Move RowCursor back

Author: Prashanth Govindarajan

src/Microsoft.Data.Analysis/DataFrame.IDataView.cs

@@ -4,7 +4,9 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using Microsoft.ML;
+using Microsoft.ML.Data;
 
 namespace Microsoft.Data.Analysis
 {
@@ -62,5 +64,91 @@ DataViewRowCursor[] IDataView.GetRowCursorSet(IEnumerable<DataViewSchema.Column>
             // TODO: change to support parallel cursors
             return new DataViewRowCursor[] { GetRowCursorCore(columnsNeeded) };
         }
+
+        private sealed class RowCursor : DataViewRowCursor
+        {
+            private bool _disposed;
+            private long _position;
+            private readonly DataFrame _dataFrame;
+            private readonly List<Delegate> _getters;
+            private Dictionary<int, int> _columnIndexToGetterIndex;
+
+            public RowCursor(DataFrame dataFrame, bool[] activeColumns)
+            {
+                Debug.Assert(dataFrame != null);
+                Debug.Assert(activeColumns != null);
+
+                _columnIndexToGetterIndex = new Dictionary<int, int>();
+                _position = -1;
+                _dataFrame = dataFrame;
+                _getters = new List<Delegate>();
+                for (int i = 0; i < Schema.Count; i++)
+                {
+                    if (!activeColumns[i])
+                    {
+                        continue;
+                    }
+
+                    Delegate getter = CreateGetterDelegate(i);
+                    _getters.Add(getter);
+                    Debug.Assert(getter != null);
+                    _columnIndexToGetterIndex[i] = _getters.Count - 1;
+                }
+            }
+
+            public override long Position => _position;
+            public override long Batch => 0;
+            public override DataViewSchema Schema => _dataFrame.DataViewSchema;
+
+            protected override void Dispose(bool disposing)
+            {
+                if (_disposed)
+                {
+                    return;
+                }
+
+                if (disposing)
+                {
+                    _position = -1;
+                }
+
+                _disposed = true;
+                base.Dispose(disposing);
+            }
+
+            private Delegate CreateGetterDelegate(int col)
+            {
+                DataFrameColumn column = _dataFrame.Columns[col];
+                return column.GetDataViewGetter(this);
+            }
+
+            public override ValueGetter<TValue> GetGetter<TValue>(DataViewSchema.Column column)
+            {
+                if (!IsColumnActive(column))
+                    throw new ArgumentOutOfRangeException(nameof(column));
+
+                return (ValueGetter<TValue>)_getters[_columnIndexToGetterIndex[column.Index]];
+            }
+
+            public override ValueGetter<DataViewRowId> GetIdGetter()
+            {
+                return (ref DataViewRowId value) => value = new DataViewRowId((ulong)_position, 0);
+            }
+
+            public override bool IsColumnActive(DataViewSchema.Column column)
+            {
+                return _getters[_columnIndexToGetterIndex[column.Index]] != null;
+            }
+
+            public override bool MoveNext()
+            {
+                if (_disposed)
+                {
+                    return false;
+                }
+                _position++;
+                return _position < _dataFrame.Rows.Count;
+            }
+        }
     }
 }