internal override void Map2ToUnchecked(VectorStorage <T> target, VectorStorage <T> other, Func <T, T, T> f, Zeros zeros, ExistingData existingData)
        {
            var processZeros = zeros == Zeros.Include || !Zero.Equals(f(Zero, Zero));

            var denseTarget = target as DenseVectorStorage <T>;
            var denseOther  = other as DenseVectorStorage <T>;

            if (denseTarget == null && (denseOther != null || processZeros))
            {
                // The handling is effectively dense but we're supposed to push
                // to a sparse target. Let's use a dense target instead,
                // then copy it normalized back to the sparse target.
                var intermediate = new DenseVectorStorage <T>(target.Length);
                Map2ToUnchecked(intermediate, other, f, zeros, ExistingData.AssumeZeros);
                intermediate.CopyTo(target, existingData);
                return;
            }

            if (denseOther != null)
            {
                T[] targetData = denseTarget.Data;
                T[] otherData  = denseOther.Data;

                int k = 0;
                for (int i = 0; i < otherData.Length; i++)
                {
                    if (k < ValueCount && Indices[k] == i)
                    {
                        targetData[i] = f(Values[k], otherData[i]);
                        k++;
                    }
                    else
                    {
                        targetData[i] = f(Zero, otherData[i]);
                    }
                }

                return;
            }

            var sparseOther = other as SparseVectorStorage <T>;

            if (sparseOther != null && denseTarget != null)
            {
                T[]   targetData      = denseTarget.Data;
                int[] otherIndices    = sparseOther.Indices;
                T[]   otherValues     = sparseOther.Values;
                int   otherValueCount = sparseOther.ValueCount;

                if (processZeros)
                {
                    int p = 0, q = 0;
                    for (int i = 0; i < targetData.Length; i++)
                    {
                        var left  = p < ValueCount && Indices[p] == i ? Values[p++] : Zero;
                        var right = q < otherValueCount && otherIndices[q] == i ? otherValues[q++] : Zero;
                        targetData[i] = f(left, right);
                    }
                }
                else
                {
                    if (existingData == ExistingData.Clear)
                    {
                        denseTarget.Clear();
                    }

                    int p = 0, q = 0;
                    while (p < ValueCount || q < otherValueCount)
                    {
                        if (q >= otherValueCount || p < ValueCount && Indices[p] < otherIndices[q])
                        {
                            targetData[Indices[p]] = f(Values[p], Zero);
                            p++;
                        }
                        else if (p >= ValueCount || q < otherValueCount && Indices[p] > otherIndices[q])
                        {
                            targetData[otherIndices[q]] = f(Zero, otherValues[q]);
                            q++;
                        }
                        else
                        {
                            Debug.Assert(Indices[p] == otherIndices[q]);
                            targetData[Indices[p]] = f(Values[p], otherValues[q]);
                            p++;
                            q++;
                        }
                    }
                }

                return;
            }

            if (sparseOther != null && target is SparseVectorStorage <T> sparseTarget)
            {
                var   indices         = new List <int>();
                var   values          = new List <T>();
                int[] otherIndices    = sparseOther.Indices;
                T[]   otherValues     = sparseOther.Values;
                int   otherValueCount = sparseOther.ValueCount;

                int p = 0, q = 0;
                while (p < ValueCount || q < otherValueCount)
                {
                    if (q >= otherValueCount || p < ValueCount && Indices[p] < otherIndices[q])
                    {
                        var value = f(Values[p], Zero);
                        if (!Zero.Equals(value))
                        {
                            indices.Add(Indices[p]);
                            values.Add(value);
                        }

                        p++;
                    }
                    else if (p >= ValueCount || q < otherValueCount && Indices[p] > otherIndices[q])
                    {
                        var value = f(Zero, otherValues[q]);
                        if (!Zero.Equals(value))
                        {
                            indices.Add(otherIndices[q]);
                            values.Add(value);
                        }

