// BSD 2-Clause License
//
// Copyright 2020 James Melville
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice,
//    this list of conditions and the following disclaimer.
//
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// Translated from the Python source code of:
//   scipy.sparse.csgraph.connected_components
//   Author: Jake Vanderplas  -- <vanderplas@astro.washington.edu>
//   License: BSD, (C) 2012

#ifndef UWOT_CONNECTED_COMPONENTS_H
#define UWOT_CONNECTED_COMPONENTS_H

#include <cstddef>
#include <utility>
#include <vector>

namespace uwot {

auto connected_components_undirected(std::size_t N,
                                     const std::vector<int> &indices1,
                                     const std::vector<int> &indptr1,
                                     const std::vector<int> &indices2,
                                     const std::vector<int> &indptr2)
    -> std::pair<unsigned int, std::vector<int>> {
  constexpr int VOID = -1;
  constexpr int END = -2;
  std::vector<int> labels(N, VOID);
  std::vector<int> SS(labels);
  unsigned int label = 0;
  auto SS_head = END;
  for (std::size_t v = 0; v < N; ++v) {
    auto vv = v;
    if (labels[vv] == VOID) {
      SS_head = vv;
      SS[vv] = END;
      while (SS_head != END) {
        vv = SS_head;
        SS_head = SS[vv];
        labels[vv] = label;
        for (auto jj = indptr1[vv]; jj < indptr1[vv + 1]; ++jj) {
          auto ww = indices1[jj];
          if (SS[ww] == VOID) {
            SS[ww] = SS_head;
            SS_head = ww;
          }
        }
        for (auto jj = indptr2[vv]; jj < indptr2[vv + 1]; ++jj) {
          auto ww = indices2[jj];
          if (SS[ww] == VOID) {
            SS[ww] = SS_head;
            SS_head = ww;
          }
        }
      }
      ++label;
    }
  }
  return {label, labels};
}
} // namespace uwot

#endif // UWOT_CONNECTED_COMPONENTS_H
