Example mappings

You can use the following examples.

Super-Tab like mapping

vim-vsnip

local has_words_before = function()
  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 feedkey = function(key, mode)
  vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes(key, true, true, true), mode, true)
end

local cmp = require('cmp')
cmp.setup {

  -- ... Your other configuration ...

  mapping = {

    -- ... Your other mappings ...

    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      elseif vim.fn["vsnip#available"](1) == 1 then
        feedkey("<Plug>(vsnip-expand-or-jump)", "")
      elseif has_words_before() then
        cmp.complete()
      else
        fallback() -- The fallback function sends a already mapped key. In this case, it's probably `<Tab>`.
      end
    end, { "i", "s" }),

    ["<S-Tab>"] = cmp.mapping(function()
      if cmp.visible() then
        cmp.select_prev_item()
      elseif vim.fn["vsnip#jumpable"](-1) == 1 then
        feedkey("<Plug>(vsnip-jump-prev)", "")
      end
    end, { "i", "s" }),

    -- ... Your other mappings ...

  }

  -- ... Your other configuration ...

}

luasnip

local has_words_before = function()
  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 luasnip = require("luasnip")
local cmp = require("cmp")

cmp.setup({

  -- ... Your other configuration ...

  mapping = {

    -- ... Your other mappings ...

    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      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" }),

    -- ... Your other mappings ...
  },

  -- ... Your other configuration ...
})

nvim-snippy

local has_words_before = function()
  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 snippy = require("snippy")

local cmp = require("cmp")

cmp.setup({

  -- ... Your other configuration ...

  mapping = {

    -- ... Your other mappings ...

    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      elseif snippy.can_expand_or_advance() then
        snippy.expand_or_advance()
      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 snippy.can_jump(-1) then
        snippy.previous()
      else
        fallback()
      end
    end, { "i", "s" }),

    -- ... Your other mappings ...
  },

  -- ... Your other configuration ...
})

Ultisnips + cmp-cmdline

• i_(S-)Tab when completion menu is visible insert next/prev entry, else jump to next/prev snippet tag
• c_(S-)Tab when completion menu is visible insert next/prev entry, else starts completion
• i_CR when completion menu is visible select completion or expand snippet, else <CR>
• c_CR when completion menu is visible select completion only, don't accept line, else <CR> (accept line)
• i_CTRL_n/p when completion menu is visible, select next/prev completion item, else <C-n>/<C-p>
• c_CTRL_n/p when completion menu is visible, select next/prev completion item, else filter history (<Up>, <Down>)

-- within packer init {{{
use {'SirVer/ultisnips',
    requires = {{'honza/vim-snippets', rtp = '.'}},
    config = function()      
      vim.g.UltiSnipsExpandTrigger = '<Plug>(ultisnips_expand)'      
      vim.g.UltiSnipsJumpForwardTrigger = '<Plug>(ultisnips_jump_forward)'
      vim.g.UltiSnipsJumpBackwardTrigger = '<Plug>(ultisnips_jump_backward)'
      vim.g.UltiSnipsListSnippets = '<c-x><c-s>'
      vim.g.UltiSnipsRemoveSelectModeMappings = 0
    end
}
-- }}}

local t = function(str)
    return vim.api.nvim_replace_termcodes(str, true, true, true)
end

