tokenization: no double BOS tokens

Author: JohannesGaessler

llama.cpp

@@ -12674,11 +12674,6 @@ static std::vector<llama_vocab::id> llama_tokenize_internal(const llama_vocab &
                 // tokenizer.encode('', add_special_tokens=True)  returns [1]
                 // tokenizer.encode('', add_special_tokens=False) returns []
 
-                if (add_special && vocab.special_add_bos != 0) {
-                    GGML_ASSERT(vocab.special_bos_id != -1);
-                    output.push_back(vocab.special_bos_id);
-                }
-
                 for (const auto & fragment : fragment_buffer) {
                     if (fragment.type == FRAGMENT_BUFFER_VARIANT_TYPE_RAW_TEXT) {
                         // without adding this leading whitespace, we do not get the same results as the original tokenizer
@@ -12705,18 +12700,18 @@ static std::vector<llama_vocab::id> llama_tokenize_internal(const llama_vocab &
                     }
                 }
 
+                if (add_special && vocab.special_add_bos != 0 && output[0] != vocab.special_bos_id) {
+                    GGML_ASSERT(vocab.special_bos_id != -1);
+                    output.insert(output.begin(), vocab.special_bos_id);
+                }
+
                 if (add_special && vocab.special_add_eos == 1) {
                     GGML_ASSERT(vocab.special_eos_id != -1);
                     output.push_back(vocab.special_eos_id);
                 }
             } break;
         case LLAMA_VOCAB_TYPE_BPE:
             {
-                if (add_special && vocab.special_add_bos != 0) {
-                    GGML_ASSERT(vocab.special_bos_id != -1);
-                    output.push_back(vocab.special_bos_id);
-                }
-
                 for (const auto & fragment : fragment_buffer) {
                     if (fragment.type == FRAGMENT_BUFFER_VARIANT_TYPE_RAW_TEXT) {
                         auto raw_text = fragment.raw_text.substr(fragment.offset, fragment.length);
@@ -12731,6 +12726,11 @@ static std::vector<llama_vocab::id> llama_tokenize_internal(const llama_vocab &
                     }
                 }
 
+                if (add_special && vocab.special_add_bos != 0 && output[0] != vocab.special_bos_id) {
+                    GGML_ASSERT(vocab.special_bos_id != -1);
+                    output.insert(output.begin(), vocab.special_bos_id);
+                }
+
                 GGML_ASSERT(vocab.special_add_eos != 1);
             } break;
         case LLAMA_VOCAB_TYPE_WPM: