Friday, March 21, 2014

Design an in-memory file system

Problem
Explain the data structures and algorithms that you would use to design an in-memory file system. Illustrate with an example in code where possible.

Solution

We will have files, which will be managed by the filesystem.Following will be classes:

File
attributes - name, extension, create time, modified time, readonly, executable, isFolder and so on.
If its folder, then it will contain set of other files.

FileSystem
It will have root level file, which will contain set of other files. Now filesystem needs couple of basic functions like remove a file , add a file , change directory of file and so on.

public class File {
    String fileName = new String();
    String extension = new String();
    long size = 0;
    Calendar createdTime = Calendar.getInstance();
    Calendar modifiedTime = Calendar.getInstance();
    Calendar lastAccessTime = Calendar.getInstance();
    boolean readOnly = false;
    boolean hidden = false;
    boolean isExecutable = false;
 
    boolean isFolder = false;
    File upperLevel = null;
    Set<File> children = new HashSet<File>();
 
    public String getName() {
        if (isFolder)
            return fileName;
        else
            return fileName + "." + extension;
    }
 
    @Override
    public String toString() {
        // String prefix = isFolder ? "/" : "";
        String appendix = isFolder ? "/" : "." + extension;
        return fileName + appendix;
    }
}

public class FileSystem {
    File root = new File();
    File current = root;
 
    public FileSystem() {
        root.fileName = "~";
        root.isFolder = true;
    }
 
    public boolean create(String name, boolean isFolder) {
        if (current.isFolder) {
            File newFile = new File();
            if (isFolder) {
                newFile.fileName = name;
            } else {
                String[] names = name.split("\\.");
                newFile.fileName = names[0];
                newFile.extension = names[1];
            }
            newFile.isFolder = isFolder;
            newFile.upperLevel = current;
            current.children.add(newFile);
            return true;
        } else {
            System.out.println("Can only create a file under a directory");
            return false;
        }
    }
 
    public void list() {
        for (File file : current.children) {
            System.out.printf("%5d %40s\n", file.size, file);
        }
    }
 
    public boolean changeDirectory(String dir) {
        if (dir.equals("..") && current.upperLevel != null) {
            current = current.upperLevel;
            return true;
        }
        for (File file : current.children) {
            if (file.getName().equals(dir)) {
                current = file;
            }
            return true;
        }
        return false;
    }
 
    public boolean remove(String[] names) {
        List<File> filesToRemove = new LinkedList<File>();
        boolean deleted = false;
        for (File file : current.children) {
            for (String name : names) {
                if (file.getName().equals(name))
                    filesToRemove.add(file);
            }
        }
        for (File file : filesToRemove) {
            current.children.remove(file);
            deleted = true;
        }
        return deleted;
    }
}
 
public static String readCommand(File current) {
    System.out.println(current + "$ ");
    String input = new String();
    InputStreamReader converter = new InputStreamReader(System.in);
    BufferedReader in = new BufferedReader(converter);
    try {
        input = in.readLine();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return input;
}
 
public static void main(String[] args) {
    System.out.println("Bruce's file system starts.");
    Q7_9 instance = new Q7_9();
    FileSystem system = instance.system;
    String input = readCommand(system.current);
 
    while (!input.equals("exit")) {
        if (input.contains("touch")) {
            String[] commands = input.split(" ");
            system.create(commands[1], false);
        } else if (input.contains("mkdir")) {
            String[] commands = input.split(" ");
            system.create(commands[1], true);
        } else if (input.contains("ls")) {
            system.list();
        } else if (input.contains("cd")) {
            String[] commands = input.split(" ");
            system.changeDirectory(commands[1]);
        } else if (input.contains("rm")) {
            String[] commands = input.split(" ");
            String[] names = new String[commands.length - 1];
            for (int i = 0; i < names.length; ++i)
                names[i] = commands[i + 1];
            system.remove(names);
        }
        input = readCommand(system.current);
    }
}

Thanks

Reactions:

0 comments:

Post a Comment