Credence.Pattern.NoRepeatedEnumTraversal (credence v0.4.5)

Performance rule: warns when the same variable is traversed multiple times using different Enum functions.

Repeated traversal of the same collection with separate Enum calls (e.g. Enum.max/1, Enum.min/1, and Enum.count/1 on the same list) allocates and iterates the data structure multiple times.

Consider combining traversals into a single Enum.reduce/3 or caching intermediate results.

Bad

def stats(list) do
  max = Enum.max(list)
  min = Enum.min(list)
  count = Enum.count(list)
  {max, min, count}
end

if Enum.member?(list, 10) and Enum.member?(list, 20) do
  Enum.count(list)
end

Good

def stats(list) do
  Enum.reduce(list, fn el, {max, min, count} ->
    {max(max, el), min(min, el), count + 1}
  end)
end

set = MapSet.new(list)
if MapSet.member?(set, 10) and MapSet.member?(set, 20) do
  MapSet.size(set)
end