Improvements to the sort routine

src/Microsoft.Data.Analysis/DataFrame.cs

@@ -335,7 +335,7 @@ public DataFrame Sample(int numberOfRows)
 
             int shuffleLowerLimit = 0;
             int shuffleUpperLimit = (int)Math.Min(Int32.MaxValue, Rows.Count);
-            
+
             int[] shuffleArray = Enumerable.Range(0, shuffleUpperLimit).ToArray();
             Random rand = new Random();
             while (shuffleLowerLimit < numberOfRows)
@@ -349,7 +349,7 @@ public DataFrame Sample(int numberOfRows)
             ArraySegment<int> segment = new ArraySegment<int>(shuffleArray, 0, shuffleLowerLimit);
 
             PrimitiveDataFrameColumn<int> indices = new PrimitiveDataFrameColumn<int>("indices", segment);
-            
+
             return Clone(indices);
         }
 
@@ -623,12 +623,16 @@ private void OnColumnsChanged()
         private DataFrame Sort(string columnName, bool isAscending)
         {
             DataFrameColumn column = Columns[columnName];
-            DataFrameColumn sortIndices = column.GetAscendingSortIndices();
+            PrimitiveDataFrameColumn<long> sortIndices = column.GetAscendingSortIndices(out Int64DataFrameColumn nullIndices);
+            for (long i = 0; i < nullIndices.Length; i++)
+            {
+                sortIndices.Append(nullIndices[i]);
+            }
             List<DataFrameColumn> newColumns = new List<DataFrameColumn>(Columns.Count);
             for (int i = 0; i < Columns.Count; i++)
             {
                 DataFrameColumn oldColumn = Columns[i];
-                DataFrameColumn newColumn = oldColumn.Clone(sortIndices, !isAscending, oldColumn.NullCount);
+                DataFrameColumn newColumn = oldColumn.Clone(sortIndices, !isAscending);
                 Debug.Assert(newColumn.NullCount == oldColumn.NullCount);
                 newColumns.Add(newColumn);
             }

src/Microsoft.Data.Analysis/DataFrameColumn.cs

@@ -331,6 +331,15 @@ public virtual StringDataFrameColumn Info()
         /// </summary>
         public virtual DataFrameColumn Description() => throw new NotImplementedException();
 
+        /// <summary>
+        /// Returns the indices of non-null values that, when applied, result in this column being sorted in ascending order. Also returns the indices of null values in <paramref name="nullIndices"/>.
+        /// </summary>
+        /// <param name="nullIndices">Indices of values that are <see langword="null"/>.</param>
+        internal virtual PrimitiveDataFrameColumn<long> GetAscendingSortIndices(out Int64DataFrameColumn nullIndices) => throw new NotImplementedException();
+
+        /// <summary>
+        /// Returns the indices of non-null values that, when applied, result in this column being sorted in ascending order
+        /// </summary>
         internal virtual PrimitiveDataFrameColumn<long> GetAscendingSortIndices() => throw new NotImplementedException();
 
         internal delegate long GetBufferSortIndex(int bufferIndex, int sortIndex);

src/Microsoft.Data.Analysis/PrimitiveDataFrameColumn.Sort.cs

@@ -21,29 +21,46 @@ public partial class PrimitiveDataFrameColumn<T> : DataFrameColumn
         internal override PrimitiveDataFrameColumn<long> GetAscendingSortIndices()
         {
             // The return sortIndices contains only the non null indices. 
-            GetSortIndices(Comparer<T>.Default, out PrimitiveDataFrameColumn<long> sortIndices);
+            Int64DataFrameColumn sortIndices = GetSortIndices(Comparer<T>.Default, out Int64DataFrameColumn _);
             return sortIndices;
         }
 
