the TAB key cycle through the offered completions.

If the offered completions are not relevant enough, the user can continue typing to further filter out unwanted completions.

A critical thing to notice is that the completion filtering is NOT based on the input being a string prefix of the completion (but that works too). The input needs to be a subsequence match of a completion. This is a fancy way of saying that any input characters need to be present in a completion string in the order in which they appear in the input. So abc is a subsequence of xaybgc, but not of xbyxaxxc. After the filter, a complicated sorting system ranks the completion strings so that the most relevant ones rise to the top of the menu (so you usually need to press TAB just once).

All of the above works with any programming language because of the identifier-based completion engine. It collects all of the identifiers in the current file and other files you visit and searches them when you type (identifiers are put into per-filetype groups).

The demo also shows the semantic engine in use. The current semantic engine supports only C-family languages. When the user presses ., -> or :: while typing in insert mode, the semantic engine is triggered (it can also be triggered with a keyboard shortcut; see the rest of the docs).

The last thing that you can see in the demo is YCM's integration with Syntastic (the little red X that shows up in the left gutter) if you are editing a file with semantic engine support. As Clang compiles your file and detects warnings or errors, they will be piped to Syntastic for display. You don't need to save your file or press any keyboard shortcut to trigger this, it "just happens" in the background.

In essence, YCM obsoletes the following Vim plugins because it has all of their features plus extra:

• clang_complete
• AutoComplPop
• Supertab
• neocomplcache

Mac OS X super-quick installation

Please refer to the full Installation Guide below; the following commands are provided on a best-effort basis and may not work for you.

Install the latest version of MacVim. Yes, MacVim. And yes, the latest.

If you don't use the MacVim GUI, it is recommended to use the Vim binary that is inside the MacVim.app package (MacVim.app/Contents/MacOS/Vim). To ensure it works correctly copy the mvim script from the MacVim download to your local binary folder (for example /usr/local/bin/mvim) and then symlink it:

ln -s /usr/local/bin/mvim vim

Install YouCompleteMe with Vundle.

Remember: YCM is a compiled plugin. If you update it using Vundle, you should rerun the install process again to rebuild it.

Install CMake. Preferably with Homebrew, but here's the stand-alone CMake installer.

If you have installed a Homebrew Python and/or Homebrew MacVim, see the FAQ for details.

Compiling YCM with semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
./install.sh --clang-completer

Compiling YCM without semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
./install.sh

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

Ubuntu Linux x64 super-quick installation

Please refer to the full Installation Guide below; the following commands are provided on a best-effort basis and may not work for you.

Make sure you have Vim 7.3.584 with python2 support. At the time of writing, the version of Vim shipping with Ubuntu is too old. You may need to compile Vim from source (don't worry, it's easy).

Install YouCompleteMe with Vundle.

Remember: YCM is a compiled plugin. If you update it using Vundle, you should rerun the install process again to rebuild it.

Install CMake. sudo apt-get install cmake

Make sure you have Python headers installed: sudo apt-get install python-dev.

Compiling YCM with semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
./install.sh --clang-completer

Compiling YCM without semantic support for C-family languages:

cd ~/.vim/bundle/YouCompleteMe
./install.sh

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

Full Installation Guide

These are the steps necessary to get YCM working on a Unix OS like Linux or Mac OS X. My apologies to Windows users, but I don't have a guide for them. The code is platform agnostic, so if everything is configured correctly, YCM should work on Windows without issues (but as of writing, it's untested on that platform).

See the FAQ if you have any issues.

Please follow the instructions carefully. Read EVERY WORD.

1. Ensure that your version of Vim is at least 7.3.584 and that it has support for python2 scripting.

   Inside Vim, type :version. Look at the first two to three lines of output; it should say Vi IMproved 7.3 and then below that, Included patches: 1-X, where X will be some number. That number needs to be 584 or higher.

   If your version of Vim is not recent enough, you may need to compile Vim from source (don't worry, it's easy).

   After you have made sure that you have Vim 7.3.584+, type the following in Vim: :echo has('python'). The output should be 1. If it's 0, then get a version of Vim with Python support.

2. Install YCM with Vundle (or Pathogen, but Vundle is a better idea). With Vundle, this would mean adding a Bundle 'Valloric/YouCompleteMe' line to your vimrc.

