added support for caching the pair of coordinates for transformation (#520)

* partial implementation

* initial implementation

* fixed pointer

* unique ptr

Author: edisonguo

worker/gdalprocess/coordinate_transform_cache.cxx

@@ -0,0 +1,36 @@
+
+#include "coordinate_transform_cache.hxx"
+#include <iostream>
+
+void CoordinateTransformCache::put(TransformKey key, void* psInfo) {
+	remove(key);
+	if( coordLookup.size() >= maxCapacity ) {
+		int minCount = std::numeric_limits<int>::max();
+		auto minIt = coordLookup.begin();
+		for(auto it = coordLookup.begin(); it != coordLookup.end(); it++ ) {
+			if( it->second->useCount < minCount ) {
+				minCount = it->second->useCount;
+				minIt = it;
+			}
+		}
+		remove(minIt->first);
+	}
+
+	coordLookup[key] = std::make_unique<CacheBlock>(psInfo);
+}	
+
+void* CoordinateTransformCache::get(TransformKey key) {
+	auto it = coordLookup.find(key);
+	if( it != coordLookup.end() ) {
+		it->second->useCount++;
+		return it->second->item;
+	}
+	return nullptr;
+}
+
+void CoordinateTransformCache::remove(TransformKey key) {
+	auto it = coordLookup.find(key);
+	if( it != coordLookup.end() ) {
+		coordLookup.erase(it);
+	}
+}

worker/gdalprocess/coordinate_transform_cache.hxx

@@ -0,0 +1,36 @@
+#ifndef COORD_TRANSFORM_H
+#define COORD_TRANSFORM_H
+
+#include "gdal_alg.h"
+#include <map>
+#include <limits>
+#include <memory>
+#include <utility>
+
+typedef std::pair<std::string, std::string> TransformKey;
+
+struct CacheBlock {
+	void* item;
+	int   useCount;
+	CacheBlock(void* item) {
+		this->item = item;
+		this->useCount = 1;
+	}
+	~CacheBlock() {
+		if( item != nullptr ) {
+			GDALDestroyGenImgProjTransformer(item);
+		}
+	}
+};
+
+class CoordinateTransformCache {
+	public:
+		void put(TransformKey, void* psInfo);
+		void* get(TransformKey key);
+		void remove(TransformKey key);
+	private:
+		std::map<TransformKey, std::unique_ptr<CacheBlock>> coordLookup;
+		size_t maxCapacity = 1024;
+};
+
+#endif