package domain

import (
	"errors"
	"fmt"
	"sort"
)

// ErrReorderInvalidIDSet means ordered_ids is not an exact permutation of the current entities in scope.
var ErrReorderInvalidIDSet = errors.New("ordered_ids must list every id in this scope exactly once, with no duplicates")

// ReorderIDsRequest is the body for batch reorder endpoints (drag-and-drop UI).
// Send "ordered_ids": [] in display order. Must include every id in that scope (use GET list) when there is at least one entity.
type ReorderIDsRequest struct {
	OrderedIDs []int64 `json:"ordered_ids"`
}

// ValidateReorderPermutation checks that ordered contains the same multiset of ids as expected (new order vs current scope).
func ValidateReorderPermutation(ordered, expected []int64) error {
	if len(expected) == 0 {
		if len(ordered) == 0 {
			return nil
		}
		return fmt.Errorf("%w: no entities exist in this scope", ErrReorderInvalidIDSet)
	}
	if len(ordered) != len(expected) {
		return fmt.Errorf("%w: want %d ids, got %d", ErrReorderInvalidIDSet, len(expected), len(ordered))
	}
	seen := make(map[int64]struct{}, len(ordered))
	for _, id := range ordered {
		if _, dup := seen[id]; dup {
			return fmt.Errorf("%w: duplicate id %d", ErrReorderInvalidIDSet, id)
		}
		seen[id] = struct{}{}
	}
	a := append([]int64(nil), expected...)
	b := append([]int64(nil), ordered...)
	sort.Slice(a, func(i, j int) bool { return a[i] < a[j] })
	sort.Slice(b, func(i, j int) bool { return b[i] < b[j] })
	for i := range a {
		if a[i] != b[i] {
			return fmt.Errorf("%w: id set does not match current scope", ErrReorderInvalidIDSet)
		}
	}
	return nil
}