From 5a778fb42dd3528846f6d61da90eb4780ff401a8 Mon Sep 17 00:00:00 2001
From: seonghobae <8172694+seonghobae@users.noreply.github.com>
Date: Tue, 30 Jun 2026 03:26:29 +0000
Subject: [PATCH] =?UTF-8?q?=E2=9A=A1=20Bolt:=20O(N^2)=20=EB=AC=B8=EC=9E=90?=
 =?UTF-8?q?=EC=97=B4=20=EC=97=B0=EA=B2=B0=20=EB=B0=8F=20O(N)=20=EC=A1=B0?=
 =?UTF-8?q?=ED=9A=8C=20=EC=84=B1=EB=8A=A5=20=EA=B0=9C=EC=84=A0?=
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디렉토리 처리 시 문자열 연결을 StringBuilder로 최적화하고, 제외 파일 목록을 Set으로 변경하여 검색 속도를 O(1)로 개선했습니다.
---
 .jules/bolt.md                    |  3 +++
 src/main/kotlin/html4tree/main.kt | 16 ++++++++--------
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..49e59dd5 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -1,3 +1,6 @@
 ## 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.
+## 2024-06-25 - Collection Lookup Performance in Loops
+**Learning:** Checking for file exclusion (`in exclude`) inside a loop iterating over directory contents can be a significant bottleneck (O(N) lookup for each file) if the `exclude` collection is a `List`.
+**Action:** Always prefer using `Set` (e.g., `mutableSetOf()`) over `List` for collections used primarily for lookup/containment checks inside loops, changing the lookup from O(N) to O(1).
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 5dca48bb..308207f4 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -54,7 +54,7 @@ fun String.urlEncodePath(): String {
     return java.net.URLEncoder.encode(this, "UTF-8").replace("+", "%20")
 }
 
-fun process_ignore_file(curr_dir: File): List<String> {
+fun process_ignore_file(curr_dir: File): Set<String> {
 
     val ignore_filename = ".html4ignore"
  
@@ -62,11 +62,12 @@ fun process_ignore_file(curr_dir: File): List<String> {
 
     val ignore_file = File(ignore_file_path)
 
-    val files_to_exclude = mutableListOf<String>()
+    val files_to_exclude = mutableSetOf<String>()
 
     if(ignore_file.exists()){
        val ignored_regexes = mutableListOf<Regex>()
 
+       // O(M) regex compilation outside of N files loop
        ignore_file.forEachLine { ignored_regexes.add(("^"+it+"$").toRegex()) }
 
        curr_dir.list().sorted().forEach {
@@ -79,15 +80,14 @@ fun process_ignore_file(curr_dir: File): List<String> {
        }
     }
 
-    if ("index.html" !in files_to_exclude)
-       files_to_exclude.add("index.html")
+    files_to_exclude.add("index.html") // Sets handle duplicates automatically, but no harm ensuring it's there
 
     return files_to_exclude
 }
  
 fun process_dir(curr_dir: File){
     
-    val exclude: List<String> = process_ignore_file(curr_dir)
+    val exclude: Set<String> = process_ignore_file(curr_dir)
 
     val css = """
               <style>
@@ -126,7 +126,7 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
+        val l = java.lang.StringBuilder()
 
         val dir_files: MutableList<File> = curr_dir.listFiles()?.toMutableList() ?: mutableListOf()
         dir_files.sortWith(compareBy ({it.name}) )
@@ -136,11 +136,11 @@ fun process_dir(curr_dir: File){
               val fileName = it.getName()
               val encodedHref = if (isLinkedDirectory) { "./${fileName.urlEncodePath()}/" } else { "./${fileName.urlEncodePath()}" }
               val ariaLabel = "${fileName} ${if (isLinkedDirectory) { "디렉토리" } else { "파일" }}".escapeHtml()
-              l += """          <li><a style="display:block; width:100%" href="${encodedHref}" aria-label="${ariaLabel}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${fileName.escapeHtml()}</a></li>"""+"\n"
+              l.append("""          <li><a style="display:block; width:100%" href="${encodedHref}" aria-label="${ariaLabel}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${fileName.escapeHtml()}</a></li>"""+"\n")
            }
         }
 
-        return l;
+        return l.toString();
      } 
 
    val index_bottom="""