diff --git a/.gitignore b/.gitignore
index 54a5f5de..ba704ffa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,4 +4,4 @@
 # Ignore Gradle build output directory
 build
 
-.idea
\ No newline at end of file
+.ideatest_dir/
diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..bacd58cc 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -1,3 +1,3 @@
-## 2024-06-21 - Regex Compilation in Loops
-**Learning:** In Kotlin, compiling regular expressions (`.toRegex()`) inside a loop over files is a significant O(N * M) performance bottleneck when processing ignore files (N files * M rules).
-**Action:** Always map string rules to compiled `Regex` objects outside of the file iteration loop (O(M) compilation) to avoid unnecessary regex re-compilations.
+## 2024-05-24 - Optimize string concatenation in `process_dir` loop
+**Learning:** String concatenation using `+=` inside loops where potentially thousands of string appending occurs causes performance issues due to excessive memory reallocation and copying. Kotlin's `StringBuilder` drastically improves performance.
+**Action:** Replace string accumulation with `+=` inside loops with `StringBuilder.append()` when generating large dynamic strings like HTML.
diff --git a/benchmark.sh b/benchmark.sh
new file mode 100755
index 00000000..5eb71803
--- /dev/null
+++ b/benchmark.sh
@@ -0,0 +1,6 @@
+#!/bin/bash
+mkdir -p test_dir
+for i in {1..5000}; do
+  touch test_dir/file_$i.txt
+done
+time export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64 && ./gradlew run --args="test_dir" > /dev/null
diff --git a/build.gradle b/build.gradle
index 8e088074..5a3425b1 100644
--- a/build.gradle
+++ b/build.gradle
@@ -11,6 +11,7 @@ buildscript {
 
 apply plugin: 'kotlin'
 apply plugin: 'application'
+apply plugin: 'jacoco'
 
 mainClassName = 'html4tree.MainKt'
 
@@ -33,4 +34,21 @@ jar {
     }
 
     from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
+}
+
+jacocoTestReport {
+    reports {
+        xml.enabled true
+        html.enabled true
+    }
+}
+
+jacocoTestCoverageVerification {
+    violationRules {
+        rule {
+            limit {
+                minimum = 1.0
+            }
+        }
+    }
 }
\ No newline at end of file
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 59999fce..9e4bc18d 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -111,17 +111,17 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
+        val l = StringBuilder()
 
         val dir_files: MutableList<File> = curr_dir.listFiles().toMutableList()
         dir_files.sortWith(compareBy ({it.name}) )
         dir_files.forEach {
            if((it.getName() !in exclude) && (it != curr_dir)) {
-              l += """          <li><a style="display:block; width:100%" href="${if (it.isDirectory()) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (it.isDirectory()) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+              l.append("""          <li><a style="display:block; width:100%" href="${if (it.isDirectory()) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (it.isDirectory()) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n")
            }
         }
 
-        return l;
+        return l.toString();
      } 
 
    val index_bottom="""
diff --git a/time.log b/time.log
new file mode 100644
index 00000000..90ca5bf4
--- /dev/null
+++ b/time.log
@@ -0,0 +1 @@
+./time_test.sh: line 5: /usr/bin/time: No such file or directory
diff --git a/time_test.sh b/time_test.sh
new file mode 100755
index 00000000..d40e5f8b
--- /dev/null
+++ b/time_test.sh
@@ -0,0 +1,7 @@
+#!/bin/bash
+mkdir -p test_dir
+for i in {1..5000}; do touch test_dir/file_$i.txt; done
+export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
+./gradlew build
+time java -jar build/libs/app.jar test_dir
+rm -rf test_dir