Source/WebCore/platform/graphics/SpringSolver.h

/*
 * Copyright (C) 2016 Apple Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 *
 * 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
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#pragma once

namespace WebCore {

class SpringSolver {
public:
    SpringSolver(double mass, double stiffness, double damping, double initialVelocity)
    {
        m_w0 = std::sqrt(stiffness / mass);
        m_zeta = damping / (2 * std::sqrt(stiffness * mass));

        if (m_zeta < 1) {
            // Under-damped.
            m_wd = m_w0 * std::sqrt(1 - m_zeta * m_zeta);
            m_A = 1;
            m_B = (m_zeta * m_w0 + -initialVelocity) / m_wd;
        } else {
            // Critically damped (ignoring over-damped case for now).
            m_A = 1;
            m_B = -initialVelocity + m_w0;
        }
    }

    double solve(double t)
    {
        if (m_zeta < 1) {
            // Under-damped
            t = std::exp(-t * m_zeta * m_w0) * (m_A * std::cos(m_wd * t) + m_B * std::sin(m_wd * t));
        } else {
            // Critically damped (ignoring over-damped case for now).
            t = (m_A + m_B * t) * std::exp(-t * m_w0);
        }

        // Map range from [1..0] to [0..1].
        return 1 - t;
    }

private:
    double m_w0;
    double m_zeta;
    double m_wd;
    double m_A;
    double m_B;
};

} // namespace WebCore