Use Longest Common Prefix (LCP) instead of LCS

Author: sasha0552

common/common.cpp

@@ -1460,12 +1460,12 @@ bool gpt_params_find_arg(int argc, char ** argv, const std::string & arg, gpt_pa
         params.chat_template = argv[i];
         return true;
     }
-    if (arg == "--lcs-similarity") {
+    if (arg == "--lcp-similarity") {
         if (++i >= argc) {
             invalid_param = true;
             return true;
         }
-        params.lcs_similarity = std::stof(argv[i]);
+        params.lcp_similarity = std::stof(argv[i]);
         return true;
     }
     if (arg == "-pps") {
@@ -1839,8 +1839,8 @@ void gpt_params_print_usage(int /*argc*/, char ** argv, const gpt_params & param
                                                                         "set custom jinja chat template (default: template taken from model's metadata)\n"
                                                                         "only commonly used templates are accepted:\n"
                                                                         "https://github.com/ggerganov/llama.cpp/wiki/Templates-supported-by-llama_chat_apply_template" });
-    options.push_back({ "server",      "       --lcs-similarity SIMILARITY",
-                                                                        "how much the prompt of a request must match the prompt of a slot in order to use that slot (default: %.2f)\n", params.lcs_similarity });
+    options.push_back({ "server",      "       --lcp-similarity SIMILARITY",
+                                                                        "how much the prompt of a request must match the prompt of a slot in order to use that slot (default: %.2f)\n", params.lcp_similarity });
 
 #ifndef LOG_DISABLE_LOGS
     options.push_back({ "logging" });

common/common.h

@@ -202,7 +202,7 @@ struct gpt_params {
 
     std::string slot_save_path;
 
-    float lcs_similarity = 0.0f;
+    float lcp_similarity = 0.0f;
 
     // batched-bench params
     bool is_pp_shared = false;

examples/server/server.cpp

@@ -647,8 +647,8 @@ struct server_context {
 
     server_metrics metrics;
 
-    // Longest Common Substring similarity for slot selection
-    float lcs_similarity = 0.0f;
+    // Longest Common Prefix similarity for slot selection
+    float lcp_similarity = 0.0f;
 
     ~server_context() {
         if (ctx) {
@@ -812,8 +812,8 @@ struct server_context {
         server_slot * ret = nullptr;
 
         // find the slot that has at least n% prompt similarity
-        if (ret == nullptr && lcs_similarity != 0.0f && !prompt.empty()) {
-            int max_lcs_len = 0;
+        if (ret == nullptr && lcp_similarity != 0.0f && !prompt.empty()) {
+            int max_lcp_len = 0;
             float similarity = 0;
 
             for (server_slot & slot : slots) {
@@ -833,23 +833,23 @@ struct server_context {
                 // length of the current slot's prompt
                 int slot_prompt_len = slot_prompt.size();
 
-                // length of the longest common substring between the current slot's prompt and the input prompt
-                int lcs_len = lcs_length(slot_prompt, prompt);
+                // length of the Longest Common Prefix between the current slot's prompt and the input prompt
+                int lcp_len = common_part(slot_prompt, prompt);
 
                 // fraction of the common substring length compared to the current slot's prompt length
-                similarity = static_cast<float>(lcs_len) / slot_prompt_len;
+                similarity = static_cast<float>(lcp_len) / slot_prompt_len;
 
                 // select the current slot if the criteria match
-                if (lcs_len > max_lcs_len && similarity > lcs_similarity) {
-                    max_lcs_len = lcs_len;
+                if (lcp_len > max_lcp_len && similarity > lcp_similarity) {
+                    max_lcp_len = lcp_len;
                     ret = &slot;
                 }
             }
 
             if (ret != nullptr) {
-                LOG_VERBOSE("selected slot by lcs similarity", {
+                LOG_VERBOSE("selected slot by lcp similarity", {
                     {"id_slot", ret->id},
-                    {"max_lcs_len", max_lcs_len},
+                    {"max_lcp_len", max_lcp_len},
                     {"similarity", similarity},
                 });
             }
@@ -2568,8 +2568,8 @@ int main(int argc, char ** argv) {
         log_data["api_key"] = "api_key: " + std::to_string(params.api_keys.size()) + " keys loaded";
     }
 
-    // Longest Common Substring similarity for slot selection
-    ctx_server.lcs_similarity = params.lcs_similarity;
+    // Longest Common Prefix similarity for slot selection
+    ctx_server.lcp_similarity = params.lcp_similarity;
 
     // load the model
     if (!ctx_server.load_model(params)) {

examples/server/utils.hpp

@@ -253,6 +253,13 @@ static size_t common_part(const std::vector<llama_token> & a, const std::vector<
     return i;
 }
 
+static size_t common_part(const std::string & a, const std::string & b) {
+    size_t i;
+    for (i = 0; i < a.size() && i < b.size() && a[i] == b[i]; i++) {}
+
+    return i;
+}
+
 static bool ends_with(const std::string & str, const std::string & suffix) {
     return str.size() >= suffix.size() && 0 == str.compare(str.size() - suffix.size(), suffix.size(), suffix);
 }
@@ -646,52 +653,3 @@ static json format_error_response(const std::string & message, const enum error_
         {"type", type_str},
     };
 }
-
-static int lcs_length(const std::string & str1, const std::string & str2) {
-    // check for empty strings
-    if (str1.empty() || str2.empty()) {
-        return 0;
-    }
-
-    // get the lengths of the input strings
-    int str1_len = str1.size();
-    int str2_len = str2.size();
-
-    // initialize the maximum length of the longest common subsequence (LCS)
-    int max_length = 0;
-
-    // use two rows instead of a 2D matrix to optimize space
-    std::vector<int> prev_row(str2_len + 1, 0);
-    std::vector<int> curr_row(str2_len + 1, 0);
-
-    // iterate through the characters of str1
-    for (int i = 1; i <= str1_len; i++) {
-        // iterate through the characters of str2
-        for (int j = 1; j <= str2_len; j++) {
-            // if characters at the current positions match
-            if (str1[i - 1] == str2[j - 1]) {
-                // if it's the first character of either string, set LCS length to 1
-                if (i == 1 || j == 1) {
-                    curr_row[j] = 1;
-                } else {
-                    // increment LCS length by 1 compared to the previous character
-                    curr_row[j] = prev_row[j - 1] + 1;
-                }
-
-                // update max_length if necessary
-                if (curr_row[j] > max_length) {
-                    max_length = curr_row[j];
-                }
-            } else {
-                // reset LCS length if characters don't match
-                curr_row[j] = 0;
-            }
-        }
-
-        // update the previous row for the next iteration
-        prev_row = curr_row;
-    }
-
-    // return the maximum length of the LCS
-    return max_length;
-}