// Copyright 2010-2025 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.

// Solves a simple LP using PDLP's direct C++ API.
//
// Note: The direct API is generally for advanced use cases. It is matrix-based,
// that is, you specify the LP using matrices and vectors instead of algebraic
// expressions. You can also use PDLP via the algebraic MPSolver API (see
// linear_solver/samples/simple_lp_program.cc).
#include <cstdint>
#include <iostream>
#include <limits>
#include <optional>
#include <vector>

#include "Eigen/Core"
#include "Eigen/SparseCore"
#include "ortools/base/init_google.h"
#include "ortools/pdlp/iteration_stats.h"
#include "ortools/pdlp/primal_dual_hybrid_gradient.h"
#include "ortools/pdlp/quadratic_program.h"
#include "ortools/pdlp/solve_log.pb.h"
#include "ortools/pdlp/solvers.pb.h"

namespace pdlp = ::operations_research::pdlp;

constexpr double kInfinity = std::numeric_limits<double>::infinity();

// Returns a small LP:
// min 5.5 x_0 - 2 x_1 - x_2 +   x_3 - 14 s.t.
//     2 x_0 +     x_1 +   x_2 + 2 x_3  = 12
//       x_0 +             x_2          <=  7
//     4 x_0                            >=  -4
//    -1 <=            1.5 x_2 -   x_3  <= 1
//   -infinity <= x_0 <= infinity
//          -2 <= x_1 <= infinity
//   -infinity <= x_2 <= 6
//         2.5 <= x_3 <= 3.5
pdlp::QuadraticProgram SimpleLp() {
  pdlp::QuadraticProgram lp(4, 4);
  // "<<" is Eigen's syntax for initialization.
  lp.constraint_lower_bounds << 12, -kInfinity, -4, -1;
  lp.constraint_upper_bounds << 12, 7, kInfinity, 1;
  lp.variable_lower_bounds << -kInfinity, -2, -kInfinity, 2.5;
  lp.variable_upper_bounds << kInfinity, kInfinity, 6, 3.5;
  const std::vector<Eigen::Triplet<double, int64_t>>
      constraint_matrix_triplets = {{0, 0, 2}, {0, 1, 1},   {0, 2, 1},
                                    {0, 3, 2}, {1, 0, 1},   {1, 2, 1},
                                    {2, 0, 4}, {3, 2, 1.5}, {3, 3, -1}};
  lp.constraint_matrix.setFromTriplets(constraint_matrix_triplets.begin(),
                                       constraint_matrix_triplets.end());
  lp.objective_vector << 5.5, -2, -1, 1;
  lp.objective_offset = -14;
  return lp;
}

int main(int argc, char* argv[]) {
  InitGoogle(argv[0], &argc, &argv, /*remove_flags=*/true);

  pdlp::PrimalDualHybridGradientParams params;
  // Below are some common parameters to modify. Here, we just re-assign the
  // defaults.
  params.mutable_termination_criteria()
      ->mutable_simple_optimality_criteria()
      ->set_eps_optimal_relative(1.0e-6);
  params.mutable_termination_criteria()
      ->mutable_simple_optimality_criteria()
      ->set_eps_optimal_absolute(1.0e-6);
  params.mutable_termination_criteria()->set_time_sec_limit(kInfinity);
  params.set_num_threads(1);
  params.set_verbosity_level(0);
  params.mutable_presolve_options()->set_use_glop(false);

  const pdlp::SolverResult result =
      pdlp::PrimalDualHybridGradient(SimpleLp(), params);
  const pdlp::SolveLog& solve_log = result.solve_log;

  if (solve_log.termination_reason() == pdlp::TERMINATION_REASON_OPTIMAL) {
    std::cout << "Solve successful" << '\n';
  } else {
    std::cout << "Solve not successful. Status: "
              << pdlp::TerminationReason_Name(solve_log.termination_reason())
              << '\n';
  }

  // Solutions vectors are always returned. *However*, their interpretation
  // depends on termination_reason! See primal_dual_hybrid_gradient.h for more
  // details on what the vectors mean if termination_reason is not
  // TERMINATION_REASON_OPTIMAL.
  std::cout << "Primal solution:\n" << result.primal_solution << '\n';
  std::cout << "Dual solution:\n" << result.dual_solution << '\n';
  std::cout << "Reduced costs:\n" << result.reduced_costs << '\n';

  const pdlp::PointType solution_type = solve_log.solution_type();
  std::cout << "Solution type: " << pdlp::PointType_Name(solution_type) << '\n';
  const std::optional<pdlp::ConvergenceInformation> ci =
      pdlp::GetConvergenceInformation(solve_log.solution_stats(),
                                      solution_type);
  if (ci.has_value()) {
    std::cout << "Primal objective: " << ci->primal_objective() << '\n';
    std::cout << "Dual objective: " << ci->dual_objective() << '\n';
  }

  std::cout << "Iterations: " << solve_log.iteration_count() << '\n';
  std::cout << "Solve time (sec): " << solve_log.solve_time_sec() << '\n';

  return 0;
}