3. [Complete this step ONLY if you care about semantic completion support for C-family languages. Otherwise it's not neccessary.]

   Download the latest version of libclang. Clang is an open-source compiler that can compile C/C++/Objective-C/Objective-C++. The libclang library it provides is used to power the YCM semantic completion engine for those languages. YCM needs libclang version 3.2 or higher.

   You can use the system libclang only if you are sure it is version 3.2 or higher, otherwise don't. Even if it is, I recommend using the official binaries from llvm.org if at all possible. Make sure you download the correct archive file for your OS.

4. Compile the ycm_core plugin plugin (ha!) that YCM needs. This is the C++ engine that YCM uses to get fast completions.

   You will need to have cmake installed in order to generate the required makefiles. Linux users can install cmake with their package manager (sudo apt-get install cmake for Ubuntu) whereas other users can download and install cmake from its project site. Mac users can also get it through Homebrew with brew install cmake.

   You also need to make sure you have Python headers installed. On a Debian-like Linux distro, this would be sudo apt-get install python-dev. On Mac they should already be present.

   Here we'll assume you installed YCM with Vundle. That means that the top-level YCM directory is in ~/.vim/bundle/YouCompleteMe.

   We'll create a new folder where build files will be placed. Run the following:

   cd ~
   mkdir ycm_build
   cd ycm_build
   

   Now we need to generate the makefiles. If you DON'T care about semantic support for C-family languages, run the following command in the ycm_build directory: cmake -G "Unix Makefiles" . ~/.vim/bundle/YouCompleteMe/cpp

   If you DO care about semantic support for C-family languages, then your cmake call will be a bit more complicated. We'll assume you downloaded a binary distribution of LLVM+Clang from llvm.org in step 3 and that you extracted the archive file to folder ~/ycm_temp/llvm_root_dir (with bin, lib, include etc. folders right inside that folder). With that in mind, run the following command in the ycm_build directory: cmake -G "Unix Makefiles" -DPATH_TO_LLVM_ROOT=~/ycm_temp/llvm_root_dir . ~/.vim/bundle/YouCompleteMe/cpp

   Now that makefiles have been generated, simply run make ycm_core.

   For those who want to use the system version of libclang, you would pass -DUSE_SYSTEM_LIBCLANG=ON to cmake instead of the -DPATH_TO_LLVM_ROOT=... flag.

   You could also force the use of a custom libclang library with -DEXTERNAL_LIBCLANG_PATH=/path/to/libclang.so flag (the library would end with .dylib on a Mac). Again, this flag would be used instead of the other flags.

5. [Complete this step ONLY if you care about semantic completion support for C-family languages. Otherwise it's not neccessary.]

   Copy the libclang library file into the YouCompleteMe/python folder. The library file is libclang.so on Linux and libclang.dylib on Mac.

   We'll assume you downloaded a binary distribution of LLVM+Clang from llvm.org in step 3 and that you extracted the archive file to folder ~/ycm_temp/llvm_root_dir (with bin, lib, include etc. folders right inside that folder).

   We'll also assume you installed YCM with Vundle. That means that the top-level YCM directory is in ~/.vim/bundle/YouCompleteMe.

   On Linux, run: cp ~/ycm_temp/llvm_root_dir/lib/libclang.so ~/.vim/bundle/YouCompleteMe/python

   On Mac, run: cp ~/ycm_temp/llvm_root_dir/lib/libclang.dylib ~/.vim/bundle/YouCompleteMe/python

   DO NOT FORGET THIS STEP. If you forget to copy over libclang.so version 3.2 into the YouCompleteMe/python folder then YCM will not work if you selected C-family support during YCM compilation.

That's it. You're done. Refer to the User Guide section on how to use YCM. Don't forget that if you want the C-family semantic completion engine to work, you will need to provide the compilation flags for your project to YCM. It's all in the User Guide.

YCM comes with sane defaults for its options, but you still may want to take a look at what's available for configuration. There are a few interesting options that are conservatively turned off by default that you may want to turn on.

User Guide

General Usage

• If the offered completions are too broad, keep typing characters; YCM will continue refining the offered completions based on your input.
• Use the TAB key to accept a completion and continue pressing TAB to cycle through the completions. Use Shift-TAB to cycle backwards. Note that if you're using console Vim (that is, not Gvim or MacVim) then it's likely that the Shift-TAB binding will not work because the console will not pass it to Vim. You can remap the keys; see the options section below.

