[ObjC] Merge extension ranges that are continues.

Can result in a small reduction in binary size as well as slight performance
improvement during serialization since it is less subcalls for writing out any
extensions in the ranges.

PiperOrigin-RevId: 504272482

src/google/protobuf/compiler/objectivec/message.cc

@@ -46,6 +46,7 @@
 #include "google/protobuf/compiler/objectivec/names.h"
 #include "google/protobuf/compiler/objectivec/oneof.h"
 #include "google/protobuf/compiler/objectivec/text_format_decode_data.h"
+#include "google/protobuf/descriptor.h"
 #include "google/protobuf/descriptor.pb.h"
 #include "google/protobuf/io/printer.h"
 
@@ -153,6 +154,39 @@ struct ExtensionRangeOrdering {
   }
 };
 
+// This is a reduced case of Descriptor::ExtensionRange with just start and end.
+struct SimpleExtensionRange {
+  SimpleExtensionRange(int start, int end) : start(start), end(end){};
+  int start;  // inclusive
+  int end;    // exclusive
+
+  // Descriptors expose extension ranges in the order they were defined in the
+  // file, but this reorders and merges the ranges that are contiguous (i.e. -
+  // [(21,30),(10,20)] -> [(10,30)])
+  static std::vector<SimpleExtensionRange> Normalize(
+      const Descriptor* descriptor) {
+    std::vector<const Descriptor::ExtensionRange*> sorted_extensions;
+    sorted_extensions.reserve(descriptor->extension_range_count());
+    for (int i = 0; i < descriptor->extension_range_count(); ++i) {
+      sorted_extensions.push_back(descriptor->extension_range(i));
+    }
+
+    std::sort(sorted_extensions.begin(), sorted_extensions.end(),
+              ExtensionRangeOrdering());
+
+    std::vector<SimpleExtensionRange> result;
+    result.reserve(sorted_extensions.size());
+    for (const auto ext : sorted_extensions) {
+      if (!result.empty() && result.back().end == ext->start) {
+        result.back().end = ext->end;
+      } else {
+        result.emplace_back(ext->start, ext->end);
+      }
+    }
+    return result;
+  }
+};
+
 // Sort the fields of the given Descriptor by number into a new[]'d array
 // and return it.
 const FieldDescriptor** SortFieldsByNumber(const Descriptor* descriptor) {
@@ -387,14 +421,8 @@ void MessageGenerator::GenerateSource(io::Printer* printer) const {
   std::unique_ptr<const FieldDescriptor*[]> size_order_fields(
       SortFieldsByStorageSize(descriptor_));
 
-  std::vector<const Descriptor::ExtensionRange*> sorted_extensions;
-  sorted_extensions.reserve(descriptor_->extension_range_count());
-  for (int i = 0; i < descriptor_->extension_range_count(); ++i) {
-    sorted_extensions.push_back(descriptor_->extension_range(i));
-  }
-
-  std::sort(sorted_extensions.begin(), sorted_extensions.end(),
-            ExtensionRangeOrdering());
+  std::vector<SimpleExtensionRange> sorted_extensions(
+      SimpleExtensionRange::Normalize(descriptor_));
 
   printer->Print(
       // clang-format off
@@ -528,10 +556,10 @@ void MessageGenerator::GenerateSource(io::Printer* printer) const {
   }
   if (!sorted_extensions.empty()) {
     printer->Print("    static const GPBExtensionRange ranges[] = {\n");
-    for (int i = 0; i < sorted_extensions.size(); i++) {
+    for (const auto& extension_range : sorted_extensions) {
       printer->Print("      { .start = $start$, .end = $end$ },\n", "start",
-                     absl::StrCat(sorted_extensions[i]->start), "end",
-                     absl::StrCat(sorted_extensions[i]->end));
+                     absl::StrCat(extension_range.start), "end",
+                     absl::StrCat(extension_range.end));
     }
     // clang-format off
     printer->Print(