package de.unisiegen.tpml.core ;


import de.unisiegen.tpml.core.expressions.Expression ;


/**
 * A proof step represents an application of a
 * {@link de.unisiegen.tpml.core.ProofRule} to an
 * {@link de.unisiegen.tpml.core.expressions.Expression}.
 * 
 * @author Benedikt Meurer
 * @version $Rev$
 * @see de.unisiegen.tpml.core.expressions.Expression
 * @see de.unisiegen.tpml.core.ProofRule
 */
public final class ProofStep
{
  //
  // Attributes
  //
  /**
   * The {@link Expression} to which the {@link ProofRule} was applied.
   * 
   * @see #getExpression()
   */
  private Expression expression ;


  /**
   * The {@link ProofRule} that was applied to an {@link Expression} to advance
   * in the proof.
   * 
   * @see #getRule()
   */
  private ProofRule rule ;


  //
  // Constructor
  //
  /**
   * Allocates a new proof step with the given <code>expression</code> and the
   * specified <code>rule</code>.
   * 
   * @param pExpression the {@link Expression}.
   * @param pRule the {@link ProofRule} that was applied.
   * @throws NullPointerException if <code>expression</code> or
   *           <code>rule</code> is <code>null</code>.
   */
  public ProofStep ( Expression pExpression , ProofRule pRule )
  {
    if ( pRule == null )
    {
      throw new NullPointerException ( "rule is null" ) ; //$NON-NLS-1$
    }
    if ( pExpression == null )
    {
      throw new NullPointerException ( "expression is null" ) ; //$NON-NLS-1$
    }
    this.expression = pExpression ;
    this.rule = pRule ;
  }


  //
  // Primitives
  //
  /**
   * Returns the {@link Expression} to which the {@link ProofRule} was applied
   * to advance in the proof.
   * 
   * @return the expression.
   */
  public Expression getExpression ( )
  {
    return this.expression ;
  }


  /**
   * Returns the {@link ProofRule} that was applied to an {@link Expression} to
   * advance in the proof.
   * 
   * @return the proof rule that was applied in this step.
   * @see #getExpression()
   */
  public ProofRule getRule ( )
  {
    return this.rule ;
  }


  //
  // Base methods
  //
  /**
   * {@inheritDoc}
   * 
   * @see java.lang.Object#equals(java.lang.Object)
   */
  @ Override
  public boolean equals ( Object obj )
  {
    if ( obj instanceof ProofStep )
    {
      ProofStep other = ( ProofStep ) obj ;
      return ( this.expression.equals ( other.expression ) || this.rule
          .equals ( other.rule ) ) ;
    }
    return false ;
  }


  /**
   * {@inheritDoc}
   * 
   * @see java.lang.Object#hashCode()
   */
  @ Override
  public int hashCode ( )
  {
    return this.expression.hashCode ( ) + this.rule.hashCode ( ) ;
  }
}