// Copyright (c) 2018, ETH Zurich and UNC Chapel Hill.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
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//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
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//     * Redistributions in binary form must reproduce the above copyright
//       notice, this list of conditions and the following disclaimer in the
//       documentation and/or other materials provided with the distribution.
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//     * Neither the name of ETH Zurich and UNC Chapel Hill nor the names of
//       its contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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// Author: Johannes L. Schoenberger (jsch at inf.ethz.ch)

#include "optim/combination_sampler.h"

#include <numeric>

#include "util/math.h"
#include "util/random.h"

namespace colmap {

CombinationSampler::CombinationSampler(const size_t num_samples)
    : num_samples_(num_samples) {}

void CombinationSampler::Initialize(const size_t total_num_samples) {
  CHECK_LE(num_samples_, total_num_samples);
  total_sample_idxs_.resize(total_num_samples);
  // Note that the samples must be in increasing order for `NextCombination`.
  std::iota(total_sample_idxs_.begin(), total_sample_idxs_.end(), 0);
}

size_t CombinationSampler::MaxNumSamples() {
  return NChooseK(total_sample_idxs_.size(), num_samples_);
}

std::vector<size_t> CombinationSampler::Sample() {
  std::vector<size_t> sampled_idxs(num_samples_);
  for (size_t i = 0; i < num_samples_; ++i) {
    sampled_idxs[i] = total_sample_idxs_[i];
  }

  if (!NextCombination(total_sample_idxs_.begin(),
                       total_sample_idxs_.begin() + num_samples_,
                       total_sample_idxs_.end())) {
    // Reached all possible combinations, so reset to original state.
    // Note that the samples must be in increasing order for `NextCombination`.
    std::iota(total_sample_idxs_.begin(), total_sample_idxs_.end(), 0);
  }

  return sampled_idxs;
}

}  // namespace colmap
