util/OptionParser: move to cmdline/

This library will depend on libfmt, and libutil shouldn't depend on
any library.

src/cmdline/OptionParser.hxx

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2003-2022 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_UTIL_OPTIONPARSER_HXX
+#define MPD_UTIL_OPTIONPARSER_HXX
+
+#include "OptionDef.hxx"
+
+#include <span>
+
+/**
+ * Command line option parser.
+ */
+class OptionParser
+{
+	std::span<const OptionDef> options;
+
+	std::span<const char *const> args;
+
+	const char **const remaining_head, **remaining_tail;
+
+public:
+	/**
+	 * Constructs #OptionParser.
+	 */
+	OptionParser(std::span<const OptionDef> _options,
+		     int _argc, char **_argv) noexcept
+		:options(_options), args(_argv + 1, _argc - 1),
+		 remaining_head(const_cast<const char **>(_argv + 1)),
+		 remaining_tail(remaining_head) {}
+
+	struct Result {
+		int index;
+		const char *value;
+
+		constexpr operator bool() const noexcept {
+			return index >= 0;
+		}
+	};
+
+	/**
+	 * Parses current command line entry.
+	 * Regardless of result, advances current position to the next
+	 * command line entry. 
+	 *
+	 * Throws on error.
+	 */
+	Result Next();
+
+	/**
+	 * Returns the remaining non-option arguments.
+	 */
+	std::span<const char *const> GetRemaining() const noexcept {
+		return {remaining_head, remaining_tail};
+	}
+
+private:
+	const char *CheckShiftValue(const char *s, const OptionDef &option);
+	Result IdentifyOption(const char *s);
+};
+
+#endif