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HTML

              
                <link rel="preconnect" href="https://fonts.gstatic.com">
<link href="https://fonts.googleapis.com/css2?family=Josefin+Sans&display=swap" rel="stylesheet">

<h1>Corona Simulator</h1>
<main>
  <section class="legend">
    <div class="legend-color healthy"></div>
    <div id="legend-healthy"> Healthy</div>
    <div class="legend-color infected"></div>
    <div id="legend-infected">Infected</div>
    <div class="legend-color immune"></div>
    <div id="legend-immune">Immune</div>
    <div class="legend-color dead"></div>
    <div id="legend-dead">Dead</div>
  </section>
  <section class="buttons">
    <button type="button" class="button" onclick="reset()">Reset</button>
    <label class="switch">
      <input type="checkbox" name="use-js" id="use-js" onchange="reloadLang()">
      <span class="slider button">
        <div id="active-lang">Rust</div>
      </span>
    </label>
    <button type="button" class="next button" onclick="advance()">Next Day</button>
    <button id="start-stop-button" type="button" class="start button" onclick="startStop()">Start Simulation</button>
  </section>
  <section>
    <form id="settings" class="config-form">
      <label for="width">Width:</label>
      <input type="number" name="width" value="20" min="0" title="Number of cells in row" onchange="reloadSettings()">
      <label for="height">Height:</label>
      <input type="number" name="height" value="5" min="0" title="Number of cells in column" onchange="reloadSettings()">
      <label for="radius">Radius:</label>
      <input type="number" name="radius" value="2" min="0" title="How many cells the virus can spread in a single day" onchange="reloadSettings()">
      <label for="infection_rate">Infection Rate:</label>
      <input type="number" name="infection_rate" value="0.02" step="0.0001" min="0.0" max="1.0" title="Chance that a person is infected by a single infected patient in the infection radius." onchange="reloadSettings()">
      <label for="death_rate">Death Rate:</label>
      <input type="number" name="death_rate" value="0.03" step="0.01" min="0.0" max="1.0" title="Chance that an infected person dies in a day." onchange="reloadSettings()">
      <label for="recovery">Days until recovered:</label>
      <input type="number" name="recovery" value="10" min="1" max="20" title="Chance that an infected person dies in a day." onchange="reloadSettings()">
      <label for="delay">Simulation delay:</label>
      <input type="number" name="delay" value="300" min="1" title="Delay between simulation steps in ms.">
    </form>
  </section>
  <!-- Dynamic JS will be inserted here -->
</main>
<footer>
  2020 - WASM Demo App by Jakob Meier - <a target="_blank" href="https://github.com/jakmeier/covid-sim">Source Code</a>
</footer>
              
            
!

CSS

              
                body {
  text-align: center;
  background-color: #e1aa7d;
  font-family: "Josefin Sans", Arial, Helvetica, sans-serif;
}

main {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  grid-template-areas: "legend buttons form" "humans humans humans";
  width: 100%;
}

.config-form {
  grid-area: form;
  display: grid;
  grid-template-columns: min-content minmax(max-content, 200px);
  align-items: center;
  justify-content: center;
  grid-gap: 10px;
  margin: 20px;
}

.buttons {
  grid-area: buttons;
  display: grid;
  grid-template-areas: "reset slider" "next next" "start start";
  grid-template-rows: 90px auto;
  grid-template-columns: 1fr 1fr;
  justify-content: center;
  justify-items: center;
  grid-gap: 0 10px;
  height: 100%;
  align-content: space-around;
}

.next {
  grid-area: next;
}

.start {
  grid-area: start;
}

.buttons button {
  margin: 10px;
  font-size: xx-large;
  width: 100%;
  height: 70px;
}

.button {
  border-radius: 1em;
  background-color: #b6d094;
  border-style: solid;
  border-color: #2e2836;
  border-width: 2px;
  color: black;
}

