From 64a13f4be2775f7a2fc8e73e8ced1feac4146216 Mon Sep 17 00:00:00 2001
From: Jacques Lucke <jacques@blender.org>
Date: Sat, 24 May 2025 21:01:05 +0200
Subject: [PATCH] Refactor: BLI: simplify extending get_default_hash function

This simplifies extending `blender::get_default_hash` to more than 4 parameters
by just adding extra factors to the list. The behavior should be the same as
before.

Pull Request: https://projects.blender.org/blender/blender/pulls/139392
---
 source/blender/blenlib/BLI_hash.hh     | 38 ++++++++++----------------
 source/blender/blenlib/BLI_hash_fwd.hh | 14 ++++++++++
 source/blender/blenlib/BLI_span.hh     |  3 +-
 source/blender/blenlib/CMakeLists.txt  |  1 +
 4 files changed, 30 insertions(+), 26 deletions(-)
 create mode 100644 source/blender/blenlib/BLI_hash_fwd.hh

diff --git a/source/blender/blenlib/BLI_hash.hh b/source/blender/blenlib/BLI_hash.hh
index e6fe832f818..738d639f519 100644
--- a/source/blender/blenlib/BLI_hash.hh
+++ b/source/blender/blenlib/BLI_hash.hh
@@ -65,6 +65,7 @@
 #include <string>
 #include <utility>
 
+#include "BLI_hash_fwd.hh"
 #include "BLI_string_ref.hh"
 
 namespace blender {
@@ -215,35 +216,24 @@ template<typename T> struct DefaultHash<T *> {
   }
 };
 
-template<typename T> uint64_t get_default_hash(const T &v)
+namespace detail {
+static constexpr std::array<uint64_t, 3> default_hash_factors = {19349669, 83492791, 3632623};
+
+template<size_t... I, typename... Args>
+inline uint64_t get_default_hash_array(std::index_sequence<I...> /*indices*/, const Args &...args)
 {
-  return DefaultHash<std::decay_t<T>>{}(v);
+  static_assert(sizeof...(Args) == sizeof...(I));
+  static_assert(sizeof...(Args) <= default_hash_factors.size());
+  return (0 ^ ... ^ (default_hash_factors[I] * DefaultHash<std::decay_t<Args>>{}(args)));
 }
 
-template<typename T1, typename T2> uint64_t get_default_hash(const T1 &v1, const T2 &v2)
-{
-  const uint64_t h1 = get_default_hash(v1);
-  const uint64_t h2 = get_default_hash(v2);
-  return h1 ^ (h2 * 19349669);
-}
+}  // namespace detail
 
-template<typename T1, typename T2, typename T3>
-uint64_t get_default_hash(const T1 &v1, const T2 &v2, const T3 &v3)
+template<typename T, typename... Args>
+inline uint64_t get_default_hash(const T &v, const Args &...args)
 {
-  const uint64_t h1 = get_default_hash(v1);
-  const uint64_t h2 = get_default_hash(v2);
-  const uint64_t h3 = get_default_hash(v3);
-  return h1 ^ (h2 * 19349669) ^ (h3 * 83492791);
-}
-
-template<typename T1, typename T2, typename T3, typename T4>
-uint64_t get_default_hash(const T1 &v1, const T2 &v2, const T3 &v3, const T4 &v4)
-{
-  const uint64_t h1 = get_default_hash(v1);
-  const uint64_t h2 = get_default_hash(v2);
-  const uint64_t h3 = get_default_hash(v3);
-  const uint64_t h4 = get_default_hash(v4);
-  return h1 ^ (h2 * 19349669) ^ (h3 * 83492791) ^ (h4 * 3632623);
+  return DefaultHash<std::decay_t<T>>{}(v) ^
+         detail::get_default_hash_array(std::make_index_sequence<sizeof...(Args)>(), args...);
 }
 
 /** Support hashing different kinds of pointer types. */
diff --git a/source/blender/blenlib/BLI_hash_fwd.hh b/source/blender/blenlib/BLI_hash_fwd.hh
new file mode 100644
index 00000000000..994cebb76a6
--- /dev/null
+++ b/source/blender/blenlib/BLI_hash_fwd.hh
@@ -0,0 +1,14 @@
+/* SPDX-FileCopyrightText: 2025 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+#include <cstdint>
+
+namespace blender {
+
+template<typename T, typename... Args>
+inline uint64_t get_default_hash(const T &v, const Args &...args);
+
+}
diff --git a/source/blender/blenlib/BLI_span.hh b/source/blender/blenlib/BLI_span.hh
index 1ddf9151a75..2eccbc99515 100644
--- a/source/blender/blenlib/BLI_span.hh
+++ b/source/blender/blenlib/BLI_span.hh
@@ -60,14 +60,13 @@
 #include <array>
 #include <vector>
 
+#include "BLI_hash_fwd.hh"
 #include "BLI_index_range.hh"
 #include "BLI_memory_utils.hh"
 #include "BLI_utildefines.h"
 
 namespace blender {
 
-template<typename T> uint64_t get_default_hash(const T &v);
-
 /**
  * References an array of type T that is owned by someone else. The data in the array cannot be
  * modified.
diff --git a/source/blender/blenlib/CMakeLists.txt b/source/blender/blenlib/CMakeLists.txt
index ecb089815c3..d1d29adca0a 100644
--- a/source/blender/blenlib/CMakeLists.txt
+++ b/source/blender/blenlib/CMakeLists.txt
@@ -251,6 +251,7 @@ set(SRC
   BLI_gsqueue.h
   BLI_hash.h
   BLI_hash.hh
+  BLI_hash_fwd.hh
   BLI_hash_md5.hh
   BLI_hash_mm2a.hh
   BLI_hash_mm3.hh