/*
 * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
 * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <LibJS/Runtime/PromiseCapability.h>
#include <LibJS/Runtime/PromiseConstructor.h>
#include <LibWeb/Bindings/ExceptionOrUtils.h>
#include <LibWeb/Streams/AbstractOperations.h>
#include <LibWeb/Streams/ReadableStream.h>
#include <LibWeb/Streams/ReadableStreamDefaultController.h>
#include <LibWeb/Streams/ReadableStreamDefaultReader.h>
#include <LibWeb/Streams/ReadableStreamGenericReader.h>
#include <LibWeb/Streams/UnderlyingSink.h>
#include <LibWeb/Streams/UnderlyingSource.h>
#include <LibWeb/Streams/WritableStream.h>
#include <LibWeb/Streams/WritableStreamDefaultController.h>
#include <LibWeb/Streams/WritableStreamDefaultWriter.h>
#include <LibWeb/WebIDL/AbstractOperations.h>
#include <LibWeb/WebIDL/ExceptionOr.h>
#include <LibWeb/WebIDL/Promise.h>

namespace Web::Streams {

// https://streams.spec.whatwg.org/#acquire-readable-stream-reader
WebIDL::ExceptionOr<JS::NonnullGCPtr<ReadableStreamDefaultReader>> acquire_readable_stream_default_reader(ReadableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Let reader be a new ReadableStreamDefaultReader.
    auto reader = TRY(realm.heap().allocate<ReadableStreamDefaultReader>(realm, realm));

    // 2. Perform ? SetUpReadableStreamDefaultReader(reader, stream).
    TRY(set_up_readable_stream_default_reader(reader, stream));

    // 3. Return reader.
    return reader;
}

// https://streams.spec.whatwg.org/#is-readable-stream-locked
bool is_readable_stream_locked(ReadableStream const& stream)
{
    // 1. If stream.[[reader]] is undefined, return false.
    if (!stream.reader())
        return false;

    // 2. Return true.
    return true;
}

// https://streams.spec.whatwg.org/#readable-stream-cancel
WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> readable_stream_cancel(ReadableStream& stream, JS::Value reason)
{
    auto& realm = stream.realm();

    // 1. Set stream.[[disturbed]] to true.
    stream.set_disturbed(true);

    // 2. If stream.[[state]] is "closed", return a promise resolved with undefined.
    if (stream.is_closed())
        return WebIDL::create_resolved_promise(realm, JS::js_undefined());

    // 3. If stream.[[state]] is "errored", return a promise rejected with stream.[[storedError]].
    if (stream.is_errored())
        return WebIDL::create_rejected_promise(realm, stream.stored_error());

    // 4. Perform ! ReadableStreamClose(stream).
    readable_stream_close(stream);

    // 5. Let reader be stream.[[reader]].
    auto reader = stream.reader();

    // FIXME:
    // 6. If reader is not undefined and reader implements ReadableStreamBYOBReader,
    //    1. Let readIntoRequests be reader.[[readIntoRequests]].
    //    2. Set reader.[[readIntoRequests]] to an empty list.
    //    3. For each readIntoRequest of readIntoRequests,
    //       1. Perform readIntoRequest’s close steps, given undefined.
    (void)reader;

    // 7. Let sourceCancelPromise be ! stream.[[controller]].[[CancelSteps]](reason).
    auto source_cancel_promise = MUST(stream.controller()->cancel_steps(reason));

    // 8. Return the result of reacting to sourceCancelPromise with a fulfillment step that returns undefined.
    auto react_result = WebIDL::react_to_promise(*source_cancel_promise,
        [](auto const&) -> WebIDL::ExceptionOr<JS::Value> { return JS::js_undefined(); },
        {});

    return WebIDL::create_resolved_promise(realm, react_result);
}

// https://streams.spec.whatwg.org/#readable-stream-fulfill-read-request
void readable_stream_fulfill_read_request(ReadableStream& stream, JS::Value chunk, bool done)
{
    // 1. Assert: ! ReadableStreamHasDefaultReader(stream) is true.
    VERIFY(readable_stream_has_default_reader(stream));

    // 2. Let reader be stream.[[reader]].
    auto& reader = *stream.reader();

    // 3. Assert: reader.[[readRequests]] is not empty.
    VERIFY(!reader.read_requests().is_empty());

    // 4. Let readRequest be reader.[[readRequests]][0].
    // 5. Remove readRequest from reader.[[readRequests]].
    auto read_request = reader.read_requests().take_first();

    // 6. If done is true, perform readRequest’s close steps.
    if (done) {
        read_request->on_close();
    }
    // 7. Otherwise, perform readRequest’s chunk steps, given chunk.
    else {
        read_request->on_chunk(chunk);
    }
}

// https://streams.spec.whatwg.org/#readable-stream-get-num-read-requests
size_t readable_stream_get_num_read_requests(ReadableStream& stream)
{
    // 1. Assert: ! ReadableStreamHasDefaultReader(stream) is true.
    VERIFY(readable_stream_has_default_reader(stream));

    // 2. Return stream.[[reader]].[[readRequests]]'s size.
    return stream.reader()->read_requests().size();
}

// https://streams.spec.whatwg.org/#readable-stream-has-default-reader
bool readable_stream_has_default_reader(ReadableStream& stream)
{
    // 1. Let reader be stream.[[reader]].
    auto reader = stream.reader();

    // 2. If reader is undefined, return false.
    if (!reader)
        return false;

    // 3. If reader implements ReadableStreamDefaultReader, return true.
    if (reader->is_default_reader())
        return true;

    // 4. Return false.
    return false;
}

// https://streams.spec.whatwg.org/#readable-stream-close
void readable_stream_close(ReadableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[state]] is "readable".
    VERIFY(stream.is_readable());

    // 2. Set stream.[[state]] to "closed".
    stream.set_stream_state(ReadableStream::State::Closed);

    // 3. Let reader be stream.[[reader]].
    auto reader = stream.reader();

    // 4. If reader is undefined, return.
    if (!reader)
        return;

    // 5. Resolve reader.[[closedPromise]] with undefined.
    WebIDL::resolve_promise(realm, *reader->closed_promise_capability());

    // 6. If reader implements ReadableStreamDefaultReader,
    if (reader->is_default_reader()) {
        // 1. Let readRequests be reader.[[readRequests]].
        // 2. Set reader.[[readRequests]] to an empty list.
        auto read_requests = move(reader->read_requests());

        // 3. For each readRequest of readRequests,
        for (auto& read_request : read_requests) {
            // 1. Perform readRequest’s close steps.
            read_request->on_close();
        }
    }
}

// https://streams.spec.whatwg.org/#readable-stream-error
void readable_stream_error(ReadableStream& stream, JS::Value error)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[state]] is "readable".
    VERIFY(stream.is_readable());

    // 2. Set stream.[[state]] to "errored".
    stream.set_stream_state(ReadableStream::State::Errored);

    // 3. Set stream.[[storedError]] to e.
    stream.set_stored_error(error);

    // 4. Let reader be stream.[[reader]].
    auto reader = stream.reader();

    // 5. If reader is undefined, return.
    if (!reader)
        return;

    // 6. Reject reader.[[closedPromise]] with e.
    WebIDL::reject_promise(realm, *reader->closed_promise_capability(), error);

    // 7. Set reader.[[closedPromise]].[[PromiseIsHandled]] to true.
    WebIDL::mark_promise_as_handled(*reader->closed_promise_capability());

    // 8. If reader implements ReadableStreamDefaultReader,
    if (reader->is_default_reader()) {
        // 1. Perform ! ReadableStreamDefaultReaderErrorReadRequests(reader, e).
        readable_stream_default_reader_error_read_requests(*reader, error);
    }
    // 9. Otherwise,
    else {
        // 1. Assert: reader implements ReadableStreamBYOBReader.
        // 2. Perform ! ReadableStreamBYOBReaderErrorReadIntoRequests(reader, e).

