From f8a2e33ee23a9711417a77d3ad61baf40236fa76 Mon Sep 17 00:00:00 2001
From: Hans-Joachim Krauch <achim.krauch@gmail.com>
Date: Tue, 17 Sep 2024 17:46:12 +0200
Subject: [PATCH] comments

---
 typescript/core/src/McapStreamReader.test.ts | 12 ++++++++----
 typescript/core/src/McapStreamReader.ts      | 17 +++++++++--------
 2 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/typescript/core/src/McapStreamReader.test.ts b/typescript/core/src/McapStreamReader.test.ts
index f4df0fbaa..4b5bdde71 100644
--- a/typescript/core/src/McapStreamReader.test.ts
+++ b/typescript/core/src/McapStreamReader.test.ts
@@ -667,9 +667,12 @@ describe("McapStreamReader", () => {
       topic: "foo",
       metadata: new Map(),
     };
+    const messageSize = 1_000;
+    const messageRecordBytes = 1 + 8 + 2 + 4 + 8 + 8 + messageSize;
+
     const makeMessage = (fillNumber: number) => ({
       channelId: 0,
-      data: new Uint8Array(1_000).fill(fillNumber),
+      data: new Uint8Array(messageSize).fill(fillNumber),
       logTime: 0n,
       publishTime: 0n,
       sequence: 0,
@@ -686,25 +689,26 @@ describe("McapStreamReader", () => {
     recordBuilder.writeMessage(makeMessage(1));
     recordBuilder.writeMessage(makeMessage(2));
     streamReader.append(recordBuilder.buffer);
-    expect(streamReader.bytesRemaining()).toBe(recordBuilder.buffer.byteLength);
-    const remainingBytes = recordBuilder.buffer.byteLength;
+    expect(streamReader.bytesRemaining()).toBe(2 * messageRecordBytes);
 
     // Add one more message. Nothing has been consumed yet, but the internal buffer should be
     // large enough to simply append the new data.
     recordBuilder.reset();
     recordBuilder.writeMessage(makeMessage(3));
     streamReader.append(recordBuilder.buffer);
-    expect(streamReader.bytesRemaining()).toBe(remainingBytes + recordBuilder.buffer.byteLength);
+    expect(streamReader.bytesRemaining()).toBe(3 * messageRecordBytes);
 
     // Read some (but not all) messages to forward the reader's internal offset
     expect(streamReader.nextRecord()).toEqual({ ...makeMessage(1), type: "Message" });
     expect(streamReader.nextRecord()).toEqual({ ...makeMessage(2), type: "Message" });
+    expect(streamReader.bytesRemaining()).toBe(1 * messageRecordBytes);
 
     // Add more messages. This will cause existing data to be shifted to the beginning of the buffer.
     recordBuilder.reset();
     recordBuilder.writeMessage(makeMessage(4));
     recordBuilder.writeMessage(makeMessage(5));
     streamReader.append(recordBuilder.buffer);
+    expect(streamReader.bytesRemaining()).toBe(3 * messageRecordBytes);
 
     expect(streamReader.nextRecord()).toEqual({ ...makeMessage(3), type: "Message" });
     expect(streamReader.nextRecord()).toEqual({ ...makeMessage(4), type: "Message" });
diff --git a/typescript/core/src/McapStreamReader.ts b/typescript/core/src/McapStreamReader.ts
index 0c10d325c..f04db4773 100644
--- a/typescript/core/src/McapStreamReader.ts
+++ b/typescript/core/src/McapStreamReader.ts
@@ -51,8 +51,8 @@ type McapReaderOptions = {
  */
 export default class McapStreamReader {
   #buffer = new ArrayBuffer(MCAP_MAGIC.length * 2);
-  #view = new DataView(this.#buffer);
-  #reader = new Reader(this.#view, MCAP_MAGIC.length * 2); // Cursor starts at end of initial buffer
+  #view = new DataView(this.#buffer, 0, 0);
+  #reader = new Reader(this.#view);
   #decompressHandlers;
   #includeChunks;
   #validateCrcs;
@@ -93,13 +93,14 @@ export default class McapStreamReader {
     }
     this.#appendOrShift(data);
   }
+
   #appendOrShift(data: Uint8Array): void {
     /** Add data to the buffer, shifting existing data or reallocating if necessary. */
     const consumedBytes = this.#reader.offset;
-    const remainingBytes = this.#view.byteLength - consumedBytes;
-    const totalNeededBytes = remainingBytes + data.byteLength;
+    const unconsumedBytes = this.#view.byteLength - consumedBytes;
+    const neededCapacity = unconsumedBytes + data.byteLength;
 
-    if (totalNeededBytes <= this.#buffer.byteLength) {
+    if (neededCapacity <= this.#buffer.byteLength) {
       // Data fits in the current buffer
       if (
         this.#view.byteOffset + this.#view.byteLength + data.byteLength <=
@@ -121,7 +122,7 @@ export default class McapStreamReader {
         const existingData = new Uint8Array(
           this.#buffer,
           this.#view.byteOffset + consumedBytes,
-          remainingBytes,
+          unconsumedBytes,
         );
         const array = new Uint8Array(this.#buffer);
         array.set(existingData, 0);
@@ -134,12 +135,12 @@ export default class McapStreamReader {
 
       // Currently, the new buffer size may be smaller than the old size. For future optimizations,
       // we could consider making the buffer size increase monotonically.
-      this.#buffer = new ArrayBuffer(totalNeededBytes * 2);
+      this.#buffer = new ArrayBuffer(neededCapacity * 2);
       const array = new Uint8Array(this.#buffer);
       const existingData = new Uint8Array(
         this.#view.buffer,
         this.#view.byteOffset + consumedBytes,
-        remainingBytes,
+        unconsumedBytes,
       );
       array.set(existingData, 0);
       array.set(data, existingData.byteLength);