Extract StringBuilder and LimitedStringBuilder with unit tests

Replace PayloadStringBuffer with LimitedStringBuilder from common.
Add thorough StringBuilder and LimitedStringBuilder test coverage.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/common/parser/ApcParser.ts

@@ -7,7 +7,7 @@ import { IApcHandler, IHandlerCollection, ApcFallbackHandlerType, IApcParser, IS
 import { ParserConstants } from 'common/parser/Constants';
 import { utf32ToString } from 'common/input/TextDecoder';
 import { IDisposable } from 'common/Types';
-import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
+import { LimitedStringBuilder } from 'common/StringBuilder';
 
 const EMPTY_HANDLERS: IApcHandler[] = [];
 
@@ -154,7 +154,7 @@ export class ApcParser implements IApcParser {
 export class ApcHandler implements IApcHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = new PayloadStringBuffer();
+  private _data = new LimitedStringBuilder(ApcHandler._payloadLimit);
   private _hitLimit: boolean = false;
 
   constructor(private _handler: (data: string) => boolean | Promise<boolean>) { }
@@ -168,7 +168,7 @@ export class ApcHandler implements IApcHandler {
     if (this._hitLimit) {
       return;
     }
-    if (this._data.add(utf32ToString(data, start, end), ApcHandler._payloadLimit)) {
+    if (this._data.append(utf32ToString(data, start, end))) {
       this._hitLimit = true;
     }
   }

src/common/parser/DcsParser.ts

@@ -8,7 +8,7 @@ import { IDcsHandler, IParams, IHandlerCollection, IDcsParser, DcsFallbackHandle
 import { utf32ToString } from 'common/input/TextDecoder';
 import { Params } from 'common/parser/Params';
 import { ParserConstants } from 'common/parser/Constants';
-import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
+import { LimitedStringBuilder } from 'common/StringBuilder';
 
 const EMPTY_HANDLERS: IDcsHandler[] = [];
 
@@ -141,7 +141,7 @@ EMPTY_PARAMS.addParam(0);
 export class DcsHandler implements IDcsHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = new PayloadStringBuffer();
+  private _data = new LimitedStringBuilder(DcsHandler._payloadLimit);
   private _params: IParams = EMPTY_PARAMS;
   private _hitLimit: boolean = false;
 
@@ -161,7 +161,7 @@ export class DcsHandler implements IDcsHandler {
     if (this._hitLimit) {
       return;
     }
-    if (this._data.add(utf32ToString(data, start, end), DcsHandler._payloadLimit)) {
+    if (this._data.append(utf32ToString(data, start, end))) {
       this._hitLimit = true;
     }
   }

src/common/parser/OscParser.ts

@@ -7,7 +7,7 @@ import { IOscHandler, IHandlerCollection, OscFallbackHandlerType, IOscParser, IS
 import { OscState, ParserConstants } from 'common/parser/Constants';
 import { utf32ToString } from 'common/input/TextDecoder';
 import { IDisposable } from 'common/Types';
-import { PayloadStringBuffer } from 'common/parser/PayloadStringBuffer';
+import { LimitedStringBuilder } from 'common/StringBuilder';
 
 const EMPTY_HANDLERS: IOscHandler[] = [];
 
@@ -195,7 +195,7 @@ export class OscParser implements IOscParser {
 export class OscHandler implements IOscHandler {
   private static _payloadLimit = ParserConstants.PAYLOAD_LIMIT;
 
-  private _data = new PayloadStringBuffer();
+  private _data = new LimitedStringBuilder(OscHandler._payloadLimit);
   private _hitLimit: boolean = false;
 
   constructor(private _handler: (data: string) => boolean | Promise<boolean>) { }
@@ -209,7 +209,7 @@ export class OscHandler implements IOscHandler {
     if (this._hitLimit) {
       return;
     }
-    if (this._data.add(utf32ToString(data, start, end), OscHandler._payloadLimit)) {
+    if (this._data.append(utf32ToString(data, start, end))) {
       this._hitLimit = true;
     }
   }

src/common/parser/PayloadStringBuffer.ts

@@ -1,34 +0,0 @@
-/**
- * Copyright (c) 2026 The xterm.js authors. All rights reserved.
- * @license MIT
- */
-
-/**
- * Accumulates string payloads from multiple chunks without O(n²) string concatenation.
- */
-export class PayloadStringBuffer {
-  private _chunks: string[] = [];
-  private _length = 0;
-
-  public reset(): void {
-    this._chunks.length = 0;
-    this._length = 0;
-  }
-
-  /**
-   * @returns true if the payload limit was exceeded (buffer is cleared in that case)
-   */
-  public add(chunk: string, limit: number): boolean {
-    this._chunks.push(chunk);
-    this._length += chunk.length;
-    if (this._length > limit) {
-      this.reset();
-      return true;
-    }
-    return false;
-  }
-
-  public toString(): string {
-    return this._chunks.join('');
-  }
-}