Implement a small cache to avoid scaling continuously while painting

YACReader/continuous_page_widget.h

@@ -4,6 +4,8 @@
 #include <QWidget>
 #include <QSize>
 #include <QVector>
+#include <QHash>
+#include <QList>
 
 #include "themable.h"
 
@@ -33,8 +35,61 @@ protected:
     void applyTheme(const Theme &theme) override;
 
 private:
+    struct ScaledPageCacheEntry {
+        qint64 sourceCacheKey = 0;
+        QSize sourceSize;
+        QSize targetSize;
+        QImage scaledImage;
+    };
+
+    struct ScaledPageCache {
+        int effectiveWidth = -1;
+        QHash<int, ScaledPageCacheEntry> pages;
+
+        void invalidateAll()
+        {
+            effectiveWidth = -1;
+            pages.clear();
+        }
+
+        void invalidateForWidth(int width)
+        {
+            if (effectiveWidth != width) {
+                effectiveWidth = width;
+                pages.clear();
+            }
+        }
+
+        void invalidatePage(int pageIndex)
+        {
+            pages.remove(pageIndex);
+        }
+
+        void keepOnlyRange(int minPageIndex, int maxPageIndex)
+        {
+            if (pages.isEmpty()) {
+                return;
+            }
+
+            QList<int> keysToRemove;
+            keysToRemove.reserve(pages.size());
+            for (auto it = pages.constBegin(); it != pages.constEnd(); ++it) {
+                if (it.key() < minPageIndex || it.key() > maxPageIndex) {
+                    keysToRemove.append(it.key());
+                }
+            }
+
+            for (int key : keysToRemove) {
+                pages.remove(key);
+            }
+        }
+    };
+
+    const QImage *scaledImageForPaint(int pageIndex, const QImage *source, const QSize &targetSize, int effectiveWidth);
+
     Render *render = nullptr;
     ContinuousViewModel *continuousViewModel = nullptr;
+    ScaledPageCache scaledPageCache;
 };
 
 #endif // CONTINUOUS_PAGE_WIDGET_H