#active-lang {
  margin: 20px;
}

input {
  background-color: #b6d094;
  border-color: #2e2836;
}

.legend {
  grid-area: legend;
  display: grid;
  justify-content: center;
  grid-template-columns: min-content min-content;
  justify-items: start;
  align-items: center;
}

.legend-color {
  width: 3em;
  height: 3em;
  margin: 0.5em;
}

.container {
  margin-top: 20px;
  grid-area: humans;
  display: grid;
  grid-gap: 1px;
}

.human {
  width: 100%;
  padding-top: 100%;
}

.healthy {
  background-color: #be8a60;
}

.infected {
  background-color: #6a2e35;
}

.immune {
  background-color: #b6d094;
}

.dead {
  background-color: #2e2836;
}

/* Slider, adapted from https://www.w3schools.com/howto/howto_css_switch.asp */

/* The switch - the box around the slider */

.switch {
  position: relative;
  display: inline-block;
  width: 100%;
  height: 100%;
  font-size: xx-large;
}

/* Hide default HTML checkbox */

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

/* The slider */

.slider {
  position: absolute;
  top: 10px;
  left: 0;
  right: 0;
  bottom: 10px;
  -webkit-transition: 0.4s;
  transition: 0.4s;
}

.slider:before {
  position: absolute;
  content: "";
  height: 50px;
  width: 50px;
  left: 10px;
  bottom: 10px;
  background-color: white;
  -webkit-transition: 0.4s;
  transition: 0.4s;
  border-radius: 50%;
}

input:checked + .slider {
  background-color: #be8a60;
}

input:checked + .slider:before {
  left: calc(100% - 60px);
}

footer {
  margin-top: 50px;
  height: 200px;
}

@media (max-width: 1200px) {
  main {
    grid-template-columns: 1fr 1fr;
    grid-template-areas: "form legend" "buttons buttons" "humans humans";
  }
}

@media (max-width: 800px) {
  main {
    grid-template-columns: 1fr;
    grid-template-areas: "form" "legend" "buttons" "humans";
  }
  .legend {
    grid-template-columns: min-content max-content min-content max-content;
  }
}

              
            
!

JS

              
                // Note: The code pen code is slightly adapted compared to the source code show in my repository at https://github.com/jakmeier/covid-sim/. 
// The reason is that here we are not using a bundler (WebPack).

// For the CodePen, I used `wasm-pack publish` to publish a package to npm.
//   1) wasm-pack build --scope jakmeier --target web
//   2) wasm-pack pack
//   3) wasm-pack publish --access public
//
// Here, we are loading this package through a public CDN. (Thanks unpkg!)
// The default export of the package gives access to a init() function that will load the WASM module.
import init, { Simulation } from "https://unpkg.com/@jakmeier/wasm-covid-sim@1.0.0/wasm_covid_sim.js";

// Note that the CDN that Skypack CDB, which CodePen uses by default, does not work. It does not export the wasm file at the time I was testing it.
// FAILS:
// import init, {Simulation} from 'https://cdn.skypack.dev/@jakmeier/wasm-covid-sim@1.0.0';

const HEALTHY = 0;
const INFECTED_DAY_0 = 1;
const INFECTED_DAY_MAX = 64;
const IMMUNE = 100;
const DEAD = 101;

let humans;
let htmlNodes;
let container;
let simulation;

let jsSimulation;
let rustSimulation;
let useJs = false;

let healthy = 0;
let infected = 0;
let immune = 0;
let dead = 0;


init()
  .then(() => {
    if (document.readyState != "loading") {
      main();
    } else {
      document.addEventListener("DOMContentLoaded", main);
    }
  })
  .catch((e) => console.log("failed loading wasm", e));

function main() {
  const mainElement = document.getElementsByTagName("main")[0];
  container = document.createElement("div");
  container.classList.add("container");
  mainElement.appendChild(container);

  rustSimulation = new Simulation();
  jsSimulation = new JsSimulation();
  simulation = rustSimulation;
  humans = new Uint8Array();

  reloadSettings();
  updateView();

  // Advance on space bar
  document.addEventListener("keydown", (evt) => {
    if (evt.key === " ") {
      advance();
    }
  });

