Address the feedback on the tokenizer's library

src/Microsoft.ML.Tokenizers/Model/BPE.cs

@@ -222,8 +222,9 @@ public override IReadOnlyList<Token> Tokenize(string sequence, bool isSpecialTok
         /// </summary>
         /// <param name="id">The Id to map to the token.</param>
         /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The mapped token of the Id.</returns>
-        public override string? IdToToken(int id, bool skipSpecialTokens = false)
+        public override string? IdToToken(int id, bool skipSpecialTokens = false, bool filterUnsupportedChars = true)
         {
             if (VocabReverse.TryGetValue(id, out string? value))
             {

src/Microsoft.ML.Tokenizers/Model/EnglishRoberta.cs

@@ -127,29 +127,30 @@ public EnglishRoberta(Stream vocabularyStream, Stream mergeStream, Stream highes
         /// <summary>
         /// Map the tokenized Id to the token.
         /// </summary>
-        /// <param name="id">The Id to map to the token.</param>
-        /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
-        /// <returns>The mapped token of the Id.</returns>
-        public override string? IdToToken(int id, bool skipSpecialTokens = false) =>
-            skipSpecialTokens && id < 0 ? null : _vocabReverse.TryGetValue(id, out var value) ? value : null;
-
-        /// <summary>
-        /// Map the tokenized Id to the original string while filtering out unsupported characters.
-        /// </summary>
         /// <param name="id">The Id to map to the string.</param>
         /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The mapped token of the Id.</returns>
-        public string? IdToFilteredToken(int id, bool skipSpecialTokens = false)
+        public override string? IdToToken(int id, bool skipSpecialTokens = false, bool filterUnsupportedChars = true)
         {
             if (skipSpecialTokens && id < 0)
+            {
                 return null;
+            }
+
             if (_vocabReverse.TryGetValue(id, out var value))
             {
-                var textChars = string.Join("", value)
-                    .Where(c => _unicodeToByte.ContainsKey(c))
-                    .Select(c => _unicodeToByte[c]);
-                var text = new string(textChars.ToArray());
-                return text;
+                if (filterUnsupportedChars)
+                {
+                    var textChars = string.Join("", value)
+                        .Where(c => _unicodeToByte.ContainsKey(c))
+                        .Select(c => _unicodeToByte[c]);
+                    return new string(textChars.ToArray());
+                }
+                else
+                {
+                    return value;
+                }
             }
 
             return null;

src/Microsoft.ML.Tokenizers/Model/Model.cs

@@ -82,8 +82,29 @@ public virtual int CountTokens(string sequence, bool isSpecialToken)
         /// </summary>
         /// <param name="id">The Id to map to the token.</param>
         /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The mapped token of the Id.</returns>
-        public abstract string? IdToToken(int id, bool skipSpecialTokens = false);
+        public abstract string? IdToToken(int id, bool skipSpecialTokens = false, bool filterUnsupportedChars = true);
+
+        /// <summary>
+        /// Decode the given ids, back to a String.
+        /// </summary>
+        /// <param name="ids">The list of ids that we want to decode.</param>
+        /// <param name="skipSpecialTokens">Whether the special tokens should be removed from the decoded string.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
+        /// <param name="decoder">The optional Decoder to merge the given list of tokens in a string.</param>
+        /// <returns>The decoded string.</returns>
+        public virtual string? Decode(IEnumerable<int> ids, TokenizerDecoder? decoder = null, bool skipSpecialTokens = false, bool filterUnsupportedChars = true)
+        {
+            List<string> tokens = new List<string>();
+
+            foreach (int id in ids)
+            {
+                tokens.Add(IdToToken(id, skipSpecialTokens, filterUnsupportedChars) ?? "");
+            }
+
+            return decoder?.Decode(tokens) ?? string.Join("", tokens);
+        }
 
         /// <summary>
         /// Gets the dictionary mapping tokens to Ids.

src/Microsoft.ML.Tokenizers/Model/Tiktoken.cs

@@ -397,8 +397,9 @@ public override int CountTokens(string sequence, bool isSpecialToken)
         /// </summary>
         /// <param name="id">The Id to map to the token.</param>
         /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The mapped token of the Id.</returns>
-        public override string? IdToToken(int id, bool skipSpecialTokens = false)
+        public override string? IdToToken(int id, bool skipSpecialTokens = false, bool filterUnsupportedChars = true)
         {
             if (!skipSpecialTokens && _specialTokensDecoder is not null && _specialTokensDecoder.TryGetValue(id, out string? token))
             {
@@ -413,8 +414,22 @@ public override int CountTokens(string sequence, bool isSpecialToken)
             return null;
         }
 
