vim.opt.completeopt = {'menu', 'menuone', 'noselect', 'noinsert'}
vim.opt.shortmess:append('c')

local function tab_complete()
    if vim.fn.pumvisible() == 1 then
        -- navigate to next item in completion menu
        return '<Down>'
    end

    local c = vim.fn.col('.') - 1
    local is_whitespace = c == 0 or vim.fn.getline('.'):sub(c, c):match('%s')

    if is_whitespace then
        -- insert tab
        return '<Tab>'
    end

    local lsp_completion = vim.bo.omnifunc == 'v:lua.vim.lsp.omnifunc'

    if lsp_completion then
        -- trigger lsp code completion
        return '<C-x><C-o>'
    end

    -- suggest words in current buffer
    return '<C-x><C-n>'
end

local function tab_prev()
    if vim.fn.pumvisible() == 1 then
        -- navigate to previous item in completion menu
        return '<Up>'
    end

    -- insert tab
    return '<Tab>'
end

vim.keymap.set('i', '<Tab>', tab_complete, {expr = true})
vim.keymap.set('i', '<S-Tab>', tab_prev, {expr = true})