PartitionPrimeNumbers.java

package lambdasinaction.chap6;

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

import static java.util.stream.Collectors.*;
import static java.util.stream.Collector.Characteristics.*;

public class PartitionPrimeNumbers {

    public static void main(String ... args) {
        System.out.println("Numbers partitioned in prime and non-prime: " + partitionPrimes(100));
        System.out.println("Numbers partitioned in prime and non-prime: " + partitionPrimesWithCustomCollector(100));

    }

    public static Map<Boolean, List<Integer>> partitionPrimes(int n) {
        return IntStream.rangeClosed(2, n).boxed()
                .collect(partitioningBy(candidate -> isPrime(candidate)));
    }

    public static boolean isPrime(int candidate) {
        return IntStream.rangeClosed(2, candidate-1)
                .limit((long) Math.floor(Math.sqrt((double) candidate)) - 1)
                .noneMatch(i -> candidate % i == 0);
    }

    public static Map<Boolean, List<Integer>> partitionPrimesWithCustomCollector(int n) {
        return IntStream.rangeClosed(2, n).boxed().collect(new PrimeNumbersCollector());
    }

    public static boolean isPrime(List<Integer> primes, Integer candidate) {
        double candidateRoot = Math.sqrt((double) candidate);
        //return takeWhile(primes, i -> i <= candidateRoot).stream().noneMatch(i -> candidate % i == 0);
        return primes.stream().takeWhile(i -> i <= candidateRoot).noneMatch(i -> candidate % i == 0);
    }
/*
    public static <A> List<A> takeWhile(List<A> list, Predicate<A> p) {
        int i = 0;
        for (A item : list) {
            if (!p.test(item)) {
                return list.subList(0, i);
            }
            i++;
        }
        return list;
    }
*/
    public static class PrimeNumbersCollector
            implements Collector<Integer, Map<Boolean, List<Integer>>, Map<Boolean, List<Integer>>> {

        @Override
        public Supplier<Map<Boolean, List<Integer>>> supplier() {
            return () -> new HashMap<Boolean, List<Integer>>() {{
                put(true, new ArrayList<Integer>());
                put(false, new ArrayList<Integer>());
            }};
        }

        @Override
        public BiConsumer<Map<Boolean, List<Integer>>, Integer> accumulator() {
            return (Map<Boolean, List<Integer>> acc, Integer candidate) -> {
                acc.get( isPrime( acc.get(true),
                        candidate) )
                        .add(candidate);
            };
        }

        @Override
        public BinaryOperator<Map<Boolean, List<Integer>>> combiner() {
            return (Map<Boolean, List<Integer>> map1, Map<Boolean, List<Integer>> map2) -> {
                map1.get(true).addAll(map2.get(true));
                map1.get(false).addAll(map2.get(false));
                return map1;
            };
        }

        @Override
        public Function<Map<Boolean, List<Integer>>, Map<Boolean, List<Integer>>> finisher() {
            return i -> i;
        }

        @Override
        public Set<Characteristics> characteristics() {
            return Collections.unmodifiableSet(EnumSet.of(IDENTITY_FINISH));
        }
    }

    public Map<Boolean, List<Integer>> partitionPrimesWithInlineCollector(int n) {
        return Stream.iterate(2, i -> i + 1).limit(n)
                .collect(
                        () -> new HashMap<Boolean, List<Integer>>() {{
                            put(true, new ArrayList<Integer>());
                            put(false, new ArrayList<Integer>());
                        }},
                        (acc, candidate) -> {
                            acc.get( isPrime(acc.get(true), candidate) )
                                    .add(candidate);
                        },
                        (map1, map2) -> {
                            map1.get(true).addAll(map2.get(true));
                            map1.get(false).addAll(map2.get(false));
                        });
    }
}