EM_Task/CoreUObject/Private/UObject/FindStronglyConnected.cpp

// Copyright Epic Games, Inc. All Rights Reserved.

#include "UObject/FindStronglyConnected.h"
#include "UObject/GarbageCollection.h"
#include "UObject/UObjectIterator.h"
#include "Serialization/ArchiveFindAllRefs.h"

void FFindStronglyConnected::FindAllCycles()
{
    UE_LOG(LogObj, Log, TEXT("Finding Edges"));
    for (FThreadSafeObjectIterator It; It; ++It)
    {
        UObject* Object = *It;
        {
            AllObjects.Add(Object);

            FArchiveFindAllRefs ArFind(Object);

            for (int32 Index = 0; Index < ArFind.References.Num(); Index++)
            {
                AllEdges.Add(Object, ArFind.References[Index]);
                if (AllEdges.Num() % 25000 == 0)
                {
                    UE_LOG(LogObj, Log, TEXT("Finding Edges %d"), AllEdges.Num());
                }
            }
        }
    }
    UE_LOG(LogObj, Log, TEXT("Finding Edges Done %d"), AllEdges.Num());

    UE_LOG(LogObj, Log, TEXT("Finding permanent objects"));
    TArray<UObject*> Fringe;
    for (int32 Index = 0; Index < AllObjects.Num(); Index++)
    {
        UObject* Object = AllObjects[Index];
        if (Object->HasAnyFlags(GARBAGE_COLLECTION_KEEPFLAGS))
        {
            PermanentObjects.Add(Object);
            Fringe.Add(Object);
        }
    }
    while (Fringe.Num())
    {
        TArray<UObject*> LastFringe;
        Exchange(LastFringe, Fringe);
        for (int32 Index = 0; Index < LastFringe.Num(); Index++)
        {
            UObject* Object = LastFringe[Index];
            TArray<UObject*> Refs;
            AllEdges.MultiFind(Object, Refs);

            for (int32 IndexRef = 0; IndexRef < Refs.Num(); IndexRef++)
            {
                UObject* RefObject = Refs[IndexRef];
                if (!PermanentObjects.Contains(RefObject))
                {
                    PermanentObjects.Add(RefObject);
                    Fringe.Add(RefObject);
                }
            }
        }
    }
    for (int32 Index = 0; Index < AllObjects.Num(); Index++)
    {
        UObject* Object = AllObjects[Index];
        if (!PermanentObjects.Contains(Object))
        {
            TempObjects.Add(Object);
        }
    }
    for (TMultiMap<UObject*, UObject*>::TIterator It(AllEdges); It; ++It)
    {
        if (!PermanentObjects.Contains(It.Key()) && !PermanentObjects.Contains(It.Value()))
        {
            Edges.Add(It.Key(), It.Value());
        }
    }
    UE_LOG(LogObj, Log, TEXT("Finding cycles"));

    for (int32 Index = 0; Index < TempObjects.Num(); Index++)
    {
        StrongConnect(TempObjects[Index]);
    }
    UE_LOG(LogObj, Log, TEXT("Finding simple cycles"));
    Stack.Empty();
    NodeIndex.Empty();
    MasterIndex = 1;

    for (int32 Index = 0; Index < Components.Num(); Index++)
    {
        TArray<UObject*>& Dest = SimpleCycles[SimpleCycles.Add(TArray<UObject*>())];
        FindSimpleCycleForComponent(Dest, Components[Index]);
    }
}

void FFindStronglyConnected::FindSimpleCycleForComponent(TArray<UObject*>& Dest, const TArray<UObject*>& Component)
{
    if (Component.Num() < 3)
    {
        Dest = Component;  // if it is only one or two items, the cycle is the component
        return;
    }
    FindSimpleCycleForComponentInner(Dest, Component, Component[0]);
    Stack.Empty();
}

bool FFindStronglyConnected::FindSimpleCycleForComponentInner(TArray<UObject*>& Dest, const TArray<UObject*>& Component, UObject* Node)
{
    Stack.Push(Node);
    TArray<UObject*> Refs;
    Edges.MultiFind(Node, Refs);

    for (int32 Index = 0; Index < Refs.Num(); Index++)
    {
        UObject* Other = Refs[Index];
        if (!Component.Contains(Other))
        {
            continue;
        }
        if (Stack.Contains(Other))
        {
            while (1)
            {
                UObject* Out = Stack.Pop(/*bAllowShrinking=*/false);
                Dest.Add(Out);
                if (Out == Other)
                {
                    return true;
                }
            }
        }
        if (FindSimpleCycleForComponentInner(Dest, Component, Other))
        {
            return true;
        }
    }
    check(0);
    return false;
}

void FFindStronglyConnected::StrongConnect(UObject* Node)
{
    if (NodeIndex.Find(Node))
    {
        return;
    }
    StrongConnectInner(Node);
}

FFindStronglyConnected::NodeInfo* FFindStronglyConnected::StrongConnectInner(UObject* Node)
{
    NodeInfo NewNode;
    NewNode.IndexValue = MasterIndex;
    NewNode.LowIndex   = MasterIndex;
    NewNode.InStack    = true;
    MasterIndex++;
    Stack.Push(Node);
    NodeInfo* CurrentIndex = &NodeIndex.Add(Node, NewNode);

    TArray<UObject*> Refs;
    Edges.MultiFind(Node, Refs);

    for (int32 Index = 0; Index < Refs.Num(); Index++)
    {
        UObject* Other          = Refs[Index];
        NodeInfo* OtherIndexVal = NodeIndex.Find(Other);
        if (!OtherIndexVal)
        {
            OtherIndexVal          = StrongConnectInner(Other);
            CurrentIndex           = NodeIndex.Find(Node);  // this could have reallocated in the recursive call
            CurrentIndex->LowIndex = FMath::Min<int32>(CurrentIndex->LowIndex, OtherIndexVal->LowIndex);
        }
        else if (OtherIndexVal->InStack)
        {
            CurrentIndex->LowIndex = FMath::Min<int32>(CurrentIndex->LowIndex, OtherIndexVal->IndexValue);
        }
    }
    if (CurrentIndex->IndexValue == CurrentIndex->LowIndex)
    {
        TArray<UObject*>& Dest = Components[Components.Add(TArray<UObject*>())];
        while (1)
        {
            UObject* Out     = Stack.Pop(/*bAllowShrinking=*/false);
            NodeInfo* OutVal = NodeIndex.Find(Out);
            OutVal->InStack  = false;
            Dest.Add(Out);
            if (Out == Node)
            {
                break;
            }
        }
    }
    return CurrentIndex;
}