SparseMatrix.h

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// Copyright Epic Games, Inc. All Rights Reserved.
#pragma once
 
#include "Chaos/Map.h"
#include "Chaos/VectorND.h"
 
namespace Chaos
{
template<class T>
class SparseMatrix
{
  public:
    SparseMatrix(const int32 size)
        : MSize(size) {}
    ~SparseMatrix() {}
 
    T operator()(const int32 i, const int32 j)
    {
        int32 Key = i * MSize + j;
        if (!MValues.Contains(Key))
        {
            MValues.Add(Key, T());
            if (!RowToIndicesMap.Contains(i))
            {
                RowToIndicesMap.Add(i, {j});
            }
            else
            {
                RowToIndicesMap[i].Add(j);
            }
        }
        return MValues[Key];
    }
    const T operator()(const int32 i, const int32 j) const
    {
        int32 Key = i * MSize + j;
        check(MValues.Contains(Key));
        return MValues[Key];
    }
    VectorND<T> operator*(const VectorND<T>& vector) const
    {
        check(vector.Num() == MSize);
        VectorND<T> result(MSize);
        for (int32 i = 0; i < MSize; ++i)
        {
            result[i] = 0;
            if (!RowToIndicesMap.Contains(i))
                continue;
            for (auto j : RowToIndicesMap[i])
            {
                result[i] += (*this)(i, j) * vector[j];
            }
        }
        return result;
    }
 
  private:
    int32 MSize;
    TMap<int32, TArray<int32>> RowToIndicesMap;
    TMap<int32, T> MValues;
};
}