sir-simulator/sir_simulator.html at main · jdrakephd/sir-simulator · GitHub

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<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width,initial-scale=1" />
  <title>SIR Simulator</title>
  <style>
    body { font-family: system-ui, -apple-system, Segoe UI, Roboto, sans-serif; margin: 16px; }
    .row { display: grid; grid-template-columns: 120px 1fr 90px; gap: 10px; align-items: center; margin: 10px 0; }
    input[type="range"] { width: 100%; }
    .panel { max-width: 980px; margin: 0 auto; }
    .stats { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 10px; margin: 10px 0 16px; }
    .card { border: 1px solid #ddd; border-radius: 10px; padding: 10px 12px; }
    .big { font-size: 18px; font-weight: 650; }
    canvas { width: 100%; height: 460px; border: 1px solid #ddd; border-radius: 10px; }
    .hint { color:#444; font-size: 13px; margin-top: 6px; }
    code { background:#f6f6f6; padding:2px 6px; border-radius:6px; }
  </style>
</head>
<body>
<div class="panel">
  <h2>SIR Simulator</h2>

  <div class="row">
    <div><label for="beta">β</label></div>
    <input id="beta" type="range" min="0" max="0.01" step="0.0001" value="0.001">
    <div id="betaVal" class="big"></div>
  </div>

  <div class="row">
    <div><label for="gamma">γ</label></div>
    <input id="gamma" type="range" min="0.01" max="1" step="0.01" value="0.2">
    <div id="gammaVal" class="big"></div>
  </div>

  <div class="row">
    <div><label for="S0">S0</label></div>
    <input id="S0" type="range" min="0" max="1000" step="1" value="500">
    <div id="S0Val" class="big"></div>
  </div>

  <div class="row">
    <div><label for="I0">I0</label></div>
    <input id="I0" type="range" min="1" max="50" step="1" value="1">
    <div id="I0Val" class="big"></div>
  </div>

  <div class="row">
    <div><label for="T">Days</label></div>
    <input id="T" type="range" min="10" max="300" step="1" value="160">
    <div id="TVal" class="big"></div>
  </div>

  <div class="stats">
    <div class="card">
      <div>R0 (≈ β·S0/γ)</div>
      <div id="R0Val" class="big"></div>
      <div class="hint">For your assignment’s parameters, disease spreads when R0 &gt; 1.</div>
    </div>
    <div class="card">
      <div>Spreads?</div>
      <div id="spreadVal" class="big"></div>
      <div class="hint">Based on whether I initially increases.</div>
    </div>
    <div class="card">
      <div>Peak I (and day)</div>
      <div id="peakVal" class="big"></div>
      <div class="hint">Useful for “small outbreak” vs “epidemic”.</div>
    </div>
  </div>

  <canvas id="plot" width="1100" height="460"></canvas>
  <div class="hint">
    Curves: <b>S</b> (blue), <b>I</b> (red), <b>R</b> (green). Values are counts (not proportions).
    Model: <code>dS/dt=-βSI</code>, <code>dI/dt=βSI-γI</code>, <code>dR/dt=γI</code>.
  </div>
</div>

<script>
  const el = id => document.getElementById(id);

  const sliders = ["beta","gamma","S0","I0","T"].map(el);
  const labels = { beta:"betaVal", gamma:"gammaVal", S0:"S0Val", I0:"I0Val", T:"TVal" };

  function fmt(x, digits=4){
    const n = Number(x);
    if (Math.abs(n) >= 100) return n.toFixed(0);
    if (Math.abs(n) >= 10) return n.toFixed(2);
    return n.toFixed(digits);
  }

  function simulate(beta, gamma, S0, I0, T){
    const dt = 0.05;               // time step (days)
    const steps = Math.floor(T/dt) + 1;
    let S = S0, I = I0, R = 0;
    const t = new Array(steps);
    const Ss = new Array(steps);
    const Is = new Array(steps);
    const Rs = new Array(steps);

    for (let k=0; k<steps; k++){
      const time = k*dt;
      t[k]=time; Ss[k]=S; Is[k]=I; Rs[k]=R;

      // Euler step
      const dS = -beta * S * I;
      const dI =  beta * S * I - gamma * I;
      const dR =  gamma * I;

      S = Math.max(0, S + dS*dt);
      I = Math.max(0, I + dI*dt);
      R = Math.max(0, R + dR*dt);
    }
    return {t,Ss,Is,Rs,dt};
  }

  function draw({t,Ss,Is,Rs}, T){
    const canvas = el("plot");
    const ctx = canvas.getContext("2d");

