/*
 * Copyright (C) 2021 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "config.h"
#include "MomentumEventDispatcher.h"

#if ENABLE(MOMENTUM_EVENT_DISPATCHER)

#include "EventDispatcher.h"
#include "Logging.h"
#include "WebProcess.h"
#include "WebProcessProxyMessages.h"
#include <WebCore/DisplayRefreshMonitor.h>
#include <WebCore/Scrollbar.h>
#include <wtf/SystemTracing.h>

namespace WebKit {

static constexpr Seconds deltaHistoryMaximumAge = 500_ms;
static constexpr Seconds deltaHistoryMaximumInterval = 150_ms;
static constexpr WebCore::FramesPerSecond idealCurveFrameRate = 60;
static constexpr Seconds idealCurveFrameInterval = 1_s / idealCurveFrameRate;

MomentumEventDispatcher::MomentumEventDispatcher(EventDispatcher& dispatcher)
    : m_observerID(DisplayLinkObserverID::generate())
    , m_dispatcher(dispatcher)
{
}

MomentumEventDispatcher::~MomentumEventDispatcher()
{
    stopDisplayLink();
}

bool MomentumEventDispatcher::eventShouldStartSyntheticMomentumPhase(WebCore::PageIdentifier pageIdentifier, const WebWheelEvent& event) const
{
    if (event.momentumPhase() != WebWheelEvent::PhaseBegan)
        return false;

    auto curveIterator = m_accelerationCurves.find(pageIdentifier);
    if (curveIterator == m_accelerationCurves.end() || !curveIterator->value) {
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher not using synthetic momentum phase: no acceleration curve");
        return false;
    }

    if (!event.rawPlatformDelta()) {
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher not using synthetic momentum phase: no raw platform delta on start event");
        return false;
    }

    return true;
}

bool MomentumEventDispatcher::handleWheelEvent(WebCore::PageIdentifier pageIdentifier, const WebWheelEvent& event, WebCore::RectEdges<bool> rubberBandableEdges)
{
    m_lastRubberBandableEdges = rubberBandableEdges;
    m_lastIncomingEvent = event;

    bool isMomentumEvent = event.momentumPhase() != WebWheelEvent::PhaseNone;

    if (m_currentGesture.active) {
        bool pageIdentifierChanged = pageIdentifier != m_currentGesture.pageIdentifier;

        // Any incoming wheel event other than a changed event for the current gesture
        // should interrupt the animation; in the usual case, it will be
        // momentumPhase == PhaseEnded that interrupts.
        bool eventShouldInterruptGesture = !isMomentumEvent || event.momentumPhase() != WebWheelEvent::PhaseChanged;

        if (event.momentumPhase() == WebWheelEvent::PhaseEnded) {
#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
            RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher saw momentum ended phase, interrupted=%d", static_cast<int>(event.momentumEndType()));
#endif

            // Ignore momentumPhase == PhaseEnded if it was due to the natural
            // end of the animation (as opposed to interruption by placing fingers
            // on the trackpad), so that our momentum is not cut short if the
            // deceleration runs longer than the system curve.
            if (event.momentumEndType() == WebWheelEvent::MomentumEndType::Natural)
                eventShouldInterruptGesture = false;
        }

        if (pageIdentifierChanged || eventShouldInterruptGesture) {
            RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher interrupting synthetic momentum phase");
            didEndMomentumPhase();
        }
    }

    if (event.phase() == WebWheelEvent::PhaseBegan || event.phase() == WebWheelEvent::PhaseChanged) {
        didReceiveScrollEvent(event);

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
        if (auto lastActivePhaseDelta = event.rawPlatformDelta())
            m_lastActivePhaseDelta = *lastActivePhaseDelta;
#endif
    }

    if (eventShouldStartSyntheticMomentumPhase(pageIdentifier, event)) {
        didStartMomentumPhase(pageIdentifier, event);
        m_isInOverriddenPlatformMomentumGesture = true;
    }

