/*

 * Copyright 2002-2003 Sun Microsystems, Inc.  All Rights Reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

class List<A> {

    /** The first element of the list, supposed to be immutable.

     */

    public A head;

    /** The remainder of the list except for its first element, supposed

     *  to be immutable.

     */

    public List<A> tail;

    /** Construct a list given its head and tail.

     */

    public List(A head, List<A> tail) {

        this.tail = tail;

        this.head = head;

    }

    /** Construct an empty list.

     */

    public List() {

        this(null, null);

    }

    /** Construct a list consisting of given element.

     */

    public static <A> List<A> make(A x1) {

        return new List<A>(x1, new List<A>());

    }

    /** Construct a list consisting of given elements.

     */

    public static <A> List<A> make(A x1, A x2) {

        return new List<A>(x1, new List<A>(x2, new List<A>()));

    }

    /** Construct a list consisting of given elements.

     */

    public static <A> List<A> make(A x1, A x2, A x3) {

        return new List<A>(x1, new List<A>(x2, new List<A>(x3, new List<A>())));

    }

    /** Construct a list consisting all elements of given array.

     */

    public static <A> List<A> make(A[] vec) {

        List<A> xs = new List<A>();

        for (int i = vec.length - 1; i >= 0; i--)

            xs = new List<A>(vec[i], xs);

        return xs;

    }

    /** Construct a list consisting of a given number of identical elements.

     *  @param len    The number of elements in the list.

     *  @param init   The value of each element.

     */

    public static <A> List<A> make(int len, A init) {

        List<A> l = new List<A>();

        for (int i = 0; i < len; i++) l = new List<A>(init, l);

        return l;

    }

    /** Does list have no elements?

     */

    public boolean isEmpty() {

        return tail == null;

    }

    /** Does list have elements?

     */

    public boolean nonEmpty() {

        return tail != null;

    }

    /** Return the number of elements in this list.

     */

    public int length() {

        List<A> l = this;

        int len = 0;

        while (l.tail != null) {

            l = l.tail;

            len++;

        }

        return len;

    }

    /** Prepend given element to front of list, forming and returning

     *  a new list.

     */

    public List<A> prepend(A x) {

        return new List<A>(x, this);

    }

    /** Prepend given list of elements to front of list, forming and returning

     *  a new list.

     */

    public List<A> prependList(List<A> xs) {

        if (this.isEmpty()) return xs;

        else if (xs.isEmpty()) return this;

        else return this.prependList(xs.tail).prepend(xs.head);

    }

    /** Reverse list, forming and returning a new list.

     */

    public List<A> reverse() {

        List<A> rev = new List<A>();

        for (List<A> l = this; l.nonEmpty(); l = l.tail)

            rev = new List<A>(l.head, rev);

        return rev;

    }

    /** Append given element at length, forming and returning

     *  a new list.

     */

    public List<A> append(A x) {

        return make(x).prependList(this);

    }

    /** Append given list at length, forming and returning

     *  a new list.

     */

    public List<A> appendList(List<A> x) {

        return x.prependList(this);

    }

    /** Copy successive elements of this list into given vector until

     *  list is exhausted or end of vector is reached.

     */

    public A[] toArray(A[] vec) {

        int i = 0;

        List<A> l = this;

        while (l.nonEmpty() && i < vec.length) {

            vec[i] = l.head;

            l = l.tail;

            i++;

        }

        return vec;

    }

    /** Form a string listing all elements with given separator character.

     */

    public String toString(String sep) {

        if (isEmpty()) {

            return "";

        } else {

            StringBuffer buf = new StringBuffer();

            buf.append(((Object)head).toString());

            for (List<A> l = tail; l.nonEmpty(); l = l.tail) {

                buf.append(sep);

                buf.append(((Object)l.head).toString());

            }

            return buf.toString();

        }

    }

    /** Form a string listing all elements with comma as the separator character.

     */

    public String toString() {

        return toString(",");

    }

    /** Compute a hash code, overrides Object

     */

    public int hashCode() {

        List<A> l = this;

        int h = 0;

        while (l.tail != null) {

            h = h * 41 + (head != null ? head.hashCode() : 0);

            l = l.tail;

        }

        return h;

    }

    /** Is this list the same as other list?

     */

    public boolean equals(Object other) {

        return other instanceof List && equals(this, (List)other);

    }

    /** Are the two lists the same?

     */

    public static boolean equals(List xs, List ys) {

        while (xs.tail != null && ys.tail != null) {

            if (xs.head == null) {

                if (ys.head != null) return false;

            } else {

                if (!xs.head.equals(ys.head)) return false;

            }

            xs = xs.tail;

            ys = ys.tail;

        }

        return xs.tail == null && ys.tail == null;

    }

    /** Does the list contain the specified element?

     */

    public boolean contains(A x) {

        List<A> l = this;

        while (l.tail != null) {

            if (x == null) {

                if (l.head == null) return true;

            } else {

                if (x.equals(l.head)) return true;

            }

            l = l.tail;

        }

        return false;

    }

}