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| ## GEOS is a port of JTS | ||
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| * The algorithms that form the core value of GEOS are developed in Java in the [JTS library](https://github.com/locationtech/jts/). C++ developers will find this annoying, but: | ||
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| * This is just history, JTS was written first and GEOS was a slavish port. | ||
| * Being memory managed, JTS is an easier language to prototype in. | ||
| * Having various visual tooling, JTS is an easier platform to debug spatial algorithms in. | ||
| * Being Java, JTS has less language legacy than GEOS, which was originally ported when STL was still not part of the standard, and therefor reflects a mix of styles and eras. | ||
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| * Ideally, new algorithms will be implemented in JTS and then ported to GEOS. | ||
| * Smaller performance optimizations in GEOS can travel back to JTS. | ||
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| * Short circuits, indexes, other non-language optimizations, should be ticketed in JTS when they are added to GEOS. | ||
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| ### Follow JTS as Much as Possible | ||
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| * Don't rename things! It makes it harder to port updates and fixes. | ||
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| * Class names | ||
| * Method names | ||
| * Variable names | ||
| * Class members | ||
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| * Yes, we know in your last job you were taught all member variables are prefixed with `m_`, but please don't. | ||
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| ### Manage Lifecycles | ||
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| * Frequently objects are only used local to a method and not returned to the caller. | ||
| * In such cases, avoid lifecycle issues entirely by **instantiating on the local stack**. | ||
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| ```java | ||
| MyObj foo = new MyObj("bar"); | ||
| ``` | ||
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| ```c++ | ||
| MyObj foo("bar"); | ||
| ``` | ||
| * Long-lived members of objects that are passed around should be held using [std::unique_ptr<>](https://en.cppreference.com/w/cpp/memory/unique_ptr). | ||
| ```java | ||
| private MyMember foo = new MyMember(); | ||
| ``` | ||
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| ```c++ | ||
| private: | ||
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| std::unique_ptr<MyMember> foo; | ||
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| public: | ||
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| MyMember() | ||
| : foo(new MyMember()) | ||
| {} | ||
| ``` | ||
| * You can pass pointers to the object to other methods using `std::unique_ptr<>.get()`. | ||
| ### Avoid Many Small Heap Allocations | ||
| * Heap allocations (objects created using `new`) are more expensive than stack allocations, but they can show up in batchs in JTS in places where structures are built, like index trees, or graphs. | ||
| * To both lower the overhead of heap allocations, and to manage the life-cycle of the objects, we recommend storing small objects in an appropriate "double-ended queue", like [std::deque<>](https://en.cppreference.com/w/cpp/container/deque). | ||
| * The implementation of `edgegraph` is an example. | ||
| * The `edgegraph` consists of a structure of many `HalfEdge` objects (two for each edge!), created in the `EdgeGraph::createEdge()` method and stored in a `std::deque<>`. | ||
| * The `std::deque<>` provides two benefits: | ||
| * It lowers the number of heap allocations, because it allocates larger blocks of space to store multiple `HalfEdge` objects. | ||
| * It handles the lifecycle of the `HalfEdge` objects that make up the `EdgeGraph`, because when the `EdgeGraph` is deallocated, the `std::deque<>` and all its contents are also automatically deallocated. | ||
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| # | ||
| SUBDIRS = \ | ||
| algorithm \ | ||
| edgegraph \ | ||
| geom \ | ||
| geomgraph \ | ||
| index \ | ||
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| /********************************************************************** | ||
| * | ||
| * GEOS - Geometry Engine Open Source | ||
| * http://geos.osgeo.org | ||
| * | ||
| * Copyright (C) 2020 Paul Ramsey <[email protected]> | ||
| * | ||
| * This is free software; you can redistribute and/or modify it under | ||
| * the terms of the GNU Lesser General Public Licence as published | ||
| * by the Free Software Foundation. | ||
| * See the COPYING file for more information. | ||
| * | ||
| **********************************************************************/ | ||
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| #pragma once | ||
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| #include <geos/edgegraph/HalfEdge.h> | ||
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| #include <geos/export.h> | ||
| #include <string> | ||
| #include <cassert> | ||
| #include <map> | ||
| #include <array> | ||
| #include <memory> | ||
| #include <vector> | ||
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| // Forward declarations | ||
| namespace geos { | ||
| namespace geom { | ||
| class Coordinate; | ||
| } | ||
| } | ||
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| #undef EDGEGRAPH_HEAPHACK | ||
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| namespace geos { | ||
| namespace edgegraph { // geos.edgegraph | ||
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| /** | ||
| * A graph comprised of {@link HalfEdge}s. | ||
| * It supports tracking the vertices in the graph | ||
| * via edges incident on them, | ||
| * to allow efficient lookup of edges and vertices. | ||
| * <p> | ||
| * This class may be subclassed to use a | ||
| * different subclass of HalfEdge, | ||
| * by overriding {@link #createEdge(Coordinate)}. | ||
| * If additional logic is required to initialize | ||
| * edges then {@link EdgeGraph#addEdge(Coordinate, Coordinate)} | ||
| * can be overridden as well. | ||
| * | ||
| * @author Martin Davis | ||
| * | ||
| */ | ||
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| class GEOS_DLL EdgeGraph { | ||
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| private: | ||
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| std::deque<HalfEdge> edges; | ||
| std::map<geom::Coordinate, HalfEdge*> vertexMap; | ||
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| HalfEdge* create(const geom::Coordinate& p0, const geom::Coordinate& p1); | ||
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| protected: | ||
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| /** | ||
| * Creates a single HalfEdge. | ||
| * Override to use a different HalfEdge subclass. | ||
| * | ||
| * @param orig the origin location | ||
| * @return a new HalfEdge with the given origin | ||
| */ | ||
| HalfEdge* createEdge(const geom::Coordinate& orig); | ||
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| /** | ||
| * Inserts an edge not already present into the graph. | ||
| * | ||
| * @param orig the edge origin location | ||
| * @param dest the edge destination location | ||
| * @param eAdj an existing edge with same orig (if any) | ||
| * @return the created edge | ||
| */ | ||
| HalfEdge* insert(const geom::Coordinate& orig, const geom::Coordinate& dest, HalfEdge* eAdj); | ||
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| public: | ||
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| /** | ||
| * Initialized | ||
| */ | ||
| EdgeGraph() {}; | ||
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| /** | ||
| * Adds an edge between the coordinates orig and dest | ||
| * to this graph. | ||
| * Only valid edges can be added (in particular, zero-length segments cannot be added) | ||
| * | ||
| * @param orig the edge origin location | ||
| * @param dest the edge destination location. | ||
| * @return the created edge | ||
| * @return null if the edge was invalid and not added | ||
| * | ||
| * @see #isValidEdge(Coordinate, Coordinate) | ||
| */ | ||
| HalfEdge* addEdge(const geom::Coordinate& orig, const geom::Coordinate& dest); | ||
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| /** | ||
| * Tests if the given coordinates form a valid edge (with non-zero length). | ||
| * | ||
| * @param orig the start coordinate | ||
| * @param dest the end coordinate | ||
| * @return true if the edge formed is valid | ||
| */ | ||
| static bool isValidEdge(const geom::Coordinate& orig, const geom::Coordinate& dest); | ||
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| void getVertexEdges(std::vector<const HalfEdge*>& edgesOut); | ||
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| /** | ||
| * Finds an edge in this graph with the given origin | ||
| * and destination, if one exists. | ||
| * | ||
| * @param orig the origin location | ||
| * @param dest the destination location. | ||
| * @return an edge with the given orig and dest, or null if none exists | ||
| */ | ||
| HalfEdge* findEdge(const geom::Coordinate& orig, const geom::Coordinate& dest); | ||
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| }; | ||
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| } // namespace geos.edgegraph | ||
| } // namespace geos | ||
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