    // Consume any incoming momentum events while we're generating a synthetic
    // momentum gesture *or* a platform momentum phase that was overridden
    // is still running after we finished the synthetic gesture.
    bool shouldIgnoreIncomingPlatformEvent = isMomentumEvent && (m_isInOverriddenPlatformMomentumGesture || m_currentGesture.active);

    if (event.momentumPhase() == WebWheelEvent::PhaseEnded)
        m_isInOverriddenPlatformMomentumGesture = false;

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    if (shouldIgnoreIncomingPlatformEvent)
        m_currentGesture.accumulatedEventOffset += event.delta();

    auto combinedPhase = (event.phase() << 8) | (event.momentumPhase());
    m_currentLogState.totalEventOffset += event.delta().height();
    if (!shouldIgnoreIncomingPlatformEvent) {
        // Log events that we don't block to the generated offsets log as well,
        // even though we didn't technically generate them, just passed them through.
        m_currentLogState.totalGeneratedOffset += event.delta().height();
        pushLogEntry(combinedPhase, combinedPhase);
    } else
        pushLogEntry(0, combinedPhase);
    
#endif

    return shouldIgnoreIncomingPlatformEvent;
}

static float appKitScrollMultiplierForEvent(const WebWheelEvent& event)
{
    auto delta = event.delta();
    auto unacceleratedDelta = event.unacceleratedScrollingDelta();
    float multiplier = 1;
    
    // FIXME: Can the AppKit multiplier ever differ by axis?
    if (delta.width() && unacceleratedDelta.width())
        multiplier = std::max(multiplier, delta.width() / unacceleratedDelta.width());
    if (delta.height() && unacceleratedDelta.height())
        multiplier = std::max(multiplier, delta.height() / unacceleratedDelta.height());

    return multiplier;
}

void MomentumEventDispatcher::dispatchSyntheticMomentumEvent(WebWheelEvent::Phase phase, WebCore::FloatSize delta)
{
    tracePoint(SyntheticMomentumEvent, static_cast<uint64_t>(phase), std::abs(delta.width()), std::abs(delta.height()));

    ASSERT(m_currentGesture.active);
    ASSERT(m_currentGesture.initiatingEvent);

    auto appKitScrollMultiplier = appKitScrollMultiplierForEvent(*m_currentGesture.initiatingEvent);
    auto appKitAcceleratedDelta = delta * appKitScrollMultiplier;
    auto wheelTicks = appKitAcceleratedDelta / WebCore::Scrollbar::pixelsPerLineStep();
    auto time = WallTime::now();

    // FIXME: Ideally we would stick legitimate rawPlatformDeltas on the event,
    // but currently nothing will consume them, and we'd have to keep track of them separately.
    WebWheelEvent syntheticEvent(
        WebEvent::Wheel,
        m_currentGesture.initiatingEvent->position(),
        m_currentGesture.initiatingEvent->globalPosition(),
        appKitAcceleratedDelta,
        wheelTicks,
        WebWheelEvent::ScrollByPixelWheelEvent,
        m_currentGesture.initiatingEvent->directionInvertedFromDevice(),
        WebWheelEvent::PhaseNone,
        phase,
        true,
        m_currentGesture.initiatingEvent->scrollCount(),
        delta,
        m_lastIncomingEvent->modifiers(),
        time,
        time,
        { },
        WebWheelEvent::MomentumEndType::Unknown);
    m_dispatcher.internalWheelEvent(m_currentGesture.pageIdentifier, syntheticEvent, m_lastRubberBandableEdges, EventDispatcher::WheelEventOrigin::MomentumEventDispatcher);