-        internal string? IdsToString(IEnumerable<int> ids, bool skipSpecialTokens = false)
+        /// <summary>
+        /// Decode the given ids, back to a String.
+        /// </summary>
+        /// <param name="ids">The list of ids that we want to decode.</param>
+        /// <param name="skipSpecialTokens">Whether the special tokens should be removed from the decoded string.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
+        /// <param name="decoder">The optional Decoder to merge the given list of tokens in a string.</param>
+        /// <returns>The decoded string.</returns>
+        public override string? Decode(IEnumerable<int> ids, TokenizerDecoder? decoder = null, bool skipSpecialTokens = false, bool filterUnsupportedChars = true)
         {
+            // Tiktoken does not ensure a one-to-one mapping between IDs and tokens. Consequently, decoding individual IDs into tokens is not supported;
+            // instead, decoding all IDs must be done collectively.
+            // Here is example of case that map one character to multiple Ids:
+            // '⭐' U-2B50 is mapped to Ids [2928, 99834] in the Tiktoken model.
+            // In other words, the character '⭐' has UTF-8 code point 0xE2, 0xAD, 0x90, Tiktoken will map 0xE2 to [2928] and 0xAD, 0x90 to [99834].
+
             if (ids is null)
             {
                 return null;

src/Microsoft.ML.Tokenizers/Tokenizer.cs

@@ -25,11 +25,13 @@ public partial class Tokenizer
         /// <param name="model">The Model in use by the Tokenizer.</param>
         /// <param name="preTokenizer">The optional PreTokenizer in use by the Tokenizer. WhiteSpace PreTokenizer will be used if this parameter is null.</param>
         /// <param name="normalizer">The optional Normalizer in use by the Tokenizer.</param>
-        public Tokenizer(Model model, PreTokenizer? preTokenizer = null, Normalizer? normalizer = null)
+        /// <param name="decoder">The optional Decoder in use by the Tokenizer during the decoding operation to merge the given list of tokens in a string.</param>
+        public Tokenizer(Model model, PreTokenizer? preTokenizer = null, Normalizer? normalizer = null, TokenizerDecoder? decoder = null)
         {
             Model = model;
             PreTokenizer = preTokenizer ?? WhiteSpace.Instance;
             Normalizer = normalizer;
+            Decoder = decoder;
         }
 
         /// <summary>
@@ -40,17 +42,17 @@ public Tokenizer(Model model, PreTokenizer? preTokenizer = null, Normalizer? nor
         /// <summary>
         /// Gets or sets the PreTokenizer used by the Tokenizer.
         /// </summary>
-        public PreTokenizer PreTokenizer { get; set; }
+        public PreTokenizer PreTokenizer { get; private set; }
 
         /// <summary>
         /// Gets or sets the Normalizer in use by the Tokenizer.
         /// </summary>
-        public Normalizer? Normalizer { get; set; }
+        public Normalizer? Normalizer { get; private set; }
 
         /// <summary>
         /// Gets or sets the Decoder in use by the Tokenizer.
         /// </summary>
-        public TokenizerDecoder? Decoder { get; set; }
+        public TokenizerDecoder? Decoder { get; private set; }
 
         /// <summary>
         /// Encodes input text to object has the tokens list, tokens Ids, tokens offset mapping.
@@ -171,53 +173,23 @@ public int CountTokens(string sequence, bool skipSpecialTokens = false)
             return idsCount;
         }
 
-        // skipSpecialTokens is used in post processing we don't support yet. We are keeping it to allow using it when we support post processing.
         /// <summary>
         /// Decodes the Id to the mapped token.
         /// </summary>
         /// <param name="id">The id to map to the token.</param>
         /// <param name="skipSpecialTokens">Indicate if want to skip the special tokens during the decoding.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The decoded string or null if there is no token mapped to the input id.</returns>
-        public string? Decode(int id, bool skipSpecialTokens = false) => Model.IdToToken(id, skipSpecialTokens);
+        public string? Decode(int id, bool skipSpecialTokens = false, bool filterUnsupportedChars = true) => Model.IdToToken(id, skipSpecialTokens, filterUnsupportedChars);
 
-        // skipSpecialTokens is used in post processing we don't support yet. We are keeping it to allow using it when we support post processing.
         /// <summary>
         /// Decode the given ids, back to a String.
         /// </summary>
         /// <param name="ids">The list of ids that we want to decode.</param>
         /// <param name="skipSpecialTokens">Whether the special tokens should be removed from the decoded string.</param>
+        /// <param name="filterUnsupportedChars">Indicate if want to filter the unsupported characters during the decoding.</param>
         /// <returns>The decoded string.</returns>
-        public string? Decode(IEnumerable<int> ids, bool skipSpecialTokens = false)
-        {
-            if (Model is Tiktoken tiktoken)
-            {
-                // Tiktoken does not ensure a one-to-one mapping between IDs and tokens. Consequently, decoding individual IDs into tokens is not supported;
-                // instead, decoding all IDs must be done collectively.
-                // Here is example of case that map one character to multiple Ids:
-                // '⭐' U-2B50 is mapped to Ids [2928, 99834] in the Tiktoken model.
-                // In other words, the character '⭐' has UTF-8 code point 0xE2, 0xAD, 0x90, Tiktoken will map 0xE2 to [2928] and 0xAD, 0x90 to [99834].
-                return tiktoken.IdsToString(ids, skipSpecialTokens);
-            }
-
-            List<string> tokens = new List<string>();
-
-            if (Model is EnglishRoberta robertaModel)
-            {
-                foreach (int id in ids)
-                {
-                    tokens.Add(robertaModel.IdToFilteredToken(id, skipSpecialTokens) ?? "");
-                }
-            }
-            else
-            {
-                foreach (int id in ids)
-                {
-                    tokens.Add(Model.IdToToken(id, skipSpecialTokens) ?? "");
-                }
-            }
-
-            return Decoder?.Decode(tokens) ?? string.Join("", tokens);
-        }
+        public string? Decode(IEnumerable<int> ids, bool skipSpecialTokens = false, bool filterUnsupportedChars = true) => Model.Decode(ids, Decoder, skipSpecialTokens, filterUnsupportedChars);
 
