db/Print: move sort/window emulation code to class DatabaseVisitorHelper

That way, each plugin can decide to implement it better.

src/db/VHelper.cxx

@@ -0,0 +1,133 @@
+/*
+ * Copyright 2003-2018 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 "config.h"
+#include "VHelper.hxx"
+#include "song/DetachedSong.hxx"
+#include "song/LightSong.hxx"
+#include "song/Filter.hxx"
+
+#include <assert.h>
+#include <string.h>
+
+DatabaseVisitorHelper::DatabaseVisitorHelper(const DatabaseSelection &_selection,
+					     VisitSong &visit_song) noexcept
+	:selection(_selection)
+{
+	// TODO: apply URI and SongFilter
+	assert(selection.uri.empty());
+	assert(selection.filter == nullptr);
+
+	if (selection.sort != TAG_NUM_OF_ITEM_TYPES) {
+		/* the client has asked us to sort the result; this is
+		   pretty expensive, because instead of streaming the
+		   result to the client, we need to copy it all into
+		   this std::vector, and then sort it */
+
+		original_visit_song = std::move(visit_song);
+		visit_song = [this](const auto &song){
+			songs.emplace_back(song);
+		};
+	} else if (selection.window != RangeArg::All()) {
+		original_visit_song = std::move(visit_song);
+		visit_song = [this](const auto &song){
+			if (selection.window.Contains(counter++))
+				original_visit_song(song);
+		};
+	}
+}
+
+DatabaseVisitorHelper::~DatabaseVisitorHelper() noexcept = default;
+
+gcc_pure
+static bool
+CompareNumeric(const char *a, const char *b) noexcept
+{
+	long a_value = strtol(a, nullptr, 10);
+	long b_value = strtol(b, nullptr, 10);
+
+	return a_value < b_value;
+}
+
+gcc_pure
+static bool
+CompareTags(TagType type, bool descending, const Tag &a, const Tag &b) noexcept
+{
+	const char *a_value = a.GetSortValue(type);
+	const char *b_value = b.GetSortValue(type);
+
+	if (descending) {
+		using std::swap;
+		swap(a_value, b_value);
+	}
+
+	switch (type) {
+	case TAG_DISC:
+	case TAG_TRACK:
+		return CompareNumeric(a_value, b_value);
+
+	default:
+		return strcmp(a_value, b_value) < 0;
+	}
+}
+
+void
+DatabaseVisitorHelper::Commit()
+{
+	/* only needed if sorting is enabled */
+	if (selection.sort == TAG_NUM_OF_ITEM_TYPES)
+		return;
+
+	assert(original_visit_song);
+
+	/* sort the song collection */
+	const auto sort = selection.sort;
+	const auto descending = selection.descending;
+
+	if (sort == TagType(SORT_TAG_LAST_MODIFIED))
+		std::stable_sort(songs.begin(), songs.end(),
+				 [descending](const DetachedSong &a, const DetachedSong &b){
+					 return descending
+						 ? a.GetLastModified() > b.GetLastModified()
+						 : a.GetLastModified() < b.GetLastModified();
+				 });
+	else
+		std::stable_sort(songs.begin(), songs.end(),
+				 [sort, descending](const DetachedSong &a,
+						    const DetachedSong &b){
+					 return CompareTags(sort, descending,
+							    a.GetTag(),
+							    b.GetTag());
+				 });
+
+	/* apply the "window" */
+	if (selection.window.end < songs.size())
+		songs.erase(std::next(songs.begin(), selection.window.end),
+			    songs.end());
+
+	if (selection.window.start >= songs.size())
+		return;
+
+	songs.erase(songs.begin(),
+		    std::next(songs.begin(), selection.window.start));
+
+	/* now pass all songs to the original visitor callback */
+	for (const auto &song : songs)
+		original_visit_song((LightSong)song);
+}