        // FIXME: Handle BYOBReader
        TODO();
    }
}

// https://streams.spec.whatwg.org/#readable-stream-add-read-request
void readable_stream_add_read_request(ReadableStream& stream, ReadRequest const& read_request)
{
    // FIXME: Check implementation type
    // 1. Assert: stream.[[reader]] implements ReadableStreamDefaultReader.
    VERIFY(stream.reader());

    // 2. Assert: stream.[[state]] is "readable".
    VERIFY(stream.is_readable());

    // 3. Append readRequest to stream.[[reader]].[[readRequests]].
    stream.reader()->read_requests().append(read_request);
}

// https://streams.spec.whatwg.org/#readable-stream-reader-generic-cancel
JS::NonnullGCPtr<WebIDL::Promise> readable_stream_reader_generic_cancel(ReadableStreamGenericReaderMixin& reader, JS::Value reason)
{
    // 1. Let stream be reader.[[stream]]
    auto stream = reader.stream();

    // 2. Assert: stream is not undefined
    VERIFY(stream);

    // 3. Return ! ReadableStreamCancel(stream, reason)
    return MUST(readable_stream_cancel(*stream, reason));
}

// https://streams.spec.whatwg.org/#readable-stream-reader-generic-initialize
void readable_stream_reader_generic_initialize(ReadableStreamGenericReaderMixin& reader, ReadableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Set reader.[[stream]] to stream.
    reader.set_stream(stream);

    // 2. Set stream.[[reader]] to reader.
    if (reader.is_default_reader()) {
        stream.set_reader(static_cast<ReadableStreamDefaultReader&>(reader));
    } else {
        // FIXME: Handle other descendents of ReadableStreamGenericReaderMixin (i.e. BYOBReader)
        TODO();
    }

    // 3. If stream.[[state]] is "readable",
    if (stream.is_readable()) {
        // 1. Set reader.[[closedPromise]] to a new promise.
        reader.set_closed_promise_capability(WebIDL::create_promise(realm));
    }
    // 4. Otherwise, if stream.[[state]] is "closed",
    else if (stream.is_closed()) {
        // 1. Set reader.[[closedPromise]] to a promise resolved with undefined.
        reader.set_closed_promise_capability(WebIDL::create_resolved_promise(realm, JS::js_undefined()));
    }
    // 5. Otherwise,
    else {
        // 1. Assert: stream.[[state]] is "errored".
        VERIFY(stream.is_errored());

        // 2. Set reader.[[closedPromise]] to a promise rejected with stream.[[storedError]].
        reader.set_closed_promise_capability(WebIDL::create_rejected_promise(realm, stream.stored_error()));

        // 3. Set reader.[[closedPromise]].[[PromiseIsHandled]] to true.
        WebIDL::mark_promise_as_handled(*reader.closed_promise_capability());
    }
}

// https://streams.spec.whatwg.org/#readable-stream-reader-generic-release
WebIDL::ExceptionOr<void> readable_stream_reader_generic_release(ReadableStreamGenericReaderMixin& reader)
{
    // 1. Let stream be reader.[[stream]].
    auto stream = reader.stream();

    // 2. Assert: stream is not undefined.
    VERIFY(stream);

    // 3. Assert: stream.[[reader]] is reader.
    VERIFY(stream->reader().ptr() == &reader);

    // 4. If stream.[[state]] is "readable", reject reader.[[closedPromise]] with a TypeError exception.
    auto exception = TRY(JS::TypeError::create(stream->realm(), "Released readable stream"sv));
    if (stream->is_readable()) {
        WebIDL::reject_promise(stream->realm(), *reader.closed_promise_capability(), exception);
    }
    // 5. Otherwise, set reader.[[closedPromise]] to a promise rejected with a TypeError exception.
    else {
        reader.set_closed_promise_capability(WebIDL::create_rejected_promise(stream->realm(), exception));
    }

    // 6. Set reader.[[closedPromise]].[[PromiseIsHandled]] to true.
    WebIDL::mark_promise_as_handled(*reader.closed_promise_capability());

    // 7. Perform ! stream.[[controller]].[[ReleaseSteps]]().
    stream->controller()->release_steps();

    // 8. Set stream.[[reader]] to undefined.
    stream->set_reader({});

    // 9. Set reader.[[stream]] to undefined.
    reader.set_stream({});

    return {};
}

// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaultreadererrorreadrequests
void readable_stream_default_reader_error_read_requests(ReadableStreamDefaultReader& reader, JS::Value error)
{
    // 1. Let readRequests be reader.[[readRequests]].
    // 2. Set reader.[[readRequests]] to a new empty list.
    auto read_requests = move(reader.read_requests());

    // 3. For each readRequest of readRequests,
    for (auto& read_request : read_requests) {
        // 1. Perform readRequest’s error steps, given e.
        read_request->on_error(error);
    }
}

// https://streams.spec.whatwg.org/#readable-stream-default-reader-read
void readable_stream_default_reader_read(ReadableStreamDefaultReader& reader, ReadRequest& read_request)
{
    // 1. Let stream be reader.[[stream]].
    auto stream = reader.stream();

    // 2. Assert: stream is not undefined.
    VERIFY(stream);

    // 3. Set stream.[[disturbed]] to true.
    stream->set_disturbed(true);

    // 4. If stream.[[state]] is "closed", perform readRequest’s close steps.
    if (stream->is_closed()) {
        read_request.on_close();
    }
    // 5. Otherwise, if stream.[[state]] is "errored", perform readRequest’s error steps given stream.[[storedError]].
    else if (stream->is_errored()) {
        read_request.on_error(stream->stored_error());
    }
    // 6. Otherwise,
    else {
        // 1. Assert: stream.[[state]] is "readable".
        VERIFY(stream->is_readable());

        // 2. Perform ! stream.[[controller]].[[PullSteps]](readRequest).
        MUST(stream->controller()->pull_steps(read_request));
    }
}

// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaultreaderrelease
WebIDL::ExceptionOr<void> readable_stream_default_reader_release(ReadableStreamDefaultReader& reader)
{
    // 1. Perform ! ReadableStreamReaderGenericRelease(reader).
    TRY(readable_stream_reader_generic_release(reader));

    // 2. Let e be a new TypeError exception.
    auto e = TRY(JS::TypeError::create(reader.realm(), "Reader has been released"sv));

    // 3. Perform ! ReadableStreamDefaultReaderErrorReadRequests(reader, e).
    readable_stream_default_reader_error_read_requests(reader, e);

    return {};
}

// https://streams.spec.whatwg.org/#set-up-readable-stream-default-reader
WebIDL::ExceptionOr<void> set_up_readable_stream_default_reader(ReadableStreamDefaultReader& reader, ReadableStream& stream)
{
    // 1. If ! IsReadableStreamLocked(stream) is true, throw a TypeError exception.
    if (is_readable_stream_locked(stream))
        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, "Cannot create stream reader for a locked stream"sv };

