Credence.Rule.NoSortThenAt (credence v0.2.0)

Performance rule: Detects sorting an entire list only to retrieve a single element by index via Enum.at/2.

Sorting is O(n log n), and if you only need one element (e.g. the kth largest or the median), the full sort is wasteful. For k=1 use Enum.min/1 or Enum.max/1 (O(n)). For small k, consider a partial sort or heap.

Bad

Enum.sort(nums, :desc) |> Enum.at(k - 1)
Enum.at(Enum.sort(nums), 0)

Good

Enum.min(nums)                       # for the smallest
Enum.max(nums)                       # for the largest
Enum.sort(nums) |> Enum.take(k)      # when you need the top-k list