MixPresentationGenerator: Infer num_tags instead of requiring use…

…r input to match.

  - b/397462369 This signal is pretty useless:
    - When it matches the number of `tags`, then encoding would succeed.
    - When it did not match the number of `tags`, then encoding would fail.
    - Therefore, it is redundant with the number of `tags`, we can simply infer the correct value.
    - N.b.: "zero" vs. "absent" tags case are distinguished via `include_mix_presentation_tags` (`CopiesMixPresentationTagsWithZeroTags` vs `IgnoresTagsWhenSetIncludeMixPresentationTagsIsFalse`).
  - Motivation (b/388577499):
    - In the future, the encoder may start inserting custom tags automatically.
    - Avoid complicating behavior (what would `num_tags` represent at the proto layer, if the encoder adds implicit ones).
    - Dropping the field means we can simply ignore and not care about the answer to this question - all we care is that the generated OBU has the final correct value.
  - Tests:
    - Add coverage that a poorly set `num_tags` is now ignored.
    - Backfill test coverage when there are more than 255 tags (spec max).

PiperOrigin-RevId: 728647443

iamf/cli/proto/mix_presentation.proto

@@ -198,7 +198,9 @@ message MixPresentationTag {
 }
 
 message MixPresentationTags {
-  optional uint32 num_tags = 1;
+  // `num_tags` is ignored. The value in the bitstream is inferred based on the
+  // number of tags.
+  optional uint32 num_tags = 1 [deprecated = true];
   repeated MixPresentationTag tags = 2;
 }
 

iamf/cli/proto_conversion/proto_to_obu/mix_presentation_generator.cc

@@ -11,6 +11,7 @@
  */
 #include "iamf/cli/proto_conversion/proto_to_obu/mix_presentation_generator.h"
 
+#include <cstddef>
 #include <cstdint>
 #include <list>
 #include <optional>
@@ -304,10 +305,14 @@ absl::Status FillLayouts(
 absl::Status FillMixPresentationTags(
     const iamf_tools_cli_proto::MixPresentationTags& mix_presentation_tags,
     std::optional<MixPresentationTags>& obu_mix_presentation_tags) {
+  if (mix_presentation_tags.has_num_tags()) {
+    LOG(WARNING) << "Ignoring deprecated `num_tags` field. Please remove it.";
+  }
   obu_mix_presentation_tags = MixPresentationTags{};
-  RETURN_IF_NOT_OK(StaticCastIfInRange<uint32_t, uint8_t>(
-      "MixPresentationTags.num_tags", mix_presentation_tags.num_tags(),
+  RETURN_IF_NOT_OK(StaticCastIfInRange<size_t, uint8_t>(
+      "Number of MixPresentationTags.tags", mix_presentation_tags.tags().size(),
       obu_mix_presentation_tags->num_tags));
+  obu_mix_presentation_tags->tags.reserve(mix_presentation_tags.tags().size());
   for (const auto& input_tag : mix_presentation_tags.tags()) {
     obu_mix_presentation_tags->tags.push_back(MixPresentationTags::Tag{
         .tag_name = input_tag.tag_name(),

iamf/cli/proto_conversion/proto_to_obu/tests/mix_presentation_generator_test.cc

@@ -39,6 +39,7 @@ namespace {
 
 using ::absl_testing::IsOk;
 using ::testing::ElementsAreArray;
+using ::testing::Not;
 typedef ::google::protobuf::RepeatedPtrField<
     iamf_tools_cli_proto::MixPresentationObuMetadata>
     MixPresentationObuMetadatas;
@@ -428,6 +429,45 @@ TEST(Generate, CopiesMixPresentationTagsWithZeroTags) {
   EXPECT_TRUE(first_obu.mix_presentation_tags_->tags.empty());
 }
 
+TEST(Generate, IgnoresDeprecatedNumTags) {
+  MixPresentationObuMetadatas mix_presentation_metadata;
+  FillMixPresentationMetadata(mix_presentation_metadata.Add());
+  auto& mix_presentation = mix_presentation_metadata.at(0);
+  mix_presentation.set_include_mix_presentation_tags(true);
+  constexpr uint32_t kIncorrectIgnoredNumTags = 1;
+  mix_presentation.mutable_mix_presentation_tags()->set_num_tags(
+      kIncorrectIgnoredNumTags);
+
+  MixPresentationGenerator generator(mix_presentation_metadata);
+
+  std::list<MixPresentationObu> generated_obus;
+  EXPECT_THAT(generator.Generate(generated_obus), IsOk());
+
+  // Ok safely ignore the deprecated `num_tags` field.
+  const auto& first_obu = generated_obus.front();
+  ASSERT_TRUE(first_obu.mix_presentation_tags_.has_value());
+  EXPECT_EQ(first_obu.mix_presentation_tags_->num_tags, 0);
+  EXPECT_TRUE(first_obu.mix_presentation_tags_->tags.empty());
+}
+
+TEST(Generate, ReturnsErrorWhenTooManyTagsArePresent) {
+  MixPresentationObuMetadatas mix_presentation_metadata;
+  FillMixPresentationMetadata(mix_presentation_metadata.Add());
+  auto& mix_presentation = mix_presentation_metadata.at(0);
+  mix_presentation.set_include_mix_presentation_tags(true);
+  constexpr int kMaxNumTags = 255;
+  for (int i = 0; i < kMaxNumTags + 1; ++i) {
+    auto* tag = mix_presentation.mutable_mix_presentation_tags()->add_tags();
+    tag->set_tag_name("tag_name");
+    tag->set_tag_value("tag_value");
+  }
+
+  MixPresentationGenerator generator(mix_presentation_metadata);
+
+  std::list<MixPresentationObu> ignored_generated_obus;
+  EXPECT_THAT(generator.Generate(ignored_generated_obus), Not(IsOk()));
+}
+
 TEST(Generate, CopiesDuplicateContentLanguageTags) {
   MixPresentationObuMetadatas mix_presentation_metadata;
   FillMixPresentationMetadata(mix_presentation_metadata.Add());