    // 2. Perform ! ReadableStreamReaderGenericInitialize(reader, stream).
    // 3. Set reader.[[readRequests]] to a new empty list.
    readable_stream_reader_generic_initialize(reader, stream);

    return {};
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-close
void readable_stream_default_controller_close(ReadableStreamDefaultController& controller)
{
    // 1. If ! ReadableStreamDefaultControllerCanCloseOrEnqueue(controller) is false, return.
    if (!readable_stream_default_controller_can_close_or_enqueue(controller))
        return;

    // 2. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 3. Set controller.[[closeRequested]] to true.
    controller.set_close_requested(true);

    // 4. If controller.[[queue]] is empty,
    if (controller.queue().is_empty()) {
        // 1. Perform ! ReadableStreamDefaultControllerClearAlgorithms(controller).
        readable_stream_default_controller_clear_algorithms(controller);

        // 2. Perform ! ReadableStreamClose(stream).
        readable_stream_close(*stream);
    }
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-enqueue
WebIDL::ExceptionOr<void> readable_stream_default_controller_enqueue(ReadableStreamDefaultController& controller, JS::Value chunk)
{
    auto& vm = controller.vm();

    // 1. If ! ReadableStreamDefaultControllerCanCloseOrEnqueue(controller) is false, return.
    if (!readable_stream_default_controller_can_close_or_enqueue(controller))
        return {};

    // 2. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 3. If ! IsReadableStreamLocked(stream) is true and ! ReadableStreamGetNumReadRequests(stream) > 0, perform ! ReadableStreamFulfillReadRequest(stream, chunk, false).
    if (is_readable_stream_locked(*stream) && readable_stream_get_num_read_requests(*stream) > 0) {
        readable_stream_fulfill_read_request(*stream, chunk, false);
    }
    // 4. Otherwise,
    else {
        // 1. Let result be the result of performing controller.[[strategySizeAlgorithm]], passing in chunk, and interpreting the result as a completion record.
        auto result = (*controller.strategy_size_algorithm())(chunk);

        // 2. If result is an abrupt completion,
        if (result.is_abrupt()) {
            // 1. Perform ! ReadableStreamDefaultControllerError(controller, result.[[Value]]).
            readable_stream_default_controller_error(controller, result.value().value());

            // 2. Return result.
            return result;
        }

        // 3. Let chunkSize be result.[[Value]].
        auto chunk_size = TRY(result.release_value().release_value().to_double(vm));

        // 4. Let enqueueResult be EnqueueValueWithSize(controller, chunk, chunkSize).
        auto enqueue_result = enqueue_value_with_size(controller, chunk, chunk_size);

        // 5. If enqueueResult is an abrupt completion,
        if (enqueue_result.is_error()) {
            auto throw_completion = Bindings::throw_dom_exception_if_needed(vm, [&] { return enqueue_result; }).throw_completion();

            // 1. Perform ! ReadableStreamDefaultControllerError(controller, enqueueResult.[[Value]]).
            readable_stream_default_controller_error(controller, throw_completion.value().value());

            // 2. Return enqueueResult.
            return enqueue_result;
        }
    }

    // 5. Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(controller).
    return readable_stream_default_controller_can_pull_if_needed(controller);
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-call-pull-if-needed
WebIDL::ExceptionOr<void> readable_stream_default_controller_can_pull_if_needed(ReadableStreamDefaultController& controller)
{
    // 1. Let shouldPull be ! ReadableStreamDefaultControllerShouldCallPull(controller).
    auto should_pull = readable_stream_default_controller_should_call_pull(controller);

    // 2. If shouldPull is false, return.
    if (!should_pull)
        return {};

    // 3. If controller.[[pulling]] is true,
    if (controller.pulling()) {
        // 1. Set controller.[[pullAgain]] to true.
        controller.set_pull_again(true);

        // 2. Return.
        return {};
    }

    // 4. Assert: controller.[[pullAgain]] is false.
    VERIFY(!controller.pull_again());

    // 5. Set controller.[[pulling]] to true.
    controller.set_pulling(true);

    // 6. Let pullPromise be the result of performing controller.[[pullAlgorithm]].
    auto pull_promise = TRY((*controller.pull_algorithm())());

    // 7. Upon fulfillment of pullPromise,
    WebIDL::upon_fulfillment(*pull_promise, [&](auto const&) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Set controller.[[pulling]] to false.
        controller.set_pulling(false);

        // 2. If controller.[[pullAgain]] is true,
        if (controller.pull_again()) {
            // 1. Set controller.[[pullAgain]] to false.
            controller.set_pull_again(false);

            // 2. Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(controller).
            TRY(readable_stream_default_controller_can_pull_if_needed(controller));
        }

        return JS::js_undefined();
    });

    // 8. Upon rejection of pullPromise with reason e,
    WebIDL::upon_rejection(*pull_promise, [&](auto const& e) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Perform ! ReadableStreamDefaultControllerError(controller, e).
        readable_stream_default_controller_error(controller, e);

        return JS::js_undefined();
    });

    return {};
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-should-call-pull
bool readable_stream_default_controller_should_call_pull(ReadableStreamDefaultController& controller)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. If ! ReadableStreamDefaultControllerCanCloseOrEnqueue(controller) is false, return false.
    if (!readable_stream_default_controller_can_close_or_enqueue(controller))
        return false;

    // 3. If controller.[[started]] is false, return false.
    if (!controller.started())
        return false;

    // 4. If ! IsReadableStreamLocked(stream) is true and ! ReadableStreamGetNumReadRequests(stream) > 0, return true.
    if (is_readable_stream_locked(*stream) && readable_stream_get_num_read_requests(*stream) > 0)
        return true;

    // 5. Let desiredSize be ! ReadableStreamDefaultControllerGetDesiredSize(controller).
    auto desired_size = readable_stream_default_controller_get_desired_size(controller);

    // 6. Assert: desiredSize is not null.
    VERIFY(desired_size.has_value());

    // 7. If desiredSize > 0, return true.
    if (desired_size.release_value() > 0)
        return true;

    // 8. Return false.
    return false;
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-clear-algorithms
void readable_stream_default_controller_clear_algorithms(ReadableStreamDefaultController& controller)
{
    // 1. Set controller.[[pullAlgorithm]] to undefined.
    controller.set_pull_algorithm({});

    // 2. Set controller.[[cancelAlgorithm]] to undefined.
    controller.set_cancel_algorithm({});

    // 3. Set controller.[[strategySizeAlgorithm]] to undefined.
    controller.set_strategy_size_algorithm({});
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-error
void readable_stream_default_controller_error(ReadableStreamDefaultController& controller, JS::Value error)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. If stream.[[state]] is not "readable", return.
    if (!stream->is_readable())
        return;

    // 3. Perform ! ResetQueue(controller).
    reset_queue(controller);

    // 4. Perform ! ReadableStreamDefaultControllerClearAlgorithms(controller).
    readable_stream_default_controller_clear_algorithms(controller);

    // 5. Perform ! ReadableStreamError(stream, e).
    readable_stream_error(*stream, error);
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-get-desired-size
Optional<float> readable_stream_default_controller_get_desired_size(ReadableStreamDefaultController& controller)
{
    auto stream = controller.stream();

    // 1. Let state be controller.[[stream]].[[state]].

