public int abc(int sum) {
return List.rangeClosed(1, sum)
.crossProduct()
.filter(t -> t._1 + t._2 < sum)
.map(t -> Tuple.of(t._1, t._2, sum - t._1 - t._2))
.filter(t -> t._1 * t._1 + t._2 * t._2 == t._3 * t._3)
.map(t -> t._1 * t._2 * t._3)
.head();
}
public class Euler39Test {
/**
* <strong>Problem 39 Integer right triangles</strong>
*
* <p>If <i>p</i> is the perimeter of a right angle triangle with integral length sides,
* {<i>a,b,c</i>}, there are exactly three solutions for <i>p</i> = 120.
*
* <p>{20,48,52}, {24,45,51}, {30,40,50}
*
* <p>For which value of <i>p</i> ≤ 1000, is the number of solutions maximised?
*
* <p>See also <a href="https://projecteuler.net/problem=39">projecteuler.net problem 39</a>.
*/
@Test
public void shouldSolveProblem39() {
assertThat(SOLUTIONS_FOR_PERIMETERS_UP_TO_1000.get(120))
.isEqualTo(some(List.of(Tuple.of(20, 48, 52), Tuple.of(24, 45, 51), Tuple.of(30, 40, 50))));
assertThat(perimeterUpTo1000WithMaximisedNumberOfSolutions()).isEqualTo(840);
}
private static int perimeterUpTo1000WithMaximisedNumberOfSolutions() {
return SOLUTIONS_FOR_PERIMETERS_UP_TO_1000
.map((perimeter, listOfSolutions) -> Tuple.of(perimeter, listOfSolutions.length()))
.maxBy(Tuple2::_2)
.get()
._1;
}
private static final Map<Integer, List<Tuple3<Integer, Integer, Integer>>>
SOLUTIONS_FOR_PERIMETERS_UP_TO_1000 =
List.rangeClosed(1, 500)
.flatMap(a -> List.rangeClosed(a, 500).map(b -> Tuple.of(a, b, hypot(a, b))))
.filter(t -> floor(t._3) == t._3)
.map(t -> t.map3(Double::intValue))
.groupBy(t -> t.apply((a, b, c) -> a + b + c))
.filterKeys(d -> d <= 1_000);
}
private static boolean isTruncatablePrime(int prime) {
return Match(prime)
.of(
Case(
$(p -> p > 7),
p -> {
final CharSeq primeSeq = CharSeq.of(Integer.toString(p));
return List.rangeClosed(1, primeSeq.length() - 1)
.flatMap(i -> List.of(primeSeq.drop(i), primeSeq.dropRight(i)))
.map(CharSeq::mkString)
.map(Long::valueOf)
.forAll(Utils.MEMOIZED_IS_PRIME::apply);
}),
Case($(), false));
}