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    m_currentLogState.totalGeneratedOffset += appKitAcceleratedDelta.height();
    pushLogEntry(phase, 0);
#endif
}

void MomentumEventDispatcher::didStartMomentumPhase(WebCore::PageIdentifier pageIdentifier, const WebWheelEvent& event)
{
    auto displayProperties = this->displayProperties(pageIdentifier);
    if (!displayProperties)
        return;

    tracePoint(SyntheticMomentumStart);

    m_currentGesture.active = true;
    m_currentGesture.pageIdentifier = pageIdentifier;
    m_currentGesture.initiatingEvent = event;
    m_currentGesture.currentOffset = { };
    m_currentGesture.startTime = MonotonicTime::now();
    m_currentGesture.displayNominalFrameRate = displayProperties->nominalFrameRate;
    m_currentGesture.accelerationCurve = [&] () -> std::optional<ScrollingAccelerationCurve> {
        auto curveIterator = m_accelerationCurves.find(m_currentGesture.pageIdentifier);
        if (curveIterator == m_accelerationCurves.end())
            return { };
        return curveIterator->value;
    }();

    startDisplayLink();

    // FIXME: The system falls back from the table to just generating deltas
    // directly when the frame interval is within 20fps of idealCurveFrameRate;
    // we should perhaps do the same.
    float idealCurveMultiplier = m_currentGesture.accelerationCurve->frameRate() / idealCurveFrameRate;
    buildOffsetTableWithInitialDelta(*event.rawPlatformDelta() * idealCurveMultiplier);

    WebCore::FloatSize consumedDelta = event.delta();
    if (m_currentGesture.initiatingEvent->directionInvertedFromDevice())
        consumedDelta.scale(-1);
    m_currentGesture.currentOffset += consumedDelta;
    
    dispatchSyntheticMomentumEvent(WebWheelEvent::PhaseBegan, event.delta());
}

void MomentumEventDispatcher::didEndMomentumPhase()
{
    ASSERT(m_currentGesture.active);

    dispatchSyntheticMomentumEvent(WebWheelEvent::PhaseEnded, { });

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher ending synthetic momentum phase with total offset %.1f %.1f, duration %f (event offset would have been %.1f %.1f) (tail index %d of %zu)", m_currentGesture.currentOffset.width(), m_currentGesture.currentOffset.height(), (MonotonicTime::now() - m_currentGesture.startTime).seconds(), m_currentGesture.accumulatedEventOffset.width(), m_currentGesture.accumulatedEventOffset.height(), m_currentGesture.currentTailDeltaIndex, m_currentGesture.tailDeltaTable.size());
    m_dispatcher.queue().dispatchAfter(1_s, [this] {
        flushLog();
    });
#endif

    stopDisplayLink();

    m_currentGesture = { };
    tracePoint(SyntheticMomentumEnd);
}

void MomentumEventDispatcher::setScrollingAccelerationCurve(WebCore::PageIdentifier pageIdentifier, std::optional<ScrollingAccelerationCurve> curve)
{
    m_accelerationCurves.set(pageIdentifier, curve);

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    WTF::TextStream stream(WTF::TextStream::LineMode::SingleLine);
    stream << curve;
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher set curve %{public}s", stream.release().utf8().data());
#endif
}

std::optional<MomentumEventDispatcher::DisplayProperties> MomentumEventDispatcher::displayProperties(WebCore::PageIdentifier pageIdentifier) const
{
    ASSERT(pageIdentifier);
    auto displayPropertiesIterator = m_displayProperties.find(pageIdentifier);
    if (displayPropertiesIterator == m_displayProperties.end())
        return std::nullopt;
    return { displayPropertiesIterator->value };
}

void MomentumEventDispatcher::startDisplayLink()
{
    auto displayProperties = this->displayProperties(m_currentGesture.pageIdentifier);
    if (!displayProperties) {
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher failed to start display link");
        return;
    }

    // FIXME: Switch down to lower-than-full-speed frame rates for the tail end of the curve.
    WebProcess::singleton().parentProcessConnection()->send(Messages::WebProcessProxy::StartDisplayLink(m_observerID, displayProperties->displayID, WebCore::FullSpeedFramesPerSecond), 0);
#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher starting display link for display %d", displayProperties->displayID);
#endif
}

void MomentumEventDispatcher::stopDisplayLink()
{
    auto displayProperties = this->displayProperties(m_currentGesture.pageIdentifier);
    if (!displayProperties) {
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher failed to stop display link");
        return;
    }

