nvim: add function to insert/update modeline

.config/nvim/lua/functions.lua

@@ -14,3 +14,42 @@ vim.api.nvim_create_autocmd(
         callback = my_remove_trailing_whitespace
     }
 )
+
+function my_insert_modeline()
+    local comment_string = vim.o.commentstring
+    local space_maybe = ''
+    if (string.match(comment_string, '%%s(.*)') ~= '') then
+        space_maybe = ' '
+    end
+
+    local fenc = vim.o.fileencoding
+    if (fenc == '') then
+        fenc = 'utf-8'
+    end
+
+    local modeline_elements = {
+        ' vim: set',
+        ' fenc=' .. fenc,
+        ' ts=' .. vim.o.tabstop,
+        ' sw=' .. vim.o.shiftwidth,
+        (vim.o.expandtab and ' et' or ' noet'),
+        ' tw=' .. vim.o.textwidth,
+        ':',
+        space_maybe
+    }
+    local modeline = comment_string:gsub('%%s', table.concat(modeline_elements))
+    modeline = modeline:gsub('  ', ' ')
+
+    local buffer = vim.api.nvim_win_get_buf(0)
+    local current = vim.api.nvim_buf_get_lines(buffer, -2, -1, true)[1]
+    if (current == modeline) then
+        print('modeline already exists')
+    elseif (string.match(current, 'vim:')) then
+        vim.api.nvim_buf_set_lines(buffer, -2, -1, true, { modeline })
+        print('modeline updated')
+    else
+        vim.api.nvim_buf_set_lines(buffer, -1, -1, true, { '', modeline })
+        print('modeline inserted')
+    end
+end
+vim.cmd([[command! ModelineInsert lua my_insert_modeline()]])