GrowOnlySpanAllocator.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "CoreMinimal.h"
 
class FGrowOnlySpanAllocator
{
public:
 
    FGrowOnlySpanAllocator() :
        MaxSize(0)
    {}
 
    // Allocate a range.  Returns allocated StartOffset.
    ENGINE_API int32 Allocate(int32 Num);
 
    // Free an already allocated range.  
    ENGINE_API void Free(int32 BaseOffset, int32 Num);
 
    int32 GetSparselyAllocatedSize() const
    {
        int32 AllocatedSize = MaxSize;
 
        for (int32 i = 0; i < FreeSpans.Num(); i++)
        {
            AllocatedSize -= FreeSpans[i].Num;
        }
 
        return AllocatedSize;
    }
 
    int32 GetMaxSize() const
    {
        return MaxSize;
    }
 
    int32 GetNumFreeSpans() const
    {
        return FreeSpans.Num();
    }
 
#if DO_CHECK
    inline bool IsFree(int32 Index) const
    {
        // If outside the max size, it is considered free as the allocator can grow at will
        if (Index >= MaxSize)
        {
            return true;
        }
 
        for (const auto& FreeSpan : FreeSpans)
        {
            if (Index >= FreeSpan.StartOffset && Index < FreeSpan.StartOffset + FreeSpan.Num)
            {
                return true;
            }
        }
 
        return false;
    }
#endif // DO_CHECK
 
    ENGINE_API void BeginDeferMerges();
    ENGINE_API void EndDeferMerges();
 
private:
    class FLinearAllocation
    {
    public:
 
        FLinearAllocation(int32 InStartOffset, int32 InNum) :
            StartOffset(InStartOffset),
            Num(InNum)
        {}
 
        int32 StartOffset;
        int32 Num;
 
        bool Contains(FLinearAllocation Other)
        {
            return StartOffset <= Other.StartOffset && (StartOffset + Num) >= (Other.StartOffset + Other.Num);
        }
 
        bool operator<(const FLinearAllocation& Other) const
        {
            return StartOffset < Other.StartOffset;
        }
    };
 
    // Size of the linear range used by the allocator
    int32 MaxSize;
 
    // Unordered free list
    TArray<FLinearAllocation, TInlineAllocator<10>> FreeSpans;
 
    bool bDeferMerges = false;
 
    ENGINE_API int32 SearchFreeList(int32 Num);
};