com.unity.uiwidgets/Runtime/ui/painting/canvas_clip.cs

using System.Collections.Generic;
using Unity.UIWidgets.foundation;
using UnityEngine;

namespace Unity.UIWidgets.ui {
    class ClipElement {
        public readonly int saveCount;
        public readonly MeshMesh mesh;
        public readonly bool convex;
        public readonly bool isRect;
        public Rect rect { get; private set; }

        uint _genId;
        bool _isIntersectionOfRects;
        Rect _bound;
        Matrix3 _invMat;

        public ClipElement(int saveCount, Path path, Matrix3 matrix, float scale) {
            this.saveCount = saveCount;

            var pathCache = path.flatten(scale);
            this.mesh = pathCache.getFillMesh(out this.convex).transform(matrix);

            var vertices = this.mesh.vertices;
            if (this.convex && vertices.Count == 4 && matrix.rectStaysRect() &&
                (Mathf.Abs(vertices[0].x - vertices[1].x) < 1e-6 && Mathf.Abs(vertices[1].y - vertices[2].y) < 1e-6 &&
                 Mathf.Abs(vertices[2].x - vertices[3].x) < 1e-6 && Mathf.Abs(vertices[3].y - vertices[0].y) < 1e-6 ||
                 Mathf.Abs(vertices[0].y - vertices[1].y) < 1e-6 && Mathf.Abs(vertices[1].x - vertices[2].x) < 1e-6 &&
                 Mathf.Abs(vertices[2].y - vertices[3].y) < 1e-6 && Mathf.Abs(vertices[3].x - vertices[0].x) < 1e-6)) {
                this.isRect = true;
                this.rect = this.mesh.bounds;
            }
            else {
                this.isRect = false;
                this.rect = null;
            }
        }

        public void setRect(Rect rect) {
            D.assert(ClipStack.invalidGenID != this._genId);
            D.assert(this.isRect && this.rect.contains(rect));
            this.rect = rect;
        }

        public void setEmpty() {
            this._genId = ClipStack.emptyGenID;
            this._isIntersectionOfRects = false;
            this._bound = Rect.zero;
        }

        public void updateBoundAndGenID(ClipElement prior) {
            this._genId = ClipStack.getNextGenID();
            this._isIntersectionOfRects = false;

            if (this.isRect) {
                this._bound = this.rect;
                if (prior == null || prior.isIntersectionOfRects()) {
                    this._isIntersectionOfRects = true;
                }
            }
            else {
                this._bound = this.mesh.bounds;
            }

            if (prior != null) {
                this._bound = this._bound.intersect(prior.getBound());
            }

            if (this._bound.isEmpty) {
                this.setEmpty();
            }
        }

        public bool isEmpty() {
            D.assert(ClipStack.invalidGenID != this._genId);
            return this.getGenID() == ClipStack.emptyGenID;
        }

        public Rect getBound() {
            D.assert(ClipStack.invalidGenID != this._genId);
            return this._bound;
        }

        public bool isIntersectionOfRects() {
            D.assert(ClipStack.invalidGenID != this._genId);
            return this._isIntersectionOfRects;
        }

        public uint getGenID() {
            D.assert(ClipStack.invalidGenID != this._genId);
            return this._genId;
        }

//        bool _convexContains(float x, float y) {
//            if (this.mesh.vertices.Count <= 2) {
//                return false;
//            }
//
//            if (this.mesh.matrix != null) {
//                if (this._invMat == null) {
//                    this._invMat = Matrix3.I();
//                    this.mesh.matrix.invert(this._invMat); // ignore if not invertible for now.
//                }
//                
//                var offset = this._invMat.mapXY(x, y);
//                x = offset.dx;
//                y = offset.dy;
//            }
//
//            for (var i = 0; i < this.mesh.vertices.Count; i++) {
//                var p0 = this.mesh.vertices[i];
//                var p1 = this.mesh.vertices[i == this.mesh.vertices.Count - 1 ? 0 : i + 1];
//
//                if (PathUtils.triarea2(p0.x, p0.y, p1.x, p1.y, x, y) < 0.0f) {
//                    return false;
//                }
//            }
//
//            return true;
//        }

        public bool contains(Rect rect) {
            if (this.isRect) {
                return this.rect.contains(rect);
            }

//            this seems to be inefficient. disable it for now.
//            if (this.convex) {
//                return this._convexContains(rect.left, rect.top) &&
//                       this._convexContains(rect.left, rect.bottom) &&
//                       this._convexContains(rect.right, rect.top) &&
//                       this._convexContains(rect.right, rect.bottom);
//            }

            return false;
        }
    }

    class ClipStack {
        static uint _genId = wideOpenGenID;

        public static uint getNextGenID() {
            return ++_genId;
        }

        public const uint invalidGenID = 0;

        public const uint emptyGenID = 1;

        public const uint wideOpenGenID = 2;

        public readonly List<ClipElement> stack = new List<ClipElement>();
        
        ClipElement _lastElement;
        Rect _bound;
        int _saveCount;

        public void save() {
            this._saveCount++;
        }

        public void restore() {
            this._saveCount--;
            this._restoreTo(this._saveCount);
        }

        void _restoreTo(int saveCount) {
            while (this._lastElement != null) {
                if (this._lastElement.saveCount <= saveCount) {
                    break;
                }

                this.stack.RemoveAt(this.stack.Count - 1);
                this._lastElement = this.stack.Count == 0 ? null : this.stack[this.stack.Count - 1];
            }
        }

        public void clipPath(Path path, Matrix3 matrix, float scale) {
            var element = new ClipElement(this._saveCount, path, matrix, scale);
            this._pushElement(element);
        }

        void _pushElement(ClipElement element) {
            ClipElement prior = this._lastElement;
            if (prior != null) {
                if (prior.isEmpty()) {
                    return;
                }

                if (prior.saveCount == this._saveCount) {
                    // can not update prior if it's cross save count.
                    if (prior.isRect && element.isRect) {
                        var isectRect = prior.rect.intersect(element.rect);
                        if (isectRect.isEmpty) {
                            prior.setEmpty();
                            return;
                        }

                        prior.setRect(isectRect);
                        var priorprior = this.stack.Count > 1 ? this.stack[this.stack.Count - 2] : null;
                        prior.updateBoundAndGenID(priorprior);
                        return;
                    }

                    if (!prior.getBound().overlaps(element.getBound())) {
                        prior.setEmpty();
                        return;
                    }
                }
            }

            this.stack.Add(element);
            this._lastElement = element;
            element.updateBoundAndGenID(prior);
        }

        public void getBounds(out Rect bound, out bool isIntersectionOfRects) {
            if (this._lastElement == null) {
                bound = null;
                isIntersectionOfRects = false;
                return;
            }

            var element = this._lastElement;
            bound = element.getBound();
            isIntersectionOfRects = element.isIntersectionOfRects();
        }
    }

    class ReducedClip {
        public readonly Rect scissor;
        public readonly List<ClipElement> maskElements = new List<ClipElement>();
        ClipElement _lastElement;

        public bool isEmpty() {
            return this.scissor != null && this.scissor.isEmpty;
        }

        public uint maskGenID() {
            var element = this._lastElement;
            if (element == null) {
                return ClipStack.wideOpenGenID;
            }

            return element.getGenID();
        }

        public ReducedClip(ClipStack stack, Rect layerBounds, Rect queryBounds) {
            Rect stackBounds;
            bool iior;
            stack.getBounds(out stackBounds, out iior);

            if (stackBounds == null) {
                this.scissor = layerBounds;
                return;
            }

            stackBounds = layerBounds.intersect(stackBounds);
            if (iior) {
                this.scissor = stackBounds;
                return;
            }

            queryBounds = stackBounds.intersect(queryBounds);
            if (queryBounds.isEmpty) {
                this.scissor = Rect.zero;
                return;
            }

            this.scissor = queryBounds;
            this._walkStack(stack, this.scissor);
        }

        void _walkStack(ClipStack stack, Rect queryBounds) {
            foreach (var element in stack.stack) {
                if (element.isRect) {
                    continue;
                }

                if (element.contains(queryBounds)) {
                    continue;
                }

                this.maskElements.Add(element);
                this._lastElement = element;
            }
        }
    }
}