text: /*
   * Copyright 2021 Google LLC
   *
   * 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 LYRA_CODEC_FILTER_BANKS_H_
  #define LYRA_CODEC_FILTER_BANKS_H_
  
  #include <cstdint>
  #include <memory>
  #include <vector>
  
  #include "absl/types/span.h"
  #include "filter_banks_interface.h"
  #include "quadrature_mirror_filter.h"
  
  namespace chromemedia {
  namespace codec {
  
  // Filter bank to split a signal into multiple bands sampled at sub-Nyquist.
  class SplitFilter {
   public:
    // Create a SplitFilter. Return nullptr if num_bands isn't a power of 2.
    static std::unique_ptr<SplitFilter> Create(int num_bands);
  
    // Split signal into multiple bands sampled at sub-Nyquist.
    // The size of the signal has to be divisible by num_bands.
    std::vector<std::vector<int16_t>> Split(absl::Span<const int16_t> signal);
  
    int num_bands() const { return num_bands_; }
  
   private:
    explicit SplitFilter(int num_bands);
  
    const int num_bands_;
    std::vector<std::vector<SplitQuadratureMirrorFilter<int16_t>>>
        filters_per_level_;
  };
  
  // Filter bank to merge multiple bands sampled at sub-Nyquist into signal.
  class MergeFilter : public MergeFilterInterface {
   public:
    // Create a MergeFilter. Return nullptr if num_bands isn't a power of 2.
    static std::unique_ptr<MergeFilter> Create(int num_bands);
  
    // Merge multiple bands sampled at sub-Nyquist into signal.
    // The size of the bands have to coincide.
    std::vector<int16_t> Merge(
        const std::vector<std::vector<int16_t>>& bands) override;
  
   private:
    explicit MergeFilter(int num_bands);
  
    std::vector<std::vector<MergeQuadratureMirrorFilter<int16_t>>>
        filters_per_level_;
  };
  
  }  // namespace codec
  }  // namespace chromemedia
  
  #endif  // LYRA_CODEC_FILTER_BANKS_H_
  