    WebProcess::singleton().parentProcessConnection()->send(Messages::WebProcessProxy::StopDisplayLink(m_observerID, displayProperties->displayID), 0);
#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher stopping display link for display %d", displayProperties->displayID);
#endif
}

void MomentumEventDispatcher::pageScreenDidChange(WebCore::PageIdentifier pageID, WebCore::PlatformDisplayID displayID, std::optional<unsigned> nominalFramesPerSecond)
{
    bool affectsCurrentGesture = (pageID == m_currentGesture.pageIdentifier);
    if (affectsCurrentGesture)
        stopDisplayLink();

    DisplayProperties properties;
    properties.displayID = displayID;
    properties.nominalFrameRate = nominalFramesPerSecond.value_or(WebCore::FullSpeedFramesPerSecond);
    m_displayProperties.set(pageID, WTFMove(properties));

    if (affectsCurrentGesture)
        startDisplayLink();
}

std::optional<WebCore::FloatSize> MomentumEventDispatcher::consumeDeltaForCurrentTime()
{
    WebCore::FloatSize delta;

    auto animationTime = MonotonicTime::now() - m_currentGesture.startTime;
    if (animationTime < m_currentGesture.tailStartDelay) {
        auto desiredOffset = offsetAtTime(animationTime);
        delta = roundedIntSize(desiredOffset - m_currentGesture.currentOffset);
    } else {
        if (m_currentGesture.currentTailDeltaIndex < m_currentGesture.tailDeltaTable.size())
            delta = -m_currentGesture.tailDeltaTable[m_currentGesture.currentTailDeltaIndex++];
        else
            return std::nullopt;
    }

    m_currentGesture.currentOffset += delta;

    if (m_currentGesture.initiatingEvent->directionInvertedFromDevice())
        delta.scale(-1);

    return delta;
}

void MomentumEventDispatcher::displayWasRefreshed(WebCore::PlatformDisplayID displayID, const WebCore::DisplayUpdate&)
{
    if (!m_currentGesture.active)
        return;

    auto displayProperties = this->displayProperties(m_currentGesture.pageIdentifier);
    if (!displayProperties || displayID != displayProperties->displayID)
        return;

    auto delta = consumeDeltaForCurrentTime();
    if (!delta) {
#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher completed synthetic momentum phase");
#endif
        didEndMomentumPhase();
        return;
    }

    dispatchSyntheticMomentumEvent(WebWheelEvent::PhaseChanged, *delta);
}

void MomentumEventDispatcher::didReceiveScrollEventWithInterval(WebCore::FloatSize size, Seconds frameInterval)
{
    auto push = [](HistoricalDeltas& deltas, Delta newDelta) {
        bool directionChanged = deltas.size() && (deltas.first().rawPlatformDelta > 0) != (newDelta.rawPlatformDelta > 0);
        if (directionChanged || newDelta.frameInterval > deltaHistoryMaximumAge)
            deltas.clear();

        deltas.prepend(newDelta);
        if (deltas.size() > deltaHistoryQueueSize)
            deltas.removeLast();
    };

    push(m_deltaHistoryX, { size.width(), frameInterval });
    push(m_deltaHistoryY, { size.height(), frameInterval });
}

void MomentumEventDispatcher::didReceiveScrollEvent(const WebWheelEvent& event)
{
    ASSERT(!m_currentGesture.active);

    if (!event.rawPlatformDelta())
        return;

    auto delta = *event.rawPlatformDelta();
    auto time = event.ioHIDEventTimestamp();

    if (!m_lastScrollTimestamp) {
        m_lastScrollTimestamp = time;
        // FIXME: Check that this matches the system (they may go with 10ms for the first frame).
        return;
    }

    auto frameInterval = time - *m_lastScrollTimestamp;
    m_lastScrollTimestamp = time;

    didReceiveScrollEventWithInterval(delta, frameInterval);
}

void MomentumEventDispatcher::buildOffsetTableWithInitialDelta(WebCore::FloatSize initialUnacceleratedDelta)
{
    m_currentGesture.offsetTable.clear();

