package com.googlecode.prolog_cafe.lang;

/**
 * Trail stack.<br>
 * The class <code>Trail</code> represents a trail stack.<br>
 * Entries pushed to this trail stack must implement
 * the <code>Undoable</code> interface.
 * @see Undoable
 * @author Mutsunori Banbara (banbara@kobe-u.ac.jp)
 * @author Naoyuki Tamura (tamura@kobe-u.ac.jp)
 * @version 1.0
 */
public final class Trail {
  /** An array of <code>Undoable</code> entries. */
  private Undoable[] buffer;

  /** the top index of this <code>Trail</code>. */
  private int top;

  /** Current timestamp of the top of {@link ChoicePointStack}. */
  long timeStamp;

  /** Constructs a new trail stack. */
  public Trail() {
    this(100);
  }

  /** Constructs a new trail stack with the given size. */
  public Trail(int n) {
    buffer = new Undoable[n];
    top = -1;
  }

  /** Discards all entries. */
  public void init() {
    deleteAll();
  }

  /** Pushes an entry to this <code>Trail</code>. */
  public void push(Undoable t) {
    if (++top >= buffer.length) {
      int len = buffer.length;
      Undoable[] new_buffer = new Undoable[2 * len];
      System.arraycopy(buffer, 0, new_buffer, 0, len);
      buffer = new_buffer;
    }
    buffer[top] = t;
  }

  /** Pops an entry from this <code>Trail</code>. */
  public Undoable pop() {
    Undoable t = buffer[top];
    buffer[top--] = null;
    return t;
  }

  /** Discards all entries. */
  protected void deleteAll() {
    while (!empty()) {
      buffer[top--] = null;
    }
  }

  /** Tests if this stack has no entry. */
  public boolean empty() {
    return top == -1;
  }

  /** Current allocation of the trail storage array.  */
  public int max() {
    return buffer.length;
  }

  /** Returns the value of <code>top</code>.
   * @see #top
   */
  public int top() {
    return top;
  }

  /** Unwinds all entries after the value of <code>i</code>. */
  public void unwind(int i) {
    Undoable t;
    while (top > i) {
      t = pop();
      t.undo();
    }
  }
}