Completion string ranking

The subsequence filter removes any completions that do not match the input, but then the sorting system kicks in. It's actually very complicated and uses lots of factors, but suffice it to say that "word boundary" (WB) subsequence character matches are "worth" more than non-WB matches. In effect, this means given an input of "gua", the completion "getUserAccount" would be ranked higher in the list than the "Fooguxa" completion (both of which are subsequence matches). A word-boundary character are all capital characters, characters preceded by an underscore and the first letter character in the completion string.

Semantic Completion Engine Usage

• You can use Ctrl+Space to trigger the completion suggestions anywhere, even without a string prefix. This is useful to see which top-level functions are available for use.
• You really also want to install the latest version of the Syntastic Vim plugin. It has support for YCM and together they will provide you with compile errors/warnings practically instantly and without saving the file.

YCM looks for a .ycm_extra_conf.py file in the directory of the opened file or in any directory above it in the hierarchy (recursively); when the file is found, it is loaded (only once!) as a Python module. YCM calls a FlagsForFile method in that module which should provide it with the information necessary to compile the current file. (You can also provide a path to a global .ycm_extra_conf.py file and override this searching behavior. See the Options section for more details.)

This system was designed this way so that the user can perform any arbitrary sequence of operations to produce a list of compilation flags YCM should hand to Clang.

See YCM's own .ycm_extra_conf.py for details on how this works. You should be able to use it as a starting point. Hint: just replace the strings in the flags variable with compilation flags necessary for your project. That should be enough for 99% of projects.

Yes, Clang's CompilationDatabase system is also supported. Again, see the above linked example file.

If Clang encounters errors when compiling the header files that your file includes, then it's probably going to take a long time to get completions. When the completion menu finally appears, it's going to have a large number of unrelated completion strings (type/function names that are not actually members). This is because Clang fails to build a precompiled preamble for your file if there are any errors in the included headers and that preamble is key to getting fast completions.

Call the :YcmDiags command to see if any errors or warnings were detected in your file. Even better, use Syntastic.

Syntastic integration

YCM has explicit support for Syntastic (and vice-versa) if you compiled YCM with Clang support; this means that any diagnostics (errors or warnings) that Clang encounters while compiling your file will be fed back to Syntastic for display.

YCM will recompile your file in the background updatetime (see :h updatetime in Vim) milliseconds after you stop typing (to be specific, on CursorHold and CursorHoldI Vim events). YCM will change your updatetime value to be 2000 milliseconds (there's an option to tell it not to do this if you wish).

The new diagnostics (if any) will be fed back to Syntastic the next time you press any key on the keyboard. So if you stop typing and just wait for the new diagnostics to come in, that will not work. You need to press some key for the GUI to update.

Having to press a key to get the updates is unfortunate, but cannot be changed due to the way Vim internals operate; there is no way that a background task can update Vim's GUI after it has finished running. You have to press a key. This will make YCM check for any pending diagnostics updates.

You can force a full, blocking compilation cycle with the :YcmForceCompileAndDiagnostics command (you may want to map that command to a key; try putting nnoremap <F5> :YcmForceCompileAndDiagnostics<CR> in your vimrc). Calling this command will force YCM to immediately recompile your file and display any new diagnostics it encounters. Do note that recompilation with this command may take a while and during this time the Vim GUI will be blocked.

After the errors are displayed by Syntastic, it will display a short diagnostic message when you move your cursor to the line with the error. You can get a detailed diagnostic message with the <leader>d key mapping (can be changed in the options) YCM provides when your cursor is on the line with the diagnostic.

You can also see the full diagnostic message for all the diagnostics in the current file in Vim's locationlist, which can be opened with the :lopen and :lclose commands. A good way to toggle the display of the locationlist with a single key mapping is provided by another (very small) Vim plugin called ListToggle (which also makes it possible to change the height of the locationlist window), also written by yours truly.

TODO: extending the semantic engine for other langs

Commands

The YcmForceCompileAndDiagnostics command

Calling this command will force YCM to immediately recompile your file and display any new diagnostics it encounters. Do note that recompilation with this command may take a while and during this time the Vim GUI will be blocked.

