Exercises.java

package exercises;

/*
 * LAMBDA PROGRAMMING LABORATORY
 *
 * For each exercise, develop a solution using Java SE 8 Lambda/Streams
 * and remove the @Ignore tag. Then run the tests.
 *
 * In NetBeans, Ctrl-F6 will run the project's tests, which default to
 * the unsolved exercises (as opposed to the solutions). Alt-F6 [PC] or
 * or Cmd-F6 [Mac] will run just the tests in the currently selected file.
 *
 * Several of the exercises read data from a text file. The field named
 * "reader" is a BufferedReader which will be opened for you on the text file.
 * In any exercise that refers to reading from the text file, you can simply
 * use the "reader" variable without worry about opening or closing it.
 * This is setup by JUnit using the @Before and @After methods at the bottom of
 * this file. The text file is "SonnetI.txt" (Shakespeare's first sonnet) which
 * is located at the root of this NetBeans project.
 */

import java.io.BufferedReader;
import java.io.IOException;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.OptionalInt;
import java.util.Random;
import java.util.Set;
import java.util.TreeMap;
import java.util.function.IntConsumer;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import org.junit.After;
import org.junit.Before;
import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;


public class Exercises {


// ========================================================
// DEFAULT METHODS
// ========================================================


    /**
     * Create a string that consists of the first letters of each
     * word in the input list.
     */
    @Test @Ignore
    public void ex01_accumulateFirstLetters() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot");

        String result = ""; // TODO

        assertEquals("abcdef", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Iterable.forEach().
    // </editor-fold>


    /**
     * Remove the words that have odd lengths from the list.
     */
    @Test @Ignore
    public void ex02_removeOddLengthWords() {
        List<String> list = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        // TODO code to modify list

        assertEquals("[alfa, echo]", list.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Collection.removeIf().
    // </editor-fold>


    /**
     * Replace every word in the list with its upper case equivalent.
     */
    @Test @Ignore
    public void ex03_upcaseAllWords() {
        List<String> list = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        //TODO code to modify list

        assertEquals("[ALFA, BRAVO, CHARLIE, DELTA, ECHO, FOXTROT]", list.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use List.replaceAll().
    // </editor-fold>


    /**
     * Convert every key-value pair of a map into a string and append them all
     * into a single string, in iteration order.
     */
    @Test @Ignore
    public void ex04_stringifyMap() {
        Map<String, Integer> input = new TreeMap<>();
        input.put("c", 3);
        input.put("b", 2);
        input.put("a", 1);

        String result = ""; // TODO

        assertEquals("a1b2c3", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Map.forEach().
    // </editor-fold>


    /**
     * Given a list of words, create a map whose keys are the first letters of
     * each words, and whose values are the sum of the lengths of those words.
     */
    @Test @Ignore
    public void ex05_mapOfStringLengths() {
        List<String> list = Arrays.asList(
            "aardvark", "bison", "capybara",
            "alligator", "bushbaby", "chimpanzee",
            "avocet", "bustard", "capuchin");
        Map<String, Integer> result = new TreeMap<>();

        //TODO code to populate result

        assertEquals("{a=23, b=20, c=26}", result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Map.merge() within Iterable.forEach().
    // </editor-fold>


// ========================================================
// SIMPLE STREAM PIPELINES
// ========================================================


    /**
     * Given a list of words, create an output list that contains
     * only the odd-length words, converted to upper case.
     */
    @Test @Ignore
    public void ex06_upcaseOddLengthWords() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        List<String> result = null; // TODO

        assertEquals("[BRAVO, CHARLIE, DELTA, FOXTROT]", result.toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use filter() and map().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use collect() to create the result list.
    // </editor-fold>


    /**
     * Join the second letters of words 1 through 4 of the list (inclusive,
     * counting from zero), separated by commas, into a single string.
     */
    @Test @Ignore
    public void ex07_joinStreamRange() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot"));

        String result = ""; // TODO

        assertEquals("r,h,e,c", result);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.skip() and Stream.limit().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.joining().
    // </editor-fold>


    /**
     * Count the number of lines in the text file. (Remember to
     * use the BufferedReader named "reader" that has already been
     * opened for you.)
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex08_countLinesInFile() throws IOException {
        long count = 0; // TODO

        assertEquals(14, count);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use BufferedReader.lines() to get a stream of lines.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.count().
    // </editor-fold>