    WebCore::FloatSize accumulatedOffset;
    WebCore::FloatSize unacceleratedDelta = initialUnacceleratedDelta;

    // Tail deltas will be dispatched at the screen refresh rate, not the momentum
    // dispatch rate, so we need to scale from 60Hz into screen refresh rate.
    float tailCurveMultiplier = static_cast<float>(idealCurveFrameRate) / m_currentGesture.displayNominalFrameRate;
    bool inTail = false;
    WebCore::FloatSize tailCarry;

    do {
        WebCore::FloatSize acceleratedDelta;
        std::tie(unacceleratedDelta, acceleratedDelta) = computeNextDelta(unacceleratedDelta);

        const float tailStartUnacceleratedDelta = 6.f;
        if (!inTail && std::abs(unacceleratedDelta.width()) < tailStartUnacceleratedDelta && std::abs(unacceleratedDelta.height()) < tailStartUnacceleratedDelta) {
            inTail = true;
            m_currentGesture.tailStartDelay = idealCurveFrameInterval * m_currentGesture.offsetTable.size();
        }

        if (inTail) {
            auto tailDelta = acceleratedDelta * tailCurveMultiplier;
            auto deltaWithCarry = tailDelta + tailCarry;
            auto quantizedDelta = roundedIntSize(deltaWithCarry);
            tailCarry = deltaWithCarry - quantizedDelta;
            m_currentGesture.tailDeltaTable.append(quantizedDelta);
        }

        accumulatedOffset += acceleratedDelta;
        m_currentGesture.offsetTable.append(accumulatedOffset);

    } while (std::abs(unacceleratedDelta.width()) > 0.5 || std::abs(unacceleratedDelta.height()) > 0.5);

    equalizeTailGaps();

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher built table with %ld frames, %.1f seconds (%ld tail frames, %.1f seconds, starting at %.1f), initial delta %.1f %.1f, distance %.1f %.1f (initial delta from last changed event %.1f %.1f)", m_currentGesture.offsetTable.size(), idealCurveFrameInterval.seconds() * m_currentGesture.offsetTable.size(), m_currentGesture.tailDeltaTable.size(), m_currentGesture.tailDeltaTable.size() * (1.f / m_currentGesture.accelerationCurve->frameRate()), m_currentGesture.tailStartDelay.seconds(), initialUnacceleratedDelta.width(), initialUnacceleratedDelta.height(), accumulatedOffset.width(), accumulatedOffset.height(), m_lastActivePhaseDelta.width(), m_lastActivePhaseDelta.height());
#endif
}

void MomentumEventDispatcher::equalizeTailGaps()
{
    // Sort the deltas up until the first zero to ensure a lack of unexpected
    // perceptual acceleration, and then inject skipped frames in order to
    // ensure that frames that have effective scroll movement (non-zero deltas)
    // are always spaced equally-or-further apart than earlier ones, but
    // never closer together.

    auto& table = m_currentGesture.tailDeltaTable;
    size_t initialTableSize = table.size();
    if (!initialTableSize)
        return;

    enum Axis { Horizontal, Vertical };
    Vector<float> deltas[2];
    unsigned firstZeroIndex[2] = { 0, 0 };
    deltas[Horizontal].reserveInitialCapacity(initialTableSize);
    deltas[Vertical].reserveInitialCapacity(initialTableSize);
    for (unsigned i = 0; i < initialTableSize; i++) {
        deltas[Horizontal].uncheckedAppend(table[i].width());
        if (!firstZeroIndex[Horizontal] && !table[i].width())
            firstZeroIndex[Horizontal] = i;

