Inefficient Membership Test (ARCH5075)

Key go.performance.inefficient-membership-test · Severity warning · Category performance

Repeatedly checking membership against a slice scans O(n) each time. Especially inside a loop, use a map[T]struct{} for O(1) lookups.

Incorrect

func selectAllowed(items, allowed []string) []string {
  var out []string
  for _, item := range items {
    for _, a := range allowed {
      if item == a {
        out = append(out, item)
      }
    }
  }
  return out
}

Correct

func selectAllowed(items, allowed []string) []string {
  set := make(map[string]struct{}, len(allowed))
  for _, a := range allowed {
    set[a] = struct{}{}
  }
  var out []string
  for _, item := range items {
    if _, ok := set[item]; ok {
      out = append(out, item)
    }
  }
  return out
}

Fix

Use a map for repeated membership checks instead of scanning a slice.