数理解析研究所講究録
第 1644 巻 2009 年 95-109

95

A Two-level Algorithm for Generating Multiset Permutations
Tadao Takaoka
Department of Computer Science, University of Canterbury
Christchurch, New Zealand
Email: tad @cosc.canterbury.ac.nz
Abstract: We present an algorithm that generates multiset permutations in $O(1)$ tiine for each
permutation, that is, by a loop-less algorithm with $O(n)$ extra memory requirement. There alreadyexist several such algorithms that generate multiset permutations in various orders. For multiset
permutations, we combine two loop-less algorithms that are designed in the same principle of tree
traversal. Our order of generation is different from any existing order, and the algorithm issimpler and faster than the previous ones.
1. Introductlon.
time generation for combinatorial objects, such as permutations and combinations, is known
also as loop-less algorithms. That is, given the current object, we generate the next object looplessly, e.g., with a fmite nunber of statements. This is also called combinatorial Gray code, since
the idea is a generalization of binary Gray code to more general combinatorial objects. Ehrich [5]was the first to investigate this topic. Since then many loop-less algorithms were invented forvarious objects, such as permutations, combinations, parenthesis strings, etc. To list up just a few,
see Bitner, et al. [1], Ehrlich [5], Lehmer [11], Eades and McKay [4], Chase [3] for combinations,Jolmson [7] and Heap [6] for pemiutations, Mikawa and Takaoka [12], Vajnovszki [18], andWalsh [20] for parenthesis strings. We focus on the recent topic of multiset permutations in this$O(1)$

paper.

There have been several algorithms that generate multiset permutations in 0(1) time perpermutation. Canfield and Williamson (1995) [2] and Korsh and Lipschutz (1997) [9] were thefirst to investigate this problem. Their algorithms achieves $O(1)$ time, but uses a linked list as acontainer ofpermutations. Thus we can go from (1, 1, 1, 2, 2, 2) to (2, 2, 2, 1, 1, 1) in $O(1)$ timeby pointer manipulations. Since these papers, there was a question whether we can do the workwith only arrays. There were two solutions by Takaoka [16] and Vajnovzski [19]. Both requireO(kn) space apart from the main container ofpermutations, where is the number of distinct
items and is the size ofpermutation. Thus we have a question whether we can do the work with
only $O(n)$ extra space. Again we have two solutions by Korsh and LaFollette (2004) [10] andTakaoka and Violich (2005) [17]. Those algorithms are based on the two-level approach, which
we use in this paper also. In [9], the upper structure is based on Johnson-Trotter algorithm and thelower structure is based on Eades and McKay [4] for combination generation. The upper structure
for the latter method [17] is the same, Jolmson-Trotter, and the lower is based on Chase’s
algorithm for combination generation.
Suppose we have a multi-set $(1,\ldots,1,2,\ldots,2,\ldots\ldots k,\ldots,k)$ , where there are $n[i]$ items of , for$i=1,$. The upper algorithm controls which item to move, and the lower algorithm controls
the movement of those $n[i]$ identical items by regarding those as ones and others as zeros.
In this paper, we use Johnson-Trotter for the upper stmcture and Takaoka’s algorithm [15] for
combination generation for the lower structure, which is less restrictive than Eades-McKay andChase. Both the upper stmcture and lower structure are based on the idea of tree traversal. Thus
our algorithm has a more transparent and consistent design methodology and the program code isshorter than existing ones. To make the problem more visible we start from an example. The
following is the list of permutations of (1, 2, 3, 4) by Johnson-Trotter, which should be readcolumn-wise.
$k$

$n$

$i$

$k$

$\ldots,$

$i$

96
1234
2134

2314

2341

3241
3214
3124
1324

1342
3142
3412
3421

4321
4312
4132
1432

1423
4123
4213
4231

2431
2413
2143
1243

Table 1. Permutations of (1, 2, 3, 4) by Jolmson-Trotter

In this list 1 moves right over (2, 3, 4). Then 2 moves right. Then 1 moves left over (3; 2, 4), etc.,
altemately. Ifwe remove 1 at the top of each column, we have permutations (2, 3, 4), (3, 2, 4), (3,4, 2), (4, 3, 2), (4, 2, 3), (2, 4, 3). This list of permutations has a similar pattem of the movementof 2 going back and forth altemately. Later we show an $O(1)$ time algorithm for Johnson-Trotter.
The next list is the list of combinations of four items, called 4-combinations out of 6 tems {1,2, 3, 4, 5, 6} in in-place expression and binary vector form. The binary vector is for illustration
purposes. In later sections, we generate only in-place forms. In combination generation, we
generate combinations one by one, which we call the forward generation. When we use
combination generation repeatedly as the lower stmcture ofmultiset permutation generation, we
use forward generation and backward generation altemately. Backward generation is to generatecombinations in reverse order of forward generation. This forward generation is ffom [15].
$i$Forward Generation
In-placebinary vector

