add sorting topics view

js/sort-topics.js

@@ -0,0 +1,45 @@
+// https://codepen.io/crouchingtigerhiddenadam/pen/qKXgap
+let selected = null;
+let container = document.getElementById("topics-container")
+
+function isBefore(el1, el2) {
+  let cur
+  if (el2.parentNode === el1.parentNode) {
+    for (cur = el1.previousSibling; cur; cur = cur.previousSibling) {
+      if (cur === el2) return true
+    }
+  }
+  return false;
+}
+
+function dragOver(e) {
+  let target = e.target.closest(".draggable-topic")
+  
+  if (!target || target === selected) {
+    return;
+  }
+  
+  if (isBefore(selected, target)) {
+    container.insertBefore(selected, target)
+  } else {
+    container.insertBefore(selected, target.nextSibling)
+  }
+}
+
+function dragEnd() {
+  if (!selected) return;
+  
+  selected.classList.remove("dragged")
+  selected = null;
+  for (let i = 0; i < container.childElementCount - 1; i++) {
+    let input = container.children[i].querySelector(".topic-input");
+    input.value = i + 1;
+  }
+}
+
+function dragStart(e) {
+  e.dataTransfer.effectAllowed = 'move'
+  e.dataTransfer.setData('text/plain', null)
+  selected = e.target
+  selected.classList.add("dragged")
+}