add chat_get_added_part

common/common.cpp

@@ -2947,3 +2947,46 @@ llama_control_vector_data llama_control_vector_load(const std::vector<llama_cont
 
     return result;
 }
+
+// apply chat template for (n_msgs - 1) and (n_msgs), then get the added part
+std::string chat_get_added_part(const std::vector<chat_message> & messages, const std::string & tmpl) {
+    auto apply_chat_template = [&tmpl](const std::vector<chat_message> & msgs, size_t delta, bool add_ass) {
+        std::vector<llama_chat_message> chat(msgs.size());
+        size_t alloc_size = 0;
+        size_t chat_size = chat.size() - delta;
+        for (size_t i = 0; i < msgs.size(); ++i) {
+            chat[i].role    = msgs[i].role.c_str();
+            chat[i].content = msgs[i].content.c_str();
+            alloc_size += msgs[i].role.size() + msgs[i].content.size();
+        }
+
+        const char * ptr_tmpl = tmpl.empty() ? nullptr : tmpl.c_str();
+        std::vector<char> buf(alloc_size * 2);
+
+        // run the first time to get the total output length
+        int32_t res = llama_chat_apply_template(nullptr, ptr_tmpl, chat.data(), chat_size, add_ass, buf.data(), buf.size());
+
+        // if it turns out that our buffer is too small, we resize it
+        if ((size_t) res > buf.size()) {
+            buf.resize(res);
+            res = llama_chat_apply_template(nullptr, ptr_tmpl, chat.data(), chat_size, add_ass, buf.data(), buf.size());
+        }
+
+        const std::string formatted_chat(buf.data(), res);
+        return formatted_chat;
+    };
+
+    std::string formatted_chat_last = messages.size() > 0
+        ? apply_chat_template(messages, 1, false) // (n_msgs - 1) messages
+        : "";
+    std::string formatted_chat_curr = apply_chat_template(messages, 0, true);
+
+    // Extract the added part (user prompt)
+    auto get_diff_part = [](const std::string & str1, const std::string & str2) {
+        size_t i = 0;
+        while (i < str1.size() && i < str2.size() && str1[i] == str2[i])
+            ++i;
+        return str2.substr(i);
+    };
+    return get_diff_part(formatted_chat_last, formatted_chat_curr);
+}

common/common.h

@@ -322,3 +322,13 @@ llama_control_vector_data llama_control_vector_load(const std::vector<llama_cont
 static const char * const LLM_KV_SPLIT_NO            = "split.no";
 static const char * const LLM_KV_SPLIT_COUNT         = "split.count";
 static const char * const LLM_KV_SPLIT_TENSORS_COUNT = "split.tensors.count";
+
+//
+// Chat templates utils
+//
+typedef struct chat_message {
+    std::string role;
+    std::string content;
+} chat_message;
+
+std::string chat_get_added_part(const std::vector<chat_message> & messages, const std::string & tmpl);

tests/test-chat-template.cpp

@@ -7,6 +7,7 @@
 #include <cassert>
 
 #include "llama.h"
+#include "common.h"
 
 int main(void) {
     llama_chat_message conversation[] = {
@@ -96,8 +97,19 @@ int main(void) {
         );
         formatted_chat.resize(res);
         std::string output(formatted_chat.data(), formatted_chat.size());
-        std::cout << output << "\n-------------------------\n";
+        std::cout << output << "\n-----\n";
         assert(output == expected);
+
+        std::vector<chat_message> v_messages;
+        for (size_t i = 0; i < message_count; ++i) {
+            v_messages.push_back({
+                conversation[i].role,
+                conversation[i].content,
+            });
+        }
+        std::cout << "chat_get_added_part(): " << chat_get_added_part(v_messages, custom_template);
+        std::cout << "\n-------------------------\n";
+        // TODO: chat_get_added_part is currently printed for debugging. Should we add tests for it in the future?
     }
     return 0;
 }