Cleanup API, update readme

Author: JimBobSquarePants

README.md

@@ -2,18 +2,30 @@
 
 [![License: Six Labors Split](https://img.shields.io/badge/license-Six%20Labors%20Split-%23e30183)](https://github.com/SixLabors/PolygonClipper/blob/master/LICENSE)
 
-## A Simple Algorithm for Boolean Operations on Polygons  
+A C# implementation of the Martínez–Rueda algorithm for performing Boolean operations on polygons. This library supports union, intersection, difference, and xor operations on complex polygons with holes, multiple contours, and self-intersections.
 
-*Francisco Martínez, Carlos Ogayar, Juan R. Jiménez, Antonio J. Rueda*
-  
-https://sci-hub.se/10.1016/j.advengsoft.2013.04.004
+## Features
 
-This repository contains the beginnings of an attempted port of the original public domain C++ implementation by the main author of the paper Francisco Martínez.   
+- Works with non-convex polygons, including holes and multiple disjoint regions
+- Handles edge cases like overlapping edges and vertical segments
+- Preserves topology: output polygons include hole/contour hierarchy
+- Deterministic and robust sweep line algorithm with O((n + k) log n) complexity
 
-The original code can be found in the reference folder.  
-  
-The plan is to implement a performant port, add additional tests and some method by which to generate renders of output clipping operations.   
-  
-This is currently an intellectual exercise but I believe a C# port could be very useful in many applications if proven successful. 
-  
-All and any assistance is gratefully accepted. :heart:
+## Usage
+
+The API centers around `Polygon` and `Contour` types. Construct input polygons using contours, then apply Boolean operations via the `PolygonClipper` class:
+
+```csharp
+Polygon result = PolygonClipper.Intersect(subject, clipping);
+```
+
+## Based On
+
+This implementation is based on the algorithm described in:
+
+> F. Martínez et al., "A simple algorithm for Boolean operations on polygons", *Advances in Engineering Software*, 64 (2013), pp. 11–19.  
+> https://sci-hub.se/10.1016/j.advengsoft.2013.04.004
+
+## License
+
+Six Labors Split License. See [`LICENSE`](https://github.com/SixLabors/PolygonClipper/blob/main/LICENSE) for details.

src/PolygonClipper/Contour.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
@@ -10,89 +11,94 @@ namespace PolygonClipper;
 /// <summary>
 /// Represents a simple polygon. The edges of the contours are interior disjoint.
 /// </summary>
-[DebuggerDisplay("Count = {VertexCount}")]
-public sealed class Contour
+[DebuggerDisplay("Count = {Count}")]
+#pragma warning disable CA1710 // Identifiers should have correct suffix
+public sealed class Contour : IReadOnlyCollection<Vertex>
+#pragma warning restore CA1710 // Identifiers should have correct suffix
 {
-    private bool precomputeCC;
-    private bool cc;
+    private bool hasCachedOrientation;
+    private bool cachedCounterClockwise;
 
     /// <summary>
-    /// Set of points conforming the external contour
+    /// Set of vertices conforming the external contour
     /// </summary>
-    private readonly List<Vertex> points = [];
+    private readonly List<Vertex> vertices = [];
 
     /// <summary>
     /// Holes of the contour. They are stored as the indexes of
     /// the holes in a polygon class
     /// </summary>
-    private readonly List<int> holes = [];
+    private readonly List<int> holeIndices = [];
 
     /// <summary>
     /// Gets the number of vertices.
     /// </summary>
-    public int VertexCount => this.points.Count;
-
-    /// <summary>
-    /// Gets the number of edges.
-    /// </summary>
-    public int EdgeCount => this.points.Count;
+    public int Count
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => this.vertices.Count;
+    }
 
     /// <summary>
     /// Gets the number of holes.
     /// </summary>
-    public int HoleCount => this.holes.Count;
+    public int HoleCount => this.holeIndices.Count;
 
     /// <summary>
     /// Gets a value indicating whether the contour is external (not a hole).
     /// </summary>
-    public bool IsExternal => this.HoleOf == null;
+    public bool IsExternal => this.ParentIndex == null;
 
     /// <summary>
-    /// Gets or sets the ID of the parent contour if this contour is a hole.
+    /// Gets or sets the index of the parent contour in the polygon if this contour is a hole.
     /// </summary>
-    public int? HoleOf { get; set; }
+    public int? ParentIndex { get; set; }
 
     /// <summary>
     /// Gets or sets the depth of the contour.
     /// </summary>
     public int Depth { get; set; }
 
     /// <summary>
-    /// Gets the vertex at the specified index of the external contour.
+    /// Gets the vertex at the specified index.
     /// </summary>
     /// <param name="index">The index of the vertex.</param>
-    /// <returns>The <see cref="Vertex"/>.</returns>
-    public Vertex GetVertex(int index) => this.points[index];
+    /// <returns>The <see cref="Vertex"/> at the specified index.</returns>
+    public Vertex this[int index]
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => this.vertices[index];
+    }
 
     /// <summary>
     /// Gets the hole index at the specified position in the contour.
     /// </summary>
     /// <param name="index">The index of the hole.</param>
     /// <returns>The hole index.</returns>
-    public int GetHoleIndex(int index) => this.holes[index];
+    public int GetHoleIndex(int index) => this.holeIndices[index];
 
     /// <summary>
     /// Gets the segment at the specified index of the contour.
     /// </summary>
     /// <param name="index">The index of the segment.</param>
-    /// <returns>The <see cref="Segment"/>.</returns>
-    internal Segment Segment(int index)
-        => (index == this.VertexCount - 1)
-        ? new Segment(this.points[^1], this.points[0])
-        : new Segment(this.points[index], this.points[index + 1]);
+    /// <returns>The <see cref="GetSegment"/>.</returns>
+    internal Segment GetSegment(int index)
+        => (index == this.Count - 1)
+        ? new Segment(this.vertices[^1], this.vertices[0])
+        : new Segment(this.vertices[index], this.vertices[index + 1]);
 
     /// <summary>
     /// Gets the bounding box of the contour.
     /// </summary>
     /// <returns>The <see cref="Box2"/>.</returns>
     public Box2 GetBoundingBox()
     {
-        if (this.VertexCount == 0)
+        if (this.Count == 0)
         {
             return default;
         }
 
-        List<Vertex> points = this.points;
+        List<Vertex> points = this.vertices;
         Box2 b = new(points[0]);
         for (int i = 1; i < points.Count; ++i)
         {
@@ -110,18 +116,18 @@ public Box2 GetBoundingBox()
     /// </returns>
     public bool IsCounterClockwise()
     {
-        if (this.precomputeCC)
+        if (this.hasCachedOrientation)
         {
-            return this.cc;
+            return this.cachedCounterClockwise;
         }
 
-        this.precomputeCC = true;
+        this.hasCachedOrientation = true;
 
         double area = 0;
         Vertex c;
         Vertex c1;
 
-        List<Vertex> points = this.points;
+        List<Vertex> points = this.vertices;
         for (int i = 0; i < points.Count - 1; i++)
         {
             c = points[i];
@@ -132,7 +138,7 @@ public bool IsCounterClockwise()
         c = points[^1];
         c1 = points[0];
         area += Vertex.Cross(c, c1);
-        return this.cc = area >= 0;
+        return this.cachedCounterClockwise = area >= 0;
     }
 
     /// <summary>
@@ -148,8 +154,8 @@ public bool IsCounterClockwise()
     /// </summary>
     public void Reverse()
     {
-        this.points.Reverse();
-        this.cc = !this.cc;
+        this.vertices.Reverse();
+        this.cachedCounterClockwise = !this.cachedCounterClockwise;
     }
 
     /// <summary>
@@ -181,7 +187,7 @@ public void SetCounterClockwise()
     /// <param name="y">The y-coordinate offset.</param>
     public void Translate(double x, double y)
     {
-        List<Vertex> points = this.points;
+        List<Vertex> points = this.vertices;
         for (int i = 0; i < points.Count; i++)
         {
             points[i] += new Vertex(x, y);
@@ -193,38 +199,46 @@ public void Translate(double x, double y)
     /// </summary>
     /// <param name="vertex">The vertex to add.</param>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public void AddVertex(in Vertex vertex) => this.points.Add(vertex);
+    public void AddVertex(in Vertex vertex) => this.vertices.Add(vertex);
 
     /// <summary>
     /// Removes the vertex at the specified index from the contour.
     /// </summary>
     /// <param name="index">The index of the vertex to remove.</param>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public void RemoveVertexAt(int index) => this.points.RemoveAt(index);
+    public void RemoveVertexAt(int index) => this.vertices.RemoveAt(index);
 
     /// <summary>
     /// Clears all vertices and holes from the contour.
     /// </summary>
     public void Clear()
     {
-        this.points.Clear();
-        this.holes.Clear();
+        this.vertices.Clear();
+        this.holeIndices.Clear();
     }
 
     /// <summary>
     /// Clears all holes from the contour.
     /// </summary>
-    public void ClearHoles() => this.holes.Clear();
+    public void ClearHoles() => this.holeIndices.Clear();
 
     /// <summary>
     /// Gets the last vertex in the contour.
     /// </summary>
     /// <returns>The last <see cref="Vertex"/> in the contour.</returns>
-    public Vertex GetLastVertex() => this.points[^1];
+    public Vertex GetLastVertex() => this.vertices[^1];
 
     /// <summary>
     /// Adds a hole index to the contour.
     /// </summary>
     /// <param name="index">The index of the hole to add.</param>
-    public void AddHoleIndex(int index) => this.holes.Add(index);
+    public void AddHoleIndex(int index) => this.holeIndices.Add(index);
+
+    /// <inheritdoc/>
+    public IEnumerator<Vertex> GetEnumerator()
+        => ((IEnumerable<Vertex>)this.vertices).GetEnumerator();
+
+    /// <inheritdoc/>
+    IEnumerator IEnumerable.GetEnumerator()
+        => ((IEnumerable)this.vertices).GetEnumerator();
 }