Resolve a regime specifier to a single Date — .resolve_regime_change

Internal helper used by .apply_regime_filter() to coerce a heterogeneous regime argument (NULL, Date scalar/vector, character coercible to Date, or a Regime object) into either a single Date scalar or a per-group data.table keyed by the caller-supplied by columns.

Usage

.resolve_regime_change_date(regime, by = NULL)

Arguments

regime: See .apply_regime_filter().
by: Optional character vector of group columns the caller wants the change date dispatched on. When NULL (default) or empty, the function always returns a scalar (the maximum change date), preserving the historical single-value contract. When non-empty and regime is a multi-group Regime whose $groups intersect by, returns a data.table with [intersect(by, regime$groups)..., change_date] (one row per group combo, holding max(change)). Otherwise falls back to scalar.

Value

One of:

NULL when no change date is specified.
A single Date (the latest change) – the scalar path.
A data.table [join_cols..., change_date] – the per-group path.