    /**
     * Find the length of the longest line in the text file.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex09_findLengthOfLongestLine() throws IOException {
        int longestLength = 0; // TODO

        assertEquals(53, longestLength);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.mapToInt() to convert to IntStream.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Look at java.util.OptionalInt to get the result.
    // </editor-fold>


    /**
     * Find the longest line in the text file.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex10_findLongestLine() throws IOException {
        String longest = ""; // TODO

        assertEquals("Feed'st thy light's flame with self-substantial fuel,", longest);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.max() with a Comparator.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use static methods on Comparator to help create a Comparator instance.
    // </editor-fold>


    /**
     * Select the set of words from the input list whose length is greater than
     * the word's position in the list (starting from zero) .
     */
    @Test @Ignore
    public void ex11_selectByLengthAndPosition() {
        List<String> input = new ArrayList<>(Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel"));

        List<String> result = null; // TODO

        assertEquals("[alfa, bravo, charlie, delta, foxtrot]", result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Instead of a stream of words (Strings), run an IntStream of positions.
    // </editor-fold>


    /**
     * Given two lists of Integer, compute a third list where each element is the
     * difference between the corresponding elements of the two input lists
     * (first minus second).
     */
    @Test @Ignore
    public void ex12_listDifference() {
        List<Integer> one = Arrays.asList(3, 1, 4, 1, 5, 9, 2, 6, 5, 3);
        List<Integer> two = Arrays.asList(2, 7, 1, 8, 2, 8, 1, 8, 2, 8);

        List<Integer> result = null; // TODO

        assertEquals("[1, -6, 3, -7, 3, 1, 1, -2, 3, -5]", result.toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Run an IntStream of list positions (indexes).
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Deal with boxed Integers either by casting or by using mapToObj().
    // </editor-fold>


// ========================================================
// INTERMEDIATE STREAM PIPELINES
// ========================================================


    /**
     * Convert a list of strings into a list of characters.
     */
    @Test @Ignore
    public void ex13_stringsToCharacters() {
        List<String> input = Arrays.asList("alfa", "bravo", "charlie");

        List<Character> result = null; // TODO

        assertEquals("[a, l, f, a, b, r, a, v, o, c, h, a, r, l, i, e]", result.toString());
        assertTrue(result.stream().allMatch(x -> x instanceof Character));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.flatMap().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Pay attention to the return type of String.chars() and boxing conversion.
    // </editor-fold>


    /**
     * Collect all the words from the text file into a list.
     * Use String.split(REGEXP) to split a string into words.
     * REGEXP is defined at the bottom of this file.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex14_listOfAllWords() throws IOException {
        List<String> output = null; // TODO

        assertEquals(
            Arrays.asList(
                "From", "fairest", "creatures", "we", "desire", "increase",
                "That", "thereby", "beauty's", "rose", "might", "never", "die",
                "But", "as", "the", "riper", "should", "by", "time", "decease",
                "His", "tender", "heir", "might", "bear", "his", "memory",
                "But", "thou", "contracted", "to", "thine", "own", "bright", "eyes",
                "Feed'st", "thy", "light's", "flame", "with", "self", "substantial", "fuel",
                "Making", "a", "famine", "where", "abundance", "lies",
                "Thy", "self", "thy", "foe", "to", "thy", "sweet", "self", "too", "cruel",
                "Thou", "that", "art", "now", "the", "world's", "fresh", "ornament",
                "And", "only", "herald", "to", "the", "gaudy", "spring",
                "Within", "thine", "own", "bud", "buriest", "thy", "content",
                "And", "tender", "churl", "mak'st", "waste", "in", "niggarding",
                "Pity", "the", "world", "or", "else", "this", "glutton", "be",
                "To", "eat", "the", "world's", "due", "by", "the", "grave", "and", "thee"),
            output);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.flatMap().
    // </editor-fold>


    /**
     * Read the words from the text file, and create a list containing the words
     * of length 8 or longer, converted to lower case, and sorted alphabetically.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex15_longLowerCaseSortedWords() throws IOException {
        List<String> output = null; // TODO

        assertEquals(
            Arrays.asList(
                "abundance", "beauty's", "contracted", "creatures",
                "increase", "niggarding", "ornament", "substantial"),
            output);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.sorted().
    // </editor-fold>


    /**
     * Read the words from the text file, and create a list containing the words
     * of length 8 or longer, converted to lower case, and sorted reverse alphabetically.
     * (Same as above except for reversed sort order.)
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex16_longLowerCaseReverseSortedWords() throws IOException {
        List<String> result = null; // TODO

        assertEquals(
            Arrays.asList(
                "substantial", "ornament", "niggarding", "increase",
                "creatures", "contracted", "beauty's", "abundance"),
            result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Comparator.reverseOrder().
    // </editor-fold>


