Add exercise parallel-letter-frequency

config.json

@@ -25,6 +25,18 @@
     },
     {
       "core": false,
+      "difficulty": 6,
+      "slug": "parallel-letter-frequency",
+      "topics": [
+        "concurrency",
+        "maps",
+        "strings"
+      ],
+      "unlocked_by": "hamming",
+      "uuid": "38a405e8-619d-400f-b53c-2f06461fdf9d"
+    },
+    {
+      "core": false,
       "difficulty": 1,
       "slug": "reverse-string",
       "topics": [

exercises/parallel-letter-frequency/.meta/HINTS.md

@@ -0,0 +1,3 @@
+Single-threaded (non-concurrent) solutions can pass all tests [but the last.](https://www.youtube.com/watch?v=mJZZNHekEQw)  Your solution will be tested for concurrency by submitting it as a [Runnable](https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html) to an [ExecutorService.](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html) Your solution must leverage multiple [Threads](https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html) to pass the final test.
+
+Java documentation on [parallel streams](https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html) may provide some help.

exercises/parallel-letter-frequency/.meta/src/reference/java/ParallelLetterFrequency.java

@@ -0,0 +1,45 @@
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+
+class ParallelLetterFrequency {
+
+    private static Map<Integer, Integer> letterFrequencyMap;
+    private final String NOT_A_LETTER = "\\P{L}+";
+
+
+    ParallelLetterFrequency(String letters) {
+        letterFrequencyMap = getMapFromLetters(letters.toLowerCase().replaceAll(NOT_A_LETTER, ""));
+    }
+
+    Map<Integer, Integer> getMapFromLetters(String letters) {
+        return letters
+                .chars()
+                .parallel()
+                .collect(Counts::new, Counts::increment, Counts::combine)
+                .buildMap();
+    }
+
+    private static class Counts {
+
+        private Map<Integer, Integer> letterCounts = new HashMap<>();
+
+        private void increment(int letter) {
+            letterCounts.put(letter, letterCounts.getOrDefault(letter, 0) + 1);
+        }
+
+        private void combine(Counts other) {
+            other.letterCounts.keySet()
+                    .forEach(letter -> letterCounts.merge(letter, other.letterCounts.get(letter), Integer::sum));
+        }
+
+        private Map<Integer, Integer> buildMap() {
+            return Collections.unmodifiableMap(letterCounts);
+        }
+    }
+
+    Map<Integer, Integer> letterCounts() {
+        return letterFrequencyMap;
+    }
+}

exercises/parallel-letter-frequency/README.md

@@ -0,0 +1,28 @@
+# Parallel Letter Frequency
+
+Count the frequency of letters in texts using parallel computation.
+
+Parallelism is about doing things in parallel that can also be done
+sequentially. A common example is counting the frequency of letters.
+Create a function that returns the total frequency of each letter in a
+list of texts and that employs parallelism.
+
+# Java Tips
+
+Single-threaded (non-concurrent) solutions can pass all tests [but the last.](https://www.youtube.com/watch?v=mJZZNHekEQw)  Your solution will be tested for concurrency by submitting it as a [Runnable](https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html) to an [ExecutorService.](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorService.html) Your solution must leverage multiple [Threads](https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html) to pass the final test.
+
+Java documentation on [parallel streams](https://docs.oracle.com/javase/tutorial/collections/streams/parallelism.html) may provide some help.
+
+# Running the tests
+
+You can run all the tests for an exercise by entering
+
+```sh
+$ gradle test
+```
+
+in your terminal.
+
+## Submitting Incomplete Solutions
+
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/parallel-letter-frequency/build.gradle

@@ -0,0 +1,18 @@
+apply plugin: "java"
+apply plugin: "eclipse"
+apply plugin: "idea"
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  testCompile "junit:junit:4.12"
+}
+
+test {
+  testLogging {
+    exceptionFormat = 'full'
+    events = ["passed", "failed", "skipped"]
+  }
+}

exercises/parallel-letter-frequency/src/test/java/ParallelLetterFrequencyTest.java

