Add MixPresentationObu::CreateFromBuffer.

This function will call a private explicit constructor which uses defaults to create an instance of the class. It will then call ValidateAndReadPayload, which will read from the provided buffer and fill in the appropriate fields within the AudioElementObu instance. This call must succeed - if it does not, we will return an error status which will signal to the caller that we were not able to read the OBU properly.

This function will be called in ProcessDescriptorObus.

PiperOrigin-RevId: 627764777

iamf/obu/BUILD

@@ -133,6 +133,7 @@ cc_library(
         "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/status",
+        "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
     ],
 )

iamf/obu/mix_presentation.cc

@@ -26,6 +26,7 @@
 #include "iamf/common/read_bit_buffer.h"
 #include "iamf/common/write_bit_buffer.h"
 #include "iamf/obu/leb128.h"
+#include "iamf/obu/obu_header.h"
 #include "iamf/obu/param_definitions.h"
 
 namespace iamf_tools {
@@ -308,6 +309,13 @@ absl::Status MixPresentationObu::ValidateAndWritePayload(
   return absl::OkStatus();
 }
 
+absl::StatusOr<MixPresentationObu> MixPresentationObu::CreateFromBuffer(
+    const ObuHeader& header, ReadBitBuffer& rb) {
+  MixPresentationObu mix_presentation_obu(header);
+  RETURN_IF_NOT_OK(mix_presentation_obu.ValidateAndReadPayload(rb));
+  return mix_presentation_obu;
+}
+
 absl::Status MixPresentationObu::ValidateAndReadPayload(ReadBitBuffer& rb) {
   return absl::UnimplementedError(
       "MixPresentationObu ValidateAndReadPayload not yet implemented.");

iamf/obu/mix_presentation.h

@@ -19,6 +19,7 @@
 #include <vector>
 
 #include "absl/status/status.h"
+#include "absl/status/statusor.h"
 #include "iamf/common/read_bit_buffer.h"
 #include "iamf/common/write_bit_buffer.h"
 #include "iamf/obu/leb128.h"
@@ -338,6 +339,20 @@ class MixPresentationObu : public ObuBase {
         num_sub_mixes_(num_sub_mixes),
         sub_mixes_(std::move(sub_mixes)) {}
 
+  /*!\brief Creates a `MixPresentationObu` from a `ReadBitBuffer`.
+   *
+   * This function is designed to be used from the perspective of the decoder.
+   * It will call `ValidateAndReadPayload` in order to read from the buffer;
+   * therefore it can fail.
+   *
+   * \param header `ObuHeader` of the OBU.
+   * \param rb `ReadBitBuffer` where the `MixPresentationObu` data is stored.
+   *     Data read from the buffer is consumed.
+   * \return an `MixPresentationObu` on success. A specific status on failure.
+   */
+  static absl::StatusOr<MixPresentationObu> CreateFromBuffer(
+      const ObuHeader& header, ReadBitBuffer& rb);
+
   /*!\brief Move Constructor. */
   MixPresentationObu(MixPresentationObu&& other) = default;
 
@@ -365,6 +380,15 @@ class MixPresentationObu : public ObuBase {
   std::vector<MixPresentationSubMix> sub_mixes_;
 
  private:
+  // Used only by the factory create function.
+  explicit MixPresentationObu(const ObuHeader& header)
+      : ObuBase(header, kObuIaAudioElement),
+        mix_presentation_id_(DecodedUleb128()),
+        count_label_(DecodedUleb128()),
+        language_labels_({}),
+        mix_presentation_annotations_({}),
+        num_sub_mixes_(DecodedUleb128()),
+        sub_mixes_({}) {}
   /*\!brief Writes the OBU payload to the buffer.
    *
    * \param wb Buffer to write to.

iamf/obu/tests/BUILD

@@ -121,6 +121,7 @@ cc_test(
     deps = [
         ":obu_test_base",
         "//iamf/cli:leb_generator",
+        "//iamf/common:read_bit_buffer",
         "//iamf/common:write_bit_buffer",
         "//iamf/obu:leb128",
         "//iamf/obu:mix_presentation",

iamf/obu/tests/mix_presentation_test.cc

@@ -19,6 +19,7 @@
 #include "absl/status/status.h"
 #include "gtest/gtest.h"
 #include "iamf/cli/leb_generator.h"
+#include "iamf/common/read_bit_buffer.h"
 #include "iamf/common/write_bit_buffer.h"
 #include "iamf/obu/leb128.h"
 #include "iamf/obu/obu_header.h"
@@ -870,5 +871,14 @@ TEST_F(GetNumChannelsFromLayoutTest, ErrorBeyondReservedSoundSystem) {
           .ok());
 }
 
+// --- Begin CreateFromBuffer tests ---
+// TODO(b/329706068): Update test once ValidateAndReadPayload is implemented.
+TEST(CreateFromBuffer, IsNotSupported) {
+  std::vector<uint8_t> source;
+  ReadBitBuffer buffer(1024, &source);
+  ObuHeader header;
+  EXPECT_FALSE(MixPresentationObu::CreateFromBuffer(header, buffer).ok());
+}
+
 }  // namespace
 }  // namespace iamf_tools