feat: add cli option to dump default config

Closes: https://github.com/aristocratos/btop/issues/1394

src/btop_cli.cpp

@@ -5,15 +5,20 @@
 #include <algorithm>
 #include <expected>
 #include <filesystem>
+#include <iterator>
 #include <optional>
+#include <ranges>
 #include <span>
 #include <stdexcept>
 #include <string>
 #include <string_view>
 
+#include <unistd.h>
+
 #include <fmt/base.h>
 #include <fmt/format.h>
 
+#include "btop_config.hpp"
 #include "btop_shared.hpp"
 #include "config.h"
 
@@ -22,6 +27,9 @@ using namespace std::string_view_literals;
 static constexpr auto BOLD = "\033[1m"sv;
 static constexpr auto BOLD_UNDERLINE = "\033[1;4m"sv;
 static constexpr auto BOLD_RED = "\033[1;31m"sv;
+static constexpr auto BOLD_GREEN = "\033[1;32m"sv;
+static constexpr auto BOLD_YELLOW = "\033[1;33m"sv;
+static constexpr auto BOLD_BRIGHT_BLACK = "\033[1;90m"sv;
 static constexpr auto YELLOW = "\033[33m"sv;
 static constexpr auto RESET = "\033[0m"sv;
 
@@ -49,6 +57,9 @@ namespace Cli {
 		for (auto it = args.begin(); it != args.end(); ++it) {
 			auto arg = *it;
 
+			if (arg == "--default-config") {
+				return default_config();
+			}
 			if (arg == "-h" || arg == "--help") {
 				usage();
 				help();
@@ -187,6 +198,50 @@ namespace Cli {
 		return cli;
 	}
 
+	auto default_config() noexcept -> Result {
+		// The idea of using `current_config` is that the CLI parser is run before loading the actual config and thus
+		// provides default values.
+		auto config = Config::current_config();
+		
+		if (isatty(STDOUT_FILENO)) {
+			std::string buffer {};
+			// The config buffer ends in `\n`. `std::views::split` will then create an empty element after the last
+			// newline, which we would write as an additional empty line at the very end.
+			auto trimmed_config = config.substr(0, config.length() - 1);
+			for (const auto line : std::views::split(trimmed_config, '\n')) {
+				auto line_view = std::string_view { line };
+				if (line_view.starts_with("#")) {
+					fmt::format_to(
+						std::back_inserter(buffer), "{1}{0}{2}\n", line_view, BOLD_BRIGHT_BLACK, RESET
+					);
+				} else if (!line_view.empty()) {
+					auto pos = line_view.find("=");
+					if (pos == line_view.npos) {
+						error("invalid default config: '=' not found");
+						return std::unexpected { 1 };
+					}
+					auto name = line_view.substr(0, pos);
+					auto value = line_view.substr(pos + 1);
+					fmt::format_to(
+						std::back_inserter(buffer),
+						"{2}{0}{4}={3}{1}{4}\n",
+						name,
+						value,
+						BOLD_YELLOW,
+						BOLD_GREEN,
+						RESET
+					);
+				} else {
+					fmt::format_to(std::back_inserter(buffer), "\n");
+				}
+			}
+			fmt::print("{}", buffer);
+		} else {
+			fmt::print("{}", config);
+		}
+		return std::unexpected { 0 };
+	}
+
 	void usage() noexcept {
 		fmt::println("{0}Usage:{1} {2}btop{1} [OPTIONS]\n", BOLD_UNDERLINE, RESET, BOLD);
 	}
@@ -204,6 +259,7 @@ namespace Cli {
 			"  {2}    --themes-dir{1} <dir>  Path to a custom themes directory\n"
 			"  {2}    --no-tty{1}            Force disable tty mode\n"
 			"  {2}-u, --update{1} <ms>       Set an initial update rate in milliseconds\n"
+			"  {2}    --default-config{1}    Print default config to standard output\n"
 			"  {2}-h, --help{1}              Show this help message and exit\n"
 			"  {2}-V, --version{1}           Show a version message and exit (more with --version)\n",
 			BOLD_UNDERLINE, RESET, BOLD