Permutation Engine

NodeJS javascript library for mapping permutations onto numbers and looping and skipping through permutations.

Installation

Permutation Engine can be installed for Node using npm.

Using npm:

npm install permutation-engine

Example problem

Consider the following table:

hor1, hor2, and hor3 are the sums for each row; ver1, ver2, and ver3 are the sums for each column; dia1 and dia2 are the sums along the diagonals. We would like to arrange the numbers in the table in such a way that:

hor1 = hor2 = hor3 = ver1 = ver2 = ver3 = dia1 = dia2 = 15

The problem is permutational

The original arrangement in the numbers is:

[ 1 2 3 4 5 6 7 8 9 ]

Here are a few other randomly-picked alternatives:

[ 1 2 9 4 3 6 8 7 9 ]
[ 9 2 3 4 6 5 7 8 1 ]
[ 1 2 8 6 5 4 7 3 9 ]

There are 9! i.e. 362 880 different permutations possible. However, only a few of these permutations will satisfy the constraint. In order to find these permutations that satisfy the constraint, we want to:

• be able to enumerate them from 0 to 362 879
• avoid checking all different possibilities

The permutation for a number

The first permutation

Each number between 0 and 362 879 maps to a different permutation.

The first number 0 is mapped to:

[ 1 2 3 4 5 6 7 8 9 ]

We can obtain the first permutation with the function engine.initialPerm():

var permutationEngine=require('permutation-engine');
var engine=new permutationEngine(9); 
var perm=engine.initialPerm();
console.log('the initial permutation is: '+perm);

Output:

the initial permutation is: 1,2,3,4,5,6,7,8,9

The initialPerm() function is equivalent to:

var perm=engine.index2perm(0);

The last permutation

The last number 362 879 is mapped to:

[ 9 8 7 6 5 4 3 2 1 ]

var engine=new permutationEngine(9); 
var perm=engine.lastPerm();
console.log('the last permutation is: '+perm);

Output:

the last permutation is: 9,8,7,6,5,4,3,2,1

Calling the lastPerm() function is equivalent to calling the index2perm() function with the last number:

var perm=engine.index2perm(362879);

Or to calling the index2perm() function with engine.indexCount:

var perm=engine.index2perm(engine.indexCount);

The permutation for any number

There is a permutation for any number between the first (zero) and the last number (n!-1). A permutation P1 is smaller than P2, if by scanning both permutations from left to right, we run into an element that is smaller in P1 than in P2.

For example:

247 [1 2 3 6 4 7 ---5--- 9 8]
248 [1 2 3 6 4 7 ---8--- 5 9]

Since 5 is smaller than 8, the combination with index 247 is smaller than the one with index 248. We can use the engine.index2perm(index) function to retrieve the permutation for a number:

var engine=new permutationEngine(9); 
var perm=engine.index2perm(247);
console.log('index 247 is mapped to: '+perm);
var perm=engine.index2perm(248);
console.log('index 248 is mapped to: '+perm);

Output:

index 247 is mapped to: 1,2,3,6,4,7,5,9,8
index 248 is mapped to: 1,2,3,6,4,7,8,5,9

You can find a full explanation for the factorial number system in Wikipedia.

The number for a permutation

We can also find the number for any particular permutation. For example, if we want the number for the permutation:

[ 1 2 8 6 5 4 7 3 9 ]

we can call the javascript function engine.perm2index(perm):

var engine=new permutationEngine(9); 
var index=engine.perm2index([ 1,2,8,6,5,4,7,3,9 ]);
console.log('permutation [ 1 2 8 6 5 4 7 3 9 ] is mapped to: '+index);

Output:

permutation [ 1 2 8 6 5 4 7 3 9 ] is mapped to: 4016

We can see that it is mapped to index 4016.

The next permutation in a row

Note: We can find the next permutation by looking up its index with index=engine.perm2index(perm) and then increment the index with index++ and then find the permutation for this next index with perm=engine.index2perm(index).

There is also a direct way through the function engine.nextPerm(perm) to find the next permutation for a given permutation:

var engine=new permutationEngine(9); 
var next=engine.nextPerm([ 1,2,8,6,5,4,7,3,9 ]);
var index=engine.perm2index(next);
console.log('the next permutation for [ 1 2 8 6 5 4 7 3 9 ] is : '+next+' with index: '+index);

Output:

the next permutation for [ 1 2 8 6 5 4 7 3 9 ] is : 1,2,8,6,5,4,7,9,3 with index: 4017

Skipping a range of permutations

Imagine we are evaluating the permutation [ 1 2 8 6 5 4 7 3 9 ]. We can see that the sum for [ 1 2 8 ] is not equal to 15. In fact, there is no point in evaluating any permutation that starts with [ 1 2 8 ]. We can use the next=skipForward([1,2,8,6,5,4,7,3,9],3) function call to skip the range with prefix [ 1 2 8 ]. The next permutation will start with the successor prefix for [ 1 2 8 ]; in this case [ 1 2 9 ].

var engine=new permutationEngine(9); 
var next=engine.skipForward([ 1,2,8,6,5,4,7,3,9 ],3);
var index=engine.perm2index(next);
console.log('the next interesting permutation for [ 1 2 8 6 5 4 7 3 9 ] is : 
    '+next+' with index: '+index);

Output:

the next interesting permutation for [ 1 2 8 6 5 4 7 3 9 ] is : 1,2,9,3,4,5,6,7,8 with index: 4320

Without skipping ranges of permutations, a permutational problem is always NP-complete. The potential total number of evaluations is n!. This usually means that the problem cannot be solved for larger dimensions. However, by judiciously skipping entire ranges of permutations, it may be possible to solve a large permutational problem anyway.

The earlier you can detect that a range is invalid, the better. For example, it is better to detect that the following prefix is invalid:

[ 1 2 8 ] [ . . . . . . ]  6!=720 possibilities skipped

than only seeing it later:

[ 1 2 8 6 5 4 ] [ . . . ]  only 3!=6 possibilities skipped 

For example, solving the Travelling salesman problem amounts to discovering a permutation-skipping strategy leaving a number of permutations to evaluate that does not grow factorially with the number of cities.

Solution for the example problem

You can find the complete solution in the file test/test-3x3.js.

var solutions=0;
var evaluations=0;
perm=engine.initialPerm();
 
while(perm!=null)
{
    evaluations++;
 
    //check if the first horizontal block is compliant; skip the entire range, if not.
    if(sum_horizontal_block(perm,1)!=15) { perm=engine.skipForward(perm,3); continue; }
 
    //check if the second horizontal block is compliant; skip the entire range, if not.
    if(sum_horizontal_block(perm,2)!=15) { perm=engine.skipForward(perm,6); continue; }
 
    if(is_solution(perm))
    {
        solutions++;
        console.log('solution:'+solutions);
        print_matrix(perm);
    }
    perm=engine.nextPerm(perm);
}
 
console.log('permutations:'+engine.indexCount);
console.log('evaluated:'+evaluations);
var evaluated_perc=(evaluations/engine.indexCount*100).toFixed(2);
console.log('evaluated perc:'+evaluated_perc+'%');
 

Output:

solution:1
2 7 6
9 5 1
4 3 8
solution:2
2 9 4
7 5 3
6 1 8
...
(There are 8 solutions in total)
...
permutations:362880
evaluated:8376
evaluated perc:2.31%

As you can see, the skipping strategy implemented, brought down the number of permutations to evaluate from 362 880 to 8 376, i.e. to around 2% of the total.

API Summary

License

Copyright (c) 2012 Erik Poupaert.
Licensed under the Library General Public License (LGPL).