BisectPerm.hpp

/***********************************************************************
Copyright (c) 2015, Carnegie Mellon University
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Authors: Chris Dellin <cdellin@gmail.com>

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#ifndef BATCHING_POMP_UTIL_BISECTPERM_HPP_
#define BATCHING_POMP_UTIL_BISECTPERM_HPP_

namespace batching_pomp {
namespace util {
 

class BisectPerm
{
public:
  BisectPerm() {}

  const std::vector< std::pair<int,int> > & get(int n)
  {
    std::map<int, const std::vector< std::pair<int,int> > >::iterator it;
    it = cache.find(n);
    if (it != cache.end())
      return it->second;

    int i;
    int last_true;
    int max_i;
    int max_val;
    std::vector< std::pair<int,int> > perm;
    std::vector<bool> done(n, false);
    std::vector<int> dist(n);

    for (;;)
    {
      last_true = -1;
      for (i=0; i<n; i++)
      {
        if (done[i]) last_true = i;
        dist[i] = (i-last_true);
      }
      last_true = n;
      for (i=n-1; i>=0; i--)
      {
        if (done[i]) last_true = i;
        dist[i] = (last_true-i) < dist[i] ? (last_true-i) : dist[i];
      }
      max_val = 0;
      max_i = 0;
      for (i=0; i<n; i++) if (max_val < dist[i])
      {
        max_val = dist[i];
        max_i = i;
      }
      if (!max_val)
        break;
      perm.push_back(std::make_pair(max_i,max_val));
      done[max_i] = true;
    }

    cache.insert(std::make_pair(n,perm));
    return cache[n];
  }

private:

  std::map<int, const std::vector< std::pair<int,int> > > cache;
};

} // namespace util
} // namespace batching_pomp

#endif // BATCHING_POMP_UTIL_BISECTPERM_HPP_