infer purpose of hidden columns as 'ignore' (dotnet#142)

src/Microsoft.ML.Auto/ColumnInference/PurposeInference.cs

@@ -280,6 +280,10 @@ public static PurposeInference.Column[] InferPurposes(MLContext context, IDataVi
                 {
                     intermediateCol = new IntermediateColumn(data, i, ColumnPurpose.Label);
                 }
+                else if (column.IsHidden)
+                {
+                    intermediateCol = new IntermediateColumn(data, i, ColumnPurpose.Ignore);
+                }
                 else if(columnOverrides != null && columnOverrides.TryGetValue(column.Name, out var columnPurpose))
                 {
                     intermediateCol = new IntermediateColumn(data, i, columnPurpose);

src/Test/PurposeInferenceTests.cs

@@ -0,0 +1,39 @@
+using System.Linq;
+using Microsoft.Data.DataView;
+using Microsoft.ML.Data;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Microsoft.ML.Auto.Test
+{
+    [TestClass]
+    public class PurposeInferenceTests
+    {
+        [TestMethod]
+        public void PurposeInferenceHiddenColumnsTest()
+        {
+            var context = new MLContext();
+
+            // build basic data view
+            var schemaBuilder = new SchemaBuilder();
+            schemaBuilder.AddColumn(DefaultColumnNames.Label, BoolType.Instance);
+            schemaBuilder.AddColumn(DefaultColumnNames.Features, NumberType.R4);
+            var schema = schemaBuilder.GetSchema();
+            IDataView data = new EmptyDataView(context, schema);
+
+            // normalize 'Features' column. this has the effect of creating 2 columns named
+            // 'Features' in the data view, the first of which gets marked as 'Hidden'
+            var normalizer = context.Transforms.Normalize(DefaultColumnNames.Features);
+            data = normalizer.Fit(data).Transform(data);
+
+            // infer purposes
+            var purposes = PurposeInference.InferPurposes(context, data, DefaultColumnNames.Label);
+
+            Assert.AreEqual(3, purposes.Count());
+            Assert.AreEqual(ColumnPurpose.Label, purposes[0].Purpose);
+            // assert first 'Features' purpose (hidden column) is Ignore
+            Assert.AreEqual(ColumnPurpose.Ignore, purposes[1].Purpose);
+            // assert second 'Features' purpose is NumericFeature
+            Assert.AreEqual(ColumnPurpose.NumericFeature, purposes[2].Purpose);
+        }
+    }
+}