feat: add `max_levels` argument to `is_fct_ish()`

[jonthegeek]

Summary

As a package developer, in order to reject vectors with too many distinct values for a factor, I would like is_fct_ish() to accept a max_levels argument that fails the entire vector when the number of unique levels exceeds the threshold.

Proposed signature

is_fct_ish(x, ..., levels = NULL, to_na = character(), max_levels = Inf)

Arguments

• max_levels (numeric(1)) — The maximum number of distinct levels (unique non-NA values, after applying to_na) allowed across the entire vector. Defaults to Inf (no limit). When finite, the check applies to every non-NA value of the vector as a whole.

Returns same as before: is_fct_ish() returns a length-1 logical.

Behavior

• When max_levels = Inf (default), behavior is identical to the current implementation.
• When max_levels is finite, count the total number of unique non-NA values across the entire vector, after first removing any values in to_na.
• If that count exceeds max_levels, return FALSE.
• to_na values must not be counted as levels — coerce them to NA or remove them from the counted vector before counting unique values, consistent with how to_na is handled elsewhere.
• This must be fast, since this is meant to be a quick predicate check for if() calls, etc. Be sure to only run this when it matters, and to be smart about how things are compared.