ChatON+Main: Add C_API wrapper for single

Add a c api wrapper for a single message tagging scenario.

Inturn to match convention followed by existing chat_apply_template
code, make it return the size expected of the tagged message string
buffer. Update internal single logic to help with same.

Explicitly check if tmpl specified is available in the loaded json
or not and then return a error if not found.

common/chaton.hpp

@@ -165,11 +165,27 @@ public:
 };
 
 inline bool chaton_meta_load(std::string &fname) {
+    if (conMeta != nullptr) {
+        LOGXLN("WARN:%s:ChatOn Meta: overwriting???", __func__);
+    }
     std::ifstream f(fname);
     conMeta = json::parse(f);
     return true;
 }
 
+inline bool chaton_tmpl_exists(const std::string &tmpl) {
+    if (conMeta == nullptr) {
+        LOG_TEELN("ERRR:%s:ChatOnMeta: Not loaded yet...", __func__);
+        return false;
+    }
+    try {
+        auto tmplData = conMeta[tmpl];
+        return true;
+    } catch (json::exception &err) {
+        LOG_TEELN("WARN:%s:ChatOnMeta: tmpl[%s] not found...", __func__, tmpl.c_str());
+        return false;
+    }
+}
 
 inline std::string chaton_tmpl_role_kv(const std::string &tmpl, const std::string &role, const std::vector<std::string> &keys) {
     std::string got = "";
@@ -230,12 +246,50 @@ inline bool chaton_tmpl_apply_single_ex(
 }
 
 // Return user-prefix + msg + user-suffix, types string and lens vector wrt the parts that make up the returned string
-inline std::string chaton_tmpl_apply_single(const std::string &tmpl, const std::string &role, const std::string &content) {
-    std::string tagged;
+inline size_t chaton_tmpl_apply_single(
+        const std::string &tmpl,
+        const std::string &role,
+        const std::string &content,
+        std::string &tagged
+        ) {
+    if (!chaton_tmpl_exists(tmpl)) {
+        return -1;
+    }
     std::string types;
     std::vector<int> lens;
     chaton_tmpl_apply_single_ex(tmpl, role, content, tagged, types, lens);
-    return tagged;
+    return tagged.size();
+}
+
+/**
+ * Apply chat-handshake-template for the specified template standard and role.
+ * If the passed char array is smaller that that required for the tagged message,
+ * * part of the tagged message which fits within dest buffer is copied
+ * * the returned value, indicates the size of the tagged message
+ * NOTE:
+ * * the passed char array should be able to fit the tagged message+0|null char.
+ * * if the return value from this function is larger than or equal to destLength,
+ *   then you will have to increase the size of the dest buffer, and call this
+ *   function a second time, to ensure that one gets the full tagged message.
+ */
+inline size_t chat_tmpl_apply_single_capi(
+        const char *tmpl,
+        const char *role,
+        const char *content,
+        char *dest,
+        const size_t destLength
+        ) {
+    std::string tagged;
+    std::string types;
+    std::vector<int> lens;
+    auto taggedLength = chaton_tmpl_apply_single(tmpl, role, content, tagged);
+    if (taggedLength <= 0) {
+        return taggedLength;
+    }
+    if (dest && (destLength > 0)) {
+        strlcpy(dest, tagged.c_str(), destLength);
+    }
+    return taggedLength;
 }
 
 // global-begin + [[role-begin] + [role-prefix] + msg + role-suffix] + global-end

examples/main/main.cpp

@@ -264,7 +264,7 @@ int main(int argc, char ** argv) {
             params.prompt = "<|im_start|>system\n" + params.prompt + "<|im_end|>";
         }
         if (params.chaton) {
-            params.prompt = chaton_tmpl_apply_single(params.chaton_template_id, K_SYSTEM, params.prompt);
+            chaton_tmpl_apply_single(params.chaton_template_id, K_SYSTEM, params.prompt, params.prompt);
         }
         embd_inp = ::llama_tokenize(ctx, params.prompt, true, true);
     } else {