tmi-practicum-2018-2019/TMI-practicum/SweepLineAlgorithm.cs

using System;
using System.Collections.Generic;
using Medallion.Collections;

namespace TMI_practicum
{
    public static class SweepLineAlgorithm
    {
        public static IEnumerable<Intersection> Solve(IList<Circle> circles)
        {
            var events = new PriorityQueue<Event>();
            var intersections = new HashSet<Intersection>();
            var active = new List<Circle>();
            
            foreach (var circle in circles)
            {
                events.Enqueue(new Event(Event.EventType.Start, circle.X - circle.R, circle));
                events.Enqueue(new Event(Event.EventType.End, circle.X + circle.R, circle));
            }

            while (events.Count != 0)
            {
                Event e = events.Dequeue();
                switch (e.Type)
                {
                    case Event.EventType.Start:
                        active.Add(e.Circle);
                        CheckIntersections(active, e.Circle, intersections);
                        break;
                    case Event.EventType.End:
                        active.Remove(e.Circle);
                        break;
                    default:
                        throw new ArgumentOutOfRangeException();
                }
            }
            
            return intersections;
        }

        private static void CheckIntersections(IEnumerable<Circle> active, Circle circle, HashSet<Intersection> intersections)
        {
            foreach (var otherCircle in active)
            {
                if (circle.Equals(otherCircle)) continue;

                var intersects = circle.FindIntersections(otherCircle);
                if (intersects == null) continue;

                foreach (var intersection in intersects)
                {
                    intersections.Add(intersection);
                }
            }
        }


        private struct Event : IComparable<Event>
        {
            public EventType Type { get; }
            public Circle Circle { get; }
            public double X { get; }

            public enum EventType
            {
                Start,
                End
            }

            public Event(EventType type, double x, Circle circle)
            {
                Type = type;
                X = x;
                Circle = circle;
            }

            public int CompareTo(Event other)
            {
                return X.CompareTo(other.X);
            }
        }
    }
}