1234
1235
1236
1246
1245

111100111010111001110101110110

1365
1345
1346
1546
2546
2346
2345
2365
4365

101011
101110101101
1001111265

Backward Generation
in-placebinary vector$\overline{4365}$

2365
2345
2346
2546
1546
1346
1345
1365
1265
1245
1246
1236
1235
1234

110011010111011101011110011011001111001111011011011110011101010111
100111101101
101110101011110011110110110101111001111010111100

Table 2. List of 4-combinations out of six items
We note that there is only one change Rom combination to combination in the in-place list, and in
the binary vector list, a one moves over a block of ones, and does not change the relative order ofzeroes. From 011110 to 011011, for example, the element and element are swapped, andelement, 1. In general, the 1 may go over several consecutive
element, 1, goes over thethe’s in a larger example. Note also that four ones at the left end finally come to the right end of thebinary vector, and vice versa for backward generation. Chase does not have this property. We canmodi the algorithm so that forward generation and backward generation altemate to fit
Johnson-Trotter. The source code is given in Appendix.
Suppose we have a list of multiset, such as (1, 1, 2, 2, 2, 3, 3, 4), we move ’s to the rightusing Takaoka’s algorithm by regarding them as ones and other items as zeroes. When all ’s$4^{th}$$4^{th}$$6^{th}$$5^{th}$$1$$\ddagger y$

$1$

$1$

97
arrive at the right end, that is, (2, 2, 2, 3, 3, 4, 1, 1), we perform one step of the movement of ’s
to the right, and ’s start to move to the left, resulting in (1, 1, 2, 2, 3, 2, 3, 4). That is, themovement of each item in Johnson-Trotter is generalized by the movements of several identicalitems by a combination generation algorithm. In Korsh and LaFollette [10], the combination
algorithm is that of Eades and McKay, and in Takaoka and Violich [17], it was Chase’s algorithm.
$2$

$1$

The following is the complete list ofpermutations of the multiset (1, 1, 2, 2, 3) in this paper.

11223
12123
12213

23211

11322

23121
23112
21132
21312
21321
12321
12312
12132
11232

12231
21231
21213
21123

22113
22131
22311

13122
13212

13221
31221
31212

31122
32112
32121
32211

Table 3. List ofpennutations of (1, 1, 2, 2, 3)

2. General framework for 2-level loop-less algorithms
We have two kinds of 2-level algorithms for combinatorial generation. Let arraybe the maincontainer for the combinatorial objects, which are obtained by combining upper and lower objects.
$a$”Type 1
1. Initialize array ”, the upper and lower objects and other data structures
2. repeat3.
repeat4.
compute the next lower object;5.
make an effect on the object in array
6. until last lower object
7.
re-initialization for lower objects
8. compute the next upper object;
9. until last upper object
$a$

$a$

”

Example [17] Mixed parenthesis strings are described briefly. Upper objects are well-formed
parenthesis strings. Lower objects are the set of binary strings in Gray code. The upper algorithm
takes care of the parenthesis strings, while the lower algorithm carries out the changes ofparentheses and brackets driven by the binary strings in Gray code. The central algorithmic issue
here is the computation ofpositions of changes between parentheses and brackets. At line 7, wecannot afford to spend more than 0(1) time, meaning we need to generate binary strings
repeatedly or reversely.
Lower

Upper

$00$

$()()$$(())$$01$$()[]$$[()]$

1110$[][]$$[[]]$$[]()$$([])$For each parenthesis string, the lower objects carry out necessary changes.

98Type 2.
1. Initialize arrayand other data stmctures. Let be current upper object
2. repeat
3.
compute the next lower object for upper object
4.
make an effect on the object in array5.
if last lower object then begln
6.
re-initialization for lower objects for7.
compute the next upper object and let it be the current
8.
end
9. until last upper object has been processed
$a$

”

$i$

$i$

$a$

”

$i$

$i$

We use type 2 for multiset permutations. Upper objects are permutations (specifically, moving
items) by Johnson-Trotter and the lower objects are combinations of items .
$i$

