neovim-config/lua/conf/langs/completion.lua

-- Credit to https://rsdlt.github.io/ for this code.

local cmp = require'cmp'

cmp.setup({
  -- Enable LSP snippets
  snippet = {
    expand = function(args)
        vim.fn["vsnip#anonymous"](args.body)
    end,
  },
  mapping = {
    ['<C-p>'] = cmp.mapping.select_prev_item(),
    ['<C-n>'] = cmp.mapping.select_next_item(),
    -- Add tab support
    ['<S-Tab>'] = cmp.mapping.select_prev_item(),
    ['<Tab>'] = cmp.mapping.select_next_item(),
    ['<C-S-f>'] = cmp.mapping.scroll_docs(-4),
    ['<C-f>'] = cmp.mapping.scroll_docs(4),
    ['<C-Space>'] = cmp.mapping.complete(),
    ['<C-e>'] = cmp.mapping.close(),
    ['<CR>'] = cmp.mapping.confirm({
      behavior = cmp.ConfirmBehavior.Insert,
      select = true,
    })
  },
  -- Installed sources:
  sources = {
    { name = 'path' },                              -- file paths
    { name = 'nvim_lsp', keyword_length = 3 },      -- from language server
    { name = 'nvim_lsp_signature_help'},            -- display function signatures with current parameter emphasized
    { name = 'nvim_lua', keyword_length = 2},       -- complete neovim's Lua runtime API such vim.lsp.*
    { name = 'buffer', keyword_length = 2 },        -- source current buffer
    { name = 'vsnip', keyword_length = 2 },         -- nvim-cmp source for vim-vsnip
    { name = 'calc'},                               -- source for math calculation
  },
  window = {
      completion = cmp.config.window.bordered(),
      documentation = cmp.config.window.bordered(),
  },
  formatting = {
      fields = {'menu', 'abbr', 'kind'},
      format = function(entry, item)
          local menu_icon ={
              nvim_lsp = 'λ',
              vsnip = '⋗',
              buffer = 'Ω',
              path = '•',
          }
          item.menu = menu_icon[entry.source.name]
          return item
      end,
  },
  -- This will let enter work like expected and tab  can be used to pick a suggestion.
  preselect = cmp.PreselectMode.None
})

-- This also prevents enter from selecting the first suggestion.
vim.o.completeopt = "menuone,noselect,preview"