即时通讯使用Java实现外部合并排序。
因此,给定一个文件,我将其拆分为较小的文件,然后对较小的部分进行排序,最后合并已排序(较小)的文件。
因此,最后一步是我遇到的麻烦。
我有一个文件列表,我想在每个步骤中,取每个文件的第一行的最小值,然后删除该行。
因此,应该是这样的:
public static void mergeSortedFiles(List<File> sorted, File output) throws IOException {
BufferedWriter wf = new BufferedWriter(new FileWriter(output));
String curLine = "";
while(!sorted.isEmpty()) {
curLine = findMinLine(sorted);
wf.write(curLine);
}
}
public static String findMinLine(List<File> sorted) throws IOException {
List<BufferedReader> brs = new ArrayList<>();
for(int i =0; i<sorted.size() ; i++) {
brs.add(new BufferedReader(new FileReader(sorted.get(i))));
}
List<String> lines = new ArrayList<>();
for(BufferedReader br : brs) {
lines.add(br.readLine());
}
Collections.sort(lines);
return lines.get(0);
}
我不确定如何更新文件,有人可以提供帮助吗?
感谢您的帮助!
您可以Comparable
围绕每个文件创建包装器,然后将包装器放置在堆中(例如PriorityQueue
)。
public class ComparableFile<T extends Comparable<T>> implements Comparable<ComparableFile<T>> {
private final Deserializer<T> deserializer;
private final Iterator<String> lines;
private T buffered;
public ComparableFile(File file, Deserializer<T> deserializer) {
this.deserializer = deserializer;
try {
this.lines = Files.newBufferedReader(file.toPath()).lines().iterator();
} catch (IOException e) {
// deal with it differently if you want, I'm just providing a working example
// and wanted to use the constructor in a lambda function
throw new UncheckedIOException(e);
}
}
@Override
public int compareTo(ComparableFile<T> that) {
T mine = peek();
T theirs = that.peek();
if (mine == null) return theirs == null ? 0 : -1;
if (theirs == null) return 1;
return mine.compareTo(theirs);
}
public T pop() {
T tmp = peek();
if (tmp != null) {
buffered = null;
return tmp;
}
throw new NoSuchElementException();
}
public boolean isEmpty() {
return peek() == null;
}
private T peek() {
if (buffered != null) return buffered;
if (!lines.hasNext()) return null;
return buffered = deserializer.deserialize(lines.next());
}
}
然后,您可以通过以下方式合并它们:
public class MergeFiles<T extends Comparable<T>> {
private final PriorityQueue<ComparableFile<T>> files;
public MergeFiles(List<File> files, Deserializer<T> deserializer) {
this.files = new PriorityQueue<>(files.stream()
.map(file -> new ComparableFile<>(file, deserializer))
.filter(comparableFile -> !comparableFile.isEmpty())
.collect(toList()));
}
public Iterator<T> getSortedElements() {
return new Iterator<T>() {
@Override
public boolean hasNext() {
return !files.isEmpty();
}
@Override
public T next() {
if (!hasNext()) throw new NoSuchElementException();
ComparableFile<T> head = files.poll();
T next = head.pop();
if (!head.isEmpty()) files.add(head);
return next;
}
};
}
}
这是一些代码来演示它的工作原理:
public static void main(String[] args) throws IOException {
List<File> files = Arrays.asList(
newTempFile(Arrays.asList("hello", "world")),
newTempFile(Arrays.asList("english", "java", "programming")),
newTempFile(Arrays.asList("american", "scala", "stackoverflow"))
);
Iterator<String> sortedElements = new MergeFiles<>(files, line -> line).getSortedElements();
while (sortedElements.hasNext()) {
System.out.println(sortedElements.next());
}
}
private static File newTempFile(List<String> words) throws IOException {
File tempFile = File.createTempFile("sorted-", ".txt");
Files.write(tempFile.toPath(), words);
tempFile.deleteOnExit();
return tempFile;
}
输出:
american
english
hello
java
programming
scala
stackoverflow
world
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