Allow rgbgfx - for stdin and stdout

Closes #1087

include/file.hpp

@@ -0,0 +1,95 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_FILE_HPP
+#define RGBDS_FILE_HPP
+
+#include <array>
+#include <assert.h>
+#include <cassert>
+#include <fcntl.h>
+#include <filesystem>
+#include <fstream>
+#include <ios>
+#include <iostream>
+#include <streambuf>
+#include <string>
+#include <string.h>
+#include <string_view>
+#include <variant>
+
+#include "helpers.h"
+#include "platform.h"
+
+#include "gfx/main.hpp"
+
+// Convenience feature for visiting the below.
+template<typename... Ts>
+struct Visitor : Ts... {
+	using Ts::operator()...;
+};
+template<typename... Ts>
+Visitor(Ts...) -> Visitor<Ts...>;
+
+class File {
+	// Construct a `std::streambuf *` by default, since it's probably lighter than a `filebuf`.
+	std::variant<std::streambuf *, std::filebuf> _file;
+
+public:
+	File() {}
+	~File() { close(); }
+
+	/**
+	 * This should only be called once, and before doing any `->` operations.
+	 * Returns `nullptr` on error, and a non-null pointer otherwise.
+	 */
+	File *open(std::filesystem::path const &path, std::ios_base::openmode mode) {
+		if (path != "-") {
+			return _file.emplace<std::filebuf>().open(path, mode) ? this : nullptr;
+		} else if (mode & std::ios_base::in) {
+			assert(!(mode & std::ios_base::out));
+			_file.emplace<std::streambuf *>(std::cin.rdbuf());
+			if (setmode(STDIN_FILENO, mode & std::ios_base::binary ? O_BINARY : O_TEXT) == -1) {
+				fatal("Failed to set stdin to %s mode: %s",
+				      mode & std::ios_base::binary ? "binary" : "text", strerror(errno));
+			}
+		} else {
+			assert(mode & std::ios_base::out);
+			_file.emplace<std::streambuf *>(std::cout.rdbuf());
+		}
+		return this;
+	}
+	std::streambuf &operator*() {
+		return std::visit(Visitor{[](std::filebuf &file) -> std::streambuf & { return file; },
+		                          [](std::streambuf *buf) -> std::streambuf & { return *buf; }},
+		                  _file);
+	}
+	std::streambuf const &operator*() const {
+		// The non-`const` version does not perform any modifications, so it's okay.
+		return **const_cast<File *>(this);
+	}
+	std::streambuf *operator->() { return &**this; }
+	std::streambuf const *operator->() const {
+		// See the `operator*` equivalent.
+		return const_cast<File *>(this)->operator->();
+	}
+	File *close() {
+		return std::visit(Visitor{[this](std::filebuf &file) {
+			                          // This is called by the destructor, and an explicit `close`
+			                          // shouldn't close twice.
+			                          _file.emplace<std::streambuf *>(nullptr);
+			                          return file.close() != nullptr;
+		                          },
+		                          [](std::streambuf *buf) { return buf != nullptr; }},
+		                  _file)
+		           ? this
+		           : nullptr;
+	}
+};
+
+#endif // RGBDS_FILE_HPP