flat_matrix.h

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// Copyright 2010-2022 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 OR_TOOLS_UTIL_FLAT_MATRIX_H_
#define OR_TOOLS_UTIL_FLAT_MATRIX_H_
 
// A very simple flattened 2D array of fixed size. It's movable, copyable.
// It can also be assigned.
// This was originally made to replace uses of vector<vector<...>> where each
// vector had a fixed size: vector<vector<>> has much worse performance in a
// highly concurrent setting, because it does a lot of memory allocations.
 
#include <memory>
#include <vector>
 
#include "absl/types/span.h"
 
namespace operations_research {
 
// NOTE(user): T=bool is not yet supported (the [] operator doesn't work).
template <typename T>
class FlatMatrix {
 public:
  FlatMatrix() : num_rows_(0), num_cols_(0) {}
  FlatMatrix(size_t num_rows, size_t num_cols)
      : num_rows_(num_rows),
        num_cols_(num_cols),
        array_(num_rows_ * num_cols_) {}
  FlatMatrix(size_t num_rows, size_t num_cols, const T& elem)
      : num_rows_(num_rows),
        num_cols_(num_cols),
        array_(num_rows_ * num_cols_, elem) {}
 
  size_t num_rows() const { return num_rows_; }
  size_t num_cols() const { return num_cols_; }
 
  absl::Span<T> operator[](size_t row) {
    return absl::Span<T>(array_.data() + row * num_cols_, num_cols_);
  }
  absl::Span<const T> operator[](size_t row) const {
    return {array_.data() + row * num_cols_, num_cols_};
  }
 
 private:
  // Those are non-const only to support the assignment operators.
  size_t num_rows_;
  size_t num_cols_;
  // NOTE(user): We could use a simpler unique_ptr<T[]> or even a self-managed
  // memory block, but we'd need to define the copy constructor.
  std::vector<T> array_;
};
 
}  // namespace operations_research
 
#endif  // OR_TOOLS_UTIL_FLAT_MATRIX_H_