    /**
     * Read words from the text file, and sort unique, lower-cased words by length,
     * then alphabetically within length, and place the result into an output list.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex17_sortedLowerCaseDistinctByLengthThenAlphabetically() throws IOException {
        List<String> result = null; // TODO

        assertEquals(
            Arrays.asList(
                "a", "as", "be", "by", "in", "or", "to", "we",
                "and", "art", "bud", "but", "die", "due", "eat", "foe",
                "his", "now", "own", "the", "thy", "too", "bear", "else",
                "eyes", "from", "fuel", "heir", "lies", "only",
                "pity", "rose", "self", "that", "thee", "this", "thou",
                "time", "with", "churl", "cruel", "flame", "fresh", "gaudy",
                "grave", "might", "never", "riper", "sweet", "thine",
                "waste", "where", "world", "bright", "desire", "famine",
                "herald", "mak'st", "making", "memory", "should", "spring",
                "tender", "within", "buriest", "content", "decease",
                "fairest", "feed'st", "glutton", "light's", "thereby", "world's", "beauty's",
                "increase", "ornament", "abundance", "creatures", "contracted", "niggarding",
                "substantial"),
            result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.distinct().
    // </editor-fold>
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Comparator.theComparing().
    // </editor-fold>


    /**
     * Count the total number of words and the number of distinct, lower case
     * words in the text file, in one pass.
     */
    @Test @Ignore
    public void ex18_countTotalAndDistinctWords() {
        long distinctCount = 0; // TODO
        long totalCount = 0; // TODO

        assertEquals("distinct count", 81, distinctCount);
        assertEquals("total count", 107, totalCount);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.peek().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use LongAdder or AtomicLong/AtomicInteger to allow peek() to have side effects.
    // </editor-fold>


// ========================================================
// ADVANCED STREAMS: REDUCTION, COLLECTORS, AND GROUPING
// ========================================================


    /**
     * Compute the value of 21!, that is, 21 factorial. This value is larger than
     * Long.MAX_VALUE, so you must use BigInteger.
     */
    @Test @Ignore
    public void ex19_bigFactorial() {
        BigInteger result = BigInteger.ONE; // TODO

        assertEquals(new BigInteger("51090942171709440000"), result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use LongStream and reduction.
    // </editor-fold>


    /**
     * Get the last word in the text file.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex20_getLastWord() throws IOException {
        List<String> result = null; // TODO

        assertEquals("thee", result);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Stream.reduce().
    // </editor-fold>


    /**
     * Categorize the words from the text file into a map, where the map's key
     * is the length of each word, and the value corresponding to a key is a
     * list of words of that length. Don't bother with uniqueness or lower-
     * casing the words.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex21_mapLengthToWordList() throws IOException {
        Map<Integer, List<String>> result = null; // TODO

        assertEquals(10, result.get(7).size());
        assertEquals(new HashSet<>(Arrays.asList("beauty's", "increase", "ornament")), new HashSet<>(result.get(8)));
        assertEquals(new HashSet<>(Arrays.asList("abundance", "creatures")), new HashSet<>(result.get(9)));
        assertEquals(new HashSet<>(Arrays.asList("contracted", "niggarding")), new HashSet<>(result.get(10)));
        assertEquals(Arrays.asList("substantial"), result.get(11));
        assertFalse(result.containsKey(12));
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.groupingBy().
    // </editor-fold>


    /**
     * Categorize the words from the text file into a map, where the map's key
     * is the length of each word, and the value corresponding to a key is a
     * count of words of that length. Don't bother with uniqueness or lower-
     * casing the words. This is the same as the previous exercise except
     * the map values are the count of words instead of a list of words.
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex22_mapLengthToWordCount() throws IOException {
        Map<Integer, Long> result = null; // TODO

        assertEquals( 1L, (long)result.get(1));
        assertEquals(11L, (long)result.get(2));
        assertEquals(28L, (long)result.get(3));
        assertEquals(21L, (long)result.get(4));
        assertEquals(16L, (long)result.get(5));
        assertEquals(12L, (long)result.get(6));
        assertEquals(10L, (long)result.get(7));
        assertEquals( 3L, (long)result.get(8));
        assertEquals( 2L, (long)result.get(9));
        assertEquals( 2L, (long)result.get(10));
        assertEquals( 1L, (long)result.get(11));

