Exercises.java

        //UNCOMMENT//Map<Integer, List<String>> result = null; // TODO
        //BEGINREMOVE
        Map<Integer, Set<String>> result =
            input.entrySet().stream()
                 .flatMap(e -> e.getValue().stream()
                                .map(v -> new SimpleEntry<>(e.getKey(), v)))
                 .collect(groupingBy(Entry::getValue, mapping(Entry::getKey, toSet())));
        //ENDREMOVE

        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
    public void ex28_selectLongerThanPreceding() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            IntStream.range(0, input.size())
                .filter(pos -> pos == 0 || input.get(pos-1).length() < input.get(pos).length())
                .mapToObj(pos -> input.get(pos))
                .collect(toList());
        //ENDREMOVE

        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
    public void ex29_concatenateAdjacent() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot");

        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        List<String> result =
            IntStream.range(1, input.size())
                .mapToObj(pos -> input.get(pos-1) + input.get(pos))
                .collect(toList());
        //ENDREMOVE

        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
    public void ex30_selectLongestWords() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

        //UNCOMMENT//List<String> result = null; // TODO
        //BEGINREMOVE
        int max = input.stream()
                       .mapToInt(String::length)
                       .max()
                       .getAsInt();

        List<String> result = input.stream()
                                   .filter(s -> s.length() == max)
                                   .collect(toList());
        //ENDREMOVE

        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
    public void ex31_selectLongestWordsOnePass() {
        Stream<String> input = Stream.of(
            "alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel");

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

        List<String> result = new ArrayList<>();
        input.forEachOrdered(s -> {
            if (result.isEmpty()) {
                result.add(s);
            } else {
                int reslen = result.get(0).length();
                int curlen = s.length();
                if (curlen > reslen) {
                    result.clear();
                    result.add(s);
                } else if (curlen == reslen) {
                    result.add(s);
                }
            }
        });
        //ENDREMOVE

        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
    public void ex32_partitionIntoSublists() {
        List<String> input = Arrays.asList(
            "alfa", "bravo", ":charlie", "delta", ":echo", ":foxtrot", "golf", "hotel");

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

        List<Integer> bounds =
            IntStream.rangeClosed(0, input.size())
                     .filter(i -> i == 0 || i == input.size() || input.get(i).startsWith(":"))
                     .boxed()
                     .collect(toList());

        List<List<String>> result =
            IntStream.range(1, bounds.size())
                     .mapToObj(i -> input.subList(bounds.get(i-1), bounds.get(i)))
                     .collect(toList());

        //ENDREMOVE

        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
    public void ex33_separateOddEvenSums() {
        IntStream input = new Random(987523).ints(20, 0, 100);

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

        Map<Boolean, Integer> sums =
            input.boxed()
                 .collect(partitioningBy(i -> (i & 1) == 1, Collectors.summingInt(i -> i)));
        int sumEvens = sums.get(false);
        int sumOdds  = sums.get(true);
        //ENDREMOVE

        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
    public void ex34_splitCharacterRuns() {
        String input = "aaaaabbccccdeeeeeeaaafff";

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

        List<Integer> bounds =
            IntStream.rangeClosed(0, input.length())
                     .filter(i -> i == 0 || i == input.length() ||
                                  input.charAt(i-1) != input.charAt(i))
                     .boxed()
                     .collect(toList());

        List<String> result =
            IntStream.range(1, bounds.size())
                     .mapToObj(i -> input.substring(bounds.get(i-1), bounds.get(i)))
                     .collect(toList());

        //ENDREMOVE

        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
    public void ex35_longestCharacterRuns() {
        String input = "aaaaabbccccdeeeeeeaaafff";

        //UNCOMMENT//String result = null; // TODO
        //BEGINREMOVE

        List<Integer> bounds =
            IntStream.rangeClosed(0, input.length())
                     .filter(i -> i == 0 || i == input.length() ||
                                  input.charAt(i-1) != input.charAt(i))
                     .boxed()
                     .collect(toList());

        String result =
            IntStream.range(1, bounds.size())
                     .boxed()
                     .max((i, j) -> Integer.compare(bounds.get(i) - bounds.get(i-1),
                                                    bounds.get(j) - bounds.get(j-1)))
                     .map(i -> input.substring(bounds.get(i-1), bounds.get(i)))
                     .get();

        //ENDREMOVE

        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
    public void ex36_reversingCollector() {
        Stream<String> input =
            IntStream.range(0, 100).mapToObj(String::valueOf).parallel();

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

        //BEGINREMOVE

        Collection<String> result =
            input.collect(Collector.of(ArrayDeque::new,
                                       ArrayDeque::addFirst,
                                       (d1, d2) -> { d2.addAll(d1); return d2; }));

        //ENDREMOVE

        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) {
        //UNCOMMENT//return null; // TODO
        //BEGINREMOVE
        Map<Integer, Long> map =
            Arrays.stream(array)
                  .boxed()
                  .collect(groupingBy(x -> x, counting()));

        return map.entrySet().stream()
                  .filter(e -> e.getValue() > array.length / 2)
                  .mapToInt(Entry::getKey)
                  .findAny();
        //ENDREMOVE
    }

    @Test
    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();
    }
}

//BEGINREMOVE
/*
 * Procedure for deriving exercise file from answers.
 * - Open a shell and change do the LambdaLab/test/solutions directory.
 * - Run the "cleanit" perl script from within this directory.
 * - This should generate the LambdaLab/test/exercises/Exercises.java file automatically.
 * - Make sure the only files open in the project are (unsolved!) Exercises.java and
 *   SonnetI.txt, then run clean, and close the NB project.
 */
//ENDREMOVE