    // layout
    const W = canvas.width, H = canvas.height;
    const padL=55, padR=20, padT=20, padB=40;
    const x0=padL, y0=H-padB, x1=W-padR, y1=padT;
    const plotW = x1-x0, plotH = y0-y1;

    // clear
    ctx.clearRect(0,0,W,H);

    // max y
    let ymax = 1;
    for (let i=0;i<t.length;i++){
      ymax = Math.max(ymax, Ss[i], Is[i], Rs[i]);
    }

    // axes
    ctx.strokeStyle="#333";
    ctx.lineWidth=1;
    ctx.beginPath();
    ctx.moveTo(x0,y1); ctx.lineTo(x0,y0); ctx.lineTo(x1,y0);
    ctx.stroke();

    // ticks
    ctx.fillStyle="#111";
    ctx.font="12px system-ui";
    const xTicks = 8, yTicks = 5;
    for (let i=0;i<=xTicks;i++){
      const tx = x0 + (i/xTicks)*plotW;
      const val = (i/xTicks)*T;
      ctx.strokeStyle="#999";
      ctx.beginPath(); ctx.moveTo(tx,y0); ctx.lineTo(tx,y0+5); ctx.stroke();
      ctx.fillText(val.toFixed(0), tx-8, y0+20);
    }
    for (let j=0;j<=yTicks;j++){
      const ty = y0 - (j/yTicks)*plotH;
      const val = (j/yTicks)*ymax;
      ctx.strokeStyle="#999";
      ctx.beginPath(); ctx.moveTo(x0-5,ty); ctx.lineTo(x0,ty); ctx.stroke();
      ctx.fillText(val.toFixed(0), 8, ty+4);
    }
    ctx.fillText("time (days)", (x0+x1)/2-35, H-10);
    ctx.fillText("people", 10, 16);

    function xy(i, series){
      const x = x0 + (t[i]/T)*plotW;
      const y = y0 - (series[i]/ymax)*plotH;
      return [x,y];
    }
    function line(series, stroke){
      ctx.strokeStyle=stroke;
      ctx.lineWidth=5;
      ctx.beginPath();
      let [x,y]=xy(0,series);
      ctx.moveTo(x,y);
      for (let i=1;i<t.length;i++){
        [x,y]=xy(i,series);
        ctx.lineTo(x,y);
      }
      ctx.stroke();
    }

    // simple distinct colors
    line(Ss, "#2b6cb0");
    line(Is, "#c53030");
    line(Rs, "#2f855a");

    // legend
    ctx.font="13px system-ui";
    const lx = x1-140, ly=y1+10;
    ctx.fillStyle="#2b6cb0"; ctx.fillText("S", lx, ly+10);
    ctx.fillStyle="#c53030"; ctx.fillText("I", lx+30, ly+10);
    ctx.fillStyle="#2f855a"; ctx.fillText("R", lx+60, ly+10);
  }

  function update(){
    const beta = Number(el("beta").value);
    const gamma = Number(el("gamma").value);
    const S0 = Number(el("S0").value);
    const I0 = Number(el("I0").value);
    const T  = Number(el("T").value);

    el(labels.beta).textContent = fmt(beta,4);
    el(labels.gamma).textContent = fmt(gamma,3);
    el(labels.S0).textContent = fmt(S0,0);
    el(labels.I0).textContent = fmt(I0,0);
    el(labels.T).textContent = fmt(T,0);

    const R0 = (beta*S0)/gamma;
    el("R0Val").textContent = fmt(R0,3);

    const sim = simulate(beta,gamma,S0,I0,T);

    // determine spread: does I increase initially?
    const inc = sim.Is[10] > sim.Is[0]; // ~0.5 days in
    el("spreadVal").textContent = inc ? "YES" : "NO";

    // peak I
    let peakI = -1, peakIdx = 0;
    for (let i=0;i<sim.Is.length;i++){
      if (sim.Is[i] > peakI){ peakI = sim.Is[i]; peakIdx = i; }
    }
    const peakDay = sim.t[peakIdx];
    el("peakVal").textContent = `${fmt(peakI,2)} (day ${fmt(peakDay,1)})`;

    draw(sim, T);
  }

  sliders.forEach(s => s.addEventListener("input", update));
  update();
</script>
</body>
</html>