15. Solution: Adapter Pattern

Solution: Adapter Pattern

MultiFileReader.java

import java.io.BufferedReader;
import java.io.Closeable;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public final class MultiFileReader implements Closeable {

  private final List<BufferedReader> readers;

  public MultiFileReader(List<Path> paths) {
    readers = new ArrayList<>(paths.size());
    try {
      for (Path path : paths) {
        readers.add(Files.newBufferedReader(path));
      }
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      close();
    }
  }

  public List<BufferedReader> getReaders() {
    return Collections.unmodifiableList(readers);
  }

  @Override
  public void close() {
    for (BufferedReader reader : readers) {
      try {
        reader.close();
      } catch (Exception ignored) {
      }
    }
  }
}

MergeShards.java

import java.io.BufferedReader;
import java.io.Writer;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;
import java.util.Objects;
import java.util.PriorityQueue;
import java.util.stream.Collectors;

public final class MergeShards {
  public static void main(String[] args) throws Exception {
    if (args.length != 2) {
      System.out.println("Usage: MergeShards [input folder] [output file]");
      return;
    }

    List<Path> inputs = Files.walk(Path.of(args[0]), 1).skip(1).collect(Collectors.toList());
    Path outputPath = Path.of(args[1]);

    try (MultiFileReader multiReader = new MultiFileReader(inputs)) {
      PriorityQueue<WordEntry> words = new PriorityQueue<>();
      for (BufferedReader reader : multiReader.getReaders()) {
        String word = reader.readLine();
        if (word != null) {
          words.add(new WordEntry(word, reader));
        }
      }

      try (Writer writer = Files.newBufferedWriter(outputPath)) {
        while (!words.isEmpty()) {
          WordEntry entry = words.poll();
          writer.write(entry.word);
          writer.write(System.lineSeparator());
          String word = entry.reader.readLine();
          if (word != null) {
            words.add(new WordEntry(word, entry.reader));
          }
        }
      }
    }
  }

  private static final class WordEntry implements Comparable<WordEntry> {
    private final String word;
    private final BufferedReader reader;

    private WordEntry(String word, BufferedReader reader) {
      this.word = Objects.requireNonNull(word);
      this.reader = Objects.requireNonNull(reader);
    }

    @Override
    public int compareTo(WordEntry other) {
      return word.compareTo(other.word);
    }
  }
}