A001923

package ch.claude_martin.playground;

import java.util.Map;
import java.util.TreeMap;
import java.util.stream.LongStream;

// see https://oeis.org/A001923
public final class SomeClass {

    /**
     * As solved by Muhammad.
     */
    static long f0(long n) {
        long sum = 0;
        for (int j = 1; j <= n; j++) {
            sum += Math.pow(j, j);
        }
        return sum;
    }

    /**
     * Recursive.
     */
    static long f1(long n) {
        if (n == 0L)
            return 0L;
        return power(n) + f1(n - 1);
    }

    /** Stream API. */
    static long f2(int n) {
        return LongStream.rangeClosed(1, n).map(SomeClass::power).sum();
    }

    /** Naive. */
    static long f3(int n) {
        long result = 0;
        for (int i = 1; i <= n; i++)
            result += power(i);
        return result;
    }

    /** Extra naive. */
    static long f4(int n) {
        if (n == 0)
            return 0;
        long result = 1;
        for (int i = 2; i <= n; i++) {
            long pow = 1;
            for (int j = 0; j < i; j++)
                pow *= i;
            result += pow;
        }
        return result;
    }

    static final Map<Integer, Long> cache = new TreeMap<>();

    /** Cached, using {@link #f1(long)}. */
    static long cached(int n) {
        return cache.computeIfAbsent(n, SomeClass::f1);
    }

    /** n<sup>n</sup>. */
    static long power(final long number) {
        long res = 1;
        long sq = number;
        long power = number;
        while (power > 0) {
            if (power % 2 == 1) {
                res *= sq;
            }
            sq = sq * sq;
            power /= 2;
        }
        return res;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 17; i++) {
            long f0 = f0(i);
            long f1 = f1(i);
            long f2 = f2(i);
            long f3 = f3(i);
            long f4 = f4(i);
            long cached = cached(i);
            assert (f0 == f1) : f0 + " != " + f1;
            assert (f0 == f2) : f1 + " != " + f2;
            assert (f0 == f3) : f1 + " != " + f3;
            assert (f0 == f4) : f1 + " != " + f4;
            assert (f0 == cached) : f1 + " != " + cached;
            System.out.print("A001923(");
            System.out.print(i);
            System.out.print(") = ");
            System.out.print(f1);
            System.out.println();
        }
        System.out.println();
    }
}