diff --git a/README.md b/README.md
index 65062d0..e4ccaa6 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@ Texinfo language support for Visual Studio Code.
 </details>
 
 <details>
-<summary>Block Folding</summary>
+<summary>Folding</summary>
 (Screenshots here...)
 </details>
 
diff --git a/package-lock.json b/package-lock.json
index 201e094..7325f68 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -61,6 +61,11 @@
                 "strip-json-comments": "^3.1.1"
             }
         },
+        "@flatten-js/interval-tree": {
+            "version": "1.0.12",
+            "resolved": "https://registry.npmjs.org/@flatten-js/interval-tree/-/interval-tree-1.0.12.tgz",
+            "integrity": "sha512-j2o14WdFPII5cI57j0XNSWQm80gM4G6RT5+NLaH8q7KmQKejR/qZiGiViMjRgMtPiiwaX6hv3hlXeyRL3yzi7g=="
+        },
         "@types/eslint-visitor-keys": {
             "version": "1.0.0",
             "resolved": "https://registry.npmjs.org/@types/eslint-visitor-keys/-/eslint-visitor-keys-1.0.0.tgz",
diff --git a/package.json b/package.json
index 7cdd430..ed53f7f 100644
--- a/package.json
+++ b/package.json
@@ -25,6 +25,7 @@
         "typescript": "^4.0.3"
     },
     "dependencies": {
+        "@flatten-js/interval-tree": "^1.0.12",
         "node-html-parser": "^1.3.1"
     },
     "scripts": {
diff --git a/src/completion.ts b/src/completion.ts
index 84960b9..dcae706 100644
--- a/src/completion.ts
+++ b/src/completion.ts
@@ -8,7 +8,7 @@
 import * as vscode from 'vscode';
 
 /**
- * Texinfo language completion item provider.
+ * Provide code completion info for Texinfo source code.
  */
 export class CompletionItemProvider implements vscode.CompletionItemProvider {
 
diff --git a/src/converter.ts b/src/converter.ts
index e22263e..95e170e 100644
--- a/src/converter.ts
+++ b/src/converter.ts
@@ -5,13 +5,13 @@
  * @license MIT
  */
 
-import { Options } from './options';
+import Options from './options';
 import * as utils from './utils';
 
 /**
  * Texinfo to HTML converter.
  */
-export class Converter {
+export default class Converter {
 
     /**
      * Convert a Texinfo document to HTML.
diff --git a/src/extension.ts b/src/extension.ts
index fa03879..76cc645 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -6,8 +6,8 @@
  */
 
 import * as vscode from 'vscode';
-import { Options } from './options';
-import { Preview } from './preview';
+import Options from './options';
+import Preview from './preview';
 import { CompletionItemProvider } from './completion';
 import { FoldingRangeProvider, FoldingRangeContext } from './folding';
 
diff --git a/src/folding.ts b/src/folding.ts
index 6284f28..ca75965 100644
--- a/src/folding.ts
+++ b/src/folding.ts
@@ -5,6 +5,7 @@
  * @license MIT
  */
 
+import IntervalTree from '@flatten-js/interval-tree';
 import * as vscode from 'vscode';
 
 /**
@@ -35,7 +36,7 @@ export class FoldingRangeContext {
         if (event.document.languageId !== 'texinfo') {
             return;
         }
-        FoldingRangeContext.get(event.document)?.update(event.contentChanges);
+        FoldingRangeContext.get(event.document).update(event.contentChanges);
     }
 
     static close(document: vscode.TextDocument) {
@@ -46,9 +47,15 @@ export class FoldingRangeContext {
         FoldingRangeContext.map.clear();
     }
 
-    foldingRanges = <vscode.FoldingRange[]>[];
+    get foldingRanges(): vscode.FoldingRange[] {
+        return this.tempFoldingRanges ?? (this.tempFoldingRanges = this.intervalTree.values);
+    }
 
-    private commentRange?: vscode.FoldingRange;
+    private intervalTree = new IntervalTree();
+
+    private tempFoldingRanges?: vscode.FoldingRange[];
+
+    private commentRange?: [number, number];
 
     private headerStart?: number;
 