    // 2. If state is "errored", return null.
    if (stream->is_errored())
        return {};

    // 3. If state is "closed", return 0.
    if (stream->is_closed())
        return 0.0f;

    // 4. Return controller.[[strategyHWM]] − controller.[[queueTotalSize]].
    return controller.strategy_hwm() - controller.queue_total_size();
}

// https://streams.spec.whatwg.org/#readable-stream-default-controller-can-close-or-enqueue
bool readable_stream_default_controller_can_close_or_enqueue(ReadableStreamDefaultController& controller)
{
    // 1. Let state be controller.[[stream]].[[state]].
    // 2. If controller.[[closeRequested]] is false and state is "readable", return true.
    // 3. Otherwise, return false.
    return !controller.close_requested() && controller.stream()->is_readable();
}

// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller
WebIDL::ExceptionOr<void> set_up_readable_stream_default_controller(ReadableStream& stream, ReadableStreamDefaultController& controller, StartAlgorithm&& start_algorithm, PullAlgorithm&& pull_algorithm, CancelAlgorithm&& cancel_algorithm, double high_water_mark, SizeAlgorithm&& size_algorithm)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[controller]] is undefined.
    VERIFY(!stream.controller());

    // 2. Set controller.[[stream]] to stream.
    controller.set_stream(stream);

    // 3. Perform ! ResetQueue(controller).
    reset_queue(controller);

    // 4. Set controller.[[started]], controller.[[closeRequested]], controller.[[pullAgain]], and controller.[[pulling]] to false.
    controller.set_started(false);
    controller.set_close_requested(false);
    controller.set_pull_again(false);
    controller.set_pulling(false);

    // 5. Set controller.[[strategySizeAlgorithm]] to sizeAlgorithm and controller.[[strategyHWM]] to highWaterMark.
    controller.set_strategy_size_algorithm(move(size_algorithm));
    controller.set_strategy_hwm(high_water_mark);

    // 6. Set controller.[[pullAlgorithm]] to pullAlgorithm.
    controller.set_pull_algorithm(move(pull_algorithm));

    // 7. Set controller.[[cancelAlgorithm]] to cancelAlgorithm.
    controller.set_cancel_algorithm(move(cancel_algorithm));

    // 8. Set stream.[[controller]] to controller.
    stream.set_controller(controller);

    // 9. Let startResult be the result of performing startAlgorithm. (This might throw an exception.)
    auto start_result = TRY(start_algorithm());

    // 10. Let startPromise be a promise resolved with startResult.
    auto start_promise = WebIDL::create_resolved_promise(realm, start_result ? start_result->promise() : JS::js_undefined());

    // 11. Upon fulfillment of startPromise,
    WebIDL::upon_fulfillment(start_promise, [&](auto const&) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Set controller.[[started]] to true.
        controller.set_started(true);

        // 2. Assert: controller.[[pulling]] is false.
        VERIFY(!controller.pulling());

        // 3. Assert: controller.[[pullAgain]] is false.
        VERIFY(!controller.pull_again());

        // 4. Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(controller).
        TRY(readable_stream_default_controller_can_pull_if_needed(controller));

        return JS::js_undefined();
    });

    // 12. Upon rejection of startPromise with reason r,
    WebIDL::upon_rejection(start_promise, [&](auto const& r) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Perform ! ReadableStreamDefaultControllerError(controller, r).
        readable_stream_default_controller_error(controller, r);

        return JS::js_undefined();
    });

    return {};
}

// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source
WebIDL::ExceptionOr<void> set_up_readable_stream_default_controller_from_underlying_source(ReadableStream& stream, JS::Value underlying_source_value, UnderlyingSource underlying_source, double high_water_mark, SizeAlgorithm&& size_algorithm)
{
    auto& realm = stream.realm();

    // 1. Let controller be a new ReadableStreamDefaultController.
    auto controller = MUST_OR_THROW_OOM(stream.heap().allocate<ReadableStreamDefaultController>(realm, realm));

    // 2. Let startAlgorithm be an algorithm that returns undefined.
    StartAlgorithm start_algorithm = [] { return JS::GCPtr<WebIDL::Promise> {}; };

    // 3. Let pullAlgorithm be an algorithm that returns a promise resolved with undefined.
    PullAlgorithm pull_algorithm = [&realm]() {
        return WebIDL::create_resolved_promise(realm, JS::js_undefined());
    };

    // 4. Let cancelAlgorithm be an algorithm that returns a promise resolved with undefined.
    CancelAlgorithm cancel_algorithm = [&realm](auto const&) {
        return WebIDL::create_resolved_promise(realm, JS::js_undefined());
    };

    // 5. If underlyingSourceDict["start"] exists, then set startAlgorithm to an algorithm which returns the result of invoking underlyingSourceDict["start"] with argument list « controller » and callback this value underlyingSource.
    if (underlying_source.start) {
        start_algorithm = [&, start = underlying_source.start]() -> WebIDL::ExceptionOr<JS::GCPtr<WebIDL::Promise>> {
            auto result = TRY(WebIDL::invoke_callback(*start, underlying_source_value, controller)).release_value();
            return WebIDL::create_resolved_promise(realm, result);
        };
    }

    // 6. If underlyingSourceDict["pull"] exists, then set pullAlgorithm to an algorithm which returns the result of invoking underlyingSourceDict["pull"] with argument list « controller » and callback this value underlyingSource.
    if (underlying_source.pull) {
        pull_algorithm = [&, pull = underlying_source.pull]() -> WebIDL::ExceptionOr<JS::GCPtr<WebIDL::Promise>> {
            auto result = TRY(WebIDL::invoke_callback(*pull, underlying_source_value, controller)).release_value();
            return WebIDL::create_resolved_promise(realm, result);
        };
    }

    // 7. If underlyingSourceDict["cancel"] exists, then set cancelAlgorithm to an algorithm which takes an argument reason and returns the result of invoking underlyingSourceDict["cancel"] with argument list « reason » and callback this value underlyingSource.
    if (underlying_source.cancel) {
        cancel_algorithm = [&, cancel = underlying_source.cancel](auto const& reason) -> WebIDL::ExceptionOr<JS::GCPtr<WebIDL::Promise>> {
            auto result = TRY(WebIDL::invoke_callback(*cancel, underlying_source_value, reason)).release_value();
            return WebIDL::create_resolved_promise(realm, result);
        };
    }

    // 8. Perform ? SetUpReadableStreamDefaultController(stream, controller, startAlgorithm, pullAlgorithm, cancelAlgorithm, highWaterMark, sizeAlgorithm).
    return set_up_readable_stream_default_controller(stream, controller, move(start_algorithm), move(pull_algorithm), move(cancel_algorithm), high_water_mark, move(size_algorithm));
}

// https://streams.spec.whatwg.org/#acquire-writable-stream-default-writer
WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStreamDefaultWriter>> acquire_writable_stream_default_writer(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Let writer be a new WritableStreamDefaultWriter.
    auto writer = MUST_OR_THROW_OOM(stream.heap().allocate<WritableStreamDefaultWriter>(realm, realm));

    // 2. Perform ? SetUpWritableStreamDefaultWriter(writer, stream).
    TRY(set_up_writable_stream_default_writer(*writer, stream));

    // 3. Return writer.
    return writer;
}

// https://streams.spec.whatwg.org/#is-writable-stream-locked
bool is_writable_stream_locked(WritableStream const& stream)
{
    // 1. If stream.[[writer]] is undefined, return false.
    if (!stream.writer())
        return false;

