MemoryStream: add support to limit size of array buffer

lib/MemoryStream.js

@@ -1,13 +1,29 @@
 const { Writable } = require('stream');
 
 class MemoryStream extends Writable {
-  constructor() {
+  constructor(maxSize = -1) {
     super();
     this.buffer = [];
+    this.size = 0;
+    this.maxSize = maxSize;
+    this.truncated = false;
   }
 
   _write(chunk, encoding, done) {
-    this.buffer.push(chunk.toString());
+    if (this.truncated) {
+      done();
+      return;
+    }
+
+    let strChunk = chunk.toString();
+
+    if (this.maxSize > 0 && (this.size + strChunk.length) >= this.maxSize) {
+      strChunk = strChunk.slice(0, this.maxSize - this.size);
+      this.truncated = true;
+    }
+
+    this.size += strChunk.length;
+    this.buffer.push(strChunk);
     done();
   }
 }

test/MemoryStream.test.js

@@ -0,0 +1,35 @@
+const expect = require('chai').expect;
+const MemoryStream = require('../lib/MemoryStream');
+
+describe('MemoryStream', () => {
+  describe('#write', () => {
+    it('should write into buffer array', () => {
+      const stream = new MemoryStream();
+      stream.write('testing');
+      stream.write('foo');
+
+      expect(stream.buffer).to.be.eql([
+        'testing',
+        'foo',
+      ]);
+      expect(stream.size).to.be.eql(10);
+      expect(stream.truncated).to.be.false;
+    });
+
+    describe('when stream have a maxSize', () => {
+      it('should limit the size of array buffer', () => {
+        const stream = new MemoryStream(9);
+        stream.write('testing');
+        stream.write('foo');
+        stream.write('bar');
+
+        expect(stream.buffer).to.be.eql([
+          'testing',
+          'fo',
+        ]);
+        expect(stream.size).to.be.eql(9);
+        expect(stream.truncated).to.be.true;
+      });
+    });
+  });
+});