penx/apps/web/lib/dnd-projection/getProjection.tsx

import { arrayMove } from '@dnd-kit/sortable'

export type UniqueIdentifier = string
export type WithFlattenedProps<T> = T & {
  parentId: string | null // parent node id
  depth: number
  index: number
}

type FlattenedItem = WithFlattenedProps<any>
// type FlattenedItem = any

function getDragDepth(offset: number, indentationWidth: number) {
  return Math.round(offset / indentationWidth)
}

function getMaxDepth({ previousItem }: { previousItem: FlattenedItem }) {
  if (previousItem) {
    return previousItem.depth + 1
  }

  return 0
}

function getMinDepth({ nextItem }: { nextItem: FlattenedItem }) {
  if (nextItem) {
    return nextItem.depth
  }

  return 0
}

export function getProjection(
  items: FlattenedItem[],
  activeId: UniqueIdentifier,
  overId: UniqueIdentifier,
  dragOffset: number,
  indentationWidth: number,
) {
  const overItemIndex = items.findIndex(({ id }) => id === overId)
  const activeItemIndex = items.findIndex(({ id }) => id === activeId)
  const activeItem = items[activeItemIndex]
  const newItems = arrayMove(items, activeItemIndex, overItemIndex)
  const previousItem = newItems[overItemIndex - 1]
  const nextItem = newItems[overItemIndex + 1]
  const dragDepth = getDragDepth(dragOffset, indentationWidth)
  const projectedDepth = activeItem.depth + dragDepth
  const maxDepth = getMaxDepth({
    previousItem,
  })
  const minDepth = getMinDepth({ nextItem })
  let depth = projectedDepth

  if (projectedDepth >= maxDepth) {
    depth = maxDepth
  } else if (projectedDepth < minDepth) {
    depth = minDepth
  }

  return { depth, maxDepth, minDepth, parentId: getParentId() }

  function getParentId() {
    if (depth === 0 || !previousItem) {
      return null
    }

    if (depth === previousItem.depth) {
      return previousItem.parentId
    }

    if (depth > previousItem.depth) {
      return previousItem.id
    }

    const newParent = newItems
      .slice(0, overItemIndex)
      .reverse()
      .find((item) => item.depth === depth)?.parentId

    return newParent ?? null
  }
}