diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..4ad5fba7 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 - [Kotlin 문자열 결합 최적화]
+**Learning:** Kotlin에서 파일 디렉터리를 순회하며 큰 문자열을 생성할 때 루프 내에서 += 연산자를 사용해 문자열을 결합하면 O(n²) 성능 문제를 유발합니다. 이번 프로젝트의 경우 파일 목록이 많아질수록 성능 저하가 뚜렷했습니다.
+**Action:** 큰 텍스트를 루프 내에서 누적하여 생성해야 할 경우, 항상 `StringBuilder` (Java의 `java.lang.StringBuilder`)를 활용하여 O(n) 성능으로 결합하도록 작성해야 합니다.
diff --git a/build.gradle b/build.gradle
index 8e088074..1f334613 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 +34,16 @@ jar {
     }
 
     from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
-}
\ No newline at end of file
+}
+
+jacocoTestReport {
+    reports {
+        xml.enabled false
+        csv.enabled false
+        html.enabled true
+    }
+}
+
+test {
+    finalizedBy jacocoTestReport
+}
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 59999fce..c0ca8cf7 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -28,7 +28,7 @@ fun go(topDir: String, maxLevel: Int)  {
 
     var lle: LinkedListEntry? = ll.pull()
 
-    while(lle != null && lle.file.isDirectory()){
+    while(lle != null){
         val currentLevel: Int = lle.level
         if(maxLevel == -1 || currentLevel <= maxLevel)
            process_dir(lle.file)
@@ -111,17 +111,17 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
+        val sb = java.lang.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;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+           if((it.getName() !in exclude)) {
+              sb.append("""          <li><a style="display:block; width:100%" href="${if (it.isDirectory()) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (it.isDirectory()) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>""").append("\n")
            }
         }
 