  // Make functions globally available for onclick handler on button
  window.advance = advance;
  window.startStop = startStop;
  window.reloadSettings = reloadSettings;
  window.reloadLang = reloadLang;
  window.reset = reset;
}

function reloadSettings() {
  const form = document.getElementById("settings");
  const data = new FormData(form);
  const w = parseInt(data.get("width"));
  const h = parseInt(data.get("height"));
  const radius = parseInt(data.get("radius"));
  const recoveryDays = parseInt(data.get("recovery"));
  const infectionRate = parseFloat(data.get("infection_rate"));
  const deathRate = parseFloat(data.get("death_rate"));

  if (w * h != humans.length) {
    // Replace current state, set all values to 0 (HEALTHY)
    humans = new Uint8Array(w * h);
    generateHtmlNodes(w, h);
  }

  jsSimulation.reconfigure(
    w,
    h,
    radius,
    recoveryDays,
    infectionRate,
    deathRate
  );
  rustSimulation.reconfigure(
    w,
    h,
    radius,
    recoveryDays,
    infectionRate,
    deathRate
  );
}

function reloadLang() {
  useJs = document.getElementById("use-js").checked;
  if (useJs) {
    simulation = jsSimulation;
    document.getElementById("active-lang").textContent = "JS";
  } else {
    simulation = rustSimulation;
    document.getElementById("active-lang").textContent = "Rust";
  }
}

function reset() {
  humans = new Uint8Array(humans.length);
  htmlNodes.forEach((node) => {
    node.classList.add("healthy");
    node.classList.remove("infected");
    node.classList.remove("dead");
    node.classList.remove("immune");
  });
  // Infect patient 0
  setState(Math.floor(humans.length / 2), INFECTED_DAY_0);
  updateView();
}

function advance() {
    let newState = new Uint8Array(humans.length);
    const t0 = performance.now();
    simulation.next_day(humans, newState);
    const t1 = performance.now();
    for (let i = 0; i < htmlNodes.length; i++) {
        if (newState[i] != humans[i]) {
            setState(i, newState[i]);
        }
    }
    updateView();
    humans = newState;
    const t2 = performance.now();
    if (useJs) {
        console.log(`Updated in ${t2 - t0}ms  (${t1 - t0}ms computing in JS, ${t2 - t1}ms DOM updates in JS)`);
    } else {
        console.log(`Updated in ${t2 - t0}ms  (${t1 - t0}ms computing in Rust, ${t2 - t1}ms DOM updates in JS)`);
    }
}

let handle;
let isRunning = false;
function startStop() {
    const form = document.getElementById("settings");
    let simulationDelay = form.elements["delay"].value;
    if (isRunning) {
        clearInterval(handle);
        document.getElementById("start-stop-button").innerText = "Start Simulation";
    } else {
        handle = setInterval(advance, simulationDelay);
        document.getElementById("start-stop-button").innerText = "Stop Simulation";
    }
    isRunning = !isRunning;
}

function generateHtmlNodes(w, h) {
  const t0 = performance.now();
  while (container.firstChild) {
    container.removeChild(container.lastChild);
  }
  container.style.gridTemplateColumns = `repeat(${w},1fr)`;
  htmlNodes = [];
  for (let y = 0; y < h; y++) {
    for (let x = 0; x < w; x++) {
      const cell = document.createElement("div");
      cell.classList.add("human");
      cell.classList.add("healthy");
      cell.addEventListener("click", () => toggleInfected(x + y * w));
      container.appendChild(cell);
      htmlNodes.push(cell);
    }
  }
  const t1 = performance.now();
  console.log(`Created HTML nodes in ${t1 - t0}ms`);

  healthy = w * h;
  infected = 0;
  immune = 0;
  dead = 0;
  
