script.js

/**
 * Interactive SDOF Vibration Simulator
 * Implements RK4 solver and real-time plotting
 */

// --- Constants & State ---
const state = {
    m: 1.0,
    k: 100,
    c: 5.0,
    F0: 10,
    p: 5.0, // Forcing frequency (rad/s)
    u0: 0.0,
    v0: 0.0,
    tMax: 10.0,
    dt: 0.001
};

// --- DOM Elements ---
const elements = {
    sliders: {
        m: document.getElementById('mass-slider'),
        k: document.getElementById('stiffness-slider'),
        c: document.getElementById('damping-slider'),
        F0: document.getElementById('force-slider'),
        p: document.getElementById('p-slider'),
        u0: document.getElementById('u0-slider'),
        v0: document.getElementById('v0-slider')
    },
    values: {
        m: document.getElementById('mass-val'),
        k: document.getElementById('stiffness-val'),
        c: document.getElementById('damping-val'),
        F0: document.getElementById('force-val'),
        p: document.getElementById('p-val'),
        u0: document.getElementById('u0-val'),
        v0: document.getElementById('v0-val')
    },
    stats: {
        omegaN: document.getElementById('omega-n-val'),
        zeta: document.getElementById('zeta-val'),
        regime: document.getElementById('regime-val'),
        peakDisp: document.getElementById('peak-disp-val'),
        peakVel: document.getElementById('peak-vel-val')
    },
    canvas: {
        disp: document.getElementById('disp-chart'),
        vel: document.getElementById('vel-chart')
    },
    equation: {
        math: document.getElementById('math-equation'),
        sol: document.getElementById('math-solution')
    }
};

// --- Physics Engine ---

/**
 * Differential equations for SDOF system
 * u' = v
 * v' = (F0 * sin(p * t) - c * v - k * u) / m
 */
function derivatives(t, u, v) {
    const u_prime = v;
    const v_prime = (state.F0 * Math.sin(state.p * t) - state.c * v - state.k * u) / state.m;
    return { u_prime, v_prime };
}

function solve() {
    let t = 0;
    let u = state.u0; // Initial displacement
    let v = state.v0; // Initial velocity

    const results = {
        t: [],
        u: [],
        v: []
    };

    const steps = Math.ceil(state.tMax / state.dt);

    for (let i = 0; i <= steps; i++) {
        // Store current state
        if (i % 10 === 0) { // Downsample for plotting (store every 10th point)
            results.t.push(t);
            results.u.push(u);
            results.v.push(v);
        }

        // RK4 Steps
        const k1 = derivatives(t, u, v);

        const t2 = t + state.dt / 2;
        const u2 = u + k1.u_prime * state.dt / 2;
        const v2 = v + k1.v_prime * state.dt / 2;
        const k2 = derivatives(t2, u2, v2);

        const t3 = t + state.dt / 2;
        const u3 = u + k2.u_prime * state.dt / 2;
        const v3 = v + k2.v_prime * state.dt / 2;
        const k3 = derivatives(t3, u3, v3);

        const t4 = t + state.dt;
        const u4 = u + k3.u_prime * state.dt;
        const v4 = v + k3.v_prime * state.dt;
        const k4 = derivatives(t4, u4, v4);

        // Update state
        u += (state.dt / 6) * (k1.u_prime + 2 * k2.u_prime + 2 * k3.u_prime + k4.u_prime);
        v += (state.dt / 6) * (k1.v_prime + 2 * k2.v_prime + 2 * k3.v_prime + k4.v_prime);
        t += state.dt;
    }

    return results;
}

/**
 * Calculate system properties
 */
function calculateProperties() {
    const omegaN = Math.sqrt(state.k / state.m);
    const zeta = state.c / (2 * Math.sqrt(state.k * state.m));

    let regime = '';
    if (zeta < 0) regime = 'Unstable';
    else if (zeta >= 0 && zeta < 1) regime = 'Underdamped';
    else if (zeta === 1) regime = 'Critically Damped';
    else regime = 'Overdamped';

    return { omegaN, zeta, regime };
}

function updateEquationDisplay(props) {
    // 1. Governing Equation
    // mu'' + cu' + ku = F0sin(pt)
    // mu'' + cu' + ku = F0sin(pt)
    const eq = `${state.m.toFixed(1)}u'' + ${state.c.toFixed(1)}u' + ${state.k.toFixed(0)}u = ${state.F0 > 0 ? `${state.F0}sin(${state.p.toFixed(1)}t)` : '0'}`;
    elements.equation.math.textContent = eq;