-        private void GetSortIndices(IComparer<T> comparer, out PrimitiveDataFrameColumn<long> columnSortIndices)
+        internal override PrimitiveDataFrameColumn<long> GetAscendingSortIndices(out Int64DataFrameColumn nullIndices)
+        {
+            Int64DataFrameColumn sortIndices = GetSortIndices(Comparer<T>.Default, out nullIndices);
+            return sortIndices;
+        }
+
+        private Int64DataFrameColumn GetSortIndices(IComparer<T> comparer, out Int64DataFrameColumn columnNullIndices)
         {
             List<List<int>> bufferSortIndices = new List<List<int>>(_columnContainer.Buffers.Count);
+            columnNullIndices = new Int64DataFrameColumn("NullIndices", NullCount);
+            long nullIndicesSlot = 0;
             // Sort each buffer first
             for (int b = 0; b < _columnContainer.Buffers.Count; b++)
             {
                 ReadOnlyDataFrameBuffer<T> buffer = _columnContainer.Buffers[b];
                 ReadOnlySpan<byte> nullBitMapSpan = _columnContainer.NullBitMapBuffers[b].ReadOnlySpan;
                 int[] sortIndices = new int[buffer.Length];
                 for (int i = 0; i < buffer.Length; i++)
+                {
                     sortIndices[i] = i;
+                }
                 IntrospectiveSort(buffer.ReadOnlySpan, buffer.Length, sortIndices, comparer);
                 // Bug fix: QuickSort is not stable. When PrimitiveDataFrameColumn has null values and default values, they move around
                 List<int> nonNullSortIndices = new List<int>();
                 for (int i = 0; i < sortIndices.Length; i++)
                 {
-                    if (_columnContainer.IsValid(nullBitMapSpan, sortIndices[i]))
+                    int localSortIndex = sortIndices[i];
+                    if (_columnContainer.IsValid(nullBitMapSpan, localSortIndex))
+                    {
                         nonNullSortIndices.Add(sortIndices[i]);
-
+                    }
+                    else
+                    {
+                        columnNullIndices[nullIndicesSlot] = localSortIndex + b * _columnContainer.Buffers[0].Length;
+                        nullIndicesSlot++;
+                    }
                 }
                 bufferSortIndices.Add(nonNullSortIndices);
             }
@@ -90,11 +107,13 @@ ValueTuple<T, int> GetFirstNonNullValueAndBufferIndexStartingAtIndex(int bufferI
                     heapOfValueAndListOfTupleOfSortAndBufferIndex.Add(valueAndBufferIndex.Item1, new List<ValueTuple<int, int>>() { (valueAndBufferIndex.Item2, i) });
                 }
             }
-            columnSortIndices = new PrimitiveDataFrameColumn<long>("SortIndices");
+            Int64DataFrameColumn columnSortIndices = new Int64DataFrameColumn("SortIndices");
             GetBufferSortIndex getBufferSortIndex = new GetBufferSortIndex((int bufferIndex, int sortIndex) => (bufferSortIndices[bufferIndex][sortIndex]) + bufferIndex * bufferSortIndices[0].Count);
             GetValueAndBufferSortIndexAtBuffer<T> getValueAndBufferSortIndexAtBuffer = new GetValueAndBufferSortIndexAtBuffer<T>((int bufferIndex, int sortIndex) => GetFirstNonNullValueAndBufferIndexStartingAtIndex(bufferIndex, sortIndex));
             GetBufferLengthAtIndex getBufferLengthAtIndex = new GetBufferLengthAtIndex((int bufferIndex) => bufferSortIndices[bufferIndex].Count);
             PopulateColumnSortIndicesWithHeap(heapOfValueAndListOfTupleOfSortAndBufferIndex, columnSortIndices, getBufferSortIndex, getValueAndBufferSortIndexAtBuffer, getBufferLengthAtIndex);
+
+            return columnSortIndices;
         }
     }
 }

src/Microsoft.Data.Analysis/StringDataFrameColumn.cs

@@ -177,19 +177,32 @@ public IEnumerator<string> GetEnumerator()
 
         internal override PrimitiveDataFrameColumn<long> GetAscendingSortIndices()
         {
-            GetSortIndices(Comparer<string>.Default, out PrimitiveDataFrameColumn<long> columnSortIndices);
+            PrimitiveDataFrameColumn<long> columnSortIndices = GetSortIndices(Comparer<string>.Default, out Int64DataFrameColumn _);
             return columnSortIndices;
         }
 
