Fixed code completion and added tab to select completion

lua/core/plugin_config/completions.lua

@@ -1,4 +1,11 @@
+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
+
 local cmp = require("cmp")
+local luasnip = require("luasnip")
 
 cmp.setup({
   mapping = cmp.mapping.preset.insert({
@@ -7,11 +14,38 @@ cmp.setup({
       ['<C-o>'] = cmp.mapping.complete(),
       ['<C-e>'] = cmp.mapping.abort(),
       ['<CR>'] = cmp.mapping.confirm({ select = true }),
+      ["<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() 
+      -- they 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 = {
+    expand = function(args)
+      require('luasnip').lsp_expand(args.body)
+    end,
   },
   sources = cmp.config.sources({
     { name = 'nvim_lsp' },
+    { name = 'luasnip' },
   }, {
     { name = 'buffer' },
   }),

lua/core/plugins.lua

@@ -37,6 +37,12 @@ local plugins = {
   -- completion
   'hrsh7th/nvim-cmp',
   'hrsh7th/cmp-nvim-lsp',
+  {
+    "L3MON4D3/LuaSnip",
+    version = "2.*", -- Replace <CurrentMajor> by the latest released major
+    -- install jsregexp (optional!).
+    build = "make install_jsregexp"
+  }
 }
 
 local opts = {}