Programming Languages Homework 8 Part I Due 11/19 Part II Due 11/26

(141 points) 141OS is a simple operating system that allows multiple users to save and print files. The 141OS manages multiple disks, multiple printers, and multiple users - all of which can be considered concurrent processes. Files can be stored on any disk and printed on any printer by any user. The goal of the system is to exploit possible parallelism to keep the devices (disks and printers) as busy as possible. This will be a two week assignment. The first part (due week 9) is to define the Java classes for UserThread, Disk, and Printer. The second part (due week 10) is to write PrintJobThread that manage printing a specified file and UserThread that saves files to disk sectors. (Note only one line of a file may be stored in a sector, so a file will typically require one sector per line.) Even though Printers and Disks can be thought of as parallel processes, I suggest you not make them Threads. If you do, you must make them Daemon threads by calling setDaemon or your program will never terminate.
Define symbolic constants for NUMBER_OF_USERS=4, NUMBER_OF_DISKS=2, and NUMBER_OF_PRINTERS=3. Store instances of the appropriate objects in three separate arrays: Users, Printers, and Disks.
A sample of the input (USER*) and output (PRINTER*) is in the hw8 directory located in my homework starts directory. The PRINTER* files will be generated by your program, but the exact files that get sent to each printer may vary. If you use a BufferedWriter, be sure you flush the buffer after each write.
Design Sketch part I: Define class UserThread, Printer, and Disk. Only UserThread will be derived from class Thread. Define class DirectoryManager that maps file names into disk sectors. You may want to define a DiskManager to keep track of the next free sector on each disk and to contain the DirectoryManager.
- Buffer: implemented as an array of char. Will be the standard Unit stored on disks, sent to printers (one line at a time) and handled by users (again, one line at a time). Write appropriate methods such as equal(String s), copyFrom(String source), and copyFrom(Buffer source).
- Disks. Each disk has a capacity specified to the constructor. The constructor must allocate all the Buffers (one per sector) when the Disk is created and must not allocate any after that.
  - Operation include the following:
```
class Disk
{
  void write(int sector, Buffer data);
  void read(int sector, Buffer data);
}
```
  - You can only read or write one line at a time, so to store a file, you must make a write for each input line. You may implement it as an array of Buffers. (We are using Buffers because Strings are not modifyable. (Note also Buffer.equals must do a deep equality if you plan to do comparisons.) Reads and writes each take 200 miliseconds. You must sleep the thread that asks to print or read a sector.
  - Note you can have only one person writing to a disk at a time or the files will be jumbled. You can have any number of Threads doing reads at a time - even when someone is writing to the disk. (Note this is not normally the case in a real OS, I had to simplify the problem somehow.)
- Printer. Each printer will write data to a file named PRINTERi where i is the index of this printer. A printer can only handle one line of text at a time. You give it a line and it takes 2750 miliseconds to print. You'll have to sleep your the thread that asks to print to simulate this delay.
- UserThreads. Each user will read from a file named USERi where i is the index of this user. Each USERi file will contain a series of the following three commands. Anything between a .save and a .end is part of the data in file X. UserThreads will handle the command evaluation.
  - .save X
  - .end
  - .print X
- The UserThread will handle saving files to disk, but it will create a new PrintJobThread to handle each print request.
- Each UserThread may have only one String or Buffer to hold the current line of text. If you read Strings from the input file, you must immediately copy that String into this single Buffer owned byt he UserThread. The UserThread interpret the command in the Buffer or Save the Buffer to a disk as appropriate.
- The DirectoryManager is a table that knows where files are stored on disk. The DirectoryManager operations are:
```
class DirectoryManager
{
  void enter(Buffer key, FileInfo file);
  FileInfo lookup(Buffer key);
}
```
- A FileInfo object will hold the disk number, length of the file (in sectors), and the index of the starting sector.
```
class FileInfo
{
  int diskNumber;
  int startingSector;
  int fileLength;
}
```
Design Sketch part II: Define class ResourceManager and PrintJobThread and finish off the 141OS.
- A ResourceManager is responsible for allocating a resource like a Disk or a Printer to a specific Thread. Note you will have two instances: one for allocating Disks and another for allocating Printers. The implementation will be something like the following (you may use mine, but be sure you understand what it does and how it works; do you know why these methods must be synchronized?):
```
class ResourceManager
{
  boolean isFree[];
  ResourceManager(int numberOfItems)
  {
    isFree = new boolean[numberOfItems];
    for (int i=0; i<isFree.length; ++i)
      isFree[i] = true;
  }
  synchronized int request()
  {
    while (true)
    {
      for (int i = 0; i < isFree.length; ++i)
        if ( isFree[i] )
        {
          isFree[i] = false;
          return i;
        }
      this.wait(); // block until someone releases a Resource
    }
  }
  synchronized void release( int index )
  {
    isFree[index] = true;
    this.notify(); // let a waiting thread run
  }
}
```
- PrintJobThread handles printing an entire print request. It must get a free printer (blocking if the are all busy) then read sectors from the disk and send to the printer - one at a time. A UserThread will create and start a new PrintJobThread for each print request.
(59 points) Build a GUI that displays the status of the 141OS. I want to see what each user is doing, what each disk is doing and what each printer is doing whenever anything changes. The details of the GUI are left up to you. If you do something impressive, I want to see a personal demo.
Submit: An electronic version of your entire program including the GUI. We will need to run this one to grade it. Also submit a listing of your entire 141OS and its GUI.