Merge remote-tracking branch 'upstream/master'

src/test/java/javaslang/collection/euler/Euler14Test.java

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+/*     / \____  _    ______   _____ / \____   ____  _____
+ *    /  \__  \/ \  / \__  \ /  __//  \__  \ /    \/ __  \   Javaslang
+ *  _/  // _\  \  \/  / _\  \\_  \/  // _\  \  /\  \__/  /   Copyright 2014-2015 Daniel Dietrich
+ * /___/ \_____/\____/\_____/____/\___\_____/_/  \_/____/    Licensed under the Apache License, Version 2.0
+ */
+package javaslang.collection.euler;
+
+import javaslang.Function1;
+import javaslang.Function2;
+import javaslang.collection.Stream;
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class Euler14Test {
+
+    /**
+     * <strong>Problem 14: Longest Collatz sequence</strong>
+     * <p>
+     *
+     * The following iterative sequence is defined for the set of positive integers:
+     *
+     *    n → n/2 (n is even)
+     *    n → 3n + 1 (n is odd)
+     *
+     * Using the rule above and starting with 13, we generate the following sequence:
+     *
+     * 13 → 40 → 20 → 10 → 5 → 16 → 8 → 4 → 2 → 1
+     * It can be seen that this sequence (starting at 13 and finishing at 1) contains 10 terms.
+     * Although it has not been proved yet (Collatz Problem), it is thought that all starting numbers finish at 1.
+     *
+     * Which starting number, under one million, produces the longest chain?
+
+     * NOTE: Once the chain starts the terms are allowed to go above one million.
+     * </p>
+     * See also <a href="https://projecteuler.net/problem=14">projecteuler.net problem 14</a>.
+     */
+
+    @Test
+    public void shouldSolveProblem14() {
+        assertThat(Stream.from(1L)
+                .take(1_000_000)
+                .maxBy(collatzSequenceLength)
+                .get()).isEqualTo(837799);
+    }
+
+    private final static Function2<Long, Long, Long> collatzRecursive = (len, n) -> {
+        if (n == 1) return len + 1;
+        else if (n % 2 == 0) {
+            return Euler14Test.collatzRecursive.apply(len + 1, n / 2);
+        } else {
+            return Euler14Test.collatzRecursive.apply(len + 1, (3 * n) + 1);
+        }
+    };
+
+    private final static Function1<Long, Long> collatzSequenceLength = collatzRecursive.memoized().curried().apply(0L);
+}