Credence.Pattern.AvoidLengthGuardLessThan2 (credence v0.8.0)

Performance rule: Detects function guards that check length(x) < 2 (or length(x) <= 1) and rewrites them into two O(1) pattern-matched clauses.

length/1 traverses the entire list just to compare with a small number. When the guard is length(x) < 2, the intent is "zero or one element", which can be expressed with two pattern-matched clauses that are O(1).

Bad

def maximumdifference(list) when length(list) < 2, do: 0

Good

def maximumdifference([]), do: 0
def maximumdifference([_]), do: 0

Pattern matching is O(1) and idiomatic, while length/1 is O(n).

Safe core — simple guards only

Fires only when the entire guard is the length comparison. A compound guard (when length(x) < 2 and P, or ... or P) is left untouched: splitting it into []/[_] clauses would silently drop P and change behaviour. Those belong with the sibling no_length_guard_to_pattern (which preserves the remaining guard), not here.