@@ -0,0 +1,235 @@
+import org.junit.*;
+
+import static org.junit.Assert.*;
+
+import java.util.concurrent.*;
+import java.util.stream.*;
+import java.util.*;
+
+public class ParallelLetterFrequencyTest {
+
+    // Poem by Friedrich Schiller. The corresponding music is the European Anthem.
+    private String OdeAnDieFreude =
+            "Freude schöner Götterfunken\n" +
+                    "Tochter aus Elysium,\n" +
+                    "Wir betreten feuertrunken,\n" +
+                    "Himmlische, dein Heiligtum!\n" +
+                    "Deine Zauber binden wieder\n" +
+                    "Was die Mode streng geteilt;\n" +
+                    "Alle Menschen werden Brüder,\n" +
+                    "Wo dein sanfter Flügel weilt.";
+
+    // Dutch national anthem
+    private String Wilhelmus =
+            "Wilhelmus van Nassouwe\n" +
+                    "ben ik, van Duitsen bloed,\n" +
+                    "den vaderland getrouwe\n" +
+                    "blijf ik tot in den dood.\n" +
+                    "Een Prinse van Oranje\n" +
+                    "ben ik, vrij, onverveerd,\n" +
+                    "den Koning van Hispanje\n" +
+                    "heb ik altijd geëerd.";
+
+    // American national anthem
+    private String StarSpangledBanner =
+            "O say can you see by the dawn's early light,\n" +
+                    "What so proudly we hailed at the twilight's last gleaming,\n" +
+                    "Whose broad stripes and bright stars through the perilous fight,\n" +
+                    "O'er the ramparts we watched, were so gallantly streaming?\n" +
+                    "And the rockets' red glare, the bombs bursting in air,\n" +
+                    "Gave proof through the night that our flag was still there;\n" +
+                    "O say does that star-spangled banner yet wave,\n" +
+                    "O'er the land of the free and the home of the brave?\n";
+
+
+    @Test
+    public void noTextsMeansNoLetters() {
+        String input = "";
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>();
+        ParallelLetterFrequency p = new ParallelLetterFrequency(input);
+
+        assertEquals(expectedOutput, p.letterCounts());
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void oneLetterIsCorrectlyCounted() {
+        String input = "a";
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>() {
+            {
+                put((int) 'a', 1);
+            }
+        };
+        ParallelLetterFrequency p = new ParallelLetterFrequency(input);
+
+        assertEquals(expectedOutput, p.letterCounts());
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void resultsAreCaseInsensitive() {
+        String input = "Aa";
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>() {
+            {
+                put((int) 'a', 2);
+            }
+        };
+        ParallelLetterFrequency p = new ParallelLetterFrequency(input);
+
+        assertEquals(expectedOutput, p.letterCounts());
+    }
+
+
+    @Ignore("Remove to run test")
+    @Test
+    public void biggerEmptyTextsStillReturnNoResults() {
+        StringBuilder b = new StringBuilder();
+        for (int i = 0; i < 10000; i++) {
+            b.append(" ");
+        }
+
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>();
+        ParallelLetterFrequency p = new ParallelLetterFrequency(b.toString());
+
+        assertEquals(expectedOutput, p.letterCounts());
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void manyRepetitionsOfTheSameTextGiveAPredictableResult() {
+        StringBuilder b = new StringBuilder();
+        for (int i = 0; i < 10000; i++) {
+            b.append("abc");
+        }
+
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>() {
+            {
+                put((int) 'a', 10000);
+                put((int) 'b', 10000);
+                put((int) 'c', 10000);
+            }
+        };
+        ParallelLetterFrequency p = new ParallelLetterFrequency(b.toString());
+
+        assertEquals(expectedOutput, p.letterCounts());
+    }
+
+
+    @Ignore("Remove to run test")
+    @Test
+    public void punctuationDoesntCount() {
+        ParallelLetterFrequency p = new ParallelLetterFrequency(OdeAnDieFreude);
+
