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+/*
+ * Copyright 2020 The libgav1 Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LIBGAV1_SRC_DECODER_STATE_H_
+#define LIBGAV1_SRC_DECODER_STATE_H_
+
+#include <array>
+#include <cstdint>
+
+#include "src/buffer_pool.h"
+#include "src/utils/constants.h"
+
+namespace libgav1 {
+
+struct DecoderState {
+  // Section 7.20. Updates frames in the reference_frame array with
+  // |current_frame|, based on the |refresh_frame_flags| bitmask.
+  void UpdateReferenceFrames(const RefCountedBufferPtr& current_frame,
+                             int refresh_frame_flags) {
+    for (int ref_index = 0, mask = refresh_frame_flags; mask != 0;
+         ++ref_index, mask >>= 1) {
+      if ((mask & 1) != 0) {
+        reference_valid[ref_index] = true;
+        reference_frame_id[ref_index] = current_frame_id;
+        reference_frame[ref_index] = current_frame;
+        reference_order_hint[ref_index] = order_hint;
+      }
+    }
+  }
+
+  // Clears all the reference frames.
+  void ClearReferenceFrames() {
+    reference_valid = {};
+    reference_frame_id = {};
+    reference_order_hint = {};
+    for (int ref_index = 0; ref_index < kNumReferenceFrameTypes; ++ref_index) {
+      reference_frame[ref_index] = nullptr;
+    }
+  }
+
+  // reference_valid and reference_frame_id are used only if
+  // sequence_header_.frame_id_numbers_present is true.
+  // The reference_valid array is indexed by a reference picture slot number.
+  // A value (boolean) in the array signifies whether the corresponding
+  // reference picture slot is valid for use as a reference picture.
+  std::array<bool, kNumReferenceFrameTypes> reference_valid = {};
+  std::array<uint16_t, kNumReferenceFrameTypes> reference_frame_id = {};
+  // A valid value of current_frame_id is an unsigned integer of at most 16
+  // bits. -1 indicates current_frame_id is not initialized.
+  int current_frame_id = -1;
+  // The RefOrderHint array variable in the spec.
+  std::array<uint8_t, kNumReferenceFrameTypes> reference_order_hint = {};
+  // The OrderHint variable in the spec. Its value comes from either the
+  // order_hint syntax element in the uncompressed header (if
+  // show_existing_frame is false) or RefOrderHint[ frame_to_show_map_idx ]
+  // (if show_existing_frame is true and frame_type is KEY_FRAME). See Section
+  // 5.9.2 and Section 7.4.
+  //
+  // NOTE: When show_existing_frame is false, it is often more convenient to
+  // just use the order_hint field of the frame header as OrderHint. So this
+  // field is mainly used to update the reference_order_hint array in
+  // UpdateReferenceFrames().
+  uint8_t order_hint = 0;
+  // reference_frame_sign_bias[i] (a boolean) specifies the intended direction
+  // of the motion vector in time for each reference frame.
+  // * |false| indicates that the reference frame is a forwards reference (i.e.
+  //   the reference frame is expected to be output before the current frame);
+  // * |true| indicates that the reference frame is a backwards reference.
+  // Note: reference_frame_sign_bias[0] (for kReferenceFrameIntra) is not used.
+  std::array<bool, kNumReferenceFrameTypes> reference_frame_sign_bias = {};
+  std::array<RefCountedBufferPtr, kNumReferenceFrameTypes> reference_frame;
+};
+
+}  // namespace libgav1
+
+#endif  // LIBGAV1_SRC_DECODER_STATE_H_