#184

frontend/src/components/TagDetailSidebar.tsx

@@ -1,4 +1,8 @@
-import { DndContext, PointerSensor, useSensor, useSensors } from '@dnd-kit/core'
+import { DndContext,
+	 PointerSensor,
+	 useDroppable,
+	 useSensor,
+	 useSensors } from '@dnd-kit/core'
 import { AnimatePresence, motion } from 'framer-motion'
 import { useEffect, useRef, useState } from 'react'
 
@@ -168,6 +172,44 @@ const findTag = (
 }
 
 
+const detachEdge = (
+    nodes:    Tag[],
+    parentId: number,
+    childId:  number,
+): Tag[] => nodes.map (t => {
+  if (t.id === parentId)
+    {
+      const children = (t.children ?? []).filter (c => c.id !== childId)
+      return { ...t, children }
+    }
+  return t.children ? { ...t, children: detachEdge (t.children, parentId, childId) } : t
+})
+
+
+const insertRootAt = (
+    roots:  Tag[],
+    index:  number,
+    tag:    Tag,
+): Tag[] => {
+  const without = roots.filter (t => t.id !== tag.id)
+  const next = without.slice ()
+  next.splice (Math.min (Math.max (index, 0), next.length), 0, tag)
+  return next
+}
+
+
+const DropSlot = ({ cat, index }: { cat: Category, index: number }) => {
+  const { setNodeRef, isOver: over } = useDroppable ({
+      id:   `slot:${ cat }:${ index }`,
+      data: { kind: 'slot', cat, index } })
+
+  return (
+      <li ref={setNodeRef} className="h-1">
+	{over && <div className="h-0.5 w-full rounded bg-sky-400"/>}
+      </li>)
+}
+
+
 type Props = { post: Post | null }
 
 
@@ -180,26 +222,58 @@ export default (({ post }: Props) => {
       useSensor (PointerSensor, { activationConstraint: { distance: 6 } }))
 
   const onDragEnd = async (e: DragEndEvent) => {
+    const activeKind = e.active.data.current?.kind
+    if (activeKind !== 'tag')
+      return
+
     const childId: number | undefined = e.active.data.current?.tagId
-    const parentId: number | undefined = e.over?.data.current?.tagId
+    const fromParentId: number | undefined = e.active.data.current?.parentTagId
 
-    if (!(childId) || !(parentId))
+    const overKind = e.over?.data.current?.kind
+
+    if (!(childId) || !(overKind))
       return
 
-    if (childId === parentId)
-      return
+    if (overKind === 'tag')
+      {
+	const parentId: number | undefined = e.over?.data.current?.tagId
+	if (!(parentId) || childId === parentId)
+	  return
 
-    setTags (prev => {
-      const child = findTag (prev, childId)
-      const parent = findTag (prev, parentId)
-      if (!(child) || !(parent))
-	return prev
+	setTags (prev => {
+	  const child = findTag (prev, childId)
+	  const parent = findTag (prev, parentId)
+	  if (!(child) || !(parent) || isDescendant (child, parentId))
+	    return prev
 
-      if (isDescendant (child, parentId))
-	return prev
+	  return attachChildOptimistic (prev, parentId, childId)
+	})
 
-      return attachChildOptimistic (prev, parentId, childId)
-    })
+	return
+      }
+
+    if (overKind === 'slot')
+      {
+	const cat: Category | undefined = e.over?.data.current?.cat
+	const index: number | undefined = e.over?.data.current?.index
+	if (!(cat) || index == null)
+	  return
+
+	setTags (prev => {
+	  const child = findTag (prev, childId)
+	  if (!(child))
+	    return prev
+
+	  const next: TagByCategory = { ...prev }
+
+	  if (fromParentId)
+	    next[cat] = detachEdge (next[cat], fromParentId, childId) as any
+
+	  next[cat] = insertRootAt (next[cat], index, child)
+
+	  return next
+	})
+      }
   }
 
   const categoryNames: Record<Category, string> = {
@@ -260,7 +334,10 @@ export default (({ post }: Props) => {
 		  <motion.ul layout>
 		    <AnimatePresence initial={false}>
 		      {tags[cat].map (tag => (
-			  renderTagTree (tag, 0, `cat-${ cat }`, suppressClickRef, undefined)))}
+			  <>
+			    {renderTagTree (tag, 0, `cat-${ cat }`, suppressClickRef, undefined)}
+			  </>))}
+		      <DropSlot cat={cat} index={0}/>
 		    </AnimatePresence>
 		  </motion.ul>
 		</motion.div>))}