    // 2. Solution Form
    let solText = "";
    if (state.F0 === 0) {
        // Free Vibration
        if (props.zeta < 1) {
            solText = `Free Underdamped: u(t) = e^(-${(props.zeta * props.omegaN).toFixed(2)}t) * (A cos(${(props.omegaN * Math.sqrt(1 - props.zeta ** 2)).toFixed(2)}t) + B sin(...))`;
        } else if (props.zeta === 1) {
            solText = `Free Critically Damped: u(t) = (A + Bt) * e^(-${props.omegaN.toFixed(2)}t)`;
        } else {
            solText = `Free Overdamped: u(t) = A * e^(s1*t) + B * e^(s2*t)`;
        }
    } else {
        // Forced Vibration
        solText = `Forced Response: u(t) = u_h(t) [Transient] + u_p(t) [Steady State]`;
    }
    elements.equation.sol.textContent = solText;
}

// --- Rendering ---

function drawChart(canvas, timeData, valueData, color, label) {
    const ctx = canvas.getContext('2d');
    const width = canvas.width;
    const height = canvas.height;

    // Clear canvas
    ctx.clearRect(0, 0, width, height);

    // Setup scaling
    const padding = 40;
    const plotWidth = width - 2 * padding;
    const plotHeight = height - 2 * padding;

    const minVal = Math.min(...valueData);
    const maxVal = Math.max(...valueData);
    const range = maxVal - minVal || 1; // Avoid division by zero

    // Helper to map coordinates
    const mapX = (t) => padding + (t / state.tMax) * plotWidth;
    const mapY = (val) => height - padding - ((val - minVal) / range) * plotHeight;

    // Draw Grid
    ctx.strokeStyle = '#334155';
    ctx.lineWidth = 1;
    ctx.beginPath();
    // Zero line
    if (minVal < 0 && maxVal > 0) {
        const yZero = mapY(0);
        ctx.moveTo(padding, yZero);
        ctx.lineTo(width - padding, yZero);
    }
    ctx.stroke();

    // Draw Curve
    ctx.strokeStyle = color;
    ctx.lineWidth = 2;
    ctx.beginPath();
    ctx.moveTo(mapX(timeData[0]), mapY(valueData[0]));

    for (let i = 1; i < timeData.length; i++) {
        ctx.lineTo(mapX(timeData[i]), mapY(valueData[i]));
    }
    ctx.stroke();

    // Draw Axes Labels (Simple)
    ctx.fillStyle = '#94a3b8';
    ctx.font = '12px Inter';
    ctx.fillText('0', padding, height - padding + 15);
    ctx.fillText(state.tMax + 's', width - padding - 20, height - padding + 15);

    ctx.fillText(maxVal.toFixed(2), 5, padding + 5);
    ctx.fillText(minVal.toFixed(2), 5, height - padding + 5);
}


let isUpdating = false;

function updateUI() {
    if (isUpdating) return;
    isUpdating = true;

    requestAnimationFrame(() => {
        // 1. Solve
        console.log("Solving with state:", JSON.stringify(state));
        const data = solve();
        const props = calculateProperties();
        console.log("Properties:", props);

        updateEquationDisplay(props);

        // 2. Update Stats
        elements.stats.omegaN.textContent = props.omegaN.toFixed(2) + ' rad/s';
        elements.stats.zeta.textContent = props.zeta.toFixed(3);
        elements.stats.regime.textContent = props.regime;

        // Peak values
        const maxDisp = Math.max(...data.u.map(Math.abs));
        const maxVel = Math.max(...data.v.map(Math.abs));
        elements.stats.peakDisp.textContent = maxDisp.toFixed(3);
        elements.stats.peakVel.textContent = maxVel.toFixed(3);

        // 3. Draw Charts
        // Resize canvas to match display size
        [elements.canvas.disp, elements.canvas.vel].forEach(canvas => {
            const rect = canvas.getBoundingClientRect();
            if (rect.width > 0 && rect.height > 0) {
                canvas.width = rect.width;
                canvas.height = rect.height;
            }
        });

        drawChart(elements.canvas.disp, data.t, data.u, '#38bdf8', 'Displacement');
        drawChart(elements.canvas.vel, data.t, data.v, '#4ade80', 'Velocity');

        isUpdating = false;
    });
}

// --- Event Listeners ---

function handleSliderChange(key) {
    return (e) => {
        const val = parseFloat(e.target.value);
        state[key] = val;
        elements.values[key].textContent = val;
        updateUI();
    };
}

function init() {
    console.log("Initializing SDOF Simulator v2");
    // Attach listeners
    elements.sliders.m.addEventListener('input', handleSliderChange('m'));
    elements.sliders.k.addEventListener('input', handleSliderChange('k'));
    elements.sliders.c.addEventListener('input', handleSliderChange('c'));
    elements.sliders.F0.addEventListener('input', handleSliderChange('F0'));
    elements.sliders.p.addEventListener('input', handleSliderChange('p'));
    elements.sliders.u0.addEventListener('input', handleSliderChange('u0'));
    elements.sliders.v0.addEventListener('input', handleSliderChange('v0'));

    // Initial render
    updateUI();

    // Handle resize
    window.addEventListener('resize', updateUI);
}

// Start
init();