Common:ChatOn: ReversePrompts, SingleMsgChatTemplate wrapper

Helper to return reverse prompts needed for a given chat template

A wrapper that will allow wrapping a given message within a tagged
chat template based on the role and chat template specified.

common/CMakeLists.txt

@@ -65,6 +65,7 @@ add_library(${TARGET} STATIC
     train.cpp
     ngram-cache.h
     ngram-cache.cpp
+    chaton.hpp
     )
 
 if (BUILD_SHARED_LIBS)

common/chaton.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <vector>
+#include <string>
+
+#include "llama.h"
+#include "log.h"
+
+inline std::string llama_chat_apply_template_simple(
+            const std::string & tmpl,
+            const std::string &role,
+            const std::string &content,
+            bool add_ass) {
+    llama_chat_message msg = { role.c_str(), content.c_str() };
+    std::vector<llama_chat_message> msgs{ msg };
+    std::vector<char> buf(content.size() * 2);
+
+    int32_t slen =  llama_chat_apply_template(nullptr, tmpl.c_str(), msgs.data(), msgs.size(), add_ass, buf.data(), buf.size());
+    if ((size_t) slen > buf.size()) {
+        buf.resize(slen);
+        slen = llama_chat_apply_template(nullptr, tmpl.c_str(), msgs.data(), msgs.size(), add_ass, buf.data(), buf.size());
+    }
+
+    const std::string tagged_msg(buf.data(), slen);
+    LOGLN("INFO:%s:%s", __func__, tagged_msg.c_str());
+    return tagged_msg;
+}
+
+// return what should be the reverse prompt for the given template id
+// ie possible end text tag(s) of specified model type's chat query response
+std::vector<std::string> llama_chat_reverse_prompt(std::string &template_id) {
+    std::vector<std::string> rends;
+
+    if (template_id == "chatml") {
+        rends.push_back("<|im_start|>user\n");
+    } else if (template_id == "llama3") {
+        rends.push_back("<|eot_id|>");
+    }
+    return rends;
+}