From c19605529975875acab04948e0d9a4c6be27ab63 Mon Sep 17 00:00:00 2001
From: Daniel Baston <dbaston@gmail.com>
Date: Tue, 14 Feb 2023 13:35:40 -0500
Subject: [PATCH] PreparedPolygon: Use std::call_once for lazy initialization

---
 include/geos/geom/prep/PreparedPolygon.h      | 10 ++++-
 src/geom/prep/PreparedPolygon.cpp             | 27 +++++++-----
 tests/unit/geom/prep/PreparedGeometryTest.cpp | 41 +++++++++++++++++++
 3 files changed, 66 insertions(+), 12 deletions(-)

diff --git a/include/geos/geom/prep/PreparedPolygon.h b/include/geos/geom/prep/PreparedPolygon.h
index a2feee6c9..f8239f4c1 100644
--- a/include/geos/geom/prep/PreparedPolygon.h
+++ b/include/geos/geom/prep/PreparedPolygon.h
@@ -50,13 +50,19 @@ namespace prep { // geos::geom::prep
  */
 class PreparedPolygon : public BasicPreparedGeometry {
 private:
-    bool isRectangle;
     mutable std::unique_ptr<noding::FastSegmentSetIntersectionFinder> segIntFinder;
     mutable std::unique_ptr<algorithm::locate::PointOnGeometryLocator> ptOnGeomLoc;
     mutable std::unique_ptr<algorithm::locate::PointOnGeometryLocator> indexedPtOnGeomLoc;
-    mutable noding::SegmentString::ConstVect segStrings;
     mutable std::unique_ptr<operation::distance::IndexedFacetDistance> indexedDistance;
 
+    mutable noding::SegmentString::ConstVect segStrings;
+
+    mutable std::once_flag segIntFinderFlag;
+    mutable std::once_flag ptOnGeomLocFlag;
+    mutable std::once_flag indexedPtOnGeomLocFlag;
+    mutable std::once_flag indexedDistanceFlag;
+    bool isRectangle;
+
 protected:
 public:
     PreparedPolygon(const geom::Geometry* geom);
diff --git a/src/geom/prep/PreparedPolygon.cpp b/src/geom/prep/PreparedPolygon.cpp
index 332c4798e..271957ab7 100644
--- a/src/geom/prep/PreparedPolygon.cpp
+++ b/src/geom/prep/PreparedPolygon.cpp
@@ -58,10 +58,11 @@ noding::FastSegmentSetIntersectionFinder*
 PreparedPolygon::
 getIntersectionFinder() const
 {
-    if(! segIntFinder) {
+    std::call_once(segIntFinderFlag, [this]() {
         noding::SegmentStringUtil::extractSegmentStrings(&getGeometry(), segStrings);
-        segIntFinder.reset(new noding::FastSegmentSetIntersectionFinder(&segStrings));
-    }
+        segIntFinder = detail::make_unique<noding::FastSegmentSetIntersectionFinder>(&segStrings);
+    });
+
     return segIntFinder.get();
 }
 
@@ -73,13 +74,18 @@ getPointLocator() const
     // instead of an IndexedPointInAreaLocator. There's a reasonable chance we will only use this locator
     // once (for example, if we get here through Geometry::intersects). So we create a simple locator for the
     // first usage and switch to an indexed locator when it is clear we're in a multiple-use scenario.
-    if(! ptOnGeomLoc) {
-        ptOnGeomLoc = detail::make_unique<algorithm::locate::SimplePointInAreaLocator>(&getGeometry());
+    if(!ptOnGeomLoc) {
+        std::call_once(ptOnGeomLocFlag, [this]() {
+            ptOnGeomLoc = detail::make_unique<algorithm::locate::SimplePointInAreaLocator>(&getGeometry());
+        });
+
         return ptOnGeomLoc.get();
-    } else if (!indexedPtOnGeomLoc) {
-        indexedPtOnGeomLoc = detail::make_unique<algorithm::locate::IndexedPointInAreaLocator>(getGeometry());
     }
 
+    std::call_once(indexedPtOnGeomLocFlag, [this]() {
+        indexedPtOnGeomLoc = detail::make_unique<algorithm::locate::IndexedPointInAreaLocator>(getGeometry());
+    });
+
     return indexedPtOnGeomLoc.get();
 }
 
@@ -157,9 +163,10 @@ operation::distance::IndexedFacetDistance*
 PreparedPolygon::
 getIndexedFacetDistance() const
 {
-    if(! indexedDistance ) {
-        indexedDistance.reset(new operation::distance::IndexedFacetDistance(&getGeometry()));
-    }
+    std::call_once(indexedDistanceFlag, [this]() {
+        indexedDistance = detail::make_unique<operation::distance::IndexedFacetDistance>(&getGeometry());
+    });
+
     return indexedDistance.get();
 }
 
diff --git a/tests/unit/geom/prep/PreparedGeometryTest.cpp b/tests/unit/geom/prep/PreparedGeometryTest.cpp
index efc5c4b82..ca9a4f949 100644
--- a/tests/unit/geom/prep/PreparedGeometryTest.cpp
+++ b/tests/unit/geom/prep/PreparedGeometryTest.cpp
@@ -12,6 +12,7 @@
 #include <geos/io/WKTReader.h>
 // std
 #include <memory>
+#include <thread>
 
 using namespace geos::geom;
 using geos::geom::prep::PreparedGeometry;
@@ -68,4 +69,44 @@ void object::test<1>
     ensure( pg1->covers(g2.get()));
 }
 
+// Check prepared geometry can be used from multiple threads
+template<>
+template<>
+void object::test<2>
+()
+{
+    std::vector<std::unique_ptr<Geometry>> geoms;
+    std::vector<std::unique_ptr<PreparedGeometry>> pgeoms;
+    std::vector<std::thread> threads;
+
+    constexpr std::size_t nrow = 10;
+    constexpr std::size_t ncol = 10;
+    constexpr std::size_t nthreads = 10;
+
+    for (std::size_t i = 0; i < ncol; i++) {
+        for (std::size_t j = 0; j < nrow; j++) {
+            CoordinateXY c(static_cast<double>(i), static_cast<double>(j));
+            auto pt = factory->createPoint(c);
+
+            geoms.emplace_back(pt->buffer(1.5));
+            pgeoms.push_back(prep::PreparedGeometryFactory::prepare(geoms.back().get()));
+        }
+    }
+
+    auto findIntersects = [&geoms](const PreparedGeometry* pg) {
+        for (const auto& geom : geoms) {
+            pg->intersects(geom.get());
+        }
+    };
+
+    for (std::size_t i = 0; i < nthreads; i++) {
+        threads.emplace_back(findIntersects, pgeoms[i].get());
+    }
+
+    for (auto& thread : threads) {
+        thread.join();
+    }
+
+}
+
 } // namespace tut