    // 2. Return true.
    return true;
}

// https://streams.spec.whatwg.org/#set-up-writable-stream-default-writer
WebIDL::ExceptionOr<void> set_up_writable_stream_default_writer(WritableStreamDefaultWriter& writer, WritableStream& stream)
{
    auto& realm = writer.realm();

    // 1. If ! IsWritableStreamLocked(stream) is true, throw a TypeError exception.
    if (is_writable_stream_locked(stream))
        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, "Stream is locked"sv };

    // 2. Set writer.[[stream]] to stream.
    writer.set_stream(stream);

    // 3. Set stream.[[writer]] to writer.
    stream.set_writer(writer);

    // 4. Let state be stream.[[state]].
    auto state = stream.state();

    // 5. If state is "writable",
    if (state == WritableStream::State::Writable) {
        // 1. If ! WritableStreamCloseQueuedOrInFlight(stream) is false and stream.[[backpressure]] is true, set writer.[[readyPromise]] to a new promise.
        if (!writable_stream_close_queued_or_in_flight(stream) && stream.backpressure()) {
            writer.set_ready_promise(WebIDL::create_promise(realm));
        }
        // 2. Otherwise, set writer.[[readyPromise]] to a promise resolved with undefined.
        else {
            writer.set_ready_promise(WebIDL::create_resolved_promise(realm, JS::js_undefined()));
        }

        // 3. Set writer.[[closedPromise]] to a new promise.
        writer.set_closed_promise(WebIDL::create_promise(realm));
    }
    // 6. Otherwise, if state is "erroring",
    else if (state == WritableStream::State::Erroring) {
        // 1. Set writer.[[readyPromise]] to a promise rejected with stream.[[storedError]].
        writer.set_ready_promise(WebIDL::create_rejected_promise(realm, stream.stored_error()));

        // 2. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
        WebIDL::mark_promise_as_handled(*writer.ready_promise());

        // 3. Set writer.[[closedPromise]] to a new promise.
        writer.set_closed_promise(WebIDL::create_promise(realm));
    }
    // 7. Otherwise, if state is "closed",
    else if (state == WritableStream::State::Closed) {
        // 1. Set writer.[[readyPromise]] to a promise resolved with undefined.
        writer.set_ready_promise(WebIDL::create_resolved_promise(realm, JS::js_undefined()));

        // 2. Set writer.[[closedPromise]] to a promise resolved with undefined.
        writer.set_closed_promise(WebIDL::create_resolved_promise(realm, JS::js_undefined()));
    }
    // 8. Otherwise,
    else {
        // 1. Assert: state is "errored".
        VERIFY(state == WritableStream::State::Errored);

        // 2. Let storedError be stream.[[storedError]].
        auto stored_error = stream.stored_error();

        // 3. Set writer.[[readyPromise]] to a promise rejected with storedError.
        writer.set_ready_promise(WebIDL::create_rejected_promise(realm, stored_error));

        // 4. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
        WebIDL::mark_promise_as_handled(*writer.ready_promise());

        // 5. Set writer.[[closedPromise]] to a promise rejected with storedError.
        writer.set_closed_promise(WebIDL::create_rejected_promise(realm, stored_error));

        // 6. Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
        WebIDL::mark_promise_as_handled(*writer.closed_promise());
    }

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-close
WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> writable_stream_close(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Let state be stream.[[state]].
    auto state = stream.state();

    // 2. If state is "closed" or "errored", return a promise rejected with a TypeError exception.
    if (state == WritableStream::State::Closed || state == WritableStream::State::Errored) {
        auto message = state == WritableStream::State::Closed ? "Cannot close a closed stream"sv : "Cannot close an errored stream"sv;
        auto exception = MUST_OR_THROW_OOM(JS::TypeError::create(realm, message));
        return WebIDL::create_rejected_promise(realm, exception);
    }

    // 3. Assert: state is "writable" or "erroring".
    VERIFY(state == WritableStream::State::Writable || state == WritableStream::State::Erroring);

    // 4. Assert: ! WritableStreamCloseQueuedOrInFlight(stream) is false.
    VERIFY(!writable_stream_close_queued_or_in_flight(stream));

    // 5. Let promise be a new promise.
    auto promise = WebIDL::create_promise(realm);

    // 6. Set stream.[[closeRequest]] to promise.
    stream.set_close_request(promise);

    // 7. Let writer be stream.[[writer]].
    auto writer = stream.writer();

    // 8. If writer is not undefined, and stream.[[backpressure]] is true, and state is "writable", resolve writer.[[readyPromise]] with undefined.
    if (writer && stream.backpressure() && state == WritableStream::State::Writable)
        WebIDL::resolve_promise(realm, *writer->ready_promise(), JS::js_undefined());

    // 9. Perform ! WritableStreamDefaultControllerClose(stream.[[controller]]).
    TRY(writable_stream_default_controller_close(*stream.controller()));

    // 10. Return promise.
    return promise;
}

// https://streams.spec.whatwg.org/#writable-stream-close-queued-or-in-flight
bool writable_stream_close_queued_or_in_flight(WritableStream const& stream)
{
    // 1. If stream.[[closeRequest]] is undefined and stream.[[inFlightCloseRequest]] is undefined, return false.
    if (!stream.close_request() && !stream.in_flight_write_request())
        return false;

    // 2. Return true.
    return true;
}

// https://streams.spec.whatwg.org/#writable-stream-deal-with-rejection
WebIDL::ExceptionOr<void> writable_stream_deal_with_rejection(WritableStream& stream, JS::Value error)
{
    // 1. Let state be stream.[[state]].
    auto state = stream.state();

    // 2. If state is "writable",
    if (state == WritableStream::State::Writable) {
        // 1. Perform ! WritableStreamStartErroring(stream, error).
        // 2. Return.
        return writable_stream_start_erroring(stream, error);
    }

    // 3. Assert: state is "erroring".
    VERIFY(state == WritableStream::State::Erroring);

    // 4. Perform ! WritableStreamFinishErroring(stream).
    return writable_stream_finish_erroring(stream);
}

// https://streams.spec.whatwg.org/#writable-stream-finish-erroring
WebIDL::ExceptionOr<void> writable_stream_finish_erroring(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[state]] is "erroring".
    VERIFY(stream.state() == WritableStream::State::Erroring);

    // 2. Assert: ! WritableStreamHasOperationMarkedInFlight(stream) is false.
    VERIFY(!writable_stream_has_operation_marked_in_flight(stream));

    // 3. Set stream.[[state]] to "errored".
    stream.set_state(WritableStream::State::Errored);

    // 4. Perform ! stream.[[controller]].[[ErrorSteps]]().
    stream.controller()->error_steps();

    // 5. Let storedError be stream.[[storedError]].
    auto stored_error = stream.stored_error();

    // 6. For each writeRequest of stream.[[writeRequests]]:
    for (auto& write_request : stream.write_requests()) {
        // 1. Reject writeRequest with storedError.
        WebIDL::reject_promise(realm, *write_request, stored_error);
    }

    // 7. Set stream.[[writeRequests]] to an empty list.
    stream.write_requests().clear();

    // 8. If stream.[[pendingAbortRequest]] is undefined,
    if (!stream.pending_abort_request().has_value()) {
        // 1. Perform ! WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
        writable_stream_reject_close_and_closed_promise_if_needed(stream);

        // 2. Return.
        return {};
    }

    // 9. Let abortRequest be stream.[[pendingAbortRequest]].
    // 10. Set stream.[[pendingAbortRequest]] to undefined.
    auto abort_request = stream.pending_abort_request().release_value();

