// <copyright file="SparseCompressedRowMatrixStorage.cs" company="Math.NET">
// Math.NET Numerics, part of the Math.NET Project
// http://numerics.mathdotnet.com
// http://github.com/mathnet/mathnet-numerics
// http://mathnetnumerics.codeplex.com
//
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using System;
using System.Collections.Generic;
using System.Linq;
using MathNet.Numerics.Properties;

namespace MathNet.Numerics.LinearAlgebra.Storage
{
    [Serializable]
    public class SparseCompressedRowMatrixStorage<T> : MatrixStorage<T>
        where T : struct, IEquatable<T>, IFormattable
    {
        // [ruegg] public fields are OK here

        /// <summary>
        /// The array containing the row indices of the existing rows. Element "j" of the array gives the index of the 
        /// element in the <see cref="Values"/> array that is first non-zero element in a row "j"
        /// </summary>
        public readonly int[] RowPointers;

        /// <summary>
        /// An array containing the column indices of the non-zero values. Element "I" of the array 
        /// is the number of the column in matrix that contains the I-th value in the <see cref="Values"/> array.
        /// </summary>
        public int[] ColumnIndices;

        /// <summary>
        /// Array that contains the non-zero elements of matrix. Values of the non-zero elements of matrix are mapped into the values 
        /// array using the row-major storage mapping described in a compressed sparse row (CSR) format.
        /// </summary>
        public T[] Values;

        /// <summary>
        /// Gets the number of non zero elements in the matrix.
        /// </summary>
        /// <value>The number of non zero elements.</value>
        public int ValueCount;

        internal SparseCompressedRowMatrixStorage(int rows, int columns)
            : base(rows, columns)
        {
            RowPointers = new int[rows];
            ColumnIndices = new int[0];
            Values = new T[0];
            ValueCount = 0;
        }

        /// <summary>
        /// True if the matrix storage format is dense.
        /// </summary>
        public override bool IsDense
        {
            get { return false; }
        }

        /// <summary>
        /// True if all fields of this matrix can be set to any value.
        /// False if some fields are fixed, like on a diagonal matrix.
        /// </summary>
        public override bool IsFullyMutable
        {
            get { return true; }
        }

        /// <summary>
        /// True if the specified field can be set to any value.
        /// False if the field is fixed, like an off-diagonal field on a diagonal matrix.
        /// </summary>
        public override bool IsMutableAt(int row, int column)
        {
            return true;
        }

        /// <summary>
        /// Retrieves the requested element without range checking.
        /// </summary>
        /// <param name="row">
        /// The row of the element.
        /// </param>
        /// <param name="column">
        /// The column of the element.
        /// </param>
        /// <returns>
        /// The requested element.
        /// </returns>
        /// <remarks>Not range-checked.</remarks>
        public override T At(int row, int column)
        {
            var index = FindItem(row, column);
            return index >= 0 ? Values[index] : Zero;
        }

        /// <summary>
        /// Sets the element without range checking.
        /// </summary>
        /// <param name="row"> The row of the element. </param>
        /// <param name="column"> The column of the element. </param>
        /// <param name="value"> The value to set the element to. </param>
        /// <remarks>WARNING: This method is not thread safe. Use "lock" with it and be sure to avoid deadlocks.</remarks>
        public override void At(int row, int column, T value)
        {
            var index = FindItem(row, column);
            if (index >= 0)
            {
                // Non-zero item found in matrix
                if (Zero.Equals(value))
                {
                    // Delete existing item
                    RemoveAtIndexUnchecked(index, row);
                }
                else
                {
                    // Update item
                    Values[index] = value;
                }
            }
            else
            {
                // Item not found. Add new value
                if (Zero.Equals(value))
                {
                    return;
                }

                index = ~index;

                // Check if the storage needs to be increased
                if ((ValueCount == Values.Length) && (ValueCount < ((long)RowCount * ColumnCount)))
                {
                    // Value array is completely full so we increase the size
                    // Determine the increase in size. We will not grow beyond the size of the matrix
                    var size = Math.Min(Values.Length + GrowthSize(), (long)RowCount * ColumnCount);
                    if (size > int.MaxValue)
                    {
                        throw new NotSupportedException(Resources.TooManyElements);
                    }

                    Array.Resize(ref Values, (int)size);
                    Array.Resize(ref ColumnIndices, (int)size);
                }