+        assertFalse(p.letterCounts().containsKey((int) ','));
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void numbersDontCount() {
+        ParallelLetterFrequency p = new ParallelLetterFrequency("Testing, 1, 2, 3");
+
+        assertFalse(p.letterCounts().containsKey((int) '1'));
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void allThreeAnthemsTogetherProduceCorrectCounts() {
+        StringBuilder b = new StringBuilder();
+        b.append(OdeAnDieFreude);
+        b.append(Wilhelmus);
+        b.append(StarSpangledBanner);
+
+        ParallelLetterFrequency p = new ParallelLetterFrequency(b.toString());
+
+        assertEquals(new Integer(49), p.letterCounts().get((int) 'a'));
+        assertEquals(new Integer(56), p.letterCounts().get((int) 't'));
+        assertEquals(new Integer(2), p.letterCounts().get((int) 'ü'));
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void multipleThreadsGetUsed()
+            throws InterruptedException, ExecutionException {
+
+
+        class MyForkJoinWorkerThread extends ForkJoinWorkerThread {
+
+            boolean wasStarted = false;
+
+            public MyForkJoinWorkerThread(ForkJoinPool pool) {
+                super(pool);
+            }
+
+            @Override
+            public void start() {
+                super.start();
+                this.wasStarted = true;
+            }
+        }
+
+        List<MyForkJoinWorkerThread> allThreads = new ArrayList<>();
+
+        class MyForkJoinThreadFactory implements ForkJoinPool.ForkJoinWorkerThreadFactory {
+            public MyForkJoinThreadFactory() {
+                super();
+            }
+
+            public MyForkJoinWorkerThread newThread(ForkJoinPool pool) {
+                final MyForkJoinWorkerThread worker = new MyForkJoinWorkerThread(pool);
+                worker.setName("Thread number: " + worker.getPoolIndex());
+                allThreads.add(worker);
+                return worker;
+            }
+        }
+
+        MyForkJoinThreadFactory factory = new MyForkJoinThreadFactory();
+        ForkJoinPool multiThreadPool = new ForkJoinPool(4, (ForkJoinPool.ForkJoinWorkerThreadFactory) factory, null, false);
+
+        String input = "abcdefghijklmnopqrstuvwxyz";
+        StringBuilder b = new StringBuilder();
+        for (int i = 1; i < 10000000; i++) {
+            b.append(input);
+        }
+
+        Map<Integer, Integer> expectedOutput = new HashMap<Integer, Integer>() {
+            {
+                put((int) 'a', 10000000);
+                put((int) 'b', 10000000);
+                put((int) 'c', 10000000);
+                put((int) 'd', 10000000);
+                put((int) 'e', 10000000);
+                put((int) 'f', 10000000);
+                put((int) 'g', 10000000);
+                put((int) 'h', 10000000);
+                put((int) 'i', 10000000);
+                put((int) 'j', 10000000);
+                put((int) 'k', 10000000);
+                put((int) 'l', 10000000);
+                put((int) 'm', 10000000);
+                put((int) 'n', 10000000);
+                put((int) 'o', 10000000);
+                put((int) 'p', 10000000);
+                put((int) 'q', 10000000);
+                put((int) 'r', 10000000);
+                put((int) 's', 10000000);
+                put((int) 't', 10000000);
+                put((int) 'u', 10000000);
+                put((int) 'v', 10000000);
+                put((int) 'w', 10000000);
+                put((int) 'x', 10000000);
+                put((int) 'y', 10000000);
+                put((int) 'z', 10000000);
+            }
+        };
+
+        Map<Integer, Integer> multiCounts = multiThreadPool.submit(
+                () -> (new ParallelLetterFrequency(b.toString()).letterCounts())
+        ).get();
+
+        boolean startedThreadCountEqualsPoolSize = allThreads.stream()
+                .filter(thread -> thread.wasStarted)
+                .count() == allThreads.size();
+
+        assertTrue(startedThreadCountEqualsPoolSize);
+    }
+}

exercises/settings.gradle

@@ -49,6 +49,7 @@ include 'ocr-numbers'
 include 'octal'
 include 'palindrome-products'
 include 'pangram'
+include 'parallel-letter-frequency'
 include 'pascals-triangle'
 include 'perfect-numbers'
 include 'phone-number'