    // 11. If abortRequest’s was already erroring is true,
    if (abort_request.was_already_erroring) {
        // 1. Reject abortRequest’s promise with storedError.
        WebIDL::reject_promise(realm, abort_request.promise, stored_error);

        // 2. Perform ! WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
        writable_stream_reject_close_and_closed_promise_if_needed(stream);

        // 3. Return.
        return {};
    }

    // 12. Let promise be ! stream.[[controller]].[[AbortSteps]](abortRequest’s reason).
    auto promise = TRY(stream.controller()->abort_steps(abort_request.reason));

    // 13. Upon fulfillment of promise,
    WebIDL::upon_fulfillment(*promise, [&, abort_promise = abort_request.promise](auto const&) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Resolve abortRequest’s promise with undefined.
        WebIDL::resolve_promise(realm, abort_promise, JS::js_undefined());

        // 2. Perform ! WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
        writable_stream_reject_close_and_closed_promise_if_needed(stream);

        return JS::js_undefined();
    });

    // 14. Upon rejection of promise with reason reason,
    WebIDL::upon_rejection(*promise, [&, abort_promise = abort_request.promise](auto const& reason) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Reject abortRequest’s promise with reason.
        WebIDL::reject_promise(realm, abort_promise, reason);

        // 2. Perform ! WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
        writable_stream_reject_close_and_closed_promise_if_needed(stream);

        return JS::js_undefined();
    });

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-close
void writable_stream_finish_in_flight_close(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[inFlightCloseRequest]] is not undefined.
    VERIFY(stream.in_flight_close_request());

    // 2. Resolve stream.[[inFlightCloseRequest]] with undefined.
    WebIDL::resolve_promise(realm, *stream.in_flight_close_request(), JS::js_undefined());

    // 3. Set stream.[[inFlightCloseRequest]] to undefined.
    stream.set_in_flight_close_request({});

    // 4. Let state be stream.[[state]].
    auto state = stream.state();

    // 5. Assert: stream.[[state]] is "writable" or "erroring".
    VERIFY(state == WritableStream::State::Writable || state == WritableStream::State::Erroring);

    // 6. If state is "erroring",
    if (state == WritableStream::State::Erroring) {
        // 1. Set stream.[[storedError]] to undefined.
        stream.set_stored_error(JS::js_undefined());

        // 2. If stream.[[pendingAbortRequest]] is not undefined,
        if (stream.pending_abort_request().has_value()) {
            // 1. Resolve stream.[[pendingAbortRequest]]'s promise with undefined.
            // 2. Set stream.[[pendingAbortRequest]] to undefined.
            WebIDL::resolve_promise(realm, stream.pending_abort_request().release_value().promise, JS::js_undefined());
        }
    }

    // 7. Set stream.[[state]] to "closed".
    stream.set_state(WritableStream::State::Closed);

    // 8. Let writer be stream.[[writer]].
    auto writer = stream.writer();

    // 9. If writer is not undefined, resolve writer.[[closedPromise]] with undefined.
    if (writer)
        WebIDL::resolve_promise(realm, *writer->closed_promise(), JS::js_undefined());

    // 10. Assert: stream.[[pendingAbortRequest]] is undefined.
    VERIFY(!stream.pending_abort_request().has_value());

    // 11. Assert: stream.[[storedError]] is undefined.
    VERIFY(stream.stored_error().is_undefined());
}

// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-close-with-error
WebIDL::ExceptionOr<void> writable_stream_finish_in_flight_close_with_error(WritableStream& stream, JS::Value error)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[inFlightCloseRequest]] is not undefined.
    VERIFY(stream.in_flight_close_request());

    // 2. Reject stream.[[inFlightCloseRequest]] with error.
    WebIDL::reject_promise(realm, *stream.in_flight_close_request(), error);

    // 3. Set stream.[[inFlightCloseRequest]] to undefined.
    stream.set_in_flight_close_request({});

    // 4. Assert: stream.[[state]] is "writable" or "erroring".
    auto state = stream.state();
    VERIFY(state == WritableStream::State::Writable || state == WritableStream::State::Erroring);

    // 5. If stream.[[pendingAbortRequest]] is not undefined,
    if (stream.pending_abort_request().has_value()) {
        // 1. Reject stream.[[pendingAbortRequest]]'s promise with error.
        // 2. Set stream.[[pendingAbortRequest]] to undefined.
        WebIDL::reject_promise(realm, stream.pending_abort_request().release_value().promise, error);
    }

    // 6. Perform ! WritableStreamDealWithRejection(stream, error).
    return writable_stream_deal_with_rejection(stream, error);
}

// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-write
void writable_stream_finish_in_flight_write(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[inFlightWriteRequest]] is not undefined.
    VERIFY(stream.in_flight_write_request());

    // 2. Resolve stream.[[inFlightWriteRequest]] with undefined.
    WebIDL::resolve_promise(realm, *stream.in_flight_write_request(), JS::js_undefined());

    // 3. Set stream.[[inFlightWriteRequest]] to undefined.
    stream.set_in_flight_write_request({});
}

// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-write-with-error
WebIDL::ExceptionOr<void> writable_stream_finish_in_flight_write_with_error(WritableStream& stream, JS::Value error)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[inFlightWriteRequest]] is not undefined.
    VERIFY(stream.in_flight_write_request());

    // 2. Reject stream.[[inFlightWriteRequest]] with error.
    WebIDL::reject_promise(realm, *stream.in_flight_write_request(), error);

    // 3. Set stream.[[inFlightWriteRequest]] to undefined.
    stream.set_in_flight_write_request({});

    // 4. Assert: stream.[[state]] is "writable" or "erroring".
    auto state = stream.state();
    VERIFY(state == WritableStream::State::Writable || state == WritableStream::State::Erroring);

    // 5. Perform ! WritableStreamDealWithRejection(stream, error).
    return writable_stream_deal_with_rejection(stream, error);
}

// https://streams.spec.whatwg.org/#writable-stream-has-operation-marked-in-flight
bool writable_stream_has_operation_marked_in_flight(WritableStream const& stream)
{
    // 1. If stream.[[inFlightWriteRequest]] is undefined and stream.[[inFlightCloseRequest]] is undefined, return false.
    if (!stream.in_flight_write_request() && !stream.in_flight_close_request())
        return false;

    // 2. Return true.
    return true;
}

// https://streams.spec.whatwg.org/#writable-stream-mark-close-request-in-flight
void writable_stream_mark_close_request_in_flight(WritableStream& stream)
{
    // 1. Assert: stream.[[inFlightCloseRequest]] is undefined.
    VERIFY(!stream.in_flight_close_request());

    // 2. Assert: stream.[[closeRequest]] is not undefined.
    VERIFY(stream.close_request());

    // 3. Set stream.[[inFlightCloseRequest]] to stream.[[closeRequest]].
    stream.set_in_flight_close_request(stream.close_request());

    // 4. Set stream.[[closeRequest]] to undefined.
    stream.set_close_request({});
}

// https://streams.spec.whatwg.org/#writable-stream-mark-first-write-request-in-flight
void writable_stream_mark_first_write_request_in_flight(WritableStream& stream)
{
    // 1. Assert: stream.[[inFlightWriteRequest]] is undefined.
    VERIFY(!stream.in_flight_write_request());

    // 2. Assert: stream.[[writeRequests]] is not empty.
    VERIFY(!stream.write_requests().is_empty());

