Automatically select the departure date when arrival changes

By default, it selects the next day, unless the new arrival date is
before the current departure, in which case is left as is, because the
idea is to help people avoid useless calendar selections when booking
for next three or four months: they would need to change the month
twice, and now only at most one.

web/templates/public/booking.gohtml

@@ -147,7 +147,8 @@
                     <label>
                         <input type="checkbox" required name="{{ .Name }}" {{ if .Checked}}checked{{ end }}
                                 {{ template "error-attrs" . }}
-                        > {{ printf ( pgettext "I have read and I accept <a href=\"%s\" target=\"_blank\">the reservation conditions</a>" "input" ) (print "/" currentLocale "/legal/reservation") | raw }}</label><br>
+                        > {{ printf ( pgettext "I have read and I accept <a href=\"%s\" target=\"_blank\">the reservation conditions</a>" "input" ) (print "/" currentLocale "/legal/reservation") | raw }}
+                    </label><br>
                     {{ template "error-message" . }}
                 {{- end }}
             </fieldset>
@@ -158,6 +159,8 @@
     {{- end }}
 
     <script>
+        (function () {
+            'use strict';
             const fieldSets = Array.from(document.querySelectorAll('fieldset[data-campsite]'));
             const options = document.querySelectorAll('input[type="radio"]');
 
@@ -178,5 +181,32 @@
                     toggleFieldSets(option);
                 }
             }
+        })();
+
+        (function () {
+            'use strict';
+
+            const arrivalDateField = document.querySelector('[name="arrival_date"]');
+            if (!arrivalDateField) {
+                return;
+            }
+
+            arrivalDateField.addEventListener('change', function (event) {
+                const arrivalDate = new Date(event.target.value);
+                if (isNaN(arrivalDate)) {
+                    return;
+                }
+                const departureDateField = document.querySelector('[name="departure_date"]');
+                if (!departureDateField) {
+                    return;
+                }
+                const departureDate = new Date(departureDateField.value);
+                if (!isNaN(departureDate) && departureDate >= arrivalDate) {
+                    return;
+                }
+                arrivalDate.setUTCDate(arrivalDate.getUTCDate() + 1);
+                departureDateField.value = `${arrivalDate.getFullYear()}-${arrivalDate.getMonth() < 9 ? '0' : ''}${(arrivalDate.getMonth() + 1)}-${arrivalDate.getDate()}`
+            })
+        })();
     </script>
 {{- end }}

web/templates/public/campsite/type.gohtml

@@ -142,6 +142,33 @@
             const right = calendar.querySelector('header button:last-of-type');
             right.addEventListener('click', () => carousel.scrollLeft += month.clientWidth);
         })();
+
+        (function () {
+            'use strict';
+
+            const arrivalDateField = document.querySelector('[name="arrival_date"]');
+            if (!arrivalDateField) {
+                return;
+            }
+
+            arrivalDateField.addEventListener('change', function (event) {
+                const arrivalDate = new Date(event.target.value);
+                if (isNaN(arrivalDate)) {
+                    return;
+                }
+                const departureDateField = document.querySelector('[name="departure_date"]');
+                if (!departureDateField) {
+                    return;
+                }
+                const departureDate = new Date(departureDateField.value);
+                if (!isNaN(departureDate) && departureDate >= arrivalDate) {
+                    return;
+                }
+                console.log('fuck you', departureDate);
+                arrivalDate.setUTCDate(arrivalDate.getUTCDate() + 1);
+                departureDateField.value = `${arrivalDate.getFullYear()}-${arrivalDate.getMonth() < 9 ? '0' : ''}${(arrivalDate.getMonth() + 1)}-${arrivalDate.getDate()}`
+            })
+        })();
     </script>
 
 {{- end }}