You may want to map this command to a key; try putting nnoremap <F5> :YcmForceCompileAndDiagnostics<CR> in your vimrc.

The YcmDiags command

Calling this command will fill Vim's locationlist with errors or warnings if any were detected in your file and then open it.

A better option would be to use Syntastic which will keep your locationlist up to date automatically and will also show error/warning notifications in Vim's gutter.

The YcmShowDetailedDiagnostic command

This command shows the full diagnostic text when the user's cursor is on the line with the diagnostic.

The YcmDebugInfo command

This will print out various debug information for the current file. Useful to see what compile commands will be used for the file if you're using the semantic completion engine.

Options

All options have reasonable defaults so if the plug-in works after installation you don't need to change any options. These options can be configured in your vimrc script by including a line like this:

let g:ycm_min_num_of_chars_for_completion = 1

Note that after changing an option in your [vimrc script] vimrc you have to restart Vim for the changes to take effect.

The g:ycm_min_num_of_chars_for_completion option

This option controls the number of characters the user needs to type before completion suggestions are triggered. For example, if the option is set to 2, then when the user types a second alphanumeric character after a whitespace character, completion suggestions will be triggered.

Default: 2

let g:ycm_min_num_of_chars_for_completion = 2

The g:ycm_filetypes_to_completely_ignore option

This option controls for which Vim filetypes (see :h filetype) should YCM be turned off. The option value should be a Vim dictionary with keys being filetype strings (like python, cpp etc) and values being unimportant (the dictionary is used like a hash set, meaning that only the keys matter). The listed filetypes will be completely ignored by YCM, meaning that neither the identifier-based completion engine nor the semantic engine will operate in files of those filetypes.

You can get the filetype of the current file in Vim with :set ft?.

Default: {notes: 1, markdown: 1, text: 1}

let g:ycm_filetypes_to_completely_ignore = {
      \ 'notes' : 1,
      \ 'markdown' : 1,
      \ 'text' : 1,
      \}

The g:ycm_filetype_specific_completion_to_disable option

This option controls for which Vim filetypes (see :h filetype) should the YCM semantic completion engine be turned off. The option value should be a Vim dictionary with keys being filetype strings (like python, cpp etc) and values being unimportant (the dictionary is used like a hash set, meaning that only the keys matter). The listed filetypes will be ignored by the YCM semantic completion engine, but the identifier-based completion engine will still trigger in files of those filetypes.

Note that even if semantic completion is not turned off for a specific filetype, you will not get semantic completion if the semantic engine does not support that filetype. Currently, the semantic engine only supports the c, cpp, objc and objcpp filetypes.

You can get the filetype of the current file in Vim with :set ft?.

Default: {}

let g:ycm_filetype_specific_completion_to_disable = {}

The g:ycm_allow_changing_updatetime option

When this option is set to 1, YCM will change the updatetime Vim option to 2000 (see :h updatetime). This may conflict with some other plugins you have (but it's unlikely). The updatetime option is the number of milliseconds that have to pass before Vim's CursorHold (see :h CursorHold) event fires. YCM runs the completion engines' "file comprehension" systems in the background on every such event; the identifier-based engine collects the identifiers whereas the semantic engine compiles the file to build an AST.

The Vim default of 4000 for updatetime is a bit long, so YCM reduces this. Set this option to 0 to force YCM to leave your updatetime setting alone.

Default: 1

let g:ycm_allow_changing_updatetime = 1

The g:ycm_add_preview_to_completeopt option

When this option is set to 1, YCM will add the preview string to Vim's completeopt option (see :h completeopt). If your completeopt option already has preview set, there will be no effect. You can see the current state of your completeopt setting with :set completeopt? (yes, the question mark is important).

When preview is present in completeopt, YCM will use the preview window at the top of the file to store detailed information about the current completion candidate (but only if the candidate came from the semantic engine). For instance, it would show the full function prototype and all the function overloads in the window if the current completion is a function name.

Default: 0

let g:ycm_add_preview_to_completeopt = 0

The g:ycm_autoclose_preview_window_after_completion option

When this option is set to 1, YCM will auto-close the preview window after the user accepts the offered completion string. If there is no preview window triggered because there is no preview string in completeopt, this option is irrelevant. See the g:ycm_add_preview_to_completeopt option for more details.

