Feat: Add buffer-specific toggles

Closes #2.

README.md

@@ -8,11 +8,13 @@ when it is not needed.
 
 ## Table of contents
 
-- [Video Demonstration](#video-demonstration)
+- [Video demonstration](#video-demonstration)
 - [Requirements](#requirements)
 - [Installation](#installation)
 - [Configuration](#configuration)
+- [Buffer-specific toggle](#buffer-specific-toggle)
 - [Commands](#commands)
+- [Licence](#licence)
 
 ## Video demonstration
 
@@ -273,6 +275,23 @@ you can use the following snippet to access it.
 require("nerd_column").default_config
 ```
 
+## Buffer-specific toggle
+
+You can control the state of the plugin via the use of the global option
+`vim.g.nerd_column_enabled` and the buffer-local option
+`vim.b.nerd_column_enabled`. If you have set the plugin to be enabled
+by default by setting `enabled` to `true` in your configuration, the global
+option `vim.g.nerd_column_enabled` will be automatically set to `true`
+when the plugin is set up.
+
+The value of the buffer-local option `vim.b.nerd_column_enabled` is checked
+first, and if it is not set, i.e. `vim.b.nerd_column_enabled` is `nil`,
+then the value of the global option `vim.g.nerd_column_enabled`
+will be used instead.
+
+This way, you can disable the plugin in specific buffers by setting
+`vim.b.nerd_column_enabled` to `false`.
+
 ## Commands
 
 Nerd Column provides 3 commands:
@@ -284,20 +303,38 @@ Nerd Column provides 3 commands:
   require("nerd_column").enable()
   ```
 
+  This command will set both `vim.g.nerd_column_enabled` and
+  `vim.b.nerd_column_enabled` to `true`, as most users would
+  expect the command to work globally.
+
 - `NerdColumnDisable` to disable the plugin.
   It can also be accessed from Lua be using:
 
   ```lua
   require("nerd_column").disable()
   ```
 
+  This command will set both `vim.g.nerd_column_enabled` and
+  `vim.b.nerd_column_enabled` to `false`, as most users would
+  expect the command to work globally.
+
 - `NerdColumnToggle` to toggle the plugin.
   It can also be accessed from Lua be using:
 
   ```lua
   require("nerd_column").toggle()
   ```
 
+  This command will get the current state of the plugin from
+  `vim.b.nerd_column_enabled` first. If `vim.b.nerd_column_enabled`
+  is not set, i.e. `vim.b.nerd_column_enabled` is `nil`, then
+  it will take the value of `vim.g.nerd_column_enabled`.
+  Then, the command will toggle the plugin state, and set both
+  `vim.g.nerd_column_enabled` and `vim.b.nerd_column_enabled` to the
+  toggled state. The reason for this is that most users would expect
+  the command to work globally, and that the toggle should act on
+  the current state of the plugin in the current buffer.
+
 ## [Licence]
 
 This plugin is licenced under the [GNU AGPL v3 licence][Licence].

lua/nerd_column.lua

@@ -217,6 +217,35 @@ local function match_file_type(given_file_type, list_of_file_types)
 	return false
 end
 
+-- The function to get whether the plugin is enabled or not
+---@return boolean nerd_column_is_enabled Whether the plugin is enabled or not
+local function nerd_column_is_enabled()
+	--
+
+	-- Get the value of buffer local variable for
+	-- whether the plugin is enabled
+	local plugin_enabled = vim.b.nerd_column_enabled
+
+	-- If the buffer local variable isn't set,
+	-- then get the value of the global variable
+	if plugin_enabled == nil then plugin_enabled = vim.g.nerd_column_enabled end
+
+	-- Return whether the plugin is enabled or not
+	return plugin_enabled
+end
+
+-- The function to set the plugin state
+---@param state boolean
+---@return nil
+local function set_nerd_column_state(state)
+	--
+
+	-- Set the plugin state in both the global variable
+	-- and the buffer local variable
+	vim.g.nerd_column_enabled = state
+	vim.b.nerd_column_enabled = state
+end
+
 -- The function to disable the colour column
 ---@param window integer The ID of the window in the buffer
 ---@return nil
@@ -231,7 +260,9 @@ local function on_change()
 
 	-- If the plugin is disabled, disable the colour column
 	-- and exit the function
-	if not config.enabled then return disable_colour_column(current_window) end
+	if not nerd_column_is_enabled() then
+		return disable_colour_column(current_window)
+	end
 
 	-- Get the current buffer
 	local current_buffer = vim.api.nvim_win_get_buf(current_window)
@@ -349,7 +380,7 @@ M.enable = function()
 	--
 
 	-- Enable the plugin
-	config.enabled = true
+	set_nerd_column_state(true)
 
 	-- Call the on change function
 	on_change()
@@ -360,7 +391,7 @@ M.disable = function()
 	--
 
 	-- Disable the plugin
-	config.enabled = false
+	set_nerd_column_state(false)
 
 	-- Disable the colour column
 	disable_colour_column(vim.api.nvim_get_current_win())
@@ -370,8 +401,11 @@ end
 M.toggle = function()
 	--
 
+	-- Get the current state of the plugin
+	local is_enabled = nerd_column_is_enabled()
+
 	-- Toggle the plugin
-	config.enabled = not config.enabled
+	set_nerd_column_state(not is_enabled)
 
 	-- Call the on change function
 	on_change()
@@ -388,6 +422,10 @@ M.setup = function(user_config)
 	-- Merge the user's configuration with the default configuration
 	config = vim.tbl_extend("force", config, user_config)
 
+	-- If the plugin is enabled, set the vim global option
+	-- for the plugin to enabled
+	if config.enabled then vim.g.nerd_column_enabled = true end
+
 	-- Create the auto command to set the colour column
 	vim.api.nvim_create_autocmd(
 		{ "BufEnter", "CursorMoved", "CursorMovedI", "WinScrolled" },