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#ifndef MOZC_PREDICTION_REALTIME_DECODER_H_
#define MOZC_PREDICTION_REALTIME_DECODER_H_

#include <functional>
#include <optional>
#include <vector>

#include "absl/log/check.h"
#include "base/container/flat_concurrent_cache.h"
#include "converter/converter_interface.h"
#include "converter/immutable_converter_interface.h"
#include "prediction/result.h"
#include "request/conversion_request.h"

namespace mozc::prediction {

// Wrapper class to perform realtime decoding with the converter.
// This wrapper hides the Segments, Converter, and ImmutableConverter from
// Predictor.
class RealtimeDecoder {
 public:
  RealtimeDecoder();
  RealtimeDecoder(const ImmutableConverterInterface& immutable_converter,
                  const ConverterInterface& converter);
  virtual ~RealtimeDecoder() = default;

  // Decodes `request`. The request type must not be CONVERSION because
  // we assume that Decode doesn't return multiple-segments.
  virtual std::vector<Result> Decode(const ConversionRequest& request) const;

  // value is reading, key is the input.
  virtual std::vector<Result> ReverseDecode(
      const ConversionRequest& request) const;

  // Decodes suffix with the prefix rid as constraint.
  // Request is used to pass configurations or flags that are not directly
  // required for the conversion. Actual conversion key is passed via `suffix`.
  // When `prefix_rid` is zero, performs the full decoding. The result is
  // automatically cached. Note that we cannot use
  // ConversionRequest::history_rid() as the prefix_rid, as history_rid is not
  // used as the constraint of realtime decoding.
  // TODO(taku): Remove `request`.
  // TODO(taku): Clarify the spec of the context information.
  virtual std::optional<Result> DecodeSuffix(const ConversionRequest& request,
                                             uint16_t prefix_rid,
                                             absl::string_view suffix) const;

 private:
  bool PushBackTopConversionResult(const ConversionRequest& request,
                                   std::vector<Result>* results) const;

  const ImmutableConverterInterface& immutable_converter() const {
    DCHECK(immutable_converter_.has_value());
    return immutable_converter_.value().get();
  }

  const ConverterInterface& converter() const {
    DCHECK(converter_.has_value());
    return converter_.value().get();
  }

  // optional<std::reference_wrapper<const T>> allows to store pure reference
  // with an undefined state. It is safer than pointer as the ownership is
  // cleaner. absl::Nonnull cannot be used as we want to use an undefined state
  // for testing.
  std::optional<std::reference_wrapper<const ImmutableConverterInterface>>
      immutable_converter_;
  std::optional<std::reference_wrapper<const ConverterInterface>> converter_;

  // Cache for DecodeSuffix.
  mutable FlatConcurrentCache<uint64_t, Result> suffix_cache_;
};
}  // namespace mozc::prediction

#endif  // MOZC_PREDICTION_REALTIME_DECODER_H_