Default: 0

let g:ycm_autoclose_preview_window_after_completion = 0

The g:ycm_max_diagnostics_to_display option

This option controls the maximum number of diagnostics shown to the user when errors or warnings are detected in the file. This option is only relevant if you are using the semantic completion engine and have installed the version of the Syntastic plugin that supports YCM.

Default: 30

let g:ycm_max_diagnostics_to_display = 30

The g:ycm_key_select_completion option

This option controls the key mapping used to select the first completion string. Invoking it repeatedly cycles forward through the completion list.

Default: <TAB>

let g:ycm_key_select_completion = '<TAB>'

The g:ycm_key_previous_completion option

This option controls the key mapping used to select the previous completion string. Invoking it repeatedly cycles backwards through the completion list.

Note that the default of <S-TAB> means Shift-TAB. Also note that the default mapping will probably only work in GUI Vim (Gvim or MacVim) and not in plain console Vim because the terminal usually does not forward modifier key combinations to Vim.

Default: <S-TAB>

let g:ycm_key_previous_completion = '<S-TAB>'

The g:ycm_key_invoke_completion option

This option controls the key mapping used to invoke the completion menu for semantic completion. By default, semantic completion is trigged automatically after typing ., -> and :: in insert mode (if semantic completion support has been compiled in). This key mapping can be used to trigger semantic completion anywhere. Useful for searching for top-level functions and classes.

Note that the default of <C-Space> means Ctrl-Space. Also note that the default mapping will probably only work in GUI Vim (Gvim or MacVim) and not in plain console Vim because the terminal usually does not forward modifier key combinations to Vim.

Setting this option to an empty string will make sure no mapping is created.

Default: <C-Space>

let g:ycm_key_invoke_completion = '<C-Space>'

The g:ycm_key_detailed_diagnostics option

This option controls the key mapping used to show the full diagnostic text when the user's cursor is on the line with the diagnostic. It basically calls :YcmShowDetailedDiagnostic.

Setting this option to an empty string will make sure no mapping is created.

Default: <leader>d

let g:ycm_key_detailed_diagnostics = '<leader>d'

The g:ycm_global_ycm_extra_conf option

Normally, YCM searches for a .ycm_extra_conf.py file for compilation flags (see the User Guide for more details on how this works). You can use this option to override this searching behavior by providing a full, absolute path to a global .ycm_extra_conf.py file (although you can call the global file whatever you want).

You can place such a global file anywhere in your filesystem.

Default: ``

let g:ycm_global_ycm_extra_conf = ''

FAQ

I get a linker warning regarding libpython on Mac when compiling YCM

If the warning is ld: warning: path '/usr/lib/libpython2.7.dylib' following -L not a directory, then feel free to ignore it; it's caused by a limitation of CMake and is not an issue. Everything should still work fine.

I get a weird window at the top of my file when I use the semantic engine

This is Vim's preview window. Vim uses it to show you extra information about something if such information is available. YCM provides Vim with such extra information. For instance, when you select a function in the completion list, the preview window will hold that function's prototype and the prototypes of any overloads of the function. It will stay there after you select the completion so that you can use the information about the parameters and their types to write the function call.

If you would like this window to auto-close after you select a completion string, set the g:ycm_autoclose_preview_window_after_completion option to 1 in your vimrc file.

If you don't want this window to ever show up, add set completeopt-=preview to your vimrc. Also make sure that the g:ycm_add_preview_to_completeopt option is set to 0.

It appears that YCM is not working

In Vim, run :messages and carefully read the output. YCM will echo messages to the message log if it encounters problems. It's likely you misconfigured something and YCM is complaining about it.

Also, you may want to run the :YcmDebugInfo command; it will make YCM spew out various debugging information, including the compile flags for the file if the file is a C-family language file and you have compiled in Clang support.

I cannot get the Syntastic integration to work

Try to update your version of Syntastic. At the time of writing (Jan 2013), the YCM integration is very recent and it's likely that your version of Syntastic does not have it.

Sometimes it takes much longer to get semantic completions than normal

This means that libclang (which YCM uses for C-family semantic completion) failed to pre-compile your file's preamble. In other words, there was an error compiling some of the source code you pulled in through your header files. I suggest calling the :YcmDiags command to see what they were (even better, have Syntastic installed and call :lopen).

