package edu.gac.max.mcs388.s2002.compiler_support;

/** The Register class is used to represent a pool from which
 * Registers can be allocated and to which they can then later be
 * deallocated.
 *
 * Some to-do items: <ul> 
 *
 * <li> signal a better exception than array out of bounds for out of
 *      registers
 *
 * <li> signal an exception for deallocating an already free register
 *
 * <li> provide enumerations of free and allocated registers
 *</ul>
 *
 * <p> Written 7 Feb 1997 by
 * <a href="http://www.gac.edu/~max/">Max Hailperin</a>
 * <a href="mailto:max@gac.edu">&lt;max@gac.edu&gt;</a> */

public class RegisterAllocator{
  Register[] pool;
  int inUse;

  /** The RegisterAllocator constructor takes a String prefix argument
   * that is used for constructing each Register.  It also takes an
   * int number argument, which specifies how many registers there
   * will be.  They are numbered consectively from 0.  For example,
   * <code>new RegisterAllocator("t", 10)</code> will allocate
   * registers <code>$t0</code> through <code>$t9</code>. */
  public RegisterAllocator(String prefix, int number){
    pool = new Register[number];
    for(int i = 0; i < number; i++)
      pool[i] = new AllocatorOwnedRegister(prefix, i, this);
    inUse = 0;
  }

  
  public Register allocate(){
    return pool[inUse++];
  }

  public void deallocate(Register r) throws NotMyRegisterException{
    if(!(r instanceof AllocatorOwnedRegister) ||
       !((AllocatorOwnedRegister) r).ownedBy(this)){
      throw new NotMyRegisterException();
    }
    pool[--inUse] = r;
  }

  public void deallocateIfYours(Register r){
    if((r instanceof AllocatorOwnedRegister) &&
       ((AllocatorOwnedRegister) r).ownedBy(this)){
      pool[--inUse] = r;
    }
  }
}

class AllocatorOwnedRegister extends Register{
  RegisterAllocator owningAllocator;

  AllocatorOwnedRegister(String prefix, int num, RegisterAllocator owner){
    super(prefix, num);
    owningAllocator = owner;
  }

  AllocatorOwnedRegister(String name, RegisterAllocator owner){
    super(name);
    owningAllocator = owner;
  }

  boolean ownedBy(RegisterAllocator ra){
    return ra == owningAllocator;
  }
}