3. General Form of Loop-less Algorithms by Tree Traversal
We borrow some materials ffom [15] to describe the concept oftree traversal used in this paper.
Let
be an alphabet for combinatorial objects. A combinatorial object is a stringof length such that each is taken ffom and satisfies some property. An order isdefined on with
. Let be the set ofall possible strings on oflength with thelexicographic order . The order on a set of combinatorial objects,, is defined byprojecting the lexicographic order ononto S.
The lexicographic tree, or lexico-tree for short, of is defined in the following way. Each$a\in S$ corresponds to a path from the root to a leaf. The root is
at leve10. If
corresponds to a node at level . We refer to as the label for the node. We sometimes do not
distinguish between node and label. If a and share the same prefix of length , they share thepath oflength in the tree. The children of each node are ordered according to the labels of thechildren. A path from the root to a leaf corresponds to a leaf itself, so a corresponds to a leaf. The
combinatorial objects at the leaves are thus ordered in lexicographic order on S.
The twisted lexico tree of a set of combinatorial objects is defined as follows together withthe parity hnction. We proceed to twist a given lexico-tree ffom the root to leaves in a breadthfirst search manner. Let the parity of the root be even. Suppose we processed up to the i-th level.
If the parity of a node at level is even, we do not twist the branches ffom to its children. If
the parity of is odd, we arrange the children of in reverse order. If we process all nodes atlevel , we give parity to all the nodes at level $i+1$ from first to last altemately starting from even.
We denote the parity of node by parity(v). When we process nodes at level in the following
algorithms, which are children of a node such that parity(v) $=p$ , we say the current parity oflevelis . Note that (labels of) nodes at level are in increasing order if the parity of the parent if even,or equivalently if the current parity of level is even. If the parity is odd, they are in decreasingorder. We draw trees lying horizontally for notational convenience. We refer to the top child of anode as the first child and the bottom as the last child.
If the labels on the paths from the root to two adjacent leaves in the twisted lexico-tree forare different at a fixed number of nodes, we can generate from object to object with the fixed
number of changes. We design an efficient algorithm that traverses the twisted lexico-tree andgenerate combinatorial objects in $O(1)$ time per object. Thus the fixed number of changes is anecessary condition for our algorithm, but not sufficient.
The current parity at level is given by parity $[i]$ , and the procedure ”output” is only to see
result;thefor $O(1)$ time this will be eliminated. We allow $O(n)$ time for initialization, but for the
repeated use of generation camot afford to re-initialize arrays in $O(n)$ time. This will be
addressed in later sections. The parity is for even and 1 for odd. Let $\Sigma=\{0, \ldots,r- 1\}$ . We
associate an object with a leaf ofa tree. The path ffom the root to the leaf identifies the object.
$\Sigma=\{\sigma_{0}, \ldots , \sigma_{r\cdot 1}\}$$n$$a_{1}\ldots a_{n}$$\Sigma$$\Sigma$$a_{i}$$\Sigma^{n}$$\sigma_{i}<\sigma_{i+1}$$<$$\Sigma$$n$$S\subseteq\Sigma^{11}$$<$$\Sigma^{\mathfrak{n}}$$S$$a=a_{1}\ldots a_{n},$$a_{i}$$i$$a_{i}$

$b$

$k$

$k$

$S$

$i$

$v$

$v$

$v$

$v$

$i$

$v$

$i$

$v$

$i$

$i$

$p$

$i$

$S$

$S$

$i$

$0$

99
Traversal from a node to the next sibling node corresponds to generating the next object, making
some changes on the object. We maintain the parity information for each level in array “parity”.
The level of the tree corresponds to various aspects of the objects as we will see in later sections.
A generic form of the algorithm follows. All algoritluns are given in Pascal style pseudo code.
Algorithm 1. Iterative tree traversal in pseudo code. keeps track of the current level of the tree.
$i$

1. initialize array a to be the first object in ;2. initialize
to be nodes on the path to the first object (top path);$S$$v_{1},$

for $i;=0$ to
4. for $i;=0$ to
5. repeat3,$\ldots,$$n$$n$$v_{n}$

do $up[i];=i$ ;do $parity[i]:=0$ ;6. output(a);7. $i:=up[n];up[n]:=n$ ;8. perform changes on a at and related positions;
9. let go to the next node at level based on the current parity;
10.
is the last child of its parent then begin11,
let further go to the next node at level ;12. up $[i];=up[i- 1]$ ; up $[i- 1];=i- 1$ ;13. parity[i]: -parity[i]14. end
15. untll $i=0$ .
$v_{i}$$i$$v_{i}$$ifv_{i}$$i$$v_{i}$$=1$