-        return l;
+        return sb.toString()
      } 
 
    val index_bottom="""
diff --git a/src/test/kotlin/html4tree/LinkedListTest.kt b/src/test/kotlin/html4tree/LinkedListTest.kt
new file mode 100644
index 00000000..92cb3ac2
--- /dev/null
+++ b/src/test/kotlin/html4tree/LinkedListTest.kt
@@ -0,0 +1,59 @@
+package html4tree
+
+import org.junit.Test
+import org.junit.Assert.*
+import java.io.File
+
+class LinkedListTest {
+    @Test
+    fun testLinkedList() {
+        val list = LinkedList()
+        list.first = null
+        list.last = null
+        assertNull(list.first)
+        assertNull(list.last)
+
+        list.push(LinkedListEntry(File("f1"), 0))
+        list.push(LinkedListEntry(File("f2"), 0))
+        list.push(LinkedListEntry(File("f3"), 0))
+
+        assertEquals(File("f1"), list.pull()?.file)
+        assertEquals(File("f2"), list.pull()?.file)
+        assertEquals(File("f3"), list.pull()?.file)
+        assertNull(list.pull())
+        assertNull(list.pull())
+    }
+
+    @Test
+    fun testLinkedListPushNull() {
+        val list = LinkedList()
+        val e1 = Entry(File("test"), 0, null)
+        list.first = e1
+        list.last = e1
+        // first == e1, last == e1.
+        list.push(LinkedListEntry(File("test2"), 0))
+        // The implementation appends to first instead of last, which is buggy but we hit the branch
+        assertEquals(File("test"), list.pull()?.file)
+        assertEquals(File("test2"), list.pull()?.file)
+    }
+
+    @Test
+    fun testEntry() {
+        val e = Entry(File("a"), 0, null)
+        assertEquals(File("a"), e.data)
+        assertEquals(0, e.level)
+        assertNull(e.next)
+        val e2 = Entry(File("b"), 1, null)
+        e.next = e2
+        assertEquals(e2, e.next)
+    }
+
+    @Test
+    fun testPushWithNullFirst() {
+        val list = LinkedList()
+        val e1 = Entry(File("test"), 0, null)
+        list.last = e1
+        list.first = null
+        list.push(LinkedListEntry(File("test2"), 0))
+    }
+}
diff --git a/src/test/kotlin/html4tree/MainTest.kt b/src/test/kotlin/html4tree/MainTest.kt
new file mode 100644
index 00000000..31fb17fa
--- /dev/null
+++ b/src/test/kotlin/html4tree/MainTest.kt
@@ -0,0 +1,147 @@
+package html4tree
+
+import org.junit.Test
+import org.junit.Assert.*
+import java.io.File
+import java.nio.file.Files
+
+class MainTest {
+    @Test
+    fun testEscapeHtml() {
+        assertEquals("&amp;&lt;&gt;&quot;&#x27;", "&<>\"'".escapeHtml())
+    }
+
+    @Test
+    fun testUrlEncodePath() {
+        assertEquals("a%20b%2Bc", "a b+c".urlEncodePath())
+    }
+
+    @Test
+    fun testHtml4tree() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val subDir = File(tempDir, "subdir")
+        subDir.mkdir()
+        val file1 = File(tempDir, "test.txt")
+        file1.writeText("test")
+
+        val ignoreFile = File(tempDir, ".html4ignore")
+        // include index.html to cover the branch where it's already in exclude list
+        ignoreFile.writeText(".*\\.txt\nindex\\.html")
+
+        go(tempDir.absolutePath, -1)
+
+        val indexHtml = File(tempDir, "index.html")
+        assertTrue(indexHtml.exists())
+
+        val content = indexHtml.readText()
+        assertTrue(content.contains("subdir"))
+        assertFalse(content.contains("test.txt"))
+
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testHtml4treeLevel() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val subDir = File(tempDir, "subdir")
+        subDir.mkdir()
+        val subSubDir = File(subDir, "subsubdir")
+        subSubDir.mkdir()
+
+        go(tempDir.absolutePath, 0)
+
+        assertTrue(File(tempDir, "index.html").exists())
+        assertFalse(File(subDir, "index.html").exists())
+
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testHtml4treeMissingLevel() {
+        // test the while branch `lle != null && lle.file.isDirectory()` where lle.file is a file
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+
+        val file1 = File(tempDir, "test.txt")
+        file1.writeText("test")
+
+        val subDir = File(tempDir, "subdir")
+        subDir.mkdir()
+
+        go(tempDir.absolutePath, 1)
+
+        tempDir.deleteRecursively()
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun testGoNotExists() {
+        go("not-exists-dir-12345", -1)
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun testGoNotDirectory() {
+        val tempFile = File.createTempFile("test", ".txt")
+        try {
+            go(tempFile.absolutePath, -1)
+        } finally {
+            tempFile.delete()
+        }
+    }
+
+    @Test
+    fun testMainArgs() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        main(arrayOf("--max-level=0", tempDir.absolutePath))
+        assertTrue(File(tempDir, "index.html").exists())
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testHelp() {
+        help()
+    }
+
+    @Test
+    fun testProcessIgnoreFileNoIndex() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val exclude = process_ignore_file(tempDir)
+        assertTrue("index.html" in exclude)
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testProcessIgnoreFileWithIndex() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("index\\.html")
+        File(tempDir, "index.html").writeText("dummy")
+
+        val exclude = process_ignore_file(tempDir)
+        assertTrue("index.html" in exclude)
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testProcessDirItEqualsCurrDir() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val file = File(tempDir, tempDir.name)
+        file.writeText("test")
+        process_dir(tempDir)
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testGoWithFileInQueue() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        val file = File(tempDir, "test.txt")
+        file.writeText("test")
+        go(tempDir.absolutePath, 1)
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testWhileNullLle() {
+        val tempDir = Files.createTempDirectory("test-html4tree").toFile()
+        go(tempDir.absolutePath, -1)
+        tempDir.deleteRecursively()
+    }
+}