nvim/lua/plugins/completions.lua

local cmp = require("cmp")
local luasnip = require("luasnip")
local has_words_before = function()
  unpack = unpack or table.unpack
  local line, col = unpack(vim.api.nvim_win_get_cursor(0))
  return col ~= 0 and vim.api.nvim_buf_get_lines(0, line - 1, line, true)[1]:sub(col, col):match("%s") == nil
end

require("mason").setup()
require("luasnip.loaders.from_vscode").lazy_load()
require("cmp").setup({
  -- keybinds
  mapping = cmp.mapping.preset.insert({
    -- unsure
    ["<C-b>"] = cmp.mapping.scroll_docs(-4),
    ["<C-f>"] = cmp.mapping.scroll_docs(4),
    ["<C-o>"] = cmp.mapping.complete(),
    ["<C-e>"] = cmp.mapping.abort(),

    -- return to confirm
    ["<CR>"] = cmp.mapping.confirm({ select = false }),

    -- tab to select prev/next option
    -- see: https://github.com/hrsh7th/nvim-cmp/wiki/Example-mappings#luasnip
    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      -- You could replace the expand_or_jumpable() calls with expand_or_locally_jumpable()
      -- that way you will only jump inside the snippet region
      elseif luasnip.expand_or_jumpable() then
        luasnip.expand_or_jump()
      elseif has_words_before() then
        cmp.complete()
      else
        fallback()
      end
    end, { "i", "s" }),
    ["<S-Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_prev_item()
      elseif luasnip.jumpable(-1) then
        luasnip.jump(-1)
      else
        fallback()
      end
    end, { "i", "s" }),
  }),

  -- snippet engine
  snippet = {
    expand = function(args)
      require("luasnip").lsp_expand(args.body)
    end,
  },

  -- snippet source
  sources = cmp.config.sources({
    { name = "nvim_lsp" },
    { name = "luasnip" },
  }, {
    { name = "buffer" },
    { name = "path" }
  }),
})