Knight.java

import javaslang.Function1;
import javaslang.Function2;
import javaslang.Tuple;
import javaslang.Tuple2;
import javaslang.collection.List;

public class Knight {
    static final Function1<Tuple2<Integer, Integer>, List<Tuple2<Integer, Integer>>> moveKnight = (start) -> {
        // c,r = column,row
        int c = start._1;
        int r = start._2;
        return List.of(
                Tuple.of(c + 2, r - 1),
                Tuple.of(c + 2, r + 1),
                Tuple.of(c - 2, r - 1),
                Tuple.of(c - 2, r + 1),
                Tuple.of(c + 1, r - 2),
                Tuple.of(c + 1, r + 2),
                Tuple.of(c - 1, r - 2),
                Tuple.of(c - 1, r + 2)
        )
                .filter(
                        t -> t._1 > 0 && t._1 < 9 && t._2 > 0 && t._2 < 9
                );
    };

    static final Function1<Tuple2<Integer, Integer>, List<Tuple2<Integer, Integer>>> in3 =
            (start) -> List.of(start).flatMap(moveKnight).flatMap(moveKnight).flatMap(moveKnight);

    static final Function2<Tuple2<Integer, Integer>, Tuple2<Integer, Integer>, Boolean> canReachIn3 =
            (start, end) -> in3.apply(start).contains(end);
}