Permutations<T> constructor
Implementation
Permutations(this.r, List<T> items)
: _items = List<T>.from(items, growable: false),
_length = nPr(items.length, r) {
if (r < 0 || r > items.length) {
throw Exception("Cannot take $r items from ${items.length}.");
}
if (!itemsAreUnique(items)) {
throw Exception("Items are not unique.");
}
}