// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

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

namespace SixLabors.PolygonClipper {
  /// <summary>
  /// Compares two <see cref="SweepEvent"/> instances for sorting in the event queue.
  /// </summary>
  internal sealed class SweepEventComparer : IComparer<SweepEvent>, IComparer
  {
      /// <inheritdoc/>
      public int Compare(SweepEvent? x, SweepEvent? y)
      {
          if (x == null)
          {
              return -1;
          }

          if (y == null)
          {
              return 1;
          }

          // Compare by x-coordinate
          if (x.Point.X > y.Point.X)
          {
              return 1;
          }

          if (x.Point.X < y.Point.X)
          {
              return -1;
          }

          // Compare by y-coordinate when x-coordinates are the same
          if (x.Point.Y != y.Point.Y)
          {
              return x.Point.Y > y.Point.Y ? 1 : -1;
          }

          // Compare left vs. right endpoint
          if (x.Left != y.Left)
          {
              return x.Left ? 1 : -1;
          }

          // Compare collinearity using signed area
          double area = PolygonUtilities.SignedArea(x.Point, x.OtherEvent.Point, y.OtherEvent.Point);
          if (area != 0)
          {
              return x.IsBelow(y.OtherEvent.Point) ? -1 : 1;
          }

          // Compare by polygon type: subject polygons have higher priority
          return x.PolygonType != PolygonType.Subject && y.PolygonType == PolygonType.Subject ? 1 : -1;
      }

      /// <inheritdoc/>
      public int Compare(object? x, object? y)
      {
          if (x is SweepEvent a && y is SweepEvent b)
          {
              return this.Compare(a, b);
          }

          throw new ArgumentException("Both arguments must be of type SweepEvent.", nameof(x));
      }
  }
}