        deltas[Vertical].uncheckedAppend(table[i].height());
        if (!firstZeroIndex[Vertical] && !table[i].height())
            firstZeroIndex[Vertical] = i;
    }

    auto sortDeltas = [&] (Axis axis) {
        if (!firstZeroIndex[axis])
            return;

        if (deltas[axis][0] > 0)
            std::sort(deltas[axis].begin(), std::next(deltas[axis].begin(), firstZeroIndex[axis]), std::greater());
        else
            std::sort(deltas[axis].begin(), std::next(deltas[axis].begin(), firstZeroIndex[axis]));
    };

    sortDeltas(Horizontal);
    sortDeltas(Vertical);

    // GapSize is a count of contiguous frames with zero deltas.
    typedef unsigned GapSize[2];
    GapSize minimumGap = { 0, 0 };
    GapSize currentGap = { 0, 0 };
    GapSize remainingGapToGenerate = { 0, 0 };
    unsigned originalTableIndex[2] = { 0, 0 };

    auto takeNextDelta = [&] (uint8_t axis) -> float {
        if (originalTableIndex[axis] >= initialTableSize)
            return 0.f;

        if (remainingGapToGenerate[axis]) {
            --remainingGapToGenerate[axis];
            ++currentGap[axis];
            return 0.f;
        }

        auto value = deltas[axis][originalTableIndex[axis]];
        if (value) {
            minimumGap[axis] = std::max(minimumGap[axis], currentGap[axis]);
            remainingGapToGenerate[axis] = minimumGap[axis] - currentGap[axis];
            if (remainingGapToGenerate[axis]) {
                --remainingGapToGenerate[axis];
                ++currentGap[axis];
                return 0.f;
            }

            currentGap[axis] = 0;
        } else
            ++currentGap[axis];

        ++originalTableIndex[axis];

        return value;
    };

    size_t finalTableSize = 0;
    table.shrink(0);

    while (originalTableIndex[Horizontal] < initialTableSize || originalTableIndex[Vertical] < initialTableSize) {
        WebCore::FloatSize delta(takeNextDelta(Horizontal), takeNextDelta(Vertical));
        table.append(delta);

        if (!delta.isZero())
            finalTableSize = table.size();
    }

    table.shrink(finalTableSize);
}

static float interpolate(float a, float b, float t)
{
    return a + t * (b - a);
}

WebCore::FloatSize MomentumEventDispatcher::offsetAtTime(Seconds time)
{
    if (!m_currentGesture.offsetTable.size())
        return { };

    float fractionalFrameNumber = time.seconds() / idealCurveFrameInterval.seconds();
    unsigned long lowerFrameNumber = std::min<unsigned long>(m_currentGesture.offsetTable.size() - 1, floor(fractionalFrameNumber));
    unsigned long upperFrameNumber = std::min<unsigned long>(m_currentGesture.offsetTable.size() - 1, lowerFrameNumber + 1);
    float amount = fractionalFrameNumber - lowerFrameNumber;
    
    return {
        -interpolate(m_currentGesture.offsetTable[lowerFrameNumber].width(), m_currentGesture.offsetTable[upperFrameNumber].width(), amount),
        -interpolate(m_currentGesture.offsetTable[lowerFrameNumber].height(), m_currentGesture.offsetTable[upperFrameNumber].height(), amount)
    };
}

static float momentumDecayRate(WebCore::FloatSize delta, Seconds frameInterval)
{
    constexpr float defaultDecay = 0.975f;
    constexpr float tailVelocity = 250.f;
    constexpr float tailDecay = 0.91f;
    auto alpha = defaultDecay;

    if (frameInterval) {
        float initialVelocity = delta.diagonalLength() / frameInterval.seconds();
        float weakMomentum = std::max<float>(0.f, (tailVelocity - initialVelocity) / tailVelocity);
        alpha = defaultDecay - (defaultDecay - tailDecay) * weakMomentum;
    }

    return std::pow(alpha, (frameInterval.seconds() / 0.008f));
}

static constexpr float fromFixedPoint(float value)
{
    return value / 65536.0f;
}

std::pair<WebCore::FloatSize, WebCore::FloatSize> MomentumEventDispatcher::computeNextDelta(WebCore::FloatSize currentUnacceleratedDelta)
{
    WebCore::FloatSize unacceleratedDelta = currentUnacceleratedDelta;

    float decayRate = momentumDecayRate(unacceleratedDelta, idealCurveFrameInterval);
    unacceleratedDelta.scale(decayRate);

