Rotate.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Templates/UnrealTemplate.h" // For GetData, GetNum, Swap
 
 
namespace AlgoImpl
{
    template <typename T>
    int32 RotateInternal(T* First, int32 Num, int32 Count)
    {
        if (Count == 0)
        {
            return Num;
        }
 
        if (Count >= Num)
        {
            return 0;
        }
 
        T* Iter = First;
        T* Mid  = First + Count;
        T* End  = First + Num;
 
        T* OldMid = Mid;
        for (;;)
        {
            Swap(*Iter++, *Mid++);
            if (Mid == End)
            {
                if (Iter == OldMid)
                {
                    return Num - Count;
                }
 
                Mid = OldMid;
            }
            else if (Iter == OldMid)
            {
                OldMid = Mid;
            }
        }
    }
}
 
namespace Algo
{
    /**
     * Rotates a given amount of elements from the front of the range to the end of the range.
     *
     * @param  Range  The range to rotate.
     * @param  Num    The number of elements to rotate from the front of the range.
     *
     * @return The new index of the element that was previously at the start of the range.
     */
    template <typename RangeType>
    FORCEINLINE int32 Rotate(RangeType& Range, int32 Count)
    {
        return AlgoImpl::RotateInternal(GetData(Range), GetNum(Range), Count);
    }
}