MD5

.gitignore

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+.idea
+.gradle
+build/

.travis.yml

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+language: java
+
+jdk:
+- oraclejdk8

build.gradle

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+group 'spbau.shevchenko'
+version '1.0-SNAPSHOT'
+
+apply plugin: 'java'
+
+//noinspection GroovyUnusedAssignment
+sourceCompatibility = 1.8
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testCompile group: 'junit', name: 'junit', version: '4.11'
+}

gradle/wrapper/gradle-wrapper.properties

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+#Thu Mar 02 20:22:38 MSK 2017
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.13-all.zip

settings.gradle

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+rootProject.name = 'Lazy'
+

src/main/java/ru/spbau/shevchenko/CLI.java

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+package ru.spbau.shevchenko;
+
+import java.io.IOException;
+import java.nio.file.Paths;
+import java.util.Arrays;
+
+public class CLI {
+    public static void main(String[] args) throws IOException {
+        for (String path : args) {
+            processSingleThreaded(path);
+            processMultiThreaded(path);
+        }
+    }
+
+    private static void processMultiThreaded(String path) throws IOException {
+        System.out.println("Multithreaded:");
+        process(path, new ForkJoinCalculator());
+    }
+    private static void processSingleThreaded(String path) throws IOException {
+        System.out.println("Singlethreaded:");
+        process(path, new SingleThreadedCalculator());
+    }
+
+    private static void process(String path, FileChecksumCalculator calculator) throws IOException {
+        long startTime = System.currentTimeMillis();
+        byte[] hash = calculator.calcChecksum(Paths.get(path));
+        long endTime = System.currentTimeMillis();
+        Double tookSeconds = (endTime - startTime) / 1000.0;
+        System.out.println("Calculated result for " + path + " in " + tookSeconds.toString());
+        System.out.println("Result is: " + Arrays.toString(hash));
+        System.out.println();
+    }
+
+}

src/main/java/ru/spbau/shevchenko/ChecksumTask.java

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+package ru.spbau.shevchenko;
+
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.nio.file.DirectoryStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.RecursiveTask;
+
+class ChecksumTask extends RecursiveTask<byte[]> {
+    private static final String MD5 = "MD5";
+    private static final String NO_SUCH_ALGORITHM_MD5 = "Well, there's no MD5 algorithm in Java...";
+    private Path path;
+
+    ChecksumTask(Path path) throws FileNotFoundException {
+        if (Files.notExists(path)) {
+            throw new FileNotFoundException(path.toString() + " not found!");
+        }
+        this.path = path;
+    }
+
+    @Override
+    protected byte[] compute() {
+        if (!Files.isDirectory(path)) {
+            try {
+                return new SingleFileChecksumCalculator().calcChecksum(path);
+            } catch (IOException e) {
+                throw new RuntimeException("Failed to calculate checksum for " + path.toString());
+            }
+        }
+        else {
+            try (DirectoryStream<Path> stream = Files.newDirectoryStream(path)) {
+                MessageDigest messageDigest = MessageDigest.getInstance(MD5);
+                messageDigest.update(path.getFileName().toString().getBytes());
+                List<ChecksumTask> subtasks = new ArrayList<>();
+                for (Path entry: stream) {
+                    ChecksumTask subtask = new ChecksumTask(entry);
+                    subtask.fork();
+                    subtasks.add(subtask);
+                }
+                for (ChecksumTask subtask : subtasks) {
+                    messageDigest.update(subtask.join());
+                }
+                return messageDigest.digest();
+            } catch (NoSuchAlgorithmException e) {
+                throw new RuntimeException(NO_SUCH_ALGORITHM_MD5);
+            } catch (IOException e) {
+                throw new RuntimeException("Failed to calculate checksum for " + path.toString());
+            }
+        }
+    }
+}

src/main/java/ru/spbau/shevchenko/FileChecksumCalculator.java

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+package ru.spbau.shevchenko;
+
+import java.io.IOException;
+import java.nio.file.Path;
+
+interface FileChecksumCalculator {
+    byte[] calcChecksum(Path path) throws IOException;
+}