                        q++;
                    }
                    else
                    {
                        var value = f(Values[p], otherValues[q]);
                        if (!Zero.Equals(value))
                        {
                            indices.Add(Indices[p]);
                            values.Add(value);
                        }

                        p++;
                        q++;
                    }
                }

                sparseTarget.Indices    = indices.ToArray();
                sparseTarget.Values     = values.ToArray();
                sparseTarget.ValueCount = values.Count;
                return;
            }

            // FALL BACK

            base.Map2ToUnchecked(target, other, f, zeros, existingData);
        }
        internal override Tuple <int, T, TOther> Find2Unchecked <TOther>(VectorStorage <TOther> other, Func <T, TOther, bool> predicate, Zeros zeros)
        {
            if (other is DenseVectorStorage <TOther> denseOther)
            {
                TOther[] otherData = denseOther.Data;
                int      k         = 0;
                for (int i = 0; i < otherData.Length; i++)
                {
                    if (k < ValueCount && Indices[k] == i)
                    {
                        if (predicate(Values[k], otherData[i]))
                        {
                            return(new Tuple <int, T, TOther>(i, Values[k], otherData[i]));
                        }
                        k++;
                    }
                    else
                    {
                        if (predicate(Zero, otherData[i]))
                        {
                            return(new Tuple <int, T, TOther>(i, Zero, otherData[i]));
                        }
                    }
                }
                return(null);
            }

            if (other is SparseVectorStorage <TOther> sparseOther)
            {
                int[]    otherIndices    = sparseOther.Indices;
                TOther[] otherValues     = sparseOther.Values;
                int      otherValueCount = sparseOther.ValueCount;
                TOther   otherZero       = BuilderInstance <TOther> .Matrix.Zero;

                // Full Scan
                int k = 0, otherk = 0;
                if (zeros == Zeros.Include && ValueCount < Length && sparseOther.ValueCount < Length && predicate(Zero, otherZero))
                {
                    for (int i = 0; i < Length; i++)
                    {
                        var left  = k < ValueCount && Indices[k] == i ? Values[k++] : Zero;
                        var right = otherk < otherValueCount && otherIndices[otherk] == i ? otherValues[otherk++] : otherZero;
                        if (predicate(left, right))
                        {
                            return(new Tuple <int, T, TOther>(i, left, right));
                        }
                    }
                    return(null);
                }

                // Sparse Scan
                k      = 0;
                otherk = 0;
                while (k < ValueCount || otherk < otherValueCount)
                {
                    if (k == ValueCount || otherk < otherValueCount && Indices[k] > otherIndices[otherk])
                    {
                        if (predicate(Zero, otherValues[otherk++]))
                        {
                            return(new Tuple <int, T, TOther>(otherIndices[otherk - 1], Zero, otherValues[otherk - 1]));
                        }
                    }
                    else if (otherk == otherValueCount || Indices[k] < otherIndices[otherk])
                    {
                        if (predicate(Values[k++], otherZero))
                        {
                            return(new Tuple <int, T, TOther>(Indices[k - 1], Values[k - 1], otherZero));
                        }
                    }
                    else
                    {
                        if (predicate(Values[k++], otherValues[otherk++]))
                        {
                            return(new Tuple <int, T, TOther>(Indices[k - 1], Values[k - 1], otherValues[otherk - 1]));
                        }
                    }
                }
                return(null);
            }

            // FALL BACK

            return(base.Find2Unchecked(other, predicate, zeros));
        }
        internal override void MapIndexedToUnchecked <TU>(VectorStorage <TU> target, Func <int, T, TU> f, Zeros zeros, ExistingData existingData)
        {
            if (target is SparseVectorStorage <TU> sparseTarget)
            {
                var indices = new List <int>();
                var values  = new List <TU>();
                if (zeros == Zeros.Include || !Zero.Equals(f(0, Zero)))
                {
                    int k = 0;
                    for (int i = 0; i < Length; i++)
                    {
                        var item = k < ValueCount && (Indices[k]) == i?f(i, Values[k++]) : f(i, Zero);

                        if (!Zero.Equals(item))
                        {
                            values.Add(item);
                            indices.Add(i);
                        }
                    }
                }
                else
                {
                    for (int i = 0; i < ValueCount; i++)
                    {
                        var item = f(Indices[i], Values[i]);
                        if (!Zero.Equals(item))
                        {
                            values.Add(item);
                            indices.Add(Indices[i]);
                        }
                    }
                }
                sparseTarget.Indices    = indices.ToArray();
                sparseTarget.Values     = values.ToArray();
                sparseTarget.ValueCount = values.Count;
                return;
            }

            if (target is DenseVectorStorage <TU> denseTarget)
            {
                if (existingData == ExistingData.Clear)
                {
                    denseTarget.Clear();
                }

                if (zeros == Zeros.Include || !Zero.Equals(f(0, Zero)))
                {
                    int k = 0;
                    for (int i = 0; i < Length; i++)
                    {
                        denseTarget.Data[i] = k < ValueCount && (Indices[k]) == i
                            ? f(i, Values[k++])
                            : f(i, Zero);
                    }
                }
                else
                {
                    CommonParallel.For(0, ValueCount, 4096, (a, b) =>
                    {
                        for (int i = a; i < b; i++)
                        {
                            denseTarget.Data[Indices[i]] = f(Indices[i], Values[i]);
                        }
                    });
                }
                return;
            }

            // FALL BACK

            base.MapIndexedToUnchecked(target, f, zeros, existingData);
        }
        // FUNCTIONAL COMBINATORS: MAP

        internal override TState Fold2Unchecked <TOther, TState>(VectorStorage <TOther> other, Func <TState, T, TOther, TState> f, TState state, Zeros zeros)
        {
            if (other is SparseVectorStorage <TOther> sparseOther)
            {
                int[]    otherIndices    = sparseOther.Indices;
                TOther[] otherValues     = sparseOther.Values;
                int      otherValueCount = sparseOther.ValueCount;
                TOther   otherZero       = BuilderInstance <TOther> .Vector.Zero;

                if (zeros == Zeros.Include)
                {
                    int p = 0, q = 0;
                    for (int i = 0; i < Length; i++)
                    {
                        var left  = p < ValueCount && Indices[p] == i ? Values[p++] : Zero;
                        var right = q < otherValueCount && otherIndices[q] == i ? otherValues[q++] : otherZero;
                        state = f(state, left, right);
                    }
                }
                else
                {
                    int p = 0, q = 0;
                    while (p < ValueCount || q < otherValueCount)
                    {
                        if (q >= otherValueCount || p < ValueCount && Indices[p] < otherIndices[q])
                        {
                            state = f(state, Values[p], otherZero);
                            p++;
                        }
                        else if (p >= ValueCount || q < otherValueCount && Indices[p] > otherIndices[q])
                        {
                            state = f(state, Zero, otherValues[q]);
                            q++;
                        }
                        else
                        {
                            Debug.Assert(Indices[p] == otherIndices[q]);
                            state = f(state, Values[p], otherValues[q]);
                            p++;
                            q++;
                        }
                    }
                }

                return(state);
            }

            if (other is DenseVectorStorage <TOther> denseOther)
            {
                TOther[] otherData = denseOther.Data;

                int k = 0;
                for (int i = 0; i < otherData.Length; i++)
                {
                    if (k < ValueCount && Indices[k] == i)
                    {
                        state = f(state, Values[k], otherData[i]);
                        k++;
                    }
                    else
                    {
                        state = f(state, Zero, otherData[i]);
                    }
                }

                return(state);
            }

            return(base.Fold2Unchecked(other, f, state, zeros));
        }