From ab9644382d14eb1254ce33e38227c874fd80a08f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Sybren=20A=2E=20St=C3=BCvel?= <sybren@blender.org>
Date: Fri, 24 Sep 2021 12:55:26 +0200
Subject: UUID: add less-than operator

Add `operator<` to C++ class to allow lexicographic ordering of UUIDs.

This will be necessary when writing asset catalogs to disk in a predictable
(i.e. ordered) manner.
---
 source/blender/blenlib/BLI_uuid.h             |  5 +++++
 source/blender/blenlib/intern/uuid.cc         | 19 +++++++++++++++++++
 source/blender/blenlib/tests/BLI_uuid_test.cc | 23 +++++++++++++++++++++++
 3 files changed, 47 insertions(+)

diff --git a/source/blender/blenlib/BLI_uuid.h b/source/blender/blenlib/BLI_uuid.h
index 98a600a5de8..ed0d31b625f 100644
--- a/source/blender/blenlib/BLI_uuid.h
+++ b/source/blender/blenlib/BLI_uuid.h
@@ -98,6 +98,11 @@ class bUUID : public ::bUUID {
 bool operator==(bUUID uuid1, bUUID uuid2);
 bool operator!=(bUUID uuid1, bUUID uuid2);
 
+/**
+ * Lexicographic comparison of the UUIDs.
+ * Equivalent to string comparison on the formatted UUIDs. */
+bool operator<(bUUID uuid1, bUUID uuid2);
+
 }  // namespace blender
 
 #endif
diff --git a/source/blender/blenlib/intern/uuid.cc b/source/blender/blenlib/intern/uuid.cc
index fe237b8aae6..3c86238036c 100644
--- a/source/blender/blenlib/intern/uuid.cc
+++ b/source/blender/blenlib/intern/uuid.cc
@@ -27,6 +27,7 @@
 #include <random>
 #include <sstream>
 #include <string>
+#include <tuple>
 
 /* Ensure the UUID struct doesn't have any padding, to be compatible with memcmp(). */
 static_assert(sizeof(bUUID) == 16, "expect UUIDs to be 128 bit exactly");
@@ -189,4 +190,22 @@ bool operator!=(const bUUID uuid1, const bUUID uuid2)
   return !(uuid1 == uuid2);
 }
 
+bool operator<(const bUUID uuid1, const bUUID uuid2)
+{
+  auto simple_fields1 = std::tie(uuid1.time_low,
+                                 uuid1.time_mid,
+                                 uuid1.time_hi_and_version,
+                                 uuid1.clock_seq_hi_and_reserved,
+                                 uuid1.clock_seq_low);
+  auto simple_fields2 = std::tie(uuid2.time_low,
+                                 uuid2.time_mid,
+                                 uuid2.time_hi_and_version,
+                                 uuid2.clock_seq_hi_and_reserved,
+                                 uuid2.clock_seq_low);
+  if (simple_fields1 == simple_fields2) {
+    return std::memcmp(uuid1.node, uuid2.node, sizeof(uuid1.node)) < 0;
+  }
+  return simple_fields1 < simple_fields2;
+}
+
 }  // namespace blender
diff --git a/source/blender/blenlib/tests/BLI_uuid_test.cc b/source/blender/blenlib/tests/BLI_uuid_test.cc
index f2160f614ba..b406a0521a1 100644
--- a/source/blender/blenlib/tests/BLI_uuid_test.cc
+++ b/source/blender/blenlib/tests/BLI_uuid_test.cc
@@ -75,6 +75,29 @@ TEST(BLI_uuid, equality)
   EXPECT_NE(uuid1, uuid2);
 }
 
+TEST(BLI_uuid, comparison_trivial)
+{
+  const bUUID uuid0{};
+  const bUUID uuid1("11111111-1111-1111-1111-111111111111");
+  const bUUID uuid2("22222222-2222-2222-2222-222222222222");
+
+  EXPECT_LT(uuid0, uuid1);
+  EXPECT_LT(uuid0, uuid2);
+  EXPECT_LT(uuid1, uuid2);
+}
+
+TEST(BLI_uuid, comparison_byte_order_check)
+{
+  const bUUID uuid0{};
+  /* Chosen to test byte ordering is taken into account correctly when comparing. */
+  const bUUID uuid12("12222222-2222-2222-2222-222222222222");
+  const bUUID uuid21("21111111-1111-1111-1111-111111111111");
+
+  EXPECT_LT(uuid0, uuid12);
+  EXPECT_LT(uuid0, uuid21);
+  EXPECT_LT(uuid12, uuid21);
+}
+
 TEST(BLI_uuid, string_formatting)
 {
   bUUID uuid;
-- 
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