gusex

defmodule MyFormatter do
	@moduledoc false
	# Custom formatter plugin that uses TAB characters for indentation instead of spaces.
	# This formatter:
	# - Uses TAB character for indentation
	# - Uses SPACE character for alignment
	# - Follows British English conventions for comments and names

	@behaviour Mix.Tasks.Format

	def features(_opts) do
		[extensions: ~w(.ex .exs)]
	end

	def format(contents, _opts) do
		contents
		|> Code.format_string!()
		|> IO.iodata_to_binary()
		|> convert_spaces_to_tabs()
	end

	# Convert leading spaces to tabs while preserving alignment spaces
	defp convert_spaces_to_tabs(formatted_code) do
		formatted_code
		|> String.split("\n")
		|> Enum.map_join("\n", &process_line/1)
	end

	# Process each line to convert indentation spaces to tabs
	defp process_line(line) do
		case Regex.run(~r/^(\s*)(.*)$/, line) do
			[_, leading_whitespace, rest] ->
				tab_indentation = convert_leading_spaces(leading_whitespace)
				tab_indentation <> rest

			_ ->
				line
		end
	end

	# Convert groups of 2 spaces at the beginning to single tab
	defp convert_leading_spaces(whitespace) do
		# Count leading spaces (ignoring any tabs that might already be there)
		space_count = String.length(String.replace(whitespace, "\t", ""))

		# Convert pairs of spaces to tabs
		tab_count = div(space_count, 2)
		remaining_spaces = rem(space_count, 2)

		# Build result with tabs for indentation and remaining space for alignment
		String.duplicate("\t", tab_count) <> String.duplicate(" ", remaining_spaces)
	end
end