macps.java~1~

野人问题的求解

package macps;

import java.util.*;
import java.io.*;

/**
* Missionaries And Cannibals Puzzle Solver.
*
* Solution to the puzzle must not violate any of the following 2 rules:
* 1) Relative headcount: cannibals can't outnumber missionaries at any point.
* 2) Boat's loading: mustn't exceed the max.
*/
public class MACPS {
public class SolutionNotFoundException extends RuntimeException { }

static final Object MISSIONARY = "m", // Simple representation
CANNIBAL = "c", // of objects
BOAT = "v"; // in the puzzle.
private int boat_max_load,
boat_min_load = 1; // Shouldn't be any other value.
private RiverScene firstScene,
finalScene;

// Recursively searches for a solution using Depth-First Search strategy.
// Takes a Stack containing only the first scene (root of tree).
// Returns a collection of scenes connecting the first scene to the final.
// Throws SolutionNotFoundException for obvious reason.
// Deploys the following optimization strategy:
// Transfers as much as possible from source side,
// as little as possible from target side.
private Collection getSolutionSteps( Stack takenSteps ) {
RiverScene lastScene = ( RiverScene ) takenSteps.peek( );
if( lastScene.equals( finalScene ) ) return takenSteps;

RiverScene newStep = lastScene.deepCopy( );

// To allow transfer in both directions to share the same chunk of code.
int start = boat_max_load,
stop = boat_min_load - 1,
step = -1;
RiverSide from = newStep.lside,
to = newStep.rside;
if( to.hasBoat( ) ) {
start = boat_min_load;
stop = boat_max_load + 1;
step = 1;
from = newStep.rside;
to = newStep.lside;
}

for( int nPassenger = start; nPassenger != stop; nPassenger += step ) {
Collection menCombs = new HashSet( // HashSet eliminates duplicates.
Combinatorics.combinations( from.getMenList( ), nPassenger ) );

nextComb:
for( Iterator comb = menCombs.iterator( ); comb.hasNext( ); ) {
Collection menList = ( Collection ) comb.next( );
try {
from.transferMen( to, menList );

// If it's a taken step, undo and try next combination.
for( Iterator i = takenSteps.iterator( ); i.hasNext( ); )
if( i.next( ).equals( newStep ) ) {
to.transferMen( from, menList );
continue nextComb;
}

takenSteps.push( newStep );
return getSolutionSteps( takenSteps );
}
catch( SecurityException e ) {
// Transfer didn't take place. Just try next combination.
}
catch( SolutionNotFoundException e ) {
// New step led to no solution in leaves. Undo, then next.
takenSteps.pop( );
to.transferMen( from, menList );
}
}
}
// All possible steps led to no solution, so
throw new SolutionNotFoundException( );
}

// Do setup, then kick-starts getSolutionSteps( Stack takenSteps ).
public Collection
getSolutionSteps( int nMissionary, int nCannibal, int boatCapacity ) {
if( nMissionary < 0 || nCannibal < 0 || boatCapacity < 0 )
throw new IllegalArgumentException( "Negative argument value." );

RiverSide sourceSide = new RiverSide( nMissionary, nCannibal, true ),
targetSide = new RiverSide( 0, 0, false );
boat_max_load = boatCapacity;
firstScene = new RiverScene( sourceSide, targetSide );
finalScene = new RiverScene( targetSide, sourceSide );

if( firstScene.lside.fatal( ) ) // First scene can be valid but fatal.
throw new SolutionNotFoundException( );

Stack steps = new Stack( );
steps.push( firstScene );
return getSolutionSteps( steps );
}

public static void main( String[ ] args ) {
int nMissionary = 3,
nCannibal = 3,
boatCapacity = 2;

System.out.println(
"\nSolving the puzzle of Missionaries And Cannibals with\n" +
nMissionary + " missionaries and " + nCannibal + " cannibals " +
"and a boat that can take up to " + boatCapacity + " creatures.." );

try {
Collection steps = new MACPS( ).
getSolutionSteps( nMissionary, nCannibal, boatCapacity );
System.out.println( "\nSolution found:\n" );
for( Iterator step = steps.iterator( ); step.hasNext( ); )
System.out.println( step.next( ) + "\n" );
}
catch( SolutionNotFoundException e ) {
System.out.println( "\nNo solution found." );
}
}
}

/**
* Represents a riverside in the puzzle.
*/
class RiverSide implements Serializable {
private ArrayList men = new ArrayList( ),
boat = new ArrayList( );

public RiverSide( int nMissionary, int nCannibal, boolean withBoat ) {
men.addAll( Collections.nCopies( nMissionary, MACPS.MISSIONARY ) );
men.addAll( Collections.nCopies( nCannibal, MACPS.CANNIBAL ) );
Collections.sort( men );
if( withBoat ) boat.add( MACPS.BOAT );
}

public RiverSide deepCopy( ) {
return ( RiverSide ) Copy.deepCopy( this );
}

public Collection getMenList( ) {
return ( Collection ) Copy.deepCopy( men );
}

public boolean equals( Object otherSide ) {
RiverSide other = ( RiverSide ) otherSide;
Collections.sort( men );
Collections.sort( other.men );
return this.men.equals( other.men ) && this.boat.equals( other.boat );
}

public String toString( ) {
return "BOAT" + boat + "\t" + "MEN" + men;
}

public boolean hasBoat( ) {
return ! boat.isEmpty( );
}

// Checks for violation of Rule #1.
public boolean fatal( ) {
int mCount = 0, cCount = 0;
for( Iterator i = men.iterator( ); i.hasNext( ); ) {
Object val = i.next( );
if( val.equals( MACPS.MISSIONARY ) ) ++mCount;
if( val.equals( MACPS.CANNIBAL ) ) ++cCount;
}
return mCount > 0 && mCount < cCount;
}

// Throws SecurityException if the transfer of all men in menList
// from this to destination *will* result in violation of Rule #1.
// Else, executes the transfer.
public void transferMen( RiverSide destination, Collection menList ) {
for( Iterator i = menList.iterator( ); i.hasNext( ); )
destination.men.add( men.remove( men.indexOf( i.next( ) ) ) );

// A nice place to automate boat transfer.
_transferBoat( destination );

// Undo the transfer if it led to violation of Rule #1.
if( fatal( ) || destination.fatal( ) ) {
destination.transferMen( this, menList );
throw new SecurityException( );
}
}

// Tansfers boat from this to destination. Called only by transferMen( ).
private void _transferBoat( RiverSide destination ) {
destination.boat.add( boat.remove( 0 ) );
}
}

/**
* Combines two riversides. Serves mainly as a data object.
*/
class RiverScene implements Serializable {
RiverSide lside, rside; // Package access.

public RiverScene( RiverSide lside, RiverSide rside ) {
this.lside = lside.deepCopy( );
this.rside = rside.deepCopy( );
}

public RiverScene deepCopy( ) {
return ( RiverScene ) Copy.deepCopy( this );
}

public boolean equals( Object otherScene ) {
RiverScene other = ( RiverScene ) otherScene;
return lside.equals( other.lside ) && rside.equals( other.rside );
}

public String toString( ) {
return "Left Side:\t" + lside + "\n" + "Right Side:\t" + rside;
}
}

/**
* Provides a static method to generate combinations of items taken r at a time.
*/
class Combinatorics {
public static Collection combinations( Collection items, int r ) {
if( r == 0 ) // Return [ [ ] ]. Note that [ ] denotes a List.
return Collections.nCopies( 1, new ArrayList( ) );

List copy = new ArrayList( items ), // To enable subListing of items.
result = new ArrayList( );
for( int i = 0; i < copy.size( ); ++i ) {
Collection subCombs =
combinations( copy.subList( i + 1, copy.size( ) ), r - 1 );
for( Iterator iter = subCombs.iterator( ); iter.hasNext( ); ) {
// Assign [ [ items.get( i ) ] ] to subComb.
List subComb = new ArrayList( copy.subList( i, i + 1 ) );

subComb.addAll( ( List ) iter.next( ) );
result.add( subComb );
}
}
return result;
}
}

/**
* Provides a static method to perform deepcopy of object via serialization.
*/
class Copy {
public static Object deepCopy( Object o ) {
try {
ByteArrayOutputStream baos = new ByteArrayOutputStream( );
ObjectOutputStream oos = new ObjectOutputStream( baos );
oos.writeObject( o );
oos.close( );

ObjectInputStream ois =
new ObjectInputStream(
new ByteArrayInputStream( baos.toByteArray( ) ) );
return ois.readObject( );
}
catch ( Exception e ) {
throw new RuntimeException( e );
}
}
}