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

@@ -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

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

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;