import type { EventCategory } from '@/services/events'

export interface CategoryTreeOption {
  id: number
  name: string
  depth: number
}

// Defends against a malformed/cyclic parentId chain reaching the client
// (shouldn't happen -- event-categories.md's re-parent guard blocks this
// server-side -- but a client-side tree builder walking `parentId`
// links needs its own bound regardless, since it has no way to verify
// the guard actually ran on this data).
const MAX_DEPTH = 32

/**
 * Depth-first flatten of a flat, parentId-linked category list into
 * actual tree order (a parent immediately followed by its children,
 * siblings ordered by `sortOrder` then name). Used to render an
 * indented tree picker (formElements/Input select + option.depth) instead of a plain flat
 * <select>.
 *
 * Also fixes a real ambiguity along the way: event_categories only
 * enforces a unique (parent_id, name) pair, not a unique name overall,
 * so two categories under different parents can share an identical
 * name. A flat name-only dropdown couldn't distinguish them; rendering
 * them at their real tree position/indentation does.
 */
export function buildCategoryTree(categories: EventCategory[]): CategoryTreeOption[] {
  const childrenByParent = new Map<number | null, EventCategory[]>()

  for (const category of categories) {
    const key = category.parentId
    const siblings = childrenByParent.get(key) ?? []

    siblings.push(category)
    childrenByParent.set(key, siblings)
  }

  for (const siblings of Array.from(childrenByParent.values())) {
    siblings.sort((a, b) => a.sortOrder - b.sortOrder || a.name.localeCompare(b.name, 'fa'))
  }

  const result: CategoryTreeOption[] = []
  const seen = new Set<number>()

  const visit = (parentId: number | null, depth: number) => {
    if (depth >= MAX_DEPTH) return

    for (const category of childrenByParent.get(parentId) ?? []) {
      if (seen.has(category.id)) continue
      seen.add(category.id)
      result.push({ id: category.id, name: category.name, depth })
      visit(category.id, depth + 1)
    }
  }

  visit(null, 0)

  return result
}