local cmp = require('cmp')
cmp.setup {
    snippet = {
        expand = function(args) vim.fn["UltiSnips#Anon"](args.body) end
    },
    -- ... Your other configuration ...
    mapping = {
        ["<Tab>"] = cmp.mapping({
            c = function()
                if cmp.visible() then
                    cmp.select_next_item({ behavior = cmp.SelectBehavior.Insert })
                else
                    cmp.complete()
                end
            end,
            i = function(fallback)
                if cmp.visible() then
                    cmp.select_next_item({ behavior = cmp.SelectBehavior.Insert })
                elseif vim.fn["UltiSnips#CanJumpForwards"]() == 1 then
                    vim.api.nvim_feedkeys(t("<Plug>(ultisnips_jump_forward)"), 'm', true)
                else
                    fallback()
                end
            end,
            s = function(fallback)
                if vim.fn["UltiSnips#CanJumpForwards"]() == 1 then
                    vim.api.nvim_feedkeys(t("<Plug>(ultisnips_jump_forward)"), 'm', true)
                else
                    fallback()
                end
            end
        }),
        ["<S-Tab>"] = cmp.mapping({
            c = function()
                if cmp.visible() then
                    cmp.select_prev_item({ behavior = cmp.SelectBehavior.Insert })
                else
                    cmp.complete()
                end
            end,
            i = function(fallback)
                if cmp.visible() then
                    cmp.select_prev_item({ behavior = cmp.SelectBehavior.Insert })
                elseif vim.fn["UltiSnips#CanJumpBackwards"]() == 1 then
                    return vim.api.nvim_feedkeys( t("<Plug>(ultisnips_jump_backward)"), 'm', true)
                else
                    fallback()
                end
            end,
            s = function(fallback)
                if vim.fn["UltiSnips#CanJumpBackwards"]() == 1 then
                    return vim.api.nvim_feedkeys( t("<Plug>(ultisnips_jump_backward)"), 'm', true)
                else
                    fallback()
                end
            end
        }),
        ['<Down>'] = cmp.mapping(cmp.mapping.select_next_item({ behavior = cmp.SelectBehavior.Select }), {'i'}),
        ['<Up>'] = cmp.mapping(cmp.mapping.select_prev_item({ behavior = cmp.SelectBehavior.Select }), {'i'}),
        ['<C-n>'] = cmp.mapping({
            c = function()
                if cmp.visible() then
                    cmp.select_next_item({ behavior = cmp.SelectBehavior.Select })
                else
                    vim.api.nvim_feedkeys(t('<Down>'), 'n', true)
                end
            end,
            i = function(fallback)
                if cmp.visible() then
                    cmp.select_next_item({ behavior = cmp.SelectBehavior.Select })
                else
                    fallback()
                end
            end
        }),
        ['<C-p>'] = cmp.mapping({
            c = function()
                if cmp.visible() then
                    cmp.select_prev_item({ behavior = cmp.SelectBehavior.Select })
                else
                    vim.api.nvim_feedkeys(t('<Up>'), 'n', true)
                end
            end,
            i = function(fallback)
                if cmp.visible() then
                    cmp.select_prev_item({ behavior = cmp.SelectBehavior.Select })
                else
                    fallback()
                end
            end
        }),
        ['<C-b>'] = cmp.mapping(cmp.mapping.scroll_docs(-4), {'i', 'c'}),
        ['<C-f>'] = cmp.mapping(cmp.mapping.scroll_docs(4), {'i', 'c'}),
        ['<C-Space>'] = cmp.mapping(cmp.mapping.complete(), {'i', 'c'}),
        ['<C-e>'] = cmp.mapping({ i = cmp.mapping.close(), c = cmp.mapping.close() }),
        ['<CR>'] = cmp.mapping({
            i = cmp.mapping.confirm({ behavior = cmp.ConfirmBehavior.Replace, select = false }),
            c = function(fallback)
                if cmp.visible() then
                    cmp.confirm({ behavior = cmp.ConfirmBehavior.Replace, select = false })
                else
                    fallback()
                end
            end
        }),
        -- ... Your other configuration ...
    },
   -- ... Your other configuration ...
}

-- Use buffer source for `/`.
cmp.setup.cmdline('/', {
    completion = { autocomplete = false },
    sources = {
        -- { name = 'buffer' }
        { name = 'buffer', opts = { keyword_pattern = [=[[^[:blank:]].*]=] } }
    }
})

-- Use cmdline & path source for ':'.
cmp.setup.cmdline(':', {
    completion = { autocomplete = false },
    sources = cmp.config.sources({
        { name = 'path' }
        }, {
        { name = 'cmdline' }
    })
})

No snippet plugin

Snippets don't expand with <Tab> in this config. Use <C-Space>.

local has_words_before = function()
  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')
cmp.setup {
  -- ... Your other configuration ...

  mapping = {

    -- ... Your other configuration ...

    ['<C-Space>'] = cmp.mapping.confirm {
      behavior = cmp.ConfirmBehavior.Insert,
      select = true,
    },

    ['<Tab>'] = function(fallback)
      if not cmp.select_next_item() then
        if vim.bo.buftype ~= 'prompt' and has_words_before() then
          cmp.complete()
        else
          fallback()
        end
      end
    end,

    ['<S-Tab>'] = function(fallback)
      if not cmp.select_prev_item() then
        if vim.bo.buftype ~= 'prompt' and has_words_before() then
          cmp.complete()
        else
          fallback()
        end
      end
    end,
  },

  snippet = {
    -- We recommend using *actual* snippet engine.
    -- It's a simple implementation so it might not work in some of the cases.
    expand = function(args)
      local line_num, col = unpack(vim.api.nvim_win_get_cursor(0))
      local line_text = vim.api.nvim_buf_get_lines(0, line_num - 1, line_num, true)[1]
      local indent = string.match(line_text, '^%s*')
      local replace = vim.split(args.body, '\n', true)
      local surround = string.match(line_text, '%S.*') or ''
      local surround_end = surround:sub(col)

      replace[1] = surround:sub(0, col - 1)..replace[1]
      replace[#replace] = replace[#replace]..(#surround_end > 1 and ' ' or '')..surround_end
      if indent ~= '' then
        for i, line in ipairs(replace) do
          replace[i] = indent..line
        end
      end

      vim.api.nvim_buf_set_lines(0, line_num - 1, line_num, true, replace)
    end,
  },

  -- ... Your other configuration ...
}

Intellij-like mapping

• If no completion is selected, insert the first one in the list.
• If a completion is selected, insert this one.

local cmp = require("cmp")

cmp.setup({
  mapping = {
    ...
    ["<Tab>"] = cmp.mapping(function(fallback)
      -- This little snippet will confirm with tab, and if no entry is selected, will confirm the first item
      if cmp.visible() then
        local entry = cmp.get_selected_entry()
	if not entry then
	  cmp.select_next_item({ behavior = cmp.SelectBehavior.Select })
	end
	cmp.confirm()
      else
        fallback()
      end
    end, {"i","s","c",}),
    ...
  }
})