util/TransparentHash: new class

Author: MaxKellermann

src/spawn/TmpfsManager.cxx

@@ -6,9 +6,7 @@
 #include "TmpfsCreate.hxx"
 #include "system/Error.hxx"
 #include "system/Mount.hxx"
-#include "util/djb_hash.hxx"
 #include "util/SharedLease.hxx"
-#include "util/SpanCast.hxx"
 
 #include <cassert>
 #include <chrono>
@@ -88,12 +86,6 @@ struct TmpfsManager::Item final
 	// TDOO void OnBroken() noexcept override;
 };
 
-inline std::size_t
-TmpfsManager::ItemHash::operator()(std::string_view name) const noexcept
-{
-	return djb_hash(AsBytes(name));
-}
-
 inline std::string_view
 TmpfsManager::ItemGetKey::operator()(const Item &item) const noexcept
 {

src/spawn/TmpfsManager.hxx

@@ -7,6 +7,7 @@
 #include "io/UniqueFileDescriptor.hxx"
 #include "util/IntrusiveHashSet.hxx"
 #include "util/IntrusiveList.hxx"
+#include "util/TransparentHash.hxx"
 
 #include <string_view>
 #include <utility> // for std::pair
@@ -21,18 +22,13 @@ class SharedLease;
 class TmpfsManager {
 	struct Item;
 
-	struct ItemHash {
-		[[gnu::pure]]
-		std::size_t operator()(std::string_view name) const noexcept;
-	};
-
 	struct ItemGetKey {
 		[[gnu::pure]]
 		std::string_view operator()(const Item &item) const noexcept;
 	};
 
 	IntrusiveHashSet<Item, 1024,
-			 IntrusiveHashSetOperators<Item, ItemGetKey, ItemHash,
+			 IntrusiveHashSetOperators<Item, ItemGetKey, TransparentHash,
 						   std::equal_to<std::string_view>>> items;
 	IntrusiveList<Item> abandoned;
 

src/util/TransparentHash.cxx

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <max.kellermann@ionos.com>
+
+#include "TransparentHash.hxx"
+#include "djb_hash.hxx"
+#include "SpanCast.hxx"
+
+std::size_t
+TransparentHash::operator()(const char *s) const noexcept
+{
+	return djb_hash_string(s);
+}
+
+std::size_t
+TransparentHash::operator()(std::span<const std::byte> s) const noexcept
+{
+	return djb_hash(s);
+}
+
+std::size_t
+TransparentHash::operator()(std::string_view s) const noexcept
+{
+	return djb_hash(AsBytes(s));
+}

src/util/TransparentHash.hxx

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <max.kellermann@ionos.com>
+
+#pragma once
+
+#include <cstddef>
+#include <span>
+#include <string_view>
+
+/**
+ * A hasher based on the DJB's cdb hash function.  Being
+ * "transparent", it allows efficient standard container lookup.
+ */
+struct TransparentHash {
+	using is_transparent = void;
+
+	[[gnu::pure]]
+	std::size_t operator()(const char *s) const noexcept;
+
+	[[gnu::pure]]
+	std::size_t operator()(std::span<const std::byte> s) const noexcept;
+
+	[[gnu::pure]]
+	std::size_t operator()(std::string_view s) const noexcept;
+};

src/util/meson.build

@@ -11,6 +11,7 @@ util_sources = [
   'StringParser.cxx',
   'StringStrip.cxx',
   'StringWithHash.cxx',
+  'TransparentHash.cxx',
   'UTF8.cxx',
 ]