@@ -56,7 +63,9 @@ export class FoldingRangeContext {
 
     private constructor(private readonly document: vscode.TextDocument) {
         FoldingRangeContext.map.set(document, this);
+        console.log(Date.now());
         this.calculateFoldingRanges();
+        console.log(Date.now());
     }
 
     private calculateFoldingRanges() {
@@ -73,8 +82,8 @@ export class FoldingRangeContext {
             this.processBlock(lineText, lineNum);
         }
         if (this.commentRange !== undefined) {
-            if (this.commentRange.end - this.commentRange.start > 1) {
-                this.foldingRanges.push(this.commentRange);
+            if (this.commentRange[1] - this.commentRange[0] > 1) {
+                this.insertRange(this.commentRange);
             }
         }
     }
@@ -89,17 +98,17 @@ export class FoldingRangeContext {
                 if (this.headerStart === undefined) {
                     this.headerStart = lineNum;
                 } else {
-                    this.foldingRanges.push(new vscode.FoldingRange(lineNum, this.headerStart));
+                    this.insertRange([lineNum, this.headerStart]);
                     this.headerStart = undefined;
                 }
             }
             if (this.commentRange === undefined) {
-                this.commentRange = new vscode.FoldingRange(lineNum, lineNum, vscode.FoldingRangeKind.Comment);
-            } else if (this.commentRange.start - 1 === lineNum) {
-                this.commentRange.start = lineNum;
+                this.commentRange = [lineNum, lineNum];
+            } else if (this.commentRange[0] - 1 === lineNum) {
+                this.commentRange[0] = lineNum;
             } else {
-                this.foldingRanges.push(this.commentRange);
-                this.commentRange = new vscode.FoldingRange(lineNum, lineNum, vscode.FoldingRangeKind.Comment);
+                this.insertRange(this.commentRange, vscode.FoldingRangeKind.Comment);
+                this.commentRange = [lineNum, lineNum];
             }
             return true;
         }
@@ -115,15 +124,28 @@ export class FoldingRangeContext {
                 return;
             }
             if (lineText.substring(1, closingBlock.name.length + 2).trim() === closingBlock.name) {
-                this.foldingRanges.push(new vscode.FoldingRange(lineNum, closingBlock.line));
+                this.insertRange([lineNum, closingBlock.line]);
             } else {
                 this.closingBlocks.push(closingBlock);
             }
         }
     }
 
+    private insertRange(range: [start: number, end: number], kind?: vscode.FoldingRangeKind) {
+        const foldingRange = new vscode.FoldingRange(range[0], range[1], kind);
+        this.intervalTree.insert(range, foldingRange);
+    }
+
     private update(events: readonly vscode.TextDocumentContentChangeEvent[]) {
-        // console.log(events);
+        console.log(events);
+    }
+
+    private clear() {
+        this.intervalTree = new IntervalTree();
+        this.tempFoldingRanges = undefined;
+        this.commentRange = undefined;
+        this.headerStart = undefined;
+        this.closingBlocks = [];
     }
 }
 
diff --git a/src/options.ts b/src/options.ts
index 42c2044..8bb93e2 100644
--- a/src/options.ts
+++ b/src/options.ts
@@ -14,7 +14,7 @@ let options: Options | undefined;
  * 
  * See `contributes.configuration` of package.json for details.
  */
-export class Options {
+export default class Options {
 
     private static get instance() {
         return options ?? (options = new Options('texinfo'));
diff --git a/src/preview.ts b/src/preview.ts
index a1c294c..c2ef413 100644
--- a/src/preview.ts
+++ b/src/preview.ts
@@ -7,14 +7,14 @@
 
 import * as path from 'path';
 import * as vscode from 'vscode';
-import { Converter } from './converter';
-import { Options } from './options';
+import Converter from './converter';
+import Options from './options';
 import * as utils from './utils';
 
 /**
  * Texinfo document preview.
  */
-export class Preview {
+export default class Preview {
 
     private static readonly map = new Map<vscode.TextDocument, Preview>();