fix: Preserve numRows for zero-column RecordBatch in IPC (#402)

## Summary

When a zero-column RecordBatch is deserialized from IPC,
`ensureSameLengthData`
in the `RecordBatch` constructor recomputes length from children via
`chunks.reduce((max, col) => Math.max(max, col.length), 0)`. With zero
children,
this always returns 0 — discarding the original length from the IPC
message header.

Other Arrow implementations (PyArrow, Arrow Go, arrow-rs) correctly
preserve
numRows for zero-column batches.

## Fix

Pass `this.data.length` to `ensureSameLengthData` as the explicit
`maxLength`
parameter, which the function already accepts as an optional third
argument.
For batches with columns, `this.data.length` already matches the max
column
length, so there is no behavior change.

## Tests

- Read a PyArrow-generated zero-column IPC stream (100 rows) and verify
numRows
- JS round-trip: write + read zero-column batch, verify numRows
preserved
- Direct constructor: verify zero-column RecordBatch preserves length

Closes #401

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rustyconover

src/recordbatch.ts

@@ -81,7 +81,7 @@ export class RecordBatch<T extends TypeMap = any> {
                 if (!(this.data instanceof Data)) {
                     throw new TypeError('RecordBatch constructor expects a [Schema, Data] pair.');
                 }
-                [this.schema, this.data] = ensureSameLengthData<T>(this.schema, this.data.children as Data<T[keyof T]>[]);
+                [this.schema, this.data] = ensureSameLengthData<T>(this.schema, this.data.children as Data<T[keyof T]>[], this.data.length);
                 break;
             }
             case 1: {

test/unit/ipc/reader/zero-column-batch-tests.ts

@@ -0,0 +1,75 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+import '../../../jest-extensions.js';
+import {
+    makeData,
+    RecordBatch,
+    RecordBatchStreamWriter,
+    Schema,
+    Struct,
+    tableFromIPC,
+} from 'apache-arrow';
+
+/** Helper to create a zero-column IPC stream buffer with the given number of rows. */
+function createZeroColumnIPCBuffer(numRows: number): Uint8Array {
+    const schema = new Schema([]);
+    const data = makeData({
+        type: new Struct([]),
+        length: numRows,
+        nullCount: 0,
+        children: [],
+    });
+    const batch = new RecordBatch(schema, data);
+    const writer = new RecordBatchStreamWriter();
+    writer.write(batch);
+    writer.finish();
+    return writer.toUint8Array(true);
+}
+
+describe('Zero-column RecordBatch numRows preservation', () => {
+
+    describe('IPC round-trip', () => {
+
+        test('should read zero-column stream and preserve numRows', () => {
+            const buffer = createZeroColumnIPCBuffer(100);
+            const table = tableFromIPC(buffer);
+
+            expect(table.numRows).toBe(100);
+            expect(table.numCols).toBe(0);
+            expect(table.batches).toHaveLength(1);
+            expect(table.batches[0].numRows).toBe(100);
+        });
+    });
+
+    describe('Direct constructor', () => {
+
+        test('RecordBatch constructor preserves length for zero-column data', () => {
+            const schema = new Schema([]);
+            const data = makeData({
+                type: new Struct([]),
+                length: 100,
+                nullCount: 0,
+                children: [],
+            });
+            const batch = new RecordBatch(schema, data);
+
+            expect(batch.numRows).toBe(100);
+            expect(batch.numCols).toBe(0);
+        });
+    });
+});