#include <XCEngine/UI/Widgets/UIScrollModel.h>

#include <algorithm>
#include <cmath>

namespace XCEngine {
namespace UI {
namespace Widgets {

float ComputeUIScrollOverflow(float contentExtent, float viewportExtent) {
    return (std::max)(0.0f, contentExtent - viewportExtent);
}

float ClampUIScrollOffset(float offset, float contentExtent, float viewportExtent) {
    return (std::max)(0.0f, (std::min)(offset, ComputeUIScrollOverflow(contentExtent, viewportExtent)));
}

UIScrollWheelResult ApplyUIScrollWheel(
    float offset,
    float wheelDelta,
    float contentExtent,
    float viewportExtent,
    float wheelStep,
    float epsilon) {
    UIScrollWheelResult result = {};
    result.offsetBefore = ClampUIScrollOffset(offset, contentExtent, viewportExtent);
    result.overflow = ComputeUIScrollOverflow(contentExtent, viewportExtent);
    result.offsetAfter = result.offsetBefore;
    if (result.overflow <= 0.0f || std::fabs(wheelDelta) <= epsilon || wheelStep <= 0.0f) {
        return result;
    }

    const float scrollUnits = wheelDelta / 120.0f;
    result.offsetAfter = ClampUIScrollOffset(
        result.offsetBefore - scrollUnits * wheelStep,
        contentExtent,
        viewportExtent);
    result.changed = std::fabs(result.offsetAfter - result.offsetBefore) > epsilon;
    return result;
}

float EnsureUIScrollOffsetVisible(
    float offset,
    float itemStart,
    float itemExtent,
    float contentExtent,
    float viewportExtent) {
    const float clampedOffset = ClampUIScrollOffset(offset, contentExtent, viewportExtent);
    if (viewportExtent <= 0.0f || itemExtent <= 0.0f) {
        return clampedOffset;
    }

    const float itemEnd = itemStart + itemExtent;
    const float viewportEnd = clampedOffset + viewportExtent;
    float nextOffset = clampedOffset;
    if (itemStart < clampedOffset) {
        nextOffset = itemStart;
    } else if (itemEnd > viewportEnd) {
        nextOffset = itemEnd - viewportExtent;
    }

    return ClampUIScrollOffset(nextOffset, contentExtent, viewportExtent);
}

} // namespace Widgets
} // namespace UI
} // namespace XCEngine