sortBy<T> function

int Function(T, T) sortBy<T>(Predicate<T> test, { int then(T, T)?, })

The sortBy function can be used to create a comparator to sort collections, comparing a and b such that:

• If both a and b satisfy the test, and then is not provided, the order between a and b undefined.

• If both a and b don't satisfy the test, and then is not provided, the order between a and b undefined.

• Otherwise, if only a or only b satisfies the test, then the one which satisfies the test comes before.

• Otherwise, if both a and b satisfy the test, and then is provided, the order between a and b with be given by then.

This comparator makes it easy to create complex comparators, using the language described below. For example, suppose you have a list of numbers which you want to sort according to the following rules:

1) If present, number 14 is always the first, followed by number 15.
2) Otherwise, odd numbers come before even ones.
3) Otherwise, numbers which are multiples of 3,
4) Otherwise, numbers which are multiples of 5,
5) Otherwise, numbers come in their natural order.

int Function(int, int) compareTo = sortBy((x) => x == 14,
    then: sortBy((x) => x == 15,
    then: sortBy((x) => x % 2 == 1,
        then: sortBy((x) => x % 3 == 0,
        then: sortBy((x) => x % 5 == 0,
            then: (int a, int b) => a.compareTo(b),
)))));

Important: When a cascade of sortBy is used, make sure you don't create inconsistencies. For example, this is inconsistent: a < b and a > c and b < c. Sorts with inconsistent rules may result in different orders for the same items depending on their initial position, and the rules may not be followed precisely.

Note sortBy can be combined with sortLike.

Implementation

int Function(T, T) sortBy<T>(
  Predicate<T> test, {
  int Function(T, T)? then,
}) =>
    (T a, T b) {
      var ta = test(a);
      var tb = test(b);
      if (ta == tb) return (then == null) ? 0 : then(a, b);
      return ta ? -1 : 1;
    };