        IntSummaryStatistics stats = result.keySet().stream().mapToInt(i -> i).summaryStatistics();
        assertEquals("min key",  1, stats.getMin());
        assertEquals("max key", 11, stats.getMax());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // Use the "downstream" overload of Collectors.groupingBy().
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.counting().
    // </editor-fold>


    /**
     * Gather the words from the text file into a map, accumulating a count of
     * the number of occurrences of each word. Don't worry about upper case and
     * lower case. Extra challenge: implement two solutions, one that uses
     * groupingBy() and the other that uses toMap().
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex23_wordFrequencies() throws IOException {
        Map<String, Long> result = null; // TODO

        assertEquals(2L, (long)result.get("tender"));
        assertEquals(6L, (long)result.get("the"));
        assertEquals(1L, (long)result.get("churl"));
        assertEquals(2L, (long)result.get("thine"));
        assertEquals(1L, (long)result.get("world"));
        assertEquals(4L, (long)result.get("thy"));
        assertEquals(3L, (long)result.get("self"));
        assertFalse(result.containsKey("lambda"));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // For Collectors.groupingBy(), consider that each word needs to
    // be categorized by itself.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // For Collectors.toMap(), the first occurrence of a word should be mapped to 1.
    // </editor-fold>


    /**
     * From the words in the text file, create nested maps, where the outer map is a
     * map from the first letter of the word to an inner map. (Use a string of length
     * one as the key.) The inner map, in turn, is a mapping from the length of the
     * word to a list of words with that length. Don't bother with any lowercasing
     * or uniquifying of the words.
     *
     * For example, given the words "foo bar baz bazz foo" the string
     * representation of the result would be:
     *     {b={3=[bar, baz], 4=[bazz]}, f={3=[foo, foo]}}
     *
     * @throws IOException
     */
    @Test @Ignore
    public void ex24_nestedMaps() throws IOException {
        Map<String, Map<Integer, List<String>>> result = null; // TODO

        assertEquals("[abundance]", result.get("a").get(9).toString());
        assertEquals("[by, be, by]", result.get("b").get(2).toString());
        assertEquals("[flame, fresh]", result.get("f").get(5).toString());
        assertEquals("[gaudy, grave]", result.get("g").get(5).toString());
        assertEquals("[should, spring]", result.get("s").get(6).toString());
        assertEquals("[substantial]", result.get("s").get(11).toString());
        assertEquals("[the, thy, thy, thy, too, the, the, thy, the, the, the]",
            result.get("t").get(3).toString());
        assertEquals("[where, waste, world]", result.get("w").get(5).toString());
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // The nested map structure that's the desired is the result of applying
    // a "downstream" collector that's the same operation as the first-level collector.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // Both collection operations are Collectors.groupingBy().
    // </editor-fold>


    /**
     * Given a stream of strings, accumulate (collect) them into the result string
     * by inserting the input string at both the beginning and end. For example, given
     * input strings "x" and "y" the result should be "yxxy". Note: the input stream
     * is a parallel stream, so you MUST write a proper combiner function to get the
     * correct result.
     */
    @Test @Ignore
    public void ex25_insertBeginningAndEnd() {
        Stream<String> input = Arrays.asList(
            "a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
            "k", "l", "m", "n", "o", "p", "q", "r", "s", "t")
            .parallelStream();

        String result = input.collect(null, null, null); // TODO

        assertEquals("tsrqponmlkjihgfedcbaabcdefghijklmnopqrst", result);
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // The combiner function must take its second argument and merge
    // it into the first argument, mutating the first argument.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // The second argument to the combiner function happens AFTER the first
    // argument in encounter order, so the second argument needs to be split
    // in half and prepended/appended to the first argument.
    // </editor-fold>


// ========================================================
// ADVANCED STREAMS: POTPOURRI
// ========================================================