                // Move all values (with a position larger than index) in the value array to the next position
                // move all values (with a position larger than index) in the columIndices array to the next position
                Array.Copy(Values, index, Values, index + 1, ValueCount - index);
                Array.Copy(ColumnIndices, index, ColumnIndices, index + 1, ValueCount - index);

                // Add the value and the column index
                Values[index] = value;
                ColumnIndices[index] = column;

                // increase the number of non-zero numbers by one
                ValueCount += 1;

                // add 1 to all the row indices for rows bigger than rowIndex
                // so that they point to the correct part of the value array again.
                for (var i = row + 1; i < RowPointers.Length; i++)
                {
                    RowPointers[i] += 1;
                }
            }
        }

        /// <summary>
        /// Delete value from internal storage
        /// </summary>
        /// <param name="itemIndex">Index of value in nonZeroValues array</param>
        /// <param name="row">Row number of matrix</param>
        /// <remarks>WARNING: This method is not thread safe. Use "lock" with it and be sure to avoid deadlocks</remarks>
        void RemoveAtIndexUnchecked(int itemIndex, int row)
        {
            // Move all values (with a position larger than index) in the value array to the previous position
            // move all values (with a position larger than index) in the columIndices array to the previous position
            Array.Copy(Values, itemIndex + 1, Values, itemIndex, ValueCount - itemIndex - 1);
            Array.Copy(ColumnIndices, itemIndex + 1, ColumnIndices, itemIndex, ValueCount - itemIndex - 1);

            // Decrease value in Row
            for (var i = row + 1; i < RowPointers.Length; i++)
            {
                RowPointers[i] -= 1;
            }

            ValueCount -= 1;

            // Check whether we need to shrink the arrays. This is reasonable to do if 
            // there are a lot of non-zero elements and storage is two times bigger
            if ((ValueCount > 1024) && (ValueCount < Values.Length / 2))
            {
                Array.Resize(ref Values, ValueCount);
                Array.Resize(ref ColumnIndices, ValueCount);
            }
        }

        /// <summary>
        /// Find item Index in nonZeroValues array
        /// </summary>
        /// <param name="row">Matrix row index</param>
        /// <param name="column">Matrix column index</param>
        /// <returns>Item index</returns>
        /// <remarks>WARNING: This method is not thread safe. Use "lock" with it and be sure to avoid deadlocks</remarks>
        public int FindItem(int row, int column)
        {
            // Determin bounds in columnIndices array where this item should be searched (using rowIndex)
            var startIndex = RowPointers[row];
            var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
            return Array.BinarySearch(ColumnIndices, startIndex, endIndex - startIndex, column);
        }

        /// <summary>
        /// Calculates the amount with which to grow the storage array's if they need to be
        /// increased in size.
        /// </summary>
        /// <returns>The amount grown.</returns>
        int GrowthSize()
        {
            int delta;
            if (Values.Length > 1024)
            {
                delta = Values.Length / 4;
            }
            else
            {
                if (Values.Length > 256)
                {
                    delta = 512;
                }
                else
                {
                    delta = Values.Length > 64 ? 128 : 32;
                }
            }

            return delta;
        }

        public override void Clear()
        {
            ValueCount = 0;
            Array.Clear(RowPointers, 0, RowPointers.Length);
        }

        public override void Clear(int rowIndex, int rowCount, int columnIndex, int columnCount)
        {
            if (rowIndex == 0 && columnIndex == 0 && rowCount == RowCount && columnCount == ColumnCount)
            {
                Clear();
                return;
            }

            for (int row = rowIndex + rowCount - 1; row >= rowIndex; row--)
            {
                var startIndex = RowPointers[row];
                var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;

                // empty row
                if (startIndex == endIndex)
                {
                    continue;
                }

                // multiple entries in row
                var first = Array.BinarySearch(ColumnIndices, startIndex, endIndex - startIndex, columnIndex);
                var last = Array.BinarySearch(ColumnIndices, startIndex, endIndex - startIndex, columnIndex + columnCount - 1);
                if (first < 0) first = ~first;
                if (last < 0) last = ~last - 1;
                int count = last - first + 1;

                if (count > 0)
                {
                    // Move all values (with a position larger than index) in the value array to the previous position
                    // move all values (with a position larger than index) in the columIndices array to the previous position
                    Array.Copy(Values, first + count, Values, first, ValueCount - first - count);
                    Array.Copy(ColumnIndices, first + count, ColumnIndices, first, ValueCount - first - count);