         private const string EndOfText = "<|endoftext|>";
         private const string FimPrefix = "<|fim_prefix|>";

test/Microsoft.ML.Tokenizers.Tests/EnglishRobertaTests.cs

@@ -30,7 +30,8 @@ public static IEnumerable<object[]> BertaData
                     new string[] { "Hello", "\u0120Bert", "a" },
                     new (int, int)[] { (0, 5), (5, 10), (10, 11) },
                     "Hello Berta",
-                    new int[] { 35245, 144292, 18759122 }
+                    new int[] { 35245, 144292, 18759122 },
+                    new string[] { "Hello", " Bert", "a" },
                 };
 
                 // Intentionally repeating the same case data to test caching.
@@ -42,7 +43,8 @@ public static IEnumerable<object[]> BertaData
                     new string[] { "Hello", "\u0120Bert", "a" },
                     new (int, int)[] { (0, 5), (5, 10), (10, 11) },
                     "Hello Berta",
-                    new int[] { 35245, 144292, 18759122 }
+                    new int[] { 35245, 144292, 18759122 },
+                    new string[] { "Hello", " Bert", "a" },
                 };
 
                 // Sentence, Expected Ids, Expected Tokens, Expected Offsets, Decoded Tokens, Token occurrence values
@@ -53,7 +55,8 @@ public static IEnumerable<object[]> BertaData
                     new string[] { "In", "\u0120the", "\u0120night", "." },
                     new (int, int)[] { (0, 2), (2, 6), (6, 12), (12, 13) },
                     "In the night.",
-                    new int[] { 2224123, 800385005, 6062347, 850314647 }
+                    new int[] { 2224123, 800385005, 6062347, 850314647 },
+                    new string[] { "In", " the", " night", "." },
                 };
 
                 // Sentence, Expected Ids, Expected Tokens, Expected Offsets, Decoded Tokens, Token occurrence values
@@ -64,7 +67,8 @@ public static IEnumerable<object[]> BertaData
                     new string[] { "He", "llo", "ĠBer", "ta" },
                     new (int, int)[] { (0, 2), (4, 7), (7, 11), (13, 15) },
                     "Hello Berta",
-                    new int[] { 2759525, 207306, 565286, 560191 }
+                    new int[] { 2759525, 207306, 565286, 560191 },
+                    new string[] { "He", "llo", " Ber", "ta" },
                 };
 
                 // Sentence, Expected Ids, Expected Tokens, Expected Offsets, Decoded Tokens, Token occurrence values
@@ -75,7 +79,8 @@ public static IEnumerable<object[]> BertaData
                     new string[] {  },
                     new (int, int)[] { },
                     "",
-                    new int[] {  }
+                    new int[] {  },
+                    new string[] {  },
                 };
             }
         }
@@ -180,9 +185,13 @@ private void TestTokenizer(Tokenizer tokenizer, CallingOrder callingOrder = Call
 
                 for (int i = 0; i < encoding.Tokens.Count; i++)
                 {
-                    Assert.Equal(encoding.Tokens[i], tokenizer.Model.IdToToken(encoding.Ids[i]));
+                    Assert.Equal(encoding.Tokens[i], tokenizer.Model.IdToToken(encoding.Ids[i], skipSpecialTokens: true, filterUnsupportedChars: false));
                     Assert.Equal(encoding.Ids[i], tokenizer.Model.TokenToId(encoding.Tokens[i]));
-                    Assert.Equal(encoding.Tokens[i], tokenizer.Decode(encoding.Ids[i]));
+                    Assert.Equal(encoding.Tokens[i], tokenizer.Decode(encoding.Ids[i], skipSpecialTokens: true, filterUnsupportedChars: false));
+
+                    string[]? filteredToken = p[6] as string[];
+
+                    Assert.Equal(filteredToken![i], tokenizer.Model.IdToToken(encoding.Ids[i], skipSpecialTokens: true, filterUnsupportedChars: true));
                 }
             }
         }