  // Infect patient 0
  setState(Math.floor(humans.length / 2), INFECTED_DAY_0);
}

function updateView() {
  document.getElementById("legend-healthy").innerText = `Healthy (${healthy})`;
  document.getElementById(
    "legend-infected"
  ).innerText = `Infected (${infected})`;
  document.getElementById("legend-immune").innerText = `Immune (${immune})`;
  document.getElementById("legend-dead").innerText = `Dead (${dead})`;
}

function setState(i, state) {
  // Remove old class
  if (humans[i] == HEALTHY) {
    htmlNodes[i].classList.remove("healthy");
    healthy -= 1;
  } else if (humans[i] >= INFECTED_DAY_0 && humans[i] <= INFECTED_DAY_MAX) {
    htmlNodes[i].classList.remove("infected");
    infected -= 1;
  } else if (humans[i] == IMMUNE) {
    htmlNodes[i].classList.remove("immune");
    immune -= 1;
  } else if (humans[i] == DEAD) {
    htmlNodes[i].classList.remove("dead");
    dead -= 1;
  }

  // Add new class
  if (state == HEALTHY) {
    htmlNodes[i].classList.add("healthy");
    healthy += 1;
  } else if (state >= INFECTED_DAY_0 && state <= INFECTED_DAY_MAX) {
    htmlNodes[i].classList.add("infected");
    infected += 1;
  } else if (state == IMMUNE) {
    htmlNodes[i].classList.add("immune");
    immune += 1;
  } else if (state == DEAD) {
    htmlNodes[i].classList.add("dead");
    dead += 1;
  }
  humans[i] = state;
}

function toggleInfected(index) {
  if (humans[index] == HEALTHY) {
    setState(index, INFECTED_DAY_0);
  } else {
    setState(index, HEALTHY);
  }
  updateView();
}

/***************
 * lib.js
 ***************/
// Mirroring the behaviour of lib.rs in JavaScript for measuring the performance difference.
class JsSimulation {
  constructor(name, year) {
    this.w = 0;
    this.h = 0;
    this.recovery = 0;
    this.infection_rate = 0.0;
    this.death_rate = 0.0;
  }
  reconfigure(w, h, radius, recovery, infection_rate, death_rate) {
    this.w = w;
    this.h = h;
    this.radius = radius;
    this.recovery = recovery;
    this.infection_rate = infection_rate;
    this.death_rate = death_rate;
  }
  next_day(input, output) {
    for (let x = 0; x < this.w; x++) {
      for (let y = 0; y < this.h; y++) {
        const current = this.get(input, x, y);
        let next = current;
        if (current == HEALTHY) {
          if (this.chance_to_catch_covid_today(input, x, y) > Math.random()) {
            next = INFECTED_DAY_0 + 1;
          }
        } else if (current >= INFECTED_DAY_0 && current <= INFECTED_DAY_MAX) {
          if (this.death_rate > Math.random()) {
            next = DEAD;
          } else if (
            current >= INFECTED_DAY_0 &&
            current <= INFECTED_DAY_MAX &&
            current - INFECTED_DAY_0 >= this.recovery
          ) {
            next = IMMUNE;
          } else {
            next = current + 1;
          }
        }
        this.set(output, x, y, next);
      }
    }
  }
  chance_to_catch_covid_today(input, x, y) {
    let infected_neighbours = 0;
    const start_x = Math.max(0, x - this.radius);
    const start_y = Math.max(0, y - this.radius);
    const end_x = Math.min(x + this.radius + 1, this.w);
    const end_y = Math.min(y + this.radius + 1, this.h);
    for (let j = start_y; j < end_y; j++) {
      for (let i = start_x; i < end_x; i++) {
        const neighbour = this.get(input, i, j);
        if (neighbour >= INFECTED_DAY_0 && neighbour <= INFECTED_DAY_MAX) {
          infected_neighbours += 1;
        }
      }
    }
    let p_healthy = Math.pow(1.0 - this.infection_rate, infected_neighbours);
    return 1.0 - p_healthy;
  }
  get(input, x, y) {
    return input[x + y * this.w];
  }
  set(output, x, y, status) {
    output[x + y * this.w] = status;
  }
}

              
            
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