libigl_Martin/sort.h

#ifndef IGL_SORT_H
#define IGL_SORT_H

#include <vector>
#include <Eigen/Core>
namespace igl
{

  // Sort the elements of a matrix X along a given dimension like matlabs sort
  // function
  //
  // Templates:
  //   T  should be a eigen matrix primitive type like int or double
  // Inputs:
  //   X  m by n matrix whose entries are to be sorted
  //   dim  dimensional along which to sort:
  //     1  sort each column (matlab default)
  //     2  sort each row
  //   ascending  sort ascending (true, matlab default) or descending (false)
  // Outputs:
  //   Y  m by n matrix whose entries are sorted
  //   IX  m by n matrix of indices so that if dim = 1, then in matlab notation
  //     for j = 1:n, Y(:,j) = X(I(:,j),j); end
  template <typename T>
  inline void sort(
    const Eigen::Matrix<T,Eigen::Dynamic,Eigen::Dynamic> & X,
    const int dim,
    const bool ascending,
    Eigen::Matrix<T,Eigen::Dynamic,Eigen::Dynamic> & Y,
    Eigen::MatrixXi & IX);

  // Act like matlab's [Y,I] = SORT(X) for std library vectors
  // Templates:
  //   T  should be a class that implements the '<' comparator operator
  // Input:
  //   unsorted  unsorted vector
  //   ascending  sort ascending (true, matlab default) or descending (false)
  // Output:
  //   sorted     sorted vector, allowed to be same as unsorted
  //   index_map  an index map such that sorted[i] = unsorted[index_map[i]]
  template <class T>
  inline void sort(
      const std::vector<T> &unsorted,
      const bool ascending,
      std::vector<T> &sorted,
      std::vector<size_t> &index_map);

}

// Implementation
#include <algorithm>
#include "reorder.h"

template <typename T>
inline void igl::sort(
  const Eigen::Matrix<T,Eigen::Dynamic,Eigen::Dynamic> & X,
  const int dim,
  const bool ascending,
  Eigen::Matrix<T,Eigen::Dynamic,Eigen::Dynamic> & Y,
  Eigen::MatrixXi & IX)
{
  // dim must be 2 or 1
  assert(dim == 1 || dim == 2);
  // Resize output
  Y.resize(X.rows(),X.cols());
  IX.resize(X.rows(),X.cols());
  // idea is to process each column (or row) as a std vector
  // get number of columns (or rows)
  int num_outer = (dim == 1 ? X.cols() : X.rows() );
  // get number of rows (or columns)
  int num_inner = (dim == 1 ? X.rows() : X.cols() );
  // loop over columns (or rows)
  for(int i = 0; i<num_outer;i++)
  {
    // Unsorted index map for this column (or row)
    std::vector<size_t> index_map(num_inner);
    std::vector<T> data(num_inner);
    for(int j = 0;j<num_inner;j++)
    {
      if(dim == 1)
      {
        data[j] = X(j,i);
      }else
      {
        data[j] = X(i,j);
      }
    }
    // sort this column (or row)
    sort<T>(
      data,
      ascending,
      data,
      index_map);
    // Copy into Y and IX
    for(int j = 0;j<num_inner;j++)
    {
      if(dim == 1)
      {
        Y(j,i) = data[j];
        IX(j,i) = index_map[j];
      }else
      {
        Y(i,j) = data[j];
        IX(i,j) = index_map[j];
      }
    }
  }
}

// Comparison struct used by sort
// http://bytes.com/topic/c/answers/132045-sort-get-index
template<class T> struct index_cmp
{
  index_cmp(const T arr) : arr(arr) {}
  bool operator()(const size_t a, const size_t b) const
  {
    return arr[a] < arr[b];
  }
  const T arr;
};

template <class T>
inline void igl::sort(
  const std::vector<T> & unsorted,
  const bool ascending,
  std::vector<T> & sorted,
  std::vector<size_t> & index_map)
{
  // Original unsorted index map
  index_map.resize(unsorted.size());
  for(size_t i=0;i<unsorted.size();i++)
  {
    index_map[i] = i;
  }
  // Sort the index map, using unsorted for comparison
  sort(
    index_map.begin(),
    index_map.end(),
    index_cmp<const std::vector<T>& >(unsorted));

  // if not ascending then reverse
  if(!ascending)
  {
    std::reverse(index_map.begin(),index_map.end());
  }
  // make space for output without clobbering
  sorted.resize(unsorted.size());
  // reorder unsorted into sorted using index map
  igl::reorder(unsorted,index_map,sorted);
}

#endif