neovim vs-code like autocompletion for lsp buffer files/path and vsnip

completion.vim

" ===========================================================================
"
" GIST: snippet / buffer / lsp / snippet vs-code like autocompletion using
"       vim-vsnip and neovim lsp plugin. synchronous completion. sorting
"       is done by length and source priority. c-n/c-p cycles items, while
"       tab select/expands/jump-next placeholder. what's missing is showing
"       a popup preview
"
" date: 2020-12-06 22:34
"
" ===========================================================================

set noshowmode
set updatetime=300
set shortmess+=c
set completeopt=menu,menuone,noselect
set pumwidth=10
set pumheight=8
set completefunc=CustomComplete

au! CursorHoldI * if getline('.')[col('.')-2] =~ '\S'
             \ && !pumvisible() | silent call feedkeys("\<c-x>\<c-u>") | endif
 
inoremap <expr> <tab>
  \ pumvisible() ?
  \   complete_info(['selected']).selected != -1 ? 
  \     "\<c-y>"
  \     :
  \     "\<c-n>\<c-y>"
  \   :
  \   "\<tab>"

imap <expr> <tab>
            \ pumvisible() ?
            \     complete_info()["selected"] != "-1" ?
            \       "\<c-y>"
            \       :
            \       "\<c-n>\<c-y>" :
            \   vsnip#jumpable(1) ?
            \         "\<Plug>(vsnip-jump-next)" :
            \       "\<tab>"

imap <expr> <S-Tab> vsnip#jumpable(-1) ? '<Plug>(vsnip-jump-prev)' : '<S-Tab>'
smap <expr> <Tab>   vsnip#jumpable(1)  ? '<Plug>(vsnip-jump-next)' : '<Tab>'
smap <expr> <S-Tab> vsnip#jumpable(-1) ? '<Plug>(vsnip-jump-prev)' : '<S-Tab>'
nmap <expr> <Tab>   vsnip#jumpable(1)  ? 'i<Plug>(vsnip-jump-next)' : '<Tab>'
nmap <expr> <S-Tab> vsnip#jumpable(1)  ? 'i<Plug>(vsnip-jump-prev)' : '<Tab>'
xmap s <Plug>(vsnip-cut-text)

" ====================== User completefunc

fun! CustomComplete(findstart, base) abort
    if a:findstart
        let keywordregex = '\k\|\(\.\?\/\?\)\+'
        let regex = '\(' . keywordregex . '\)\+\%' . col(".") . 'c'
        return match(getline("."), regex)
    endif
    echo a:base
    let limit = 4
    let buffers  = s:sort(BufferSources(a:base))[0:limit]
    let snippets = s:sort(SnippetSources(a:base))[0:limit]
    let lsp      = s:sort(LspSources(a:base))[0:limit]
    let files    = s:sort(FileSources(a:base))[0:2*limit]
    let lists = [
        \   { 'list': buffers,  'priority': 1  },
        \   { 'list': snippets, 'priority': 3  },
        \   { 'list': files,    'priority': 2  },
        \   { 'list': lsp,      'priority': 4  }
        \ ]
    return s:mix(lists)
endfun

fun! s:sort(list)
    return sort(a:list, { a,b -> len(a.word) > len(b.word) })
endf

fun! s:mix(lists)
    let res = []
    for group in a:lists
        for item in group.list
            let item.priority = group.priority
            let res += [item]
        endfor
    endfor
    return sort(res,  { a,b -> len(a.word) > len(b.word) ? 1 : 
                \ len(a.word) < len(b.word) ? 0 : 
                \ a.priority < b.priority })
endfun

" priorities -> [ 1, 5, 2, 3 ]
fun! s:best(priorities)
    let best = 0
    for i in a:priorities
        let best = i > best ? i : best
    endfor
endfun


" ================================== COMPLETION SOURCES

" 1. BUFFER
fun! BufferSources(word)
    return  
    \ map(
    \     uniq(
    \       sort( 
    \           filter(
    \               split(
    \                   join(
    \                       getline(1,'$'), 
    \                   "\n"), 
    \               '\<\|\>'), 
    \           { _,s -> s =~ '^\(\k\+\)$' 
    \                 && s !~ '[#_\-1-9]' 
    \                 && len(s) > 1 
    \                 && s =~ '^'.a:word.'.*'}))),
    \ { _,s -> { 'word': s, 'kind': 'buffer' } })
    
endf

" 2. SNIPPETS
fun! SnippetSources(word)
    try
        return 
            \   filter(
            \       vsnip#get_complete_items(bufnr()), 
            \   { _,s -> s.word =~ '^'.a:word.'.*' })
    catch /.*/
        return []
    endtry
endf

" 3. FILES
func! FileSources(word) 
  let files = filter(getcompletion(a:word, "file"), 'match(v:val, "//") < 0')
  return map(files, { _,file -> { "kind": "file", "word": file } })
endfunc

" 4. LSP
fun! LspSources(word)
    try
        return v:lua.omnifunc_sync(a:word)
    catch /.*/
        return []
    endtry
endf

lua << EOF
function _G.omnifunc_sync(base)
  local params = vim.lsp.util.make_position_params()
  local result = vim.lsp.buf_request_sync(bufnr, 'textDocument/completion', params, 2000)
  local items = {}
  if result then
    for _, item in ipairs(result) do
      if not item.err then
        local matches = vim.lsp.util.text_document_completion_list_to_complete_items(item.result, base)
        vim.list_extend(items, matches)
      end
    end
  end
  return items
end
EOF