Fix legacy behaviour

Sources/Tokenizers/Tokenizer.swift

@@ -278,9 +278,9 @@ public class PreTrainedTokenizer: Tokenizer {
     public var unknownTokenId: Int? { model.unknownTokenId }
     public var fuseUnknownTokens: Bool { model.fuseUnknownTokens }
 
-    private let addedTokens: Set<String>
-    private let specialTokens: [String: Int]
-    private let addedTokensRegex: NSRegularExpression?
+    let addedTokens: Set<String>
+    let specialTokens: [String: Int]
+    let addedTokensRegex: NSRegularExpression?
 
     private let preTokenizer: PreTokenizer?
     private let normalizer: Normalizer?
@@ -721,4 +721,18 @@ class LlamaPreTrainedTokenizer: PreTrainedTokenizer {
         let updatedData = Config(configDictionary)
         try super.init(tokenizerConfig: tokenizerConfig, tokenizerData: updatedData, strict: strict)
     }
+
+    /// If `isLegacy` is `False`, a prefix token is added unless the first token is special.
+    /// https://github.com/huggingface/transformers/blob/e6dcf8abd6f65bb4b6dfc1831b20d9ba49ce00e2/src/transformers/models/t5/tokenization_t5.py#L374-L387
+    override func tokenize(text: String) -> [String] {
+        if isLegacy || text.isEmpty {
+            return super.tokenize(text: text)
+        }
+
+        let tokens = super.tokenize(text: sentencePieceUnderline + text.replacingOccurrences(of: sentencePieceUnderline, with: " "))
+        if tokens.first == sentencePieceUnderline, let second = tokens.dropFirst().first, specialTokens[second] != nil {
+            return Array(tokens[1...])
+        }
+        return tokens
+    }
 }

Tests/TokenizersTests/TokenizerTests.swift

@@ -118,6 +118,17 @@ class PhiSimpleTests: XCTestCase {
         XCTAssertEqual(tokenizer.encode(text: "hello world"), [15339, 1917])
         XCTAssertEqual(tokenizer.encode(text: "<|im_start|>user<|im_sep|>Who are you?<|im_end|><|im_start|>assistant<|im_sep|>"), [100264, 882, 100266, 15546, 527, 499, 30, 100265, 100264, 78191, 100266])
     }
+
+    /// https://github.com/huggingface/swift-transformers/issues/96
+    func testLegacyLlamaBehaviour() async throws {
+        guard let tokenizer = try await AutoTokenizer.from(pretrained: "mlx-community/Phi-3-mini-4k-instruct-4bit-no-q-embed") as? PreTrainedTokenizer else {
+            XCTFail()
+            return
+        }
+
+        let inputIds = tokenizer(" Hi")
+        XCTAssertEqual(inputIds, [1, 29871, 6324])
+    }
 }
 
 class UnregisteredTokenizerTests: XCTestCase {