linesearch-nonmonotone.hpp

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#pragma once
 
#include "linesearch-base.hpp"
#include <functional>
 
namespace aligator {

template <typename Scalar> struct NonmonotoneLinesearch : Linesearch<Scalar> {
  using typename Linesearch<Scalar>::FunctionSample;
  using typename Linesearch<Scalar>::Options;
  using fun_t = std::function<Scalar(Scalar)>;
 
  Scalar run(fun_t f, Scalar phi0, Scalar dphi0, Scalar &a_opt);
  NonmonotoneLinesearch(const Options &options);
 
  void reset() {
    mov_avg = Scalar(0.);
    avg_weight = Scalar(0.);
  }

  Scalar avg_eta = 0.85;
  Scalar beta_dec = 0.5;
 
private:
  Scalar mov_avg;
  Scalar avg_weight;
};
 
template <typename Scalar>
NonmonotoneLinesearch<Scalar>::NonmonotoneLinesearch(const Options &options)
    : Linesearch<Scalar>(options)
    , mov_avg(0.)
    , avg_weight(0.) {}
 
template <typename Scalar>
Scalar NonmonotoneLinesearch<Scalar>::run(fun_t f, Scalar phi0, Scalar dphi0,
                                          Scalar &a_opt) {
  const Options &opts = this->options_;
  mov_avg = avg_eta * avg_weight * mov_avg + phi0;
  avg_weight = avg_eta * avg_weight + 1;
  mov_avg /= avg_weight;
 
  while (a_opt > opts.alpha_min) {
    try {
      const Scalar phia = f(a_opt);
      bool suff_decrease = phia <= mov_avg + opts.armijo_c1 * a_opt * dphi0;
      if (suff_decrease)
        return phia;
    } catch (const std::runtime_error &) {
    }
    a_opt *= beta_dec;
  }
 
  // then, a_opt <= opts.alpha_min
  a_opt = std::max(a_opt, opts.alpha_min);
  return f(a_opt);
}
 
} // namespace aligator