    // The delta queue operates on pre-acceleration deltas, so insert the new event *before* accelerating.
    didReceiveScrollEventWithInterval(unacceleratedDelta, idealCurveFrameInterval);

    auto accelerateAxis = [&] (HistoricalDeltas& deltas, float value) {
        float totalDelta = 0;
        Seconds totalTime;
        unsigned count = 0;

        for (const auto& delta : deltas) {
            totalDelta += std::abs(delta.rawPlatformDelta);
            count++;

            if (delta.frameInterval > deltaHistoryMaximumInterval) {
                totalTime += deltaHistoryMaximumInterval;
                break;
            }

            totalTime += delta.frameInterval;

            if (totalTime >= deltaHistoryMaximumAge)
                break;
        }

        auto averageFrameInterval = std::clamp(totalTime / count, 1_ms, deltaHistoryMaximumInterval);
        float averageFrameIntervalMS = averageFrameInterval.milliseconds();
        float averageDelta = totalDelta / count;

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
        if (!m_currentGesture.didLogInitialQueueState)
            RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher initial historical deltas: average delta %f, average time %fms, event count %d", averageDelta, averageFrameIntervalMS, count);
#endif

        constexpr float velocityGainA = fromFixedPoint(2.f);
        constexpr float velocityGainB = fromFixedPoint(955.f);
        constexpr float velocityConstant = fromFixedPoint(98369.f);
        constexpr float minimumVelocity = fromFixedPoint(1.f);

        float velocity = (velocityGainA * averageFrameIntervalMS * averageFrameIntervalMS - velocityGainB * averageFrameIntervalMS + velocityConstant) * averageDelta;
        velocity = std::max<float>(velocity, minimumVelocity);

        // FIXME: This math is incomplete if we need to support acceleration tables as well.
        float multiplier = m_currentGesture.accelerationCurve->accelerationFactor(velocity) / velocity;
        return value * multiplier;
    };

    WebCore::FloatSize acceleratedDelta(
        accelerateAxis(m_deltaHistoryX, unacceleratedDelta.width()),
        accelerateAxis(m_deltaHistoryY, unacceleratedDelta.height())
    );

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)
    m_currentGesture.didLogInitialQueueState = true;
#endif

    return { unacceleratedDelta, acceleratedDelta };
}

#if ENABLE(MOMENTUM_EVENT_DISPATCHER_TEMPORARY_LOGGING)

void MomentumEventDispatcher::pushLogEntry(uint32_t generatedPhase, uint32_t eventPhase)
{
    m_currentLogState.time = MonotonicTime::now();
    m_currentLogState.generatedPhase = generatedPhase;
    m_currentLogState.eventPhase = eventPhase;
    m_log.append(m_currentLogState);
}

void MomentumEventDispatcher::flushLog()
{
    if ((MonotonicTime::now() - m_currentLogState.time) < 500_ms)
        return;

    if (m_log.isEmpty())
        return;

    auto startTime = m_log[0].time;
    RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher event log: time,generatedOffset,generatedPhase,eventOffset,eventPhase");
    for (const auto& entry : m_log)
        RELEASE_LOG(ScrollAnimations, "MomentumEventDispatcher event log: %f,%f,%d,%f,%d", (entry.time - startTime).seconds(), entry.totalGeneratedOffset, entry.generatedPhase, entry.totalEventOffset, entry.eventPhase);

    m_log.clear();
    m_currentLogState = { };
}

#endif

} // namespace WebKit

#endif