Prefill public completion form with current timer if present, and allow refresh

app/views/completions/_form.html.slim

@@ -48,19 +48,26 @@
           = form.check_box :completed, class: "form-check-input"
           = form.label :completed
         javascript:
+          savetime = "";
+          ispatch = #{method == :patch};
           completedEl = document.getElementById('completion_completed');
           completedEl.addEventListener('change', (e) => {
             const timeEl = document.getElementById('time');
             const missingPiecesEl = document.getElementById('missing_pieces');
             const remainingPiecesEl = document.getElementById('remaining_pieces');
+            const refreshEl = document.getElementById('refresh');
             if (e.target.checked) {
-              timeEl.value = '#{@completion.display_time_from_start}';
+              if (ispatch) timeEl.value = '#{@completion.display_time_from_start}';
+              else timeEl.value = savetime;
               missingPiecesEl.style.display = 'block';
               remainingPiecesEl.style.display = 'none';
+              if (refreshEl) refreshEl.style.display = '';
             } else {
+              savetime = timeEl.value;
               timeEl.value = '#{display_time(@contest.duration_seconds)}';
               missingPiecesEl.style.display = 'none';
               remainingPiecesEl.style.display = 'block';
+              if (refreshEl) refreshEl.style.display = 'none';
             }
           })
     .row.mb-3
@@ -68,8 +75,32 @@
         .form-floating
           = form.text_field :display_time_from_start, autocomplete: "off", class: "form-control", id: "time"
           = form.label :display_time_from_start, class: "required"
+      - unless method == :patch
+        .col.mt-2
+          a.btn.btn-primary id="refresh"
+            = t("helpers.buttons.refresh")
       .form-text
         = t("activerecord.attributes.completion.display_time_from_start_description")
+      - if @contest.start_time.present? && method != :patch
+        javascript:
+          startTime = #{@contest.start_time.present? ? @contest.start_time.to_i : "null"};
+          pauseTime = #{@contest.pause_time.present? ? @contest.pause_time.to_i : "null"};
+          function updateTime() {
+            const timeEl = document.getElementById('time');
+            if (timeEl) {
+              if (startTime) {
+                let s = Math.floor((Date.now() - 1000 * startTime) / 1000);
+                if (pauseTime) s = Math.floor(pauseTime - startTime);
+                let ss = s % 60;
+                let mm = Math.floor(s / 60) % 60;
+                let hh = Math.floor(s / 3600);
+                timeEl.value = `${hh}:${mm < 10 ? `0${mm}` : mm}:${ss < 10 ? `0${ss}` : ss}`;
+              }
+            }
+          }
+          refreshButton = document.getElementById("refresh");
+          refreshButton?.addEventListener('click', updateTime);
+          updateTime();
     .row.mb-3 id="missing_pieces"
       .col
         .form-floating