<style> table { font-family: arial, sans-serif; font-size: 24px; border-collapse: collapse; width: 80%; } td, th { border: 1px solid #dddddd; text-align: left; padding: 8px; }

  tr:nth-child(even) {
    background-color: #dddddd;
  }
  button {
    background-color: #4CAF50;
    border: none;
    color: white;
    padding: 15px 32px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 24px;
    margin: 4px 2px;
    cursor: pointer;
  }
  input {
    font-size: 24px;
  }
</style>

COVID-19 Rapid Antigen Test Probabilities

<p>
  <b>Specificity</b> is how likely a test is to return a positive result if
  you have COVID. By default, this is set to 80%, the minimum required for
  an RAT to be sold.
</p>

<p>
  <b>Sensitivity</b> is how likely a test is to return a negative result if
  you don't have COVID. By default, this is set to 98%, the minimum required
  for an RAT to be sold.
</p>

<p>
  <b>Probability</b> is how likely you are to have COVID before the test.
  This should be the percentage of people in your area who have COVID if you have no symptoms. If you do have symptoms, increase the probability accordingly. I don't have the maths yet.
</p>

<table>
  <tr>
    <th>Variable</th>
    <th>Number</th>
  </tr>
  <tr>
    <td><b>Specificity</b> (0-100%)</td>
    <td>
      <input type="text" id="specificity" name="specificity" value="80" />
      <td></td>
    </td>
  </tr>

  <tr>
    <td><b>Sensitivity</b> (0-100%)</td>
    <td>
      <input type="text" id="sensitivity" name="sensitivity" value="98" />
      <td></td>
    </td>
  </tr>

  <tr>
    <td><b>Probability</b> (0-100%)</td>
    <td>
      <input type="text" id="probability" name="probability" value="1" />
      <td></td>
    </td>
  </tr>
</table>
<button onclick=createParagraph()>Calculate</button>
<p id=calculationText></p>

<script>

    const createParagraph = () => {
      const html = getCalculation();
      document.getElementById("calculationText").innerHTML = html;
    }

    const roundCalculation = (calculation) => {
      console.log(calculation)
      roundTo = (calculation < 10 || calculation > 98) ? 1 : 0
      console.log(roundTo)
      return calculation.toFixed(roundTo);
    }

    const getCalculation = () => {

      let specificity = document.getElementById("specificity").value;
      let sensitivity = document.getElementById("sensitivity").value;
      let probability = document.getElementById("probability").value;

      let inputList = new Array(specificity, sensitivity, probability);

      for (const element of inputList) {
        if (isNaN(element)) {
          return `All inputs must be numbers.`
        }
        if (element <= 0 || element > 100) {
          return `All inputs must be between 0 and 100. Probability cannot be 0.`
        }
      }

      specificity = specificity / 100;
      sensitivity = sensitivity / 100;
      probability = probability / 100;

      let true_positive = specificity * probability
      let false_positive = (1 - specificity) * probability
      let true_negative = sensitivity * (1 - probability)
      let false_negative = (1 - sensitivity) * (1 - probability)

      let positive_chance = true_positive / (true_positive + false_negative) * 100
      let negative_chance = false_positive / (false_positive + true_negative) * 100

      positive_text = `If you test positive for COVID-19, the chance you actually have it is ${roundCalculation(positive_chance)}%.`
      negative_text = `If you test negative for COVID-19, the chance you actually have it is ${roundCalculation(negative_chance)}%.`

      details_header = `Details:`

      tp_text = `True Positive: The chance you have COVID-19 and test positive is ${roundCalculation(true_positive * 100)}%.`
      fp_text = `False Positive: The chance you have COVID-19 but test negative is ${roundCalculation(false_positive * 100)}%.`
      tn_text = `True Negative: The chance you don't have COVID-19 and test negative is ${roundCalculation(true_negative * 100)}%.`
      fn_text = `False Negative: The chance you don't have COVID-19 but test positive is ${roundCalculation(false_negative * 100)}%.`

      text_html = `${positive_text}<br>${negative_text}<br><br>${details_header}<br><br>${tp_text}<br>${fp_text}<br>${tn_text}<br>${fn_text}`

      return text_html;
    }
</script>