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diff --git a/dom/streams/QueueWithSizes.h b/dom/streams/QueueWithSizes.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_QueueWithSizes_h
+#define mozilla_dom_QueueWithSizes_h
+
+#include <cmath>
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/UniquePtr.h"
+#include "nsTArray.h"
+
+namespace mozilla::dom {
+
+// Note: Consumers of QueueWithSize need to ensure it is traced. See
+// ReadableStreamDefaultController.cpp for an example.
+
+struct ValueWithSize : LinkedListElement<ValueWithSize> {
+  ValueWithSize(JS::Handle<JS::Value> aValue, double aSize)
+      : mValue(aValue), mSize(aSize){};
+
+  JS::Heap<JS::Value> mValue;
+  double mSize = 0.0f;
+};
+
+// This type is a little tricky lifetime wise: Despite the fact that we're
+// talking about linked list of VaueWithSize, what is actually stored in the
+// list are dynamically allocated ValueWithSize*. As a result, these have to be
+// deleted. There are two ways this lifetime is managed:
+//
+// 1. In DequeueValue we pop the first element into a UniquePtr, so that it is
+//    correctly cleaned up at destruction time.
+// 2. We use an AutoCleanLinkedList which will delete elements when destroyed
+//    or `clear`ed.
+using QueueWithSizes = AutoCleanLinkedList<ValueWithSize>;
+
+// https://streams.spec.whatwg.org/#is-non-negative-number
+inline bool IsNonNegativeNumber(double v) {
+  // Step 1. Implicit.
+  // Step 2.
+  if (std::isnan(v)) {
+    return false;
+  }
+
+  // Step 3.
+  return !(v < 0);
+}
+
+// https://streams.spec.whatwg.org/#enqueue-value-with-size
+template <class QueueContainingClass>
+inline void EnqueueValueWithSize(QueueContainingClass aContainer,
+                                 JS::Handle<JS::Value> aValue, double aSize,
+                                 ErrorResult& aRv) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots. (Implicit by template instantiation.)
+  // Step 2. If ! IsNonNegativeNumber(size) is false, throw a RangeError
+  // exception.
+  if (!IsNonNegativeNumber(aSize)) {
+    aRv.ThrowRangeError("invalid size");
+    return;
+  }
+
+  // Step 3. If size is +∞, throw a RangeError exception.
+  if (std::isinf(aSize)) {
+    aRv.ThrowRangeError("Infinite queue size");
+    return;
+  }
+
+  // Step 4. Append a new value-with-size with value value and size size to
+  // container.[[queue]].
+  // (See the comment on QueueWithSizes for the lifetime reasoning around this
+  // allocation.)
+  ValueWithSize* valueWithSize = new ValueWithSize(aValue, aSize);
+  aContainer->Queue().insertBack(valueWithSize);
+  // Step 5. Set container.[[queueTotalSize]] to container.[[queueTotalSize]] +
+  // size.
+  aContainer->SetQueueTotalSize(aContainer->QueueTotalSize() + aSize);
+}
+
+// https://streams.spec.whatwg.org/#dequeue-value
+template <class QueueContainingClass>
+inline void DequeueValue(QueueContainingClass aContainer,
+                         JS::MutableHandle<JS::Value> aResultValue) {
+  // Step 1. Implicit via template instantiation.
+  // Step 2.
+  MOZ_ASSERT(!aContainer->Queue().isEmpty());
+
+  // Step 3+4
+  // UniquePtr to ensure memory is freed.
+  UniquePtr<ValueWithSize> valueWithSize(aContainer->Queue().popFirst());
+
+  // Step 5.
+  aContainer->SetQueueTotalSize(aContainer->QueueTotalSize() -
+                                valueWithSize->mSize);
+
+  // Step 6.
+  if (aContainer->QueueTotalSize() < 0) {
+    aContainer->SetQueueTotalSize(0);
+  }
+
+  // Step 7.
+  aResultValue.set(valueWithSize->mValue);
+}
+
+// https://streams.spec.whatwg.org/#peek-queue-value
+template <class QueueContainingClass>
+inline void PeekQueueValue(QueueContainingClass aContainer,
+                           JS::MutableHandle<JS::Value> aResultValue) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots.
+  // Step 2. Assert: container.[[queue]] is not empty.
+  MOZ_ASSERT(!aContainer->Queue().isEmpty());
+
+  // Step 3. Let valueWithSize be container.[[queue]][0].
+  ValueWithSize* valueWithSize = aContainer->Queue().getFirst();
+
+  // Step 4. Return valueWithSize’s value.
+  aResultValue.set(valueWithSize->mValue);
+}
+
+// https://streams.spec.whatwg.org/#reset-queue
+template <class QueueContainingClass>
+inline void ResetQueue(QueueContainingClass aContainer) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots. (implicit)
+
+  // Step 2. Set container.[[queue]] to a new empty list.
+  aContainer->Queue().clear();
+
+  // Step 3. Set container.[[queueTotalSize]] to 0.
+  aContainer->SetQueueTotalSize(0.0);
+}
+
+}  // namespace mozilla::dom
+
+#endif