Bottom line, if libclang can't pre-compile your file's preamble because there were errors in it, you're going to get slow completions because there's no AST cache.

YCM auto-inserts completion strings I don't want!

This means you probably have some mappings that interfere with YCM's internal ones. Make sure you don't have something mapped to <C-p>, <C-x> or <C-u> (in insert mode).

YCM never selects something for you; it just shows you a menu and the user has to explicitly select something. If something is being selected automatically, this means there's a bug or a misconfiguration somewhere.

I get a E227: mapping already exists for <blah> error when I start Vim

This means that YCM tried to set up a key mapping but failed because you already had something mapped to that key combination. The <blah> part of the message will tell you what was the key combination that failed.

Look in the options section and see if which of the default mappings conflict with your own. Then change that option value to something else so that the conflict goes away.

I'm trying to use a Homebrew Vim with YCM and I'm getting segfaults

Something (I don't know what) is wrong with the way that Homebrew configures and builds Vim. I recommend using MacVim. Even if you don't like the MacVim GUI, you can use the Vim binary that is inside the MacVim.app package (it's MacVim.app/Contents/MacOS/Vim) and get the Vim console experience.

I have a Homebrew Python and/or MacVim; can't compile/SIGABRT when starting

You should probably run brew rm python; brew install python to get the latest fixes that should make YCM work with such a configuration. Also rebuild Macvim then. If you still get problems with this, see issue #18 for suggestions.

I get LONG_BIT definition appears wrong for platform when compiling

Look at the output of your CMake call. There should be a line in it like the following (with .dylib in place of .so on a Mac):

-- Found PythonLibs: /usr/lib/libpython2.7.so (Required is at least version "2.5")

That would be the correct output. An example of incorrect output would be the following:

-- Found PythonLibs: /usr/lib/libpython2.7.so (found suitable version "2.5.1", minimum required is "2.5")

Notice how there's an extra bit of output there, the found suitable version "<version>" part, where <version> is not the same as the version of the dynamic library. In the example shown, the library is version 2.7 but the second string is version 2.5.1.

This means that CMake found one version of Python headers and a different version for the library. This is wrong. It can happen when you have multiple versions of Python installed on your machine.

You should probably add the following flags to your cmake call (again, dylib instead of so on a Mac):

-DPYTHON_INCLUDE_DIR=/usr/include/python2.7 -DPYTHON_LIBRARY=/usr/lib/libpython2.7.so

This will force the paths to the Python include directory and the Python library to use. You may need to set these flags to something else, but you need to make sure you use the same version of Python that your Vim binary is built against, which is highly likely to be the system's default Python.

Why isn't YCM just written in plain VimScript, FFS?

Because of the identifier completion engine and subsequence-based filtering. Let's say you have many dozens of files open in a single Vim instance (I often do); the identifier-based engine then needs to store thousands (if not tens of thousands) of identifiers in its internal data-structures. When the user types, YCM needs to perform subsequence-based filtering on all of those identifiers (every single one!) in less than 10 milliseconds.

I'm sorry, but that level of performance is just plain impossible to achieve with VimScript. I've tried, and the language is just too slow. No, you can't get acceptable performance even if you limit yourself to just the identifiers in the current file and simple prefix-based fitering.

Why does YCM demand such a recent version of Vim?

During YCM's development several show-stopper bugs where encountered in Vim. Those needed to be fixed upstream (and were). A few months after those bugs were fixed, Vim trunk landed the pyeval() function which improved YCM performance even more since less time was spent serializing and deserializing data between Vim and the embedded Python interpreter. A few critical bugfixes for pyeval() landed in Vim 7.3.584 (and a few commits before that).

I get annoying messages in Vim's status area when I type

If you're referring to the User defined completion <bla bla> back at original and similar, then sadly there's no fix for those. Vim will emit them no matter how hard I try to silence them.

You'll have to learn to ignore them. It's a shitty "solution", I know.

Contact

If you have questions, bug reports, suggestions, etc. please use the issue tracker. The latest version is available at http://valloric.github.com/YouCompleteMe/.

The author's homepage is http://val.markovic.io.

License

This software is licensed under the GPL v3 license. © 2012 Strahinja Val Markovic <val@markovic.io>.