                    // Decrease value in Row
                    for (var k = row + 1; k < RowPointers.Length; k++)
                    {
                        RowPointers[k] -= count;
                    }

                    ValueCount -= count;
                }
            }

            // Check whether we need to shrink the arrays. This is reasonable to do if
            // there are a lot of non-zero elements and storage is two times bigger
            if ((ValueCount > 1024) && (ValueCount < Values.Length / 2))
            {
                Array.Resize(ref Values, ValueCount);
                Array.Resize(ref ColumnIndices, ValueCount);
            }
        }

        /// <summary>
        /// Indicates whether the current object is equal to another object of the same type.
        /// </summary>
        /// <param name="other">
        /// An object to compare with this object.
        /// </param>
        /// <returns>
        /// <c>true</c> if the current object is equal to the <paramref name="other"/> parameter; otherwise, <c>false</c>.
        /// </returns>
        public override bool Equals(MatrixStorage<T> other)
        {
            // Reject equality when the argument is null or has a different shape.
            if (other == null || ColumnCount != other.ColumnCount || RowCount != other.RowCount)
            {
                return false;
            }

            // Accept if the argument is the same object as this.
            if (ReferenceEquals(this, other))
            {
                return true;
            }

            var sparse = other as SparseCompressedRowMatrixStorage<T>;
            if (sparse == null)
            {
                return base.Equals(other);
            }

            if (ValueCount != sparse.ValueCount)
            {
                // TODO: this is not always correct
                return false;
            }

            // If all else fails, perform element wise comparison.
            for (var index = 0; index < ValueCount; index++)
            {
                // TODO: AlmostEquals
                if (!Values[index].Equals(sparse.Values[index]) || ColumnIndices[index] != sparse.ColumnIndices[index])
                {
                    return false;
                }
            }

            return true;
        }

        /// <summary>
        /// Returns a hash code for this instance.
        /// </summary>
        /// <returns>
        /// A hash code for this instance, suitable for use in hashing algorithms and data structures like a hash table. 
        /// </returns>
        public override int GetHashCode()
        {
            var values = Values;
            var hashNum = Math.Min(ValueCount, 25);
            int hash = 17;
            unchecked
            {
                for (var i = 0; i < hashNum; i++)
                {
                    hash = hash * 31 + values[i].GetHashCode();
                }
            }
            return hash;
        }

        // INITIALIZATION

        public static SparseCompressedRowMatrixStorage<T> OfMatrix(MatrixStorage<T> matrix)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(matrix.RowCount, matrix.ColumnCount);
            matrix.CopyToUnchecked(storage, skipClearing: true);
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfInit(int rows, int columns, Func<int, int, T> init)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int row = 0; row < rows; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < columns; col++)
                {
                    var x = init(row, col);
                    if (!Zero.Equals(x))
                    {
                        values.Add(x);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfDiagonalInit(int rows, int columns, Func<int, T> init)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int i = 0; i < Math.Min(rows, columns); i++)
            {
                rowPointers[i] = values.Count;
                var x = init(i);
                if (!Zero.Equals(x))
                {
                    values.Add(x);
                    columnIndices.Add(i);
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfArray(T[,] array)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(array.GetLength(0), array.GetLength(1));
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int row = 0; row < storage.RowCount; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < storage.ColumnCount; col++)
                {
                    if (!Zero.Equals(array[row,col]))
                    {
                        values.Add(array[row, col]);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfRowArrays(T[][] data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(data.Length, data[0].Length);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int row = 0; row < storage.RowCount; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < storage.ColumnCount; col++)
                {
                    T x = data[row][col];
                    if (!Zero.Equals(x))
                    {
                        values.Add(x);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfColumnArrays(T[][] data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(data[0].Length, data.Length);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int row = 0; row < storage.RowCount; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < storage.ColumnCount; col++)
                {
                    T x = data[col][row];
                    if (!Zero.Equals(x))
                    {
                        values.Add(x);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfRowVectors(VectorStorage<T>[] data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(data.Length, data[0].Length);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            // TODO PERF: Optimize for sparse and dense cases
            for (int row = 0; row < storage.RowCount; row++)
            {
                var vector = data[row];
                rowPointers[row] = values.Count;
                for (int col = 0; col < storage.ColumnCount; col++)
                {
                    var x = vector.At(col);
                    if (!Zero.Equals(x))
                    {
                        values.Add(x);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfColumnVectors(VectorStorage<T>[] data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(data[0].Length, data.Length);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            // TODO PERF: Optimize for sparse and dense cases
            for (int row = 0; row < storage.RowCount; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < storage.ColumnCount; col++)
                {
                    var x = data[col].At(row);
                    if (!Zero.Equals(x))
                    {
                        values.Add(x);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfIndexedEnumerable(int rows, int columns, IEnumerable<Tuple<int, int, T>> data)
        {
            var trows = new List<Tuple<int, T>>[rows];
            foreach (var item in data)
            {
                if (!Zero.Equals(item.Item3))
                {
                    var row = trows[item.Item1] ?? (trows[item.Item1] = new List<Tuple<int, T>>());
                    row.Add(new Tuple<int, T>(item.Item2, item.Item3));
                }
            }