    // 3. Let writeRequest be stream.[[writeRequests]][0].
    // 4. Remove writeRequest from stream.[[writeRequests]].
    auto write_request = stream.write_requests().take_first();

    // 5. Set stream.[[inFlightWriteRequest]] to writeRequest.
    stream.set_in_flight_write_request(write_request);
}

// https://streams.spec.whatwg.org/#writable-stream-reject-close-and-closed-promise-if-needed
void writable_stream_reject_close_and_closed_promise_if_needed(WritableStream& stream)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[state]] is "errored".
    VERIFY(stream.state() == WritableStream::State::Errored);

    // 2. If stream.[[closeRequest]] is not undefined,
    if (stream.close_request()) {
        // 1. Assert: stream.[[inFlightCloseRequest]] is undefined.
        VERIFY(!stream.in_flight_close_request());

        // 2. Reject stream.[[closeRequest]] with stream.[[storedError]].
        WebIDL::reject_promise(realm, *stream.close_request(), stream.stored_error());

        // 3. Set stream.[[closeRequest]] to undefined.
        stream.set_close_request({});
    }

    // 3. Let writer be stream.[[writer]].
    auto writer = stream.writer();

    // 4. If writer is not undefined,
    if (writer) {
        // 1. Reject writer.[[closedPromise]] with stream.[[storedError]].
        WebIDL::reject_promise(realm, *writer->closed_promise(), stream.stored_error());

        // 2. Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
        WebIDL::mark_promise_as_handled(*writer->closed_promise());
    }
}

// https://streams.spec.whatwg.org/#writable-stream-start-erroring
WebIDL::ExceptionOr<void> writable_stream_start_erroring(WritableStream& stream, JS::Value reason)
{
    // 1. Assert: stream.[[storedError]] is undefined.
    VERIFY(stream.stored_error().is_undefined());

    // 2. Assert: stream.[[state]] is "writable".
    VERIFY(stream.state() == WritableStream::State::Writable);

    // 3. Let controller be stream.[[controller]].
    auto controller = stream.controller();

    // 4. Assert: controller is not undefined.
    VERIFY(controller);

    // 5. Set stream.[[state]] to "erroring".
    stream.set_state(WritableStream::State::Erroring);

    // 6. Set stream.[[storedError]] to reason.
    stream.set_stored_error(reason);

    // 7. Let writer be stream.[[writer]].
    auto writer = stream.writer();

    // 8. If writer is not undefined, perform ! WritableStreamDefaultWriterEnsureReadyPromiseRejected(writer, reason).
    if (writer)
        writable_stream_default_writer_ensure_ready_promise_rejected(*writer, reason);

    // 9. If ! WritableStreamHasOperationMarkedInFlight(stream) is false and controller.[[started]] is true, perform ! WritableStreamFinishErroring(stream).
    if (!writable_stream_has_operation_marked_in_flight(stream) && controller->started())
        TRY(writable_stream_finish_erroring(stream));

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-update-backpressure
void writable_stream_update_backpressure(WritableStream& stream, bool backpressure)
{
    auto& realm = stream.realm();

    // 1. Assert: stream.[[state]] is "writable".
    VERIFY(stream.state() == WritableStream::State::Writable);

    // 2. Assert: ! WritableStreamCloseQueuedOrInFlight(stream) is false.
    VERIFY(!writable_stream_close_queued_or_in_flight(stream));

    // 3. Let writer be stream.[[writer]].
    auto writer = stream.writer();

    // 4. If writer is not undefined and backpressure is not stream.[[backpressure]],
    if (writer && backpressure != stream.backpressure()) {
        // 1. If backpressure is true, set writer.[[readyPromise]] to a new promise.
        if (backpressure) {
            writer->set_ready_promise(WebIDL::create_promise(realm));
        }
        // 2. Otherwise,
        else {
            // 1. Assert: backpressure is false.

            // 2. Resolve writer.[[readyPromise]] with undefined.
            WebIDL::resolve_promise(realm, *writer->ready_promise(), JS::js_undefined());
        }
    }

    // 5. Set stream.[[backpressure]] to backpressure.
    stream.set_backpressure(backpressure);
}

// https://streams.spec.whatwg.org/#writable-stream-default-writer-ensure-ready-promise-rejected
void writable_stream_default_writer_ensure_ready_promise_rejected(WritableStreamDefaultWriter& writer, JS::Value error)
{
    auto& realm = writer.realm();

    // 1. If writer.[[readyPromise]].[[PromiseState]] is "pending", reject writer.[[readyPromise]] with error.
    auto& ready_promise = verify_cast<JS::Promise>(*writer.ready_promise()->promise());
    if (ready_promise.state() == JS::Promise::State::Pending) {
        WebIDL::reject_promise(realm, *writer.ready_promise(), error);
    }
    // 2. Otherwise, set writer.[[readyPromise]] to a promise rejected with error.
    else {
        writer.set_ready_promise(WebIDL::create_rejected_promise(realm, error));
    }

    // 3. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
    WebIDL::mark_promise_as_handled(*writer.ready_promise());
}

// https://streams.spec.whatwg.org/#writable-stream-default-writer-get-desired-size
Optional<double> writable_stream_default_writer_get_desired_size(WritableStreamDefaultWriter const& writer)
{
    // 1. Let stream be writer.[[stream]].
    auto stream = writer.stream();

    // 2. Let state be stream.[[state]].
    auto state = stream->state();

    // 3. If state is "errored" or "erroring", return null.
    if (state == WritableStream::State::Errored || state == WritableStream::State::Erroring)
        return {};

    // 4. If state is "closed", return 0.
    if (state == WritableStream::State::Closed)
        return 0.0;

    // 5. Return ! WritableStreamDefaultControllerGetDesiredSize(stream.[[controller]]).
    return writable_stream_default_controller_get_desired_size(*stream->controller());
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-advance-queue-if-needed
WebIDL::ExceptionOr<void> writable_stream_default_controller_advance_queue_if_needed(WritableStreamDefaultController& controller)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. If controller.[[started]] is false, return.
    if (!controller.started())
        return {};

    // 3. If stream.[[inFlightWriteRequest]] is not undefined, return.
    if (stream->in_flight_write_request())
        return {};

    // 4. Let state be stream.[[state]].
    auto state = stream->state();

    // 5. Assert: state is not "closed" or "errored".
    VERIFY(state != WritableStream::State::Closed && state != WritableStream::State::Errored);

    // 6. If state is "erroring",
    if (state == WritableStream::State::Erroring) {
        // 1. Perform ! WritableStreamFinishErroring(stream).
        // 2. Return.
        return writable_stream_finish_erroring(*stream);
    }

    // 7. If controller.[[queue]] is empty, return.
    if (controller.queue().is_empty())
        return {};

    // 8. Let value be ! PeekQueueValue(controller).
    auto value = peek_queue_value(controller);

    // 9. If value is the close sentinel, perform ! WritableStreamDefaultControllerProcessClose(controller).
    if (is_close_sentinel(value)) {
        TRY(writable_stream_default_controller_process_close(controller));
    }
    // 10. Otherwise, perform ! WritableStreamDefaultControllerProcessWrite(controller, value).
    else {
        TRY(writable_stream_default_controller_process_write(controller, value));
    }

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-clear-algorithms
void writable_stream_default_controller_clear_algorithms(WritableStreamDefaultController& controller)
{
    // 1. Set controller.[[writeAlgorithm]] to undefined.
    controller.set_write_algorithm({});

