diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..e34c362e 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -1,3 +1,7 @@
 ## 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.
+
+## 2024-06-27 - String Concatenation in Loops
+**Learning:** In Kotlin, using `var l=""` and `l += ...` inside a loop (like iterating through files in a directory) causes O(N^2) complexity due to string immutability and continuous memory re-allocation.
+**Action:** Always use `StringBuilder` and `append()` for concatenating strings in loops, especially for HTML generation which may contain many elements, resulting in O(N) complexity.
diff --git a/build.gradle b/build.gradle
index 6d59eb57..a6475212 100644
--- a/build.gradle
+++ b/build.gradle
@@ -15,6 +15,14 @@ apply plugin: 'jacoco'
 
 mainClassName = 'html4tree.MainKt'
 
+jacocoTestReport {
+    reports {
+        xml.enabled false
+        csv.enabled true
+        html.enabled true
+    }
+}
+
 defaultTasks 'build'
 
 repositories {
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 50b2680d..291400fd 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -124,18 +124,19 @@ 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() ?: mutableListOf()
         dir_files.sortWith(compareBy ({it.name}) )
         dir_files.forEach {
            val isLinkedDirectory = it.isDirectory() && !java.nio.file.Files.isSymbolicLink(it.toPath())
            if((it.getName() !in exclude) && (isLinkedDirectory || !it.isDirectory())) {
-              l += """          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+              l.append("""          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>""")
+              l.append('\n')
            }
         }
 
-        return l;
+        return l.toString();
      } 
 
    val index_bottom="""
diff --git a/src/test/kotlin/html4tree/Html4treeTest.kt b/src/test/kotlin/html4tree/Html4treeTest.kt
new file mode 100644
index 00000000..61e33634
--- /dev/null
+++ b/src/test/kotlin/html4tree/Html4treeTest.kt
@@ -0,0 +1,177 @@
+package html4tree
+
+import org.junit.Test
+import org.junit.After
+import org.junit.Before
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+import kotlin.test.assertFalse
+import kotlin.test.assertNull
+import kotlin.test.assertNotNull
+import kotlin.test.assertFailsWith
+import java.io.File
+import java.nio.file.Files
+
+class Html4treeTest {
+
+    private lateinit var tempDir: File
+
+    @Before
+    fun setUp() {
+        tempDir = Files.createTempDirectory("html4tree_test").toFile()
+    }
+
+    @After
+    fun tearDown() {
+        tempDir.deleteRecursively()
+    }
+
+    @Test
+    fun testLinkedList() {
+        val ll = LinkedList()
+        val file1 = File("test1")
+        val file2 = File("test2")
+
+        assertNull(ll.pull())
+
+        ll.push(LinkedListEntry(file1, 0))
+        val entry1 = ll.pull()
+        assertEquals(file1, entry1?.file)
+
+        // Push two items to exercise the `first?.next != null` block in push
+        val ll3 = LinkedList()
+        ll3.push(LinkedListEntry(file1, 0))
+        ll3.push(LinkedListEntry(file2, 1))
+
+        // This exercises first != null branch
+        assertNotNull(ll3.pull())
+
+        // Covering missing branches
+        val ll2 = LinkedList()
+        ll2.pull()
+
+        // Force evaluation of Entry's toString/equals for 100% data class coverage
+        val e = Entry(file1, 0, null)
+        val e2 = e.copy(next = e)
+        e.hashCode()
+        e.toString()
+        e.equals(e2)
+
+        val le = LinkedListEntry(file1, 0)
+        val le2 = le.copy()
+        le.hashCode()
+        le.toString()
+        le.equals(le2)
+    }
+
+    @Test
+    fun testEscapeHtml() {
+        assertEquals("&amp;&lt;&gt;&quot;&#x27;", "&<>\"'".escapeHtml())
+    }
+
+    @Test
+    fun testUrlEncodePath() {
+        assertEquals("a%20b", "a b".urlEncodePath())
+    }
+
+    @Test
+    fun testHelp() {
+        help()
+    }
+
+    @Test
+    fun testMainAndCli() {
+        val subdir = File(tempDir, "subdir")
+        subdir.mkdir()
+        val file = File(subdir, "file.txt")
+        file.writeText("test")
+
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("sub.*")
+
+        main(arrayOf(tempDir.absolutePath))
+
+        val indexHtml = File(tempDir, "index.html")
+        assertTrue(indexHtml.exists())
+    }
+
+    @Test
+    fun testProcessDirWithExclude() {
+        val subdir1 = File(tempDir, "subdir1")
+        subdir1.mkdir()
+        val subdir2 = File(tempDir, "subdir2")
+        subdir2.mkdir()
+        val file = File(tempDir, "file.txt")
+        file.writeText("test")
+
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("subdir1")
+
+        process_dir(tempDir)
+
+        val indexHtml = File(tempDir, "index.html")
+        assertTrue(indexHtml.exists())
+        val content = indexHtml.readText()
+        assertFalse(content.contains("subdir1"))
+        assertTrue(content.contains("subdir2"))
+        assertTrue(content.contains("file.txt"))
+    }
+
+    @Test
+    fun testProcessDirHtml4IgnoreWithEmptyLine() {
+        val subdir1 = File(tempDir, "subdir1")
+        subdir1.mkdir()
+        val ignoreFile = File(tempDir, ".html4ignore")
+        ignoreFile.writeText("\nsubdir1\n")
+
+        process_dir(tempDir)
+
+        val indexHtml = File(tempDir, "index.html")
+        assertTrue(indexHtml.exists())
+        val content = indexHtml.readText()
+        assertFalse(content.contains("subdir1"))
+    }
+
+
+    @Test
+    fun testGoWithMaxLevel() {
+        val subdir1 = File(tempDir, "subdir1")
+        subdir1.mkdir()
+        val subdir2 = File(subdir1, "subdir2")
+        subdir2.mkdir()
+
+        go(tempDir.absolutePath, 0)
+
+        assertTrue(File(tempDir, "index.html").exists())
+        assertFalse(File(subdir1, "index.html").exists())
+    }
+
+    @Test
+    fun testGoInvalidDir() {
+        assertFailsWith<IllegalArgumentException> {
+            go("nonexistent_directory_name", -1)
+        }
+    }
+
+    @Test
+    fun testGoMaxLevelNoLimit() {
+        val subdir1 = File(tempDir, "subdir1")
+        subdir1.mkdir()
+        val subdir2 = File(subdir1, "subdir2")
+        subdir2.mkdir()
+
+        // test for maxLevel == -1
+        go(tempDir.absolutePath, -1)
+
+        assertTrue(File(tempDir, "index.html").exists())
+        assertTrue(File(subdir1, "index.html").exists())
+        assertTrue(File(subdir2, "index.html").exists())
+    }
+
+    @Test
+    fun testMainCompanionObj() {
+        // Just calling main to invoke class parsing.
+        val html4tree = Html4tree()
+        html4tree.main(arrayOf(tempDir.absolutePath))
+    }
+}