-        private void GetSortIndices(Comparer<string> comparer, out PrimitiveDataFrameColumn<long> columnSortIndices)
+        internal override PrimitiveDataFrameColumn<long> GetAscendingSortIndices(out Int64DataFrameColumn nullIndices)
+        {
+            PrimitiveDataFrameColumn<long> columnSortIndices = GetSortIndices(Comparer<string>.Default, out nullIndices);
+            return columnSortIndices;
+        }
+
+        private PrimitiveDataFrameColumn<long> GetSortIndices(Comparer<string> comparer, out Int64DataFrameColumn columnNullIndices)
         {
             List<int[]> bufferSortIndices = new List<int[]>(_stringBuffers.Count);
+            columnNullIndices = new Int64DataFrameColumn("NullIndices", NullCount);
+            long nullIndicesSlot = 0;
             foreach (List<string> buffer in _stringBuffers)
             {
                 var sortIndices = new int[buffer.Count];
                 for (int i = 0; i < buffer.Count; i++)
                 {
                     sortIndices[i] = i;
+                    if (buffer[i] == null)
+                    {
+                        columnNullIndices[nullIndicesSlot] = i + bufferSortIndices.Count * int.MaxValue;
+                        nullIndicesSlot++;
+                    }
                 }
                 // TODO: Refactor the sort routine to also work with IList?
                 string[] array = buffer.ToArray();
@@ -227,11 +240,12 @@ ValueTuple<string, int> GetFirstNonNullValueStartingAtIndex(int stringBufferInde
                     heapOfValueAndListOfTupleOfSortAndBufferIndex.Add(valueAndBufferSortIndex.Item1, new List<ValueTuple<int, int>>() { (valueAndBufferSortIndex.Item2, i) });
                 }
             }
-            columnSortIndices = new PrimitiveDataFrameColumn<long>("SortIndices");
+            PrimitiveDataFrameColumn<long> columnSortIndices = new PrimitiveDataFrameColumn<long>("SortIndices");
             GetBufferSortIndex getBufferSortIndex = new GetBufferSortIndex((int bufferIndex, int sortIndex) => (bufferSortIndices[bufferIndex][sortIndex]) + bufferIndex * bufferSortIndices[0].Length);
             GetValueAndBufferSortIndexAtBuffer<string> getValueAtBuffer = new GetValueAndBufferSortIndexAtBuffer<string>((int bufferIndex, int sortIndex) => GetFirstNonNullValueStartingAtIndex(bufferIndex, sortIndex));
             GetBufferLengthAtIndex getBufferLengthAtIndex = new GetBufferLengthAtIndex((int bufferIndex) => bufferSortIndices[bufferIndex].Length);
             PopulateColumnSortIndicesWithHeap(heapOfValueAndListOfTupleOfSortAndBufferIndex, columnSortIndices, getBufferSortIndex, getValueAtBuffer, getBufferLengthAtIndex);
+            return columnSortIndices;
         }
 
         public new StringDataFrameColumn Clone(DataFrameColumn mapIndices, bool invertMapIndices, long numberOfNullsToAppend)

test/Microsoft.Data.Analysis.Tests/DataFrameTests.cs

@@ -920,6 +920,43 @@ public void TestPrimitiveColumnSort(int numberOfNulls)
             Assert.Null(sortedIntColumn[9]);
         }
 
+        [Fact]
+        public void TestSortWithDifferentNullCountsInColumns()
+        {
+            DataFrame dataFrame = MakeDataFrameWithAllMutableColumnTypes(10);
+            dataFrame["Int"][3] = null;
+            dataFrame["String"][3] = null;
+            DataFrame sorted = dataFrame.OrderBy("Int");
+            void Verify(DataFrame sortedDataFrame)
+            {
+                Assert.Equal(10, sortedDataFrame.Rows.Count);
+                DataFrameRow lastRow = sortedDataFrame.Rows[sortedDataFrame.Rows.Count - 1];
+                DataFrameRow penultimateRow = sortedDataFrame.Rows[sortedDataFrame.Rows.Count - 2];
+                foreach (object value in lastRow)
+                {
+                    Assert.Null(value);
+                }
+
+                for (int i = 0; i < sortedDataFrame.Columns.Count; i++)
+                {
+                    string columnName = sortedDataFrame.Columns[i].Name;
+                    if (columnName != "String" && columnName != "Int")
+                    {
+                        Assert.Equal(dataFrame[columnName][3], penultimateRow[i]);
+                    }
+                    else if (columnName == "String" || columnName == "Int")
+                    {
+                        Assert.Null(penultimateRow[i]);
+                    }
+                }
+            }
+
+            Verify(sorted);
+
+            sorted = dataFrame.OrderBy("String");
+            Verify(sorted);
+        }
+
         private void VerifyJoin(DataFrame join, DataFrame left, DataFrame right, JoinAlgorithm joinAlgorithm)
         {
             Int64DataFrameColumn mapIndices = new Int64DataFrameColumn("map", join.Rows.Count);