    /**
     * Denormalize this map. The input is a map whose keys are the number of legs of an animal
     * and whose values are lists of names of animals. Run through the map and generate a
     * "denormalized" list of strings describing the animal, with the animal's name separated
     * by a colon from the number of legs it has. The ordering in the output list is not
     * considered significant.
     *
     * Input is Map<Integer, List<String>>:
     *   { 4=["ibex", "hedgehog", "wombat"],
     *     6=["ant", "beetle", "cricket"],
     *     ...
     *   }
     *
     * Output should be a List<String>:
     *   [ "ibex:4",
     *     "hedgehog:4",
     *     "wombat:4",
     *     "ant:6",
     *     "beetle:6",
     *     "cricket:6",
     *     ...
     *   ]
     */
    @Test @Ignore
    public void ex26_denormalizeMap() {
        Map<Integer, List<String>> input = new HashMap<>();
        input.put(4, Arrays.asList("ibex", "hedgehog", "wombat"));
        input.put(6, Arrays.asList("ant", "beetle", "cricket"));
        input.put(8, Arrays.asList("octopus", "spider", "squid"));
        input.put(10, Arrays.asList("crab", "lobster", "scorpion"));
        input.put(750, Arrays.asList("millipede"));

        List<String> result = null; // TODO

        assertEquals(13, result.size());
        assertTrue(result.contains("ibex:4"));
        assertTrue(result.contains("hedgehog:4"));
        assertTrue(result.contains("wombat:4"));
        assertTrue(result.contains("ant:6"));
        assertTrue(result.contains("beetle:6"));
        assertTrue(result.contains("cricket:6"));
        assertTrue(result.contains("octopus:8"));
        assertTrue(result.contains("spider:8"));
        assertTrue(result.contains("squid:8"));
        assertTrue(result.contains("crab:10"));
        assertTrue(result.contains("lobster:10"));
        assertTrue(result.contains("scorpion:10"));
        assertTrue(result.contains("millipede:750"));
    }
    // Hint 1:
    // <editor-fold defaultstate="collapsed">
    // There are several ways to approach this. You could use a stream of map keys,
    // a stream of map entries, or nested forEach() methods.
    // </editor-fold>
    // Hint 2:
    // <editor-fold defaultstate="collapsed">
    // If you use streams, consider using Stream.flatMap().
    // </editor-fold>


// ========================================================
// NEW FOR 2016
// ========================================================

    /**
     * Invert a "multi-map". (P. Sandoz)
     *
     * Given a Map<X, Set<Y>>, convert it to Map<Y, Set<X>>.
     */
    @Test @Ignore
    public void ex27_invertMultiMap() {
        Map<String, List<Integer>> input = new HashMap<>();
        input.put("a", Arrays.asList(1, 2));
        input.put("b", Arrays.asList(2, 3));
        input.put("c", Arrays.asList(1, 3));
        input.put("d", Arrays.asList(1, 4));
        input.put("e", Arrays.asList(2, 4));
        input.put("f", Arrays.asList(3, 4));

        Map<Integer, List<String>> result = null; // TODO

        assertEquals(new HashSet<>(Arrays.asList("a", "c", "d")), result.get(1));
        assertEquals(new HashSet<>(Arrays.asList("a", "b", "e")), result.get(2));
        assertEquals(new HashSet<>(Arrays.asList("b", "c", "f")), result.get(3));
        assertEquals(new HashSet<>(Arrays.asList("d", "e", "f")), result.get(4));
        assertEquals(4, result.size());
    }


    /**
     * Select from the input list each word that longer than the immediately
     * preceding word. Include the first word, since it is longer than the
     * (nonexistent) word that precedes it.
     *
     * XXX - compare to ex11
     */
    @Test @Ignore
    public void ex28_selectLongerThanPreceding() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

        List<String> result = null; // TODO

        assertEquals("[alfa, bravo, charlie, foxtrot, hotel]", result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Instead of a stream of words (Strings), run an IntStream of positions.
    // </editor-fold>


    /**
     * Generate a list of words that is the concatenation of each adjacent
     * pair of words in the input list. For example, if the input is
     *     [x, y, z]
     * the output should be
     *     [xy, yz]
     *
     * XXX - compare to ex11
     */
    @Test @Ignore
    public void ex29_concatenateAdjacent() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot");

        List<String> result = null; // TODO

        assertEquals("[alfabravo, bravocharlie, charliedelta, deltaecho, echofoxtrot]",
                     result.toString());
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Instead of a stream of words (Strings), run an IntStream of positions.
    // </editor-fold>

    /**
     * Select the longest words from the input list. That is, select the words
     * whose lengths are equal to the maximum word length. For this exercise,
     * it's easiest to perform two passes over the input list.
     *
     * XXX - compare to ex09 and ex10
     */
    @Test @Ignore
    public void ex30_selectLongestWords() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