    // 2. Set controller.[[closeAlgorithm]] to undefined.
    controller.set_close_algorithm({});

    // 3. Set controller.[[abortAlgorithm]] to undefined.
    controller.set_abort_algorithm({});

    // 4. Set controller.[[strategySizeAlgorithm]] to undefined.
    controller.set_strategy_size_algorithm({});
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-close
WebIDL::ExceptionOr<void> writable_stream_default_controller_close(WritableStreamDefaultController& controller)
{
    // 1. Perform ! EnqueueValueWithSize(controller, close sentinel, 0).
    TRY(enqueue_value_with_size(controller, create_close_sentinel(), 0.0));

    // 2. Perform ! WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
    TRY(writable_stream_default_controller_advance_queue_if_needed(controller));

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-error
WebIDL::ExceptionOr<void> writable_stream_default_controller_error(WritableStreamDefaultController& controller, JS::Value error)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. Assert: stream.[[state]] is "writable".
    VERIFY(stream->state() == WritableStream::State::Writable);

    // 3. Perform ! WritableStreamDefaultControllerClearAlgorithms(controller).
    writable_stream_default_controller_clear_algorithms(controller);

    // 4. Perform ! WritableStreamStartErroring(stream, error).
    return writable_stream_start_erroring(stream, error);
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-get-backpressure
bool writable_stream_default_controller_get_backpressure(WritableStreamDefaultController const& controller)
{
    // 1. Let desiredSize be ! WritableStreamDefaultControllerGetDesiredSize(controller).
    auto desired_size = writable_stream_default_controller_get_desired_size(controller);

    // 2. Return true if desiredSize ≤ 0, or false otherwise.
    return desired_size <= 0.0;
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-get-desired-size
double writable_stream_default_controller_get_desired_size(WritableStreamDefaultController const& controller)
{
    // 1. Return controller.[[strategyHWM]] − controller.[[queueTotalSize]].
    return controller.strategy_hwm() - controller.queue_total_size();
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-process-close
WebIDL::ExceptionOr<void> writable_stream_default_controller_process_close(WritableStreamDefaultController& controller)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. Perform ! WritableStreamMarkCloseRequestInFlight(stream).
    writable_stream_mark_close_request_in_flight(*stream);

    // 3. Perform ! DequeueValue(controller).
    dequeue_value(controller);

    // 4. Assert: controller.[[queue]] is empty.
    VERIFY(controller.queue().is_empty());

    // 5. Let sinkClosePromise be the result of performing controller.[[closeAlgorithm]].
    auto sink_close_promise = TRY((*controller.close_algorithm())());

    // 6. Perform ! WritableStreamDefaultControllerClearAlgorithms(controller).
    writable_stream_default_controller_clear_algorithms(controller);

    // 7. Upon fulfillment of sinkClosePromise,
    WebIDL::upon_fulfillment(*sink_close_promise, [&, stream = stream](auto const&) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Perform ! WritableStreamFinishInFlightClose(stream).
        writable_stream_finish_in_flight_close(*stream);

        return JS::js_undefined();
    });

    // 8. Upon rejection of sinkClosePromise with reason reason,
    WebIDL::upon_rejection(*sink_close_promise, [&, stream = stream](auto const& reason) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Perform ! WritableStreamFinishInFlightCloseWithError(stream, reason).
        TRY(writable_stream_finish_in_flight_close_with_error(*stream, reason));

        return JS::js_undefined();
    });

    return {};
}

// https://streams.spec.whatwg.org/#writable-stream-default-controller-process-write
WebIDL::ExceptionOr<void> writable_stream_default_controller_process_write(WritableStreamDefaultController& controller, JS::Value chunk)
{
    // 1. Let stream be controller.[[stream]].
    auto stream = controller.stream();

    // 2. Perform ! WritableStreamMarkFirstWriteRequestInFlight(stream).
    writable_stream_mark_first_write_request_in_flight(*stream);

    // 3. Let sinkWritePromise be the result of performing controller.[[writeAlgorithm]], passing in chunk.
    auto sink_write_promise = TRY((*controller.write_algorithm())(chunk));

    // 4. Upon fulfillment of sinkWritePromise,
    WebIDL::upon_fulfillment(*sink_write_promise, [&, stream = stream](auto const&) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. Perform ! WritableStreamFinishInFlightWrite(stream).
        writable_stream_finish_in_flight_write(*stream);

        // 2. Let state be stream.[[state]].
        auto state = stream->state();

        // 3. Assert: state is "writable" or "erroring".
        VERIFY(state == WritableStream::State::Writable || state == WritableStream::State::Erroring);

        // 4. Perform ! DequeueValue(controller).
        dequeue_value(controller);

        // 5. If ! WritableStreamCloseQueuedOrInFlight(stream) is false and state is "writable",
        if (!writable_stream_close_queued_or_in_flight(*stream) && state == WritableStream::State::Writable) {
            // 1. Let backpressure be ! WritableStreamDefaultControllerGetBackpressure(controller).
            auto backpressure = writable_stream_default_controller_get_backpressure(controller);

            // 2. Perform ! WritableStreamUpdateBackpressure(stream, backpressure).
            writable_stream_update_backpressure(*stream, backpressure);
        }

        // 6 .Perform ! WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
        TRY(writable_stream_default_controller_advance_queue_if_needed(controller));

        return JS::js_undefined();
    });

    // 5. Upon rejection of sinkWritePromise with reason,
    WebIDL::upon_rejection(*sink_write_promise, [&, stream = stream](auto const& reason) -> WebIDL::ExceptionOr<JS::Value> {
        // 1. If stream.[[state]] is "writable", perform ! WritableStreamDefaultControllerClearAlgorithms(controller).
        if (stream->state() == WritableStream::State::Writable)
            writable_stream_default_controller_clear_algorithms(controller);

        // 2. Perform ! WritableStreamFinishInFlightWriteWithError(stream, reason).
        TRY(writable_stream_finish_in_flight_write_with_error(*stream, reason));

        return JS::js_undefined();
    });

    return {};
}

// https://streams.spec.whatwg.org/#close-sentinel
// Non-standard function that implements the "close sentinel" value.
JS::Value create_close_sentinel()
{
    // The close sentinel is a unique value enqueued into [[queue]], in lieu of a chunk, to signal that the stream is closed. It is only used internally, and is never exposed to web developers.
    // Note: We use the empty Value to signal this as, similarly to the note above, the empty value is not exposed to nor creatable by web developers.
    return {};
}

// https://streams.spec.whatwg.org/#close-sentinel
// Non-standard function that implements the "If value is a close sentinel" check.
bool is_close_sentinel(JS::Value value)
{
    return value.is_empty();
}

// Non-standard function to aid in converting a user-provided function into a WebIDL::Callback. This is essentially
// what the Bindings generator would do at compile time, but at runtime instead.
JS::ThrowCompletionOr<JS::Handle<WebIDL::CallbackType>> property_to_callback(JS::VM& vm, JS::Value value, JS::PropertyKey const& property_key)
{
    auto property = TRY(value.get(vm, property_key));

    if (property.is_undefined())
        return JS::Handle<WebIDL::CallbackType> {};

    if (!property.is_function())
        return vm.throw_completion<JS::TypeError>(JS::ErrorType::NotAFunction, TRY_OR_THROW_OOM(vm, property.to_string_without_side_effects()));

    return vm.heap().allocate_without_realm<WebIDL::CallbackType>(property.as_object(), HTML::incumbent_settings_object());
}

}