// Quality.h

#ifndef Quality_H
#define Quality_H

// Quality is an abtract base class for indicating quality.  The
// meaning of quality and the number and types of objects for which
// the quality is evaluated are defined by derived classes.
//
// Quality provides two public methods:
// 1. is_valid() returns true if the quality is acceptable
// 2. is_better_than(const Quality&) returns true if the current
//    quality is better than the arguemnt.
//
// A quality is typically constructed from two objects but more
// or less are possible.  The quality might indicate the nearness
// or overlap between the objects.
//
// Examples include the distance between two points, the distance
// between a point and a line and the overlap volume between
// N spheres.  In any case, better coul be defined as a smaller
// or larger value.
//
// For most applications where it is not neccessary to discriminate
// between qualities of different types, the concrete classes 
// InvalidQuality and SimpleQuality can be used instead of introducing
// a new subclass.  The former is always invalid and the latter is
// always valid and uses a double (smaller is better) as the measure
// of quality.
//

class Quality {

private:  // methods

  // User-supplied comparison.
  // Return true if the current quality is better than the argument.
  // This is used by the method is_better_than.
  // It is guaranteed that both qualities are valid and of the same
  // type.
  virtual bool _is_better_than(const Quality& quality) const =0;

public:  // methods

  // return the type
  virtual void* get_type() const =0;

  // Is the distance between the pair in the allowed range?
  // Override this if neccessary.
  virtual bool is_valid() const;

  // Is this quality better than the specified one?
  // A valid quality is always better than an invalid one.
  // Return false if both are invalid.
  // Return false if the qualities are valid and of different type.
  // Do not override this method--use _is_better_than.
  bool is_better_than(const Quality& quality) const;

};

#endif