When we come to the last child of a parent ($w$ in the above figure), we have to update up $[i]$ toup[i-l] so that when we visit the last leaf of the sub-tree rooted at $w$ , we can come back directly

ffom the leaf to $w$ or its ancestor ifw itself is a last child. We refer to the paths from to a andfrom next to as the current path and the opposite path. A current path and opposite pathconsist oflast children and first children respectively except for the left ends. Ifnext in theabove fgure is a last child, we fiirther set to the next node ofnext , say , so we can avoid$0(n)$ time to set up the environment for such ’s later. This is illustrated in the above figure by thepath from “nextto “next object ’ ”. That is, when we traverse down the current path, we
prepare for the opposite path so that we can jump over the opposite path from level to the leaf.
In this situation a and ’ share the same prefix from position 1 to i-l. We call the difference
point. The two strings also share the same suffix (possibly empty). Let the longest such is fromposition j $+1$ to . Then we callj the solution point. Inmitively this is the point where thedifference caused upstream is solved. Algorithms for permutations and for combinations in thispaper are designed along the paradigm in this section.
$v_{i}$$v_{i}$$a$’$v_{i}$$v_{i}$$v_{i}$

$u$

$u$

”

$a$

$v_{i}$

$i$

$a$

$i$

$n$

4. Review of Johnson-Trotter
In the iterative algorithm for Johmson-Trotter, the level of the tree corresponds to the item we aretrying to move. Parity corresponds to the direction of movement, left or right, represented by-l

100
is to hold the position of item . Array element $c[i]$is to count the number of nodes at level and check the last child condition. Procedure “move(x)”is to move item to the direction given by $d[x]$ , and update the positions of affected items. The
comment output shows the point where a new permutation is generated. Arrayplays therole of parity in Algorithm 1. In the program, level variable corresponds to item . If we regard
the position of each item as the label of the node, we have a close correspondence with thetwisted lexico-tree. In the following figure, level numbers are reversed to allow item moves first.
(odd parity) or $+1$ (even parity). Array

”

”

$x$

$p$
$i$

$x$

$*/$

$/*$

$d$

$i$

leve13

leve14”$i$level 1level.2

move(1)move(l)move(l)move(l)move(l)move $(i)$move(l)move(l)move(l)move(1)move(l)move(l)move(l)move(l)move(1)move(l)move(1)move(l)

1234
2134
2314
2341
3241
3214
3124
1324
1342
3142
3412
3421
4321
4312
4132
1432
1423
4123
4213
4231
2431
2413
2143
1243

Figure 1 Tree for Johnson-Trotter
In this figure levels increase from leaves to the root. Variable keeps track of the current level oftree traversal. The action move(x) is attached to each node of the tree. Variable $c[i]$ is called thechild counter, which can tell if we are hitting a last child at the current level. In the following
sections, we say moves actively in the procedure call “move(x)”, and the item neighboring
$i$

$x$

$x$

moves passively by swapping.

Algorithm 2. Iterative algorithm for Johnson-Trotter
procedure move(x);begin var $w$ ;$w;=a[p[x]+d[x]];a[p[x]+d[x]];=x;a[p[x]]:=w$ ;$p[w]:=p[x];p[x]:=p[x]+d[x];c[i]:=c[i]+1;/*$ output
end;

begin {main program}

$*/$

101
for $i:=1$ to do begin$a[i]:=i,$ $up[i];=i;p[i]:=i;c[i]:=1;d[i];=1$ ;
end;
$n$

repeat$i:=up[1]$ ; up $[1]:=1$ ;move(i);if $c[i]=n- i+1$ then begin
currently hitting a last child at level
up $[i];=up[i+1]$ ; up $[i];=i+1;/*$ inherit the up-value of the parent$c[i];=1$ ;$d[i]:=- d[i]$ ;parity (direction) reversed
end
until $i=n$ ;
end.
$/*$

$/*$

$i^{*}/$

$*/$

$*/$

5. The Two Level Approach
Now we combine the loop-less algoritlun in Section 4 for permutations and the combination
generation algorithm described later in the next section in detail. Let us generate permutations ofmultiset $(1,1,\ldots,1,2,2,\ldots,2,\ldots,k,k,\ldots,k)$ . Let $n[i]$ be the number of items in the multiset, that is,
the multiplicity of . Let $n=n[1]+\ldots+n[k]$ , that is, the total size of the multiset.
When items 1 move in the container array $a[1..n]$ , the range ofmovement is 1.. $n]$ , thewhole span of array ”. The range of items 2 is limited to $[1.. n- n[1]]$ when all ls are at the rightend, or range $[n[1]+1..n]$ if all ls are at the left end. If we defme the capsule of 2 as range$[1.. n- n[1]]$ , the latter range of the capsule can be obtained by adding the base, base