Improve TOTP form

Based on https://bbbootstrap.com/snippets/bootstrap-5-verify-otp-validation-form-inputs-78878858

templates/login/opt_input.html

@@ -1,17 +1,27 @@
 {% extends "base_login_page.html" %}
 {% block content %}
 
+<style>
+#otp input {
+  padding-left: 0px;
+  padding-right: 0px;
+}
+</style>
+
 <div>
     <p>Please go to your authenticator app <i>{{ factor.name }}</i>, generate a new code and enter it here:</p>
-    <form method="post" action="{{ factor.login_url(_p.redirect_uri) }}">
+    <form id="totp_form" method="post" action="{{ factor.login_url(_p.redirect_uri) }}">
-
+        <input type="hidden" id="code" name="code" />
         <div class="form-group">
-            <label for="code" class="form-label mt-4">Generated code</label>
+            <div id="otp" class="inputs d-flex flex-row justify-content-center mt-2">
-            <input type="text" name="code" minlength="6" maxlength="6" class="form-control" id="code"
+                <input class="m-2 text-center form-control rounded" type="text" id="i1" maxlength="1" autofocus/>
-                   placeholder="XXXXXX" autofocus>
+                <input class="m-2 text-center form-control rounded" type="text" id="i2" maxlength="1"/>
+                <input class="m-2 text-center form-control rounded" type="text" id="i3" maxlength="1"/>
+                <input class="m-2 text-center form-control rounded" type="text" id="i4" maxlength="1"/>
+                <input class="m-2 text-center form-control rounded" type="text" id="i5" maxlength="1"/>
+                <input class="m-2 text-center form-control rounded" type="text" id="i6" maxlength="1"/>
+            </div>
         </div>
-
-        <button class="w-100 btn btn-primary" type="submit" style="margin: 20px 0px;">Login</button>
     </form>
 </div>
 
@@ -21,4 +31,53 @@
     <a href="/logout">Sign out</a>
 </div>
 
+<script>
+function OTPInput() {
+    const inputs = document.querySelectorAll('#otp > *[id]');
+    for (let i = 0; i < inputs.length; i++) {
+        // Reset form on init
+        inputs[i].value = "";
+
+        inputs[i].addEventListener('keydown', (event) => {
+            if (event.key === "Backspace") {
+                if (inputs[i].value != "") {
+                    inputs[i].value = '';
+                } else if (i > 0) {
+                    inputs[i - 1].value = "";
+                    inputs[i - 1].focus();
+                }
+            }
+
+            // Code has already been typed entirely
+            else if (i === inputs.length - 1 && inputs[i].value !== '')
+            {
+                return;
+            }
+
+            // Add new digit
+            else if (event.keyCode >= 48 && event.keyCode <= 57) {
+                inputs[i].value = event.key;
+                if (i !== inputs.length - 1)
+                    inputs[i + 1].focus();
+                else
+                    submitCode();
+            }
+
+            event.preventDefault();
+        });
+    }
+}
+
+function submitCode() {
+    const code = [...document.querySelectorAll('#otp > *[id]')]
+        .map((i) => i.value)
+        .join("");
+
+    document.getElementById("code").value = code;
+    document.getElementById("totp_form").submit();
+}
+
+document.addEventListener("DOMContentLoaded", () => OTPInput());
+</script>
+
 {% endblock content %}