Credence.Pattern.AvoidGraphemesEnumCount (credence v0.4.0)

Performance rule: Detects the use of Enum.count/1 or Enum.count/2 on the result of String.graphemes/1.

Calling String.graphemes/1 eagerly allocates a list containing every grapheme in the string. If your goal is simply to count the characters, String.length/1 accomplishes this without the intermediate list allocation.

If you are counting characters that match a specific condition (using Enum.count/2), you can avoid allocating the entire list in memory by using a lazy stream backed by String.next_grapheme/1.

Bad

# Counting all characters
string
|> String.graphemes()
|> Enum.count()

# Counting characters with a predicate
string
|> String.graphemes()
|> Enum.count(fn char -> char == "a" end)

Good

# Counting all characters
String.length(string)

# Or in a pipeline:
string
|> String.length()

# Counting characters with a predicate
string
|> Stream.unfold(&String.next_grapheme/1)
|> Enum.count(fn char -> char == "a" end)