        List<String> result = null; // TODO

        assertEquals("[charlie, foxtrot]", result.toString());
    }

    /**
     * Select the longest words from the input stream. That is, select the words
     * whose lengths are equal to the maximum word length. For this exercise,
     * you must compute the result in a single pass over the input stream.
     *
     * XXX - compare to ex30
     */
    @Test @Ignore
    public void ex31_selectLongestWordsOnePass() {
        Stream<String> input = Stream.of(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

        List<String> result = null; // TODO

        assertEquals("[charlie, foxtrot]", result.toString());
    }

    /**
     * Create a list of non-overlapping sublists of the input list, where each
     * sublist (except for the first one) starts with a word whose first character is a ":".
     * For example, given the input list
     *     [w, x, :y, z]
     * the output should be
     *     [[w, x], [:y, z]]
     */
    @Test @Ignore
    public void ex32_partitionIntoSublists() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", ":charlie", "delta", ":echo", ":foxtrot", "golf", "hotel");

        List<List<String>> result = null; // TODO

        assertEquals("[[alfa, bravo], [:charlie, delta], [:echo], [:foxtrot, golf, hotel]]",
                     result.toString());
    }

    /**
     * Given a stream of integers, compute separate sums of the even and odd values
     * in this stream. Since the input is a stream, this necessitates making a single
     * pass over the input.
     */
    @Test @Ignore
    public void ex33_separateOddEvenSums() {
        IntStream input = new Random(987523).ints(20, 0, 100);

        int sumEvens = 0; // TODO
        int sumOdds  = 0; // TODO

        assertEquals(516, sumEvens);
        assertEquals(614, sumOdds);
    }
    // Hint:
    // <editor-fold defaultstate="collapsed">
    // Use Collectors.partitioningBy().
    // </editor-fold>

    /**
     * Given a string, split it into a list of strings consisting of
     * consecutive characters from the original string. Note: this is
     * similar to Python's itertools.groupby function, but it differs
     * from Java's Collectors.groupingBy() collector.
     *
     * XXX - compare to ex32
     */
    @Test @Ignore
    public void ex34_splitCharacterRuns() {
        String input = "aaaaabbccccdeeeeeeaaafff";

        List<String> result = null; // TODO

        assertEquals("[aaaaa, bb, cccc, d, eeeeee, aaa, fff]", result.toString());
    }

    /**
     * Given a string, find the substring containing the longest run of consecutive,
     * equal characters.
     *
     * XXX - compare to ex34
     */
    @Test @Ignore
    public void ex35_longestCharacterRuns() {
        String input = "aaaaabbccccdeeeeeeaaafff";

        String result = null; // TODO

        assertEquals("eeeeee", result);
    }

    /**
     * Given a parallel stream of strings, collect them into a collection in reverse order.
     * Since the stream is parallel, you MUST write a proper combiner function in order to get
     * the correct result.
     */
    @Test @Ignore
    public void ex36_reversingCollector() {
        Stream<String> input =
            IntStream.range(0, 100).mapToObj(String::valueOf).parallel();

        Collection<String> result =
            input.collect(Collector.of(null, null, null)); // TODO


        assertEquals(
            IntStream.range(0, 100)
                     .map(i -> 99 - i)
                     .mapToObj(String::valueOf)
                     .collect(Collectors.toList()),
            new ArrayList<>(result));
    }

    /**
     * Given an array of int, find the int value that occurs a majority
     * of times in the array (that is, strictly more than half of the
     * elements are that value), and return it in an OptionalInt. If there
     * is no majority value, return an empty OptionalInt.
     */

    OptionalInt majority(int[] array) {
        return null; // TODO
    }

    @Test @Ignore
    public void ex37_majority() {
        int[] array1 = { 3, 3, 4, 2, 4, 4, 2, 4, 4 };
        int[] array2 = { 3, 3, 4, 2, 4, 4, 2, 4 };

        OptionalInt result1 = majority(array1);
        OptionalInt result2 = majority(array2);

        assertTrue(result1.isPresent() && result1.getAsInt() == 4);
        assertFalse(result2.isPresent());
    }


// ========================================================
// END OF EXERCISES -- CONGRATULATIONS!
// TEST INFRASTRUCTURE IS BELOW
// ========================================================

    static final String REGEXP = "[- .:,]+"; // for splitting into words

    private BufferedReader reader;

    @Before
    public void z_setUpBufferedReader() throws IOException {
        reader = Files.newBufferedReader(
                Paths.get("SonnetI.txt"), StandardCharsets.UTF_8);
    }

    @After
    public void z_closeBufferedReader() throws IOException {
        reader.close();
    }
}