tag/ParseName: generate an optimized tag_name_parse() at build time

src/tag/GenParseName.cxx

+/*
+ * Copyright 2003-2020 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.
+ */
+
+#include "Type.h"
+
+#include <map>
+#include <string_view>
+
+#include <stdio.h>
+
+/*
+
+  This program generates an optimized parser for tag names, by doign
+  switch() on the first character.  This reduces the number of
+  strcmp() calls.
+
+ */
+
+int
+main(int argc, char **argv)
+{
+	if (argc != 2)
+		return EXIT_FAILURE;
+
+	FILE *out = fopen(argv[1], "w");
+
+	std::map<std::string_view, TagType> names;
+	for (unsigned i = 0; i < unsigned(TAG_NUM_OF_ITEM_TYPES); ++i)
+		names[tag_item_names[i]] = TagType(i);
+
+	fprintf(out,
+		"#include \"ParseName.hxx\"\n"
+		"\n"
+		"#include <assert.h>\n"
+		"#include <string.h>\n"
+		"\n"
+		"TagType\n"
+		"tag_name_parse(const char *name) noexcept\n"
+		"{\n"
+		"  assert(name != nullptr);\n"
+		"\n"
+		"  switch (*name) {\n");
+
+	char first = 0;
+
+	for (const auto &i : names) {
+		const std::string_view name = i.first;
+		const TagType tag = i.second;
+
+		if (name.front() != first) {
+			if (first != 0)
+				fprintf(out, "    break;\n\n");
+			first = name.front();
+			fprintf(out, "  case '%c':\n", first);
+		}
+
+		fprintf(out,
+			"    if (strcmp(name + 1, \"%.*s\") == 0) return TagType(%u);\n",
+			int(name.size() - 1), name.data() + 1, unsigned(tag));
+	}
+
+	fprintf(out, "    break;\n\n");
+
+	fprintf(out,
+		"  }\n"
+		"\n"
+		"  return TAG_NUM_OF_ITEM_TYPES;\n"
+		"}\n");
+
+	return EXIT_SUCCESS;
+}

src/tag/ParseName.cxx

@@ -26,21 +26,6 @@
 #include <string.h>
 
 TagType
-tag_name_parse(const char *name) noexcept
-{
-	assert(name != nullptr);
-
-	for (unsigned i = 0; i < TAG_NUM_OF_ITEM_TYPES; ++i) {
-		assert(tag_item_names[i] != nullptr);
-
-		if (strcmp(name, tag_item_names[i]) == 0)
-			return (TagType)i;
-	}
-
-	return TAG_NUM_OF_ITEM_TYPES;
-}
-
-TagType
 tag_name_parse(StringView name) noexcept
 {
 	assert(name != nullptr);

src/tag/meson.build

+generate_parse_name = executable(
+  'GenParseName',
+  'GenParseName.cxx',
+  'Names.c',
+  native: true,
+)
+
+parse_name_cxx = custom_target(
+  'RunGenParseName',
+  output: ['ParseName2.cxx'],
+  command : [
+    generate_parse_name, '@OUTPUT@',
+  ],
+)
+
 tag_sources = [
   'Tag.cxx',
   'Builder.cxx',
@@ -5,6 +20,7 @@ tag_sources = [
   'Settings.cxx',
   'Config.cxx',
   'ParseName.cxx',
+  parse_name_cxx,
   'Names.c',
   'FixString.cxx',
   'Pool.cxx',