#pragma region CPL License
/*
Nuclex Native Framework
Copyright (C) 2002-2015 Nuclex Development Labs

This library is free software; you can redistribute it and/or
modify it under the terms of the IBM Common Public License as
published by the IBM Corporation; either version 1.0 of the
License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
IBM Common Public License for more details.

You should have received a copy of the IBM Common Public
License along with this library
*/
#pragma endregion // CPL License

#ifndef NUCLEX_GEOMETRY_AREAS_TRACERS_POLYGON2TRACER_H
#define NUCLEX_GEOMETRY_AREAS_TRACERS_POLYGON2TRACER_H

#include "Nuclex/Geometry/Config.h"
#include "Nuclex/Geometry/Areas/Polygon2.h"
#include "Nuclex/Geometry/Lines/Line2.h"
#include "Nuclex/Geometry/LineContacts.h"

namespace Nuclex { namespace Geometry { namespace Areas { namespace Tracers {

  // ------------------------------------------------------------------------------------------- //

  /// <summary>Determines contacts between lines and 2D polygons</summary>
  template <typename TScalar>
  class Polygon2Tracer {

    /// <summary>Determines where a line enters and where it exits a polygon</summary>
    /// <param name="line">
    ///   Line whose entry and exit through the polygon will be determined
    /// </param>
    /// <param name="polygon">Polygon the line will be checked against</param>
    /// <returns>The times at which the line enters and leaves the polygon</returns>
    /// <remarks>
    ///   If the polygon is concave or self-intersects, this method will report
    ///   the times of first entry and last exit.
    /// </remarks>
    public: static LineContacts<TScalar> FindLineContacts(
      const Lines::Line2<TScalar> &line, const Polygon2<TScalar> &polygon
    ) {
      std::size_t vertexCount = polygon.Vertices.size();
      if(vertexCount < 2) { // Point or empty polygon?
        return LineContacts<TScalar>::None;
      }

      // Initialize the line contacts with the closing line segment from the polygon's
      // last vertex to its first
      LineContacts<TScalar> contacts;
      contacts.EntryTime = contacts.ExitTime = findLineContact(
        line, polygon.Vertices[vertexCount - 1], polygon.Vertices[0]
      );
      if(vertexCount == 2) {
        return contacts;
      }

      // Now check for collisions with each of the polygon's edges and track the time
      // of first entry and last exit.
      for(std::size_t index = 0; index < vertexCount - 1; ++index) {
        TScalar time = findLineContact(
          line, polygon.Vertices[index], polygon.Vertices[index + 1]
        );
        if(Math<TScalar>::IsNan(contacts.EntryTime) || (time < contacts.EntryTime)) {
          contacts.EntryTime = time;
        }
        if(Math<TScalar>::IsNan(contacts.ExitTime) || (time >= contacts.ExitTime)) {
          contacts.ExitTime = time;
        }
      }

      return contacts;
    }

    /// <summary>Determines where a line crosses a line segment</summary>
    /// <param name="line">Line whose time of crossing will be determined</param>
    /// <param name="start">Point at which the line segment begins</param>
    /// <param name="end">Point at which the line segment ends</param>
    /// <returns>The time at which the line crosses the line segment</returns>
    private: static TScalar findLineContact(
      const Lines::Line2<TScalar> &line,
      const Point2<TScalar> &start, const Point2<TScalar> &end
    ) {
      Vector2<TScalar> segmentDirection = (end - start);

      // Time of intersection of the segment with the traced line. This allows us to see
      // if the segment hits the traced line inside its length.
      TScalar s = (
        line.Direction.X * (line.Origin.Y - start.Y) -
        line.Direction.Y * (line.Origin.X - start.X)
      ) / (line.Direction.X * segmentDirection.Y - segmentDirection.X * line.Direction.Y);

      if((s >= 0) && (s < 1)) {
        TScalar t = (
          segmentDirection.X * (line.Origin.Y - start.Y) -
          segmentDirection.Y * (line.Origin.X - start.X)
        ) / (line.Direction.X * segmentDirection.Y - segmentDirection.X * line.Direction.Y);

        return t;
      } else {
        return std::numeric_limits<TScalar>::signaling_NaN();
      }
    }

  };

  // ------------------------------------------------------------------------------------------- //

}}}} // namespace Nuclex::Geometry::Areas::Tracers

#endif // NUCLEX_GEOMETRY_AREAS_TRACERS_POLYGON2TRACER_H