#include "s2.h"
#include "s2cap.h"
#include "s2cellid.h"
#include "s2latlng.h"
#include "s2regioncoverer.h"
#include "strings/strutil.h"

namespace {

const double kEarthCircumferenceMeters = 1000 * 40075.017;

double EarthMetersToRadians(double meters) {
  return (2 * M_PI) * (meters / kEarthCircumferenceMeters);
}

double RadiansToEarthMeters(double radians) {
  return (radians * kEarthCircumferenceMeters) / (2 * M_PI);
}

string CellToString(const S2CellId& id) {
  return StringPrintf("%d:%s", id.level(), id.ToToken().c_str());
}

}  // namespace

// Returns the cell level with a side most closely matching the
// specified number of meters.
int GetClosestLevel(double meters) {
  return S2::kAvgEdge.GetClosestLevel(EarthMetersToRadians(meters));
}

// Returns the distance between two locations.
double DistanceBetweenLocations(double lat1, double lng1,
                                double lat2, double lng2) {
  S2LatLng latlng1 = S2LatLng::FromDegrees(lat1, lng1);
  S2LatLng latlng2 = S2LatLng::FromDegrees(lat2, lng2);
  return RadiansToEarthMeters(latlng1.GetDistance(latlng2).radians());
}

// Generates a list of cells covering lat,lng between the target
// s2 cell levels (min_level, max_level).
vector<string> IndexCells(double lat, double lng,
                          int min_level, int max_level) {
  S2CellId id(S2CellId::FromLatLng(S2LatLng::FromDegrees(lat, lng)));
  vector<string> v;
  do {
    if (id.level() >= min_level && id.level() <= max_level) {
      v.push_back(CellToString(id));
    }
    id = id.parent();
  } while (id.level() > 0);
  return v;
}

// Generates a list of cells at the target s2 cell levels which cover
// a cap of radius 'radius_meters' with center at lat & lng.
vector<string> SearchCells(double lat, double lng, double radius_meters,
                           int min_level, int max_level) {
  const double radius_radians = EarthMetersToRadians(radius_meters);
  const S2Cap region = S2Cap::FromAxisHeight(
      S2LatLng::FromDegrees(lat, lng).Normalized().ToPoint(),
      (radius_radians * radius_radians) / 2);
  S2RegionCoverer coverer;
  coverer.set_min_level(min_level);
  coverer.set_max_level(max_level);

  vector<S2CellId> covering;
  coverer.GetCovering(region, &covering);
  vector<string> v(covering.size());
  for (size_t i = 0; i < covering.size(); ++i) {
    v[i] = CellToString(covering[i]);
  }
  return v;
}