Program Assignment $3$: Implement a Simple Virtual File System IN JAVA

Purpose

Computer users are familiar with the abstract concept of a $$file$,$ but the operating system has to do a great deal of work to map logical files to a physical storage device. The low level details of how an operating system keeps track of a file's data, which may be scattered across the disk, are taken for granted for most computer users. In this assignment you will implement a simple file system on a virtual $256$ byte disk.

Instructions

The Disk

The virtual disk drive is implemented by the Drive class in the provided code. The $256$ bytes of the drive are divided into $16$ blocks. Each block is $16$ bytes in size. This mimics the structure of an actual disk drive, where blocks are typically $4$K in size.

The File Format

The file system used on this virtual disk is Tiny File System $($TFS$).$ It allows for the creation of both files and subdirectories. The meaning of the data stored within a block depends on that block's type. The first block of the file system, block $0,$ is always used to hold the root directory. Initially, all of the other blocks are part of the free space list. File names in the virtual disk must be in lowercase letters, and directories uppercase letters.$$

Root Directory

Root Directory$00010203040506070809101112131415$nextNn$1$s$1$n$2$s$2$n$3$s$3$n$4$s$4$n$5$s$5$n$6$s$6$n$7$s$7$

The first byte of the root directory contains the block number of the first block of the free space list. The second byte is a count of how many files or directories are stored inside the root directory. The rest of the bytes are file$/$directory names $($nX$),$ or the block in which the corresponding file$/$directory is indexed $($sX$).$ Unneeded entries are set to $0.$

Free Space

Free Space$00010203040506070809101112131415$next$000000000000000$

The first byte of a block in the free space list contains the block number of the next block of the free space list. The last block on the free space list should contain $-1$ in this position.

Directories

Directories$00010203040506070809101112131415$PNn$1$s$1$n$2$s$2$n$3$s$3$n$4$s$4$n$5$s$5$n$6$s$6$n$7$s$7$

The first byte in a directory is the block number of its parent directory. The second byte is a count of how many files or directories are stored inside that directory. The rest of the bytes are file$/$directory names $($nX$),$ or the block in which the corresponding file$/$directory is indexed $($sX$).$ Unneeded entries are set to $0.$

File Index

File Index$00010203040506070809101112131415$Psizedatadatadatadatadatadatadatadatadatadatadatadatadatadata

File Data

The first byte in a file index is the block number of its parent directory.The second byte of a file index block contains the size of the file, in bytes. The remaining bytes are the block number of actual file data. Since there are at most $14$ of these, the maximum file size is $14$x$16=224.$

Descriptions of Commands

CommandDescription

import

Copy a file stored in the regular file system to the memory resident virtual disk. The file is assumed to be a text file. If the file already exists in the virtual file system, it is overwritten.

export

Copy a file stored in the virtual disk to the regular file system. The file is assumed to be a text file. If the file already exists on the Linux$/$Windows$/$OSX file system, an error is reported, and no write occurs.

ls

List the contents $($file name and size$)$ of the given TFS directory.

mkdir

Create a new virtual files system directory at $.$

raw

Display the raw contents of the disk, using the Drive.contentsAsByte$()$ method.

exit

End the program, causing all the data in the virtual disk to be lost.

Other Descriptions

ItemDescription

A legal path name in Linux$/$Windows$/$OSX; can be absolute or relative. Example: $"/$bin$/$stuff$/$Mine$.$txt$"$

A legal absolute path in the virtual file system. Example: $"/$c$"$ or $"/$X$/$q$"$

Sample Interaction

Here is some sample interaction with your shell. This is only a simple, possible example. Use any prompt you desire, display more info with ls$,$ etc.

:$>$ import $./$myfile$.$txt a :$>$ import $./$myfile$.$txt c :$>$ export c $./$myfile$.$txt Error, file exists. :$>$ ls $/$ a $12$ c $12$ :$>$ exit

Provided code:

import java.util.Scanner;

class Main $\{$

public static void main$($String$[]$ args$)$

$\{$

Drive disk $=$ new Drive$()$;

boolean exit $=$ false;

Scanner in $=$ new Scanner$($System$.$in$)$;

System.out.println$("$Virtual file system"$)$;

System.out.print$("$:$>")$;

while $(!$exit$)$

$\{$

String command $=$ in$.$nextLine$()$;

String$[]$ tokens $=$ UI$.$parseCommand$($command$)$;

exit $=($tokens$[0].$equals$("$exit$"))$;

if $($tokens$[0].$equals$("$import$"))$

UI$.$importFile$($tokens$,$ disk$)$;

else if $($tokens$[0].$equals$("$export$"))$

UI$.$exportFile$($tokens$,$ disk$)$;

else if $($tokens$[0].$equals$("$ls$"))$

UI$.$ls$($tokens$,$disk$)$;

else if $($tokens$[0].$equals$("$raw$"))$

UI$.$raw$($disk$)$;

else if $($tokens$[0].$equals$("$mkdir$"))$

UI$.$mkdir$($tokens$,$ disk$)$;

else if $(!$exit$)$

$\{$

System.out.println$("$command not recognized."$)$;

$\}$

if $(!$exit$)$

$\{$

System.out.print$("$:$>")$;

$\}$

$\}$

System.out.println$("$Program terminated."$)$;

$\}$

$\}$

public class UI

public static String$[]$ parseCommand$($String command$)$

$\{$

String$[]$ a $=\{""\}$;

return a;

$\}$

public static voi