#pragma once

#include "../../Widgets/Widget.h"
#include <cstddef>
#include <functional>
#include <memory>
#include <vector>
#include "../Engine/Geometry.h"

namespace UI {

enum class VirtualizingStackSizing {
    Fixed, // use size provider
    Fill // use available space/page layout
};

class VirtualizingStackLayout : public Widget {
    std::size_t itemCount;
    float spacing;
    float overscan;
    std::function<Size(std::size_t)> itemSizeProvider;
    std::function<std::unique_ptr<Widget>(std::size_t)> itemFactory;
    VirtualizingStackSizing sizing;
    std::vector<Size> itemSizes;
    std::vector<float> itemOffsets;
    std::size_t visibleStart = 0;
    std::size_t visibleEnd = 0;
    float scrollOffset = 0.0f;
    bool hasVisibleRange = false;
    Size metricsSpace{-1.0f, -1.0f}; // available space

    Size ViewportExtent() const;
    Size ResolveAvailableSpace(Size availableSpace) const;
    void RebuildMetrics(Size availableSpace);
    bool UpdateVisibleChildren();
    std::size_t FindFirstVisibleIndex(float start) const;
    std::size_t FindLastVisibleIndex(float end) const;

public:
    VirtualizingStackLayout(
        std::size_t itemCount,
        float spacing,
        float overscan,
        std::function<Size(std::size_t)> itemSizeProvider,
        std::function<std::unique_ptr<Widget>(std::size_t)> itemFactory,
        VirtualizingStackSizing sizing = VirtualizingStackSizing::Fixed
    );

    void Measure(Size availableSpace) override;
    void Arrange(Rect finalBounds) override;
    void SetScrollOffset(Point offset) override;
};

}