Ngram hashing to estimator (dotnet#1811)

src/Microsoft.ML.Data/Transforms/ConversionsExtensionsCatalog.cs

@@ -23,7 +23,10 @@ public static class ConversionsExtensionsCatalog
         /// <param name="inputColumn">Name of the input column.</param>
         /// <param name="outputColumn">Name of the column to be transformed. If this is null '<paramref name="inputColumn"/>' will be used.</param>
         /// <param name="hashBits">Number of bits to hash into. Must be between 1 and 31, inclusive.</param>
-        /// <param name="invertHash">Limit the number of keys used to generate the slot name to this many. 0 means no invert hashing, -1 means no limit.</param>
+        /// <param name="invertHash">During hashing we constuct mappings between original values and the produced hash values.
+        /// Text representation of original values are stored in the slot names of the  metadata for the new column.Hashing, as such, can map many initial values to one.
+        /// <paramref name="invertHash"/> specifies the upper bound of the number of distinct input values mapping to a hash that should be retained.
+        /// <value>0</value> does not retain any input values. <value>-1</value> retains all input values mapping to each hash.</param>
         public static HashingEstimator Hash(this TransformsCatalog.ConversionTransforms catalog, string inputColumn, string outputColumn = null,
             int hashBits = HashDefaults.HashBits, int invertHash = HashDefaults.InvertHash)
             => new HashingEstimator(CatalogUtils.GetEnvironment(catalog), inputColumn, outputColumn, hashBits, invertHash);
@@ -99,8 +102,8 @@ public static KeyToVectorMappingEstimator MapKeyToVector(this TransformsCatalog.
         /// <param name="inputColumn">Name of the column to be transformed.</param>
         /// <param name="outputColumn">Name of the output column. If this is null '<paramref name="inputColumn"/>' will be used.</param>
         /// <param name="maxNumTerms">Maximum number of keys to keep per column when auto-training.</param>
-        /// <param name="sort">How items should be ordered when vectorized. By default, they will be in the order encountered.
-        /// If by value items are sorted according to their default comparison, for example, text sorting will be case sensitive (for example, 'A' then 'Z' then 'a').</param>
+        /// <param name="sort">How items should be ordered when vectorized. If <see cref="ValueToKeyMappingTransformer.SortOrder.Occurrence"/> choosen they will be in the order encountered.
+        /// If <see cref="ValueToKeyMappingTransformer.SortOrder.Value"/>, items are sorted according to their default comparison, for example, text sorting will be case sensitive (for example, 'A' then 'Z' then 'a').</param>
         public static ValueToKeyMappingEstimator MapValueToKey(this TransformsCatalog.ConversionTransforms catalog,
             string inputColumn,
             string outputColumn = null,

src/Microsoft.ML.Data/Transforms/DropSlotsTransform.cs

@@ -200,7 +200,7 @@ public sealed class ColumnInfo
             /// Describes how the transformer handles one input-output column pair.
             /// </summary>
             /// <param name="input">Name of the input column.</param>
-            /// <param name="output">Name of the column resulting from the transformation of <paramref name="input"/>. Null means <paramref name="input"/> is replaced. </param>
+            /// <param name="output">Name of the column resulting from the transformation of <paramref name="input"/>. Null means <paramref name="input"/> is replaced.</param>
             /// <param name="slots">Ranges of indices in the input column to be dropped. Setting max in <paramref name="slots"/> to null sets max to int.MaxValue.</param>
             public ColumnInfo(string input, string output = null, params (int min, int? max)[] slots)
             {
@@ -252,7 +252,7 @@ private static VersionInfo GetVersionInfo()
         /// </summary>
         /// <param name="env">The environment to use.</param>
         /// <param name="input">Name of the input column.</param>
-        /// <param name="output">Name of the column resulting from the transformation of <paramref name="input"/>. Null means <paramref name="input"/> is replaced. </param>
+        /// <param name="output">Name of the column resulting from the transformation of <paramref name="input"/>. Null means <paramref name="input"/> is replaced.</param>
         /// <param name="min">Specifies the lower bound of the range of slots to be dropped. The lower bound is inclusive. </param>
         /// <param name="max">Specifies the upper bound of the range of slots to be dropped. The upper bound is exclusive.</param>
         public SlotsDroppingTransformer(IHostEnvironment env, string input, string output = null, int min = default, int? max = null)

src/Microsoft.ML.Data/Transforms/Hashing.cs

@@ -132,7 +132,10 @@ public sealed class ColumnInfo
             /// <param name="hashBits">Number of bits to hash into. Must be between 1 and 31, inclusive.</param>
             /// <param name="seed">Hashing seed.</param>
             /// <param name="ordered">Whether the position of each term should be included in the hash.</param>
-            /// <param name="invertHash">Limit the number of keys used to generate the slot name to this many. 0 means no invert hashing, -1 means no limit.</param>
+            /// <param name="invertHash">During hashing we constuct mappings between original values and the produced hash values.
+            /// Text representation of original values are stored in the slot names of the  metadata for the new column.Hashing, as such, can map many initial values to one.
+            /// <paramref name="invertHash"/> specifies the upper bound of the number of distinct input values mapping to a hash that should be retained.
+            /// <value>0</value> does not retain any input values. <value>-1</value> retains all input values mapping to each hash.</param>
             public ColumnInfo(string input, string output,
                 int hashBits = HashingEstimator.Defaults.HashBits,
                 uint seed = HashingEstimator.Defaults.Seed,
@@ -1211,7 +1214,10 @@ internal static bool IsColumnTypeValid(ColumnType type)
         /// <param name="inputColumn">Name of the column to be transformed.</param>
         /// <param name="outputColumn">Name of the output column. If this is null '<paramref name="inputColumn"/>' will be used.</param>
         /// <param name="hashBits">Number of bits to hash into. Must be between 1 and 31, inclusive.</param>
-        /// <param name="invertHash">Limit the number of keys used to generate the slot name to this many. 0 means no invert hashing, -1 means no limit.</param>
+        /// <param name="invertHash">During hashing we constuct mappings between original values and the produced hash values.
+        /// Text representation of original values are stored in the slot names of the  metadata for the new column.Hashing, as such, can map many initial values to one.
+        /// <paramref name="invertHash"/> specifies the upper bound of the number of distinct input values mapping to a hash that should be retained.
+        /// <value>0</value> does not retain any input values. <value>-1</value> retains all input values mapping to each hash.</param>
         public HashingEstimator(IHostEnvironment env, string inputColumn, string outputColumn = null,
             int hashBits = Defaults.HashBits, int invertHash = Defaults.InvertHash)
             : this(env, new HashingTransformer.ColumnInfo(inputColumn, outputColumn ?? inputColumn, hashBits: hashBits, invertHash: invertHash))