            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            int index = 0;
            for (int row = 0; row < rows; row++)
            {
                rowPointers[row] = index;
                var trow = trows[row];
                if (trow != null)
                {
                    trow.Sort();
                    foreach (var item in trow)
                    {
                        values.Add(item.Item2);
                        columnIndices.Add(item.Item1);
                        index++;
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfRowEnumerables(int rows, int columns, IEnumerable<IEnumerable<T>> data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            using (var rowIterator = data.GetEnumerator())
            {
                for (int row = 0; row < rows; row++)
                {
                    if (!rowIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, rows));
                    rowPointers[row] = values.Count;
                    using (var columnIterator = rowIterator.Current.GetEnumerator())
                    {
                        for (int col = 0; col < columns; col++)
                        {
                            if (!columnIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, columns));
                            if (!Zero.Equals(columnIterator.Current))
                            {
                                values.Add(columnIterator.Current);
                                columnIndices.Add(col);
                            }
                        }
                        if (columnIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, columns));
                    }
                }
                if (rowIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, rows));
            }
            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfColumnEnumerables(int rows, int columns, IEnumerable<IEnumerable<T>> data)
        {
            var trows = new List<Tuple<int, T>>[rows];
            using (var columnIterator = data.GetEnumerator())
            {
                for (int column = 0; column < columns; column++)
                {
                    if (!columnIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, columns));
                    using (var rowIterator = columnIterator.Current.GetEnumerator())
                    {
                        for (int row = 0; row < rows; row++)
                        {
                            if (!rowIterator.MoveNext()) throw new ArgumentOutOfRangeException("data", string.Format(Resources.ArgumentArrayWrongLength, rows));
                            if (!Zero.Equals(rowIterator.Current))
                            {
                                var trow = trows[row] ?? (trows[row] = new List<Tuple<int, T>>());
                                trow.Add(new Tuple<int, T>(column, rowIterator.Current));
                            }
                        }
                    }
                }
            }

            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            int index = 0;
            for (int row = 0; row < rows; row++)
            {
                rowPointers[row] = index;
                var trow = trows[row];
                if (trow != null)
                {
                    trow.Sort();
                    foreach (var item in trow)
                    {
                        values.Add(item.Item2);
                        columnIndices.Add(item.Item1);
                        index++;
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfRowMajorEnumerable(int rows, int columns, IEnumerable<T> data)
        {
            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            using (var iterator = data.GetEnumerator())
            {
                for (int row = 0; row < rows; row++)
                {
                    rowPointers[row] = values.Count;
                    for (int col = 0; col < columns; col++)
                    {
                        iterator.MoveNext();
                        if (!Zero.Equals(iterator.Current))
                        {
                            values.Add(iterator.Current);
                            columnIndices.Add(col);
                        }
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        public static SparseCompressedRowMatrixStorage<T> OfColumnMajorList(int rows, int columns, IList<T> data)
        {
            if (rows * columns != data.Count)
            {
                throw new ArgumentOutOfRangeException(Resources.ArgumentMatrixDimensions);
            }

            var storage = new SparseCompressedRowMatrixStorage<T>(rows, columns);
            var rowPointers = storage.RowPointers;
            var columnIndices = new List<int>();
            var values = new List<T>();

            for (int row = 0; row < rows; row++)
            {
                rowPointers[row] = values.Count;
                for (int col = 0; col < columns; col++)
                {
                    var item = data[row + (col*rows)];
                    if (!Zero.Equals(item))
                    {
                        values.Add(item);
                        columnIndices.Add(col);
                    }
                }
            }

            storage.ColumnIndices = columnIndices.ToArray();
            storage.Values = values.ToArray();
            storage.ValueCount = values.Count;
            return storage;
        }

        // MATRIX COPY

        internal override void CopyToUnchecked(MatrixStorage<T> target, bool skipClearing = false)
        {
            var sparseTarget = target as SparseCompressedRowMatrixStorage<T>;
            if (sparseTarget != null)
            {
                CopyToUnchecked(sparseTarget);
                return;
            }

            var denseTarget = target as DenseColumnMajorMatrixStorage<T>;
            if (denseTarget != null)
            {
                CopyToUnchecked(denseTarget, skipClearing);
                return;
            }

            // FALL BACK

            if (!skipClearing)
            {
                target.Clear();
            }

            if (ValueCount != 0)
            {
                for (int row = 0; row < RowCount; row++)
                {
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        target.At(row, ColumnIndices[j], Values[j]);
                    }
                }
            }
        }

        void CopyToUnchecked(SparseCompressedRowMatrixStorage<T> target)
        {
            target.ValueCount = ValueCount;
            target.Values = new T[ValueCount];
            target.ColumnIndices = new int[ValueCount];

            if (ValueCount != 0)
            {
                Array.Copy(Values, target.Values, ValueCount);
                Buffer.BlockCopy(ColumnIndices, 0, target.ColumnIndices, 0, ValueCount * Constants.SizeOfInt);
                Buffer.BlockCopy(RowPointers, 0, target.RowPointers, 0, RowCount * Constants.SizeOfInt);
            }
        }

        void CopyToUnchecked(DenseColumnMajorMatrixStorage<T> target, bool skipClearing)
        {
            if (!skipClearing)
            {
                target.Clear();
            }

            if (ValueCount != 0)
            {
                for (int row = 0; row < RowCount; row++)
                {
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        target.At(row, ColumnIndices[j], Values[j]);
                    }
                }
            }
        }

        internal override void CopySubMatrixToUnchecked(MatrixStorage<T> target,
            int sourceRowIndex, int targetRowIndex, int rowCount,
            int sourceColumnIndex, int targetColumnIndex, int columnCount,
            bool skipClearing = false)
        {
            if (target == null)
            {
                throw new ArgumentNullException("target");
            }

            var sparseTarget = target as SparseCompressedRowMatrixStorage<T>;
            if (sparseTarget != null)
            {
                CopySubMatrixToUnchecked(sparseTarget, 
                    sourceRowIndex, targetRowIndex, rowCount,
                    sourceColumnIndex, targetColumnIndex, columnCount,
                    skipClearing);
                return;
            }

            // FALL BACK

            if (!skipClearing)
            {
                target.Clear(targetRowIndex, rowCount, targetColumnIndex, columnCount);
            }

            for (int i = sourceRowIndex, row = 0; i < sourceRowIndex + rowCount; i++, row++)
            {
                var startIndex = RowPointers[i];
                var endIndex = i < RowPointers.Length - 1 ? RowPointers[i + 1] : ValueCount;

                for (int j = startIndex; j < endIndex; j++)
                {
                    // check if the column index is in the range
                    if ((ColumnIndices[j] >= sourceColumnIndex) && (ColumnIndices[j] < sourceColumnIndex + columnCount))
                    {
                        var column = ColumnIndices[j] - sourceColumnIndex;
                        target.At(targetRowIndex + row, targetColumnIndex + column, Values[j]);
                    }
                }
            }
        }

        void CopySubMatrixToUnchecked(SparseCompressedRowMatrixStorage<T> target,
            int sourceRowIndex, int targetRowIndex, int rowCount,
            int sourceColumnIndex, int targetColumnIndex, int columnCount,
            bool skipClearing)
        {
            var rowOffset = targetRowIndex - sourceRowIndex;
            var columnOffset = targetColumnIndex - sourceColumnIndex;

            // special case for empty target - much faster
            if (target.ValueCount == 0)
            {
                // note: ValueCount is maximum resulting ValueCount (just using max to avoid internal copying)
                // resulting arrays will likely be smaller - unless all values fit in the chosen range.
                var values = new List<T>(ValueCount);
                var columnIndices = new List<int>(ValueCount);
                var rowPointers = target.RowPointers;

                for (int i = sourceRowIndex, row = 0; i < sourceRowIndex + rowCount; i++, row++)
                {
                    rowPointers[i + rowOffset] = values.Count;

                    var startIndex = RowPointers[i];
                    var endIndex = i < RowPointers.Length - 1 ? RowPointers[i + 1] : ValueCount;

                    // note: we might be able to replace this loop with Array.Copy (perf)
                    for (int j = startIndex; j < endIndex; j++)
                    {
                        // check if the column index is in the range
                        if ((ColumnIndices[j] >= sourceColumnIndex) && (ColumnIndices[j] < sourceColumnIndex + columnCount))
                        {
                            values.Add(Values[j]);
                            columnIndices.Add(ColumnIndices[j] + columnOffset);
                        }
                    }
                }

                for(int i=targetRowIndex + rowCount; i<rowPointers.Length; i++)
                {
                    rowPointers[i] = values.Count;
                }

                target.ValueCount = values.Count;
                target.Values = values.ToArray();
                target.ColumnIndices = columnIndices.ToArray();

                return;
            }

            if (!skipClearing)
            {
                target.Clear(targetRowIndex, rowCount, targetColumnIndex, columnCount);
            }

            // NOTE: potential for more efficient implementation
            for (int i = sourceRowIndex, row = 0; i < sourceRowIndex + rowCount; i++, row++)
            {
                var startIndex = RowPointers[i];
                var endIndex = i < RowPointers.Length - 1 ? RowPointers[i + 1] : ValueCount;

                for (int j = startIndex; j < endIndex; j++)
                {
                    // check if the column index is in the range
                    if ((ColumnIndices[j] >= sourceColumnIndex) && (ColumnIndices[j] < sourceColumnIndex + columnCount))
                    {
                        var column = ColumnIndices[j] - sourceColumnIndex;
                        target.At(targetRowIndex + row, targetColumnIndex + column, Values[j]);
                    }
                }
            }
        }

        // ROW COPY

        internal override void CopySubRowToUnchecked(VectorStorage<T> target, int rowIndex,
            int sourceColumnIndex, int targetColumnIndex, int columnCount,
            bool skipClearing = false)
        {
            if (!skipClearing)
            {
                target.Clear(targetColumnIndex, columnCount);
            }

            // Determine bounds in columnIndices array where this item should be searched (using rowIndex)
            var startIndex = RowPointers[rowIndex];
            var endIndex = rowIndex < RowPointers.Length - 1 ? RowPointers[rowIndex + 1] : ValueCount;

            if (startIndex == endIndex)
            {
                return;
            }

            // If there are non-zero elements use base class implementation
            for (int i = sourceColumnIndex, j = 0; i < sourceColumnIndex + columnCount; i++, j++)
            {
                var index = FindItem(rowIndex, i);
                target.At(j, index >= 0 ? Values[index] : Zero);
            }
        }

        // EXTRACT

        public override T[] ToRowMajorArray()
        {
            var ret = new T[RowCount * ColumnCount];
            if (ValueCount != 0)
            {
                for (int row = 0; row < RowCount; row++)
                {
                    var offset = row * ColumnCount;
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        ret[offset + ColumnIndices[j]] = Values[j];
                    }
                }
            }
            return ret;
        }

        public override T[] ToColumnMajorArray()
        {
            var ret = new T[RowCount * ColumnCount];
            if (ValueCount != 0)
            {
                for (int row = 0; row < RowCount; row++)
                {
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        ret[(ColumnIndices[j]) * RowCount + row] = Values[j];
                    }
                }
            }
            return ret;
        }

        public override T[,] ToArray()
        {
            var ret = new T[RowCount, ColumnCount];
            if (ValueCount != 0)
            {
                for (int row = 0; row < RowCount; row++)
                {
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        ret[row, ColumnIndices[j]] = Values[j];
                    }
                }
            }
            return ret;
        }

        // ENUMERATION

        public override IEnumerable<T> Enumerate()
        {
            int k = 0;
            for (int row = 0; row < RowCount; row++)
            {
                for (int col = 0; col < ColumnCount; col++)
                {
                    yield return k < (row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount) && (ColumnIndices[k]) == col
                        ? Values[k++]
                        : Zero;
                }
            }
        }

        public override IEnumerable<Tuple<int, int, T>> EnumerateIndexed()
        {
            int k = 0;
            for (int row = 0; row < RowCount; row++)
            {
                for (int col = 0; col < ColumnCount; col++)
                {
                    yield return k < (row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount) && (ColumnIndices[k]) == col
                        ? new Tuple<int, int, T>(row, col, Values[k++])
                        : new Tuple<int, int, T>(row, col, Zero);
                }
            }
        }

        public override IEnumerable<T> EnumerateNonZero()
        {
            return Values.Take(ValueCount).Where(x => !Zero.Equals(x));
        }

        public override IEnumerable<Tuple<int, int, T>> EnumerateNonZeroIndexed()
        {
            for (int row = 0; row < RowCount; row++)
            {
                var startIndex = RowPointers[row];
                var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                for (var j = startIndex; j < endIndex; j++)
                {
                    if (!Zero.Equals(Values[j]))
                    {
                        yield return new Tuple<int, int, T>(row, ColumnIndices[j], Values[j]);
                    }
                }
            }
        }

        // FUNCTIONAL COMBINATORS

        public override void MapInplace(Func<T, T> f, bool forceMapZeros = false)
        {
            var newRowPointers = new int[RowCount];
            var newColumnIndices = new List<int>();
            var newValues = new List<T>();

            if (forceMapZeros || !Zero.Equals(f(Zero)))
            {
                int k = 0;
                for (int row = 0; row < RowCount; row++)
                {
                    newRowPointers[row] = newValues.Count;
                    for (int col = 0; col < ColumnCount; col++)
                    {
                        var item = k < (row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount) && (ColumnIndices[k]) == col
                            ? f(Values[k++])
                            : f(Zero);
                        if (!Zero.Equals(item))
                        {
                            newValues.Add(item);
                            newColumnIndices.Add(col);
                        }
                    }
                }
            }
            else
            {
                for (int row = 0; row < RowCount; row++)
                {
                    newRowPointers[row] = newValues.Count;
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        var item = f(Values[j]);
                        if (!Zero.Equals(item))
                        {
                            newValues.Add(item);
                            newColumnIndices.Add(ColumnIndices[j]);
                        }
                    }
                }
            }

            ColumnIndices = newColumnIndices.ToArray();
            Values = newValues.ToArray();
            ValueCount = newValues.Count;
            Array.Copy(newRowPointers, RowPointers, RowCount);
        }

        public override void MapIndexedInplace(Func<int, int, T, T> f, bool forceMapZeros = false)
        {
            var newRowPointers = new int[RowCount];
            var newColumnIndices = new List<int>();
            var newValues = new List<T>();

            if (forceMapZeros || !Zero.Equals(f(0,0,Zero)))
            {
                int k = 0;
                for (int row = 0; row < RowCount; row++)
                {
                    newRowPointers[row] = newValues.Count;
                    for (int col = 0; col < ColumnCount; col++)
                    {
                        var item = k < (row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount) && (ColumnIndices[k]) == col
                            ? f(row, col, Values[k++])
                            : f(row, col, Zero);
                        if (!Zero.Equals(item))
                        {
                            newValues.Add(item);
                            newColumnIndices.Add(col);
                        }
                    }
                }
            }
            else
            {
                for (int row = 0; row < RowCount; row++)
                {
                    newRowPointers[row] = newValues.Count;
                    var startIndex = RowPointers[row];
                    var endIndex = row < RowPointers.Length - 1 ? RowPointers[row + 1] : ValueCount;
                    for (var j = startIndex; j < endIndex; j++)
                    {
                        var item = f(row, ColumnIndices[j], Values[j]);
                        if (!Zero.Equals(item))
                        {
                            newValues.Add(item);
                            newColumnIndices.Add(ColumnIndices[j]);
                        }
                    }
                }
            }

            ColumnIndices = newColumnIndices.ToArray();
            Values = newValues.ToArray();
            ValueCount = newValues.Count;
            Array.Copy(newRowPointers, RowPointers, RowCount);
        }
    }
}