GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t > Class Template Reference

Nearest-neighbor calculations. More...

#include <GeographicLib/NearestNeighbor.hpp>

Public Member Functions
	NearestNeighbor ()
	NearestNeighbor (const std::vector< pos_t > &pts, const distfun_t &dist, int bucket=4)
void	Initialize (const std::vector< pos_t > &pts, const distfun_t &dist, int bucket=4)
dist_t	Search (const std::vector< pos_t > &pts, const distfun_t &dist, const pos_t &query, std::vector< int > &ind, int k=1, dist_t maxdist=std::numeric_limits< dist_t >::max(), dist_t mindist=-1, bool exhaustive=true, dist_t tol=0) const
int	NumPoints () const
void	Save (std::ostream &os, bool bin=true) const
void	Load (std::istream &is, bool bin=true)
void	swap (NearestNeighbor &t)
void	Statistics (int &setupcost, int &numsearches, int &searchcost, int &mincost, int &maxcost, double &mean, double &sd) const
void	ResetStatistics () const

Friends
std::ostream &	operator<< (std::ostream &os, const NearestNeighbor &t)
std::istream &	operator>> (std::istream &is, NearestNeighbor &t)

Detailed Description

template<typename dist_t, typename pos_t, class distfun_t>
class GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >

Nearest-neighbor calculations.

This class solves the nearest-neighbor problm using a vantage-point tree as described in Finding nearest neighbors.

This class is templated so that it can handle arbitrary metric spaces as follows:

Template Parameters

dist_t	the type used for measuring distances; it can be a real or signed integer type; in typical geodetic applications, dist_t might be double.
pos_t	the type for specifying the positions of points; geodetic application might bundle the latitude and longitude into a std::pair<dist_t, dist_t>.
distfun_t	the type of a function object which takes takes two positions (of type pos_t) and returns the distance (of type dist_t); in geodetic applications, this might be a class which is constructed with a Geodesic object and which implements a member function with a signature dist_t operator() (const pos_t&, const pos_t&) const, which returns the geodesic distance between two points.

Note

The distance measure must satisfy the triangle inequality, \( d(a,c) \le d(a,b) + d(b,c) \) for all points a, b, c. The geodesic distance (given by Geodesic::Inverse) does, while the great ellipse distance and the rhumb line distance do not. If you use the ordinary Euclidean distance, i.e., \( \sqrt{(x_a-x_b)^2 + (y_a-y_b)^2} \) for two dimensions, don't be tempted to leave out the square root in the interests of "efficiency"; the squared distance does not satisfy the triangle inequality!

This is a "header-only" implementation and, as such, depends in a minimal way on the rest of GeographicLib (the only dependency is through the use of GeographicLib::GeographicErr for handling and run-time exceptions). Therefore, it is easy to extract this class from the rest of GeographicLib and use it as a stand-alone facility.

The dist_t type must support numeric_limits queries (specifically: is_signed, is_integer, max(), digits).

The NearestNeighbor object is constructed with a vector of points (type pos_t) and a distance function (type distfun_t). However the object does not store the points. When querying the object with Search(), it's necessary to supply the same vector of points and the same distance function.

There's no capability in this implementation to add or remove points from the set. Instead Initialize() should be called to re-initialize the object with the modified vector of points.

Because of the overhead in constructing a NearestNeighbor object for a large set of points, functions Save() and Load() are provided to save the object to an external file. operator<<(), operator>>() and Boost serialization can also be used to save and restore a NearestNeighbor object. This is illustrated in the example.

Example of use:

// Example of using the GeographicLib::NearestNeighbor class.  WARNING: this
// creates a file, pointset.xml or pointset.txt, in the current directory.
// Read lat/lon locations from locations.txt and lat/lon queries from
// queries.txt.  For each query print to standard output: the index for the
// closest location and the distance to it.  Print statistics to standard error
// at the end.
 
#include <iostream>
#include <exception>
#include <vector>
#include <fstream>
#include <string>
 
#if !defined(GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION)
#define GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION 0
#endif
 
#if GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION
// If Boost serialization is available, use it.
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#endif
 
#include <GeographicLib/NearestNeighbor.hpp>
#include <GeographicLib/Geodesic.hpp>
#include <GeographicLib/DMS.hpp>
 
using namespace std;
using namespace GeographicLib;
 
// A structure to hold a geographic coordinate.
struct pos {
  double _lat, _lon;
  pos(double lat = 0, double lon = 0) : _lat(lat), _lon(lon) {}
};
 
// A class to compute the distance between 2 positions.
class DistanceCalculator {
private:
  Geodesic _geod;
public:
  explicit DistanceCalculator(const Geodesic& geod) : _geod(geod) {}
  double operator() (const pos& a, const pos& b) const {
    double d;
    _geod.Inverse(a._lat, a._lon, b._lat, b._lon, d);
    if ( !(d >= 0) )
      // Catch illegal positions which result in d = NaN
      throw GeographicErr("distance doesn't satisfy d >= 0");
    return d;
  }
};
 
int main() {
  try {
    // Read in locations
    vector<pos> locs;
    double lat, lon;
    string sa, sb;
    {
      ifstream is("locations.txt");
      if (!is.good())
        throw GeographicErr("locations.txt not readable");
      while (is >> sa >> sb) {
        DMS::DecodeLatLon(sa, sb, lat, lon);
        locs.push_back(pos(lat, lon));
      }
      if (locs.size() == 0)
        throw GeographicErr("need at least one location");
    }
 
    // Define a distance function object
    DistanceCalculator distance(Geodesic::WGS84());
 
    // Create NearestNeighbor object
    NearestNeighbor<double, pos, DistanceCalculator> pointset;
 
    {
      // Used saved object if it is available
#if GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION
      ifstream is("pointset.xml");
      if (is.good()) {
        boost::archive::xml_iarchive ia(is);
        ia >> BOOST_SERIALIZATION_NVP(pointset);
      }
#else
      ifstream is("pointset.txt");
      if (is.good())
        is >> pointset;
#endif
    }
    // Is the saved pointset up-to-date?
    if (pointset.NumPoints() != int(locs.size())) {
      // else initialize it
      pointset.Initialize(locs, distance);
      // and save it
#if GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION
      ofstream os("pointset.xml");
      if (!os.good())
        throw GeographicErr("cannot write to pointset.xml");
      boost::archive::xml_oarchive oa(os);
      oa << BOOST_SERIALIZATION_NVP(pointset);
#else
      ofstream os("pointset.txt");
      if (!os.good())
        throw GeographicErr("cannot write to pointset.txt");
      os << pointset << "\n";
#endif
    }
 
    ifstream is("queries.txt");
    double d;
    int count = 0;
    vector<int> k;
    while (is >> sa >> sb) {
      ++count;
      DMS::DecodeLatLon(sa, sb, lat, lon);
      d = pointset.Search(locs, distance, pos(lat, lon), k);
      if (k.size() != 1)
          throw GeographicErr("unexpected number of results");
      cout << k[0] << " " << d << "\n";
    }
    int setupcost, numsearches, searchcost, mincost, maxcost;
    double mean, sd;
    pointset.Statistics(setupcost, numsearches, searchcost,
                        mincost, maxcost, mean, sd);
    int
      totcost = setupcost + searchcost,
      exhaustivecost = count * pointset.NumPoints();
    cerr
      << "Number of distance calculations = " << totcost << "\n"
      << "With an exhaustive search = " << exhaustivecost << "\n"
      << "Ratio = " << double(totcost) / exhaustivecost << "\n"
      << "Efficiency improvement = "
      << 100 * (1 - double(totcost) / exhaustivecost) << "%\n";
  }
  catch (const exception& e) {
    cerr << "Caught exception: " << e.what() << "\n";
    return 1;
  }
}

Definition at line 103 of file NearestNeighbor.hpp.

Constructor & Destructor Documentation

NearestNeighbor() [1/2]

template<typename dist_t , typename pos_t , class distfun_t >

GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::NearestNeighbor ( )

inline

Default constructor for NearestNeighbor.

This is equivalent to specifying an empty set of points.

Definition at line 118 of file NearestNeighbor.hpp.

NearestNeighbor() [2/2]

template<typename dist_t , typename pos_t , class distfun_t >

GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::NearestNeighbor ( const std::vector< pos_t > &  pts, const distfun_t &  dist, int  bucket = 4  )

inline

Constructor for NearestNeighbor.

Parameters

[in]	pts	a vector of points to include in the set.
[in]	dist	the distance function object.
[in]	bucket	the size of the buckets at the leaf nodes; this must lie in [0, 2 + 4*sizeof(dist_t)/sizeof(int)] (default 4).

Exceptions

GeographicErr	if the value of bucket is out of bounds or the size of pts is too big for an int.
std::bad_alloc	if memory for the tree can't be allocated.

pts may contain coincident points (i.e., the distance between them vanishes); these are treated as distinct.

The choice of bucket is a tradeoff between space and efficiency. A larger bucket decreases the size of the NearestNeighbor object which scales as pts.size() / max(1, bucket) and reduces the number of distance calculations to construct the object by log2(bucket) * pts.size(). However each search then requires about bucket additional distance calculations.

Warning

The distances computed by dist must satisfy the standard metric conditions. If not, the results are undefined. Neither the data in pts nor the query points should contain NaNs or infinities because such data violates the metric conditions.

The same arguments pts and dist must be provided to the Search() function.

Definition at line 149 of file NearestNeighbor.hpp.

References GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Initialize().

Member Function Documentation

Initialize()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Initialize ( const std::vector< pos_t > &  pts, const distfun_t &  dist, int  bucket = 4  )

inline

Initialize or re-initialize NearestNeighbor.

Parameters

[in]	pts	a vector of points to include in the tree.
[in]	dist	the distance function object.
[in]	bucket	the size of the buckets at the leaf nodes; this must lie in [0, 2 + 4*sizeof(dist_t)/sizeof(int)] (default 4).

Exceptions

GeographicErr	if the value of bucket is out of bounds or the size of pts is too big for an int.
std::bad_alloc	if memory for the tree can't be allocated.

See also the documentation on the constructor.

If an exception is thrown, the state of the NearestNeighbor is unchanged.

Definition at line 170 of file NearestNeighbor.hpp.

Referenced by GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::NearestNeighbor().

Search()

template<typename dist_t , typename pos_t , class distfun_t >

dist_t GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Search ( const std::vector< pos_t > &  pts, const distfun_t &  dist, const pos_t &  query, std::vector< int > &  ind, int  k = 1, dist_t  maxdist = std::numeric_limits<dist_t>::max(), dist_t  mindist = -1, bool  exhaustive = true, dist_t  tol = 0  ) const

inline

Search the NearestNeighbor.

Parameters

[in]	pts	the vector of points used for initialization.
[in]	dist	the distance function object used for initialization.
[in]	query	the query point.
[out]	ind	a vector of indices to the closest points found.
[in]	k	the number of points to search for (default = 1).
[in]	maxdist	only return points with distances of maxdist or less from query (default is the maximum dist_t).
[in]	mindist	only return points with distances of more than mindist from query (default = −1).
[in]	exhaustive	whether to do an exhaustive search (default true).
[in]	tol	the tolerance on the results (default 0).

Returns

the distance to the closest point found (−1 if no points are found).

Exceptions

GeographicErr

if pts has a different size from that used to construct the object.

The indices returned in ind are sorted by distance from query (closest first).

The simplest invocation is with just the 4 non-optional arguments. This returns the closest distance and the index to the closest point in ind₀. If there are several points equally close, then ind₀ gives the index of an arbirary one of them. If there's no closest point (because the set of points is empty), then ind is empty and −1 is returned.

With exhaustive = true and tol = 0 (their default values), this finds the indices of k closest neighbors to query whose distances to query are in (mindist, maxdist]. If mindist and maxdist have their default values, then these bounds have no effect. If query is one of the points in the tree, then set mindist = 0 to prevent this point (and other coincident points) from being returned.

If exhaustive = false, exit as soon as k results satisfying the distance criteria are found. If less than k results are returned then the search was exhaustive even if exhaustive = false.

If tol is positive, do an approximate search; in this case the results are to be interpreted as follows: if the k'th distance is dk, then all results with distances less than or equal dk − tol are correct; all others are suspect — there may be other closer results with distances greater or equal to dk − tol. If less than k results are found, then the search is exact.

mindist should be used to exclude a "small" neighborhood of the query point (relative to the average spacing of the data). If mindist is large, the efficiency of the search deteriorates.

Note

Only the shortest distance is returned (as as the function value). The distances to other points (indexed by ind_j for j > 0) can be found by invoking dist again.

Warning

The arguments pts and dist must be identical to those used to initialize the NearestNeighbor; if not, this function will return some meaningless result (however, if the size of pts is wrong, this function throw an exception).

The query point cannot be a NaN or infinite because then the metric conditions are violated.

Definition at line 258 of file NearestNeighbor.hpp.

NumPoints()

template<typename dist_t , typename pos_t , class distfun_t >

int GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::NumPoints ( ) const

inline

Returns

the total number of points in the set.

Definition at line 355 of file NearestNeighbor.hpp.

Save()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Save ( std::ostream &  os, bool  bin = true  ) const

inline

Write the object to an I/O stream.

Parameters

[in,out]	os	the stream to write to.
[in]	bin	if true (the default) save in binary mode.

Exceptions

std::bad_alloc

if memory for the string representation of the object can't be allocated.

The counters tracking the statistics of searches are not saved; however the initializtion cost is saved. The format of the binary saves is not portable.

Note

Boost serialization can also be used to save and restore a NearestNeighbor object. This requires that the GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION macro be defined.

Definition at line 374 of file NearestNeighbor.hpp.

Load()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Load ( std::istream &  is, bool  bin = true  )

inline

Read the object from an I/O stream.

Parameters

[in,out]	is	the stream to read from
[in]	bin	if true (the default) load in binary mode.

Exceptions

GeographicErr	if the state read from is is illegal.
std::bad_alloc	if memory for the tree can't be allocated.

The counters tracking the statistics of searches are reset by this operation. Binary data must have been saved on a machine with the same architecture. If an exception is thrown, the state of the NearestNeighbor is unchanged.

Note

Boost serialization can also be used to save and restore a NearestNeighbor object. This requires that the GEOGRAPHICLIB_HAVE_BOOST_SERIALIZATION macro be defined.

Warning

The same arguments pts and dist used for initialization must be provided to the Search() function.

Definition at line 452 of file NearestNeighbor.hpp.

swap()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::swap ( NearestNeighbor< dist_t, pos_t, distfun_t > &  t )

inline

Swap with another NearestNeighbor object.

Parameters

[in,out]

t

the NearestNeighbor object to swap with.

Definition at line 560 of file NearestNeighbor.hpp.

References std::swap().

Referenced by std::swap().

Statistics()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::Statistics ( int &  setupcost, int &  numsearches, int &  searchcost, int &  mincost, int &  maxcost, double &  mean, double &  sd  ) const

inline

The accumulated statistics on the searches so far.

Parameters

[out]	setupcost	the cost of initializing the NearestNeighbor.
[out]	numsearches	the number of calls to Search().
[out]	searchcost	the total cost of the calls to Search().
[out]	mincost	the minimum cost of a Search().
[out]	maxcost	the maximum cost of a Search().
[out]	mean	the mean cost of a Search().
[out]	sd	the standard deviation in the cost of a Search().

Here "cost" measures the number of distance calculations needed. Note that the accumulation of statistics is not thread safe.

Definition at line 587 of file NearestNeighbor.hpp.

ResetStatistics()

template<typename dist_t , typename pos_t , class distfun_t >

void GeographicLib::NearestNeighbor< dist_t, pos_t, distfun_t >::ResetStatistics ( ) const

inline

Reset the counters for the accumulated statistics on the searches so far.

Definition at line 599 of file NearestNeighbor.hpp.

Friends And Related Function Documentation

operator<<

template<typename dist_t , typename pos_t , class distfun_t >

std::ostream & operator<< ( std::ostream &  os, const NearestNeighbor< dist_t, pos_t, distfun_t > &  t  )

friend

Write the object to stream os as text.

Parameters

[in,out]	os	the output stream.
[in]	t	the NearestNeighbor object to be saved.

Exceptions

std::bad_alloc

if memory for the string representation of the object can't be allocated.

Definition at line 541 of file NearestNeighbor.hpp.

operator>>

template<typename dist_t , typename pos_t , class distfun_t >

std::istream & operator>> ( std::istream &  is, NearestNeighbor< dist_t, pos_t, distfun_t > &  t  )

friend

Read the object from stream is as text.

Parameters

[in,out]	is	the input stream.
[out]	t	the NearestNeighbor object to be loaded.

Exceptions

GeographicErr	if the state read from is is illegal.
std::bad_alloc	if memory for the tree can't be allocated.

Definition at line 552 of file NearestNeighbor.hpp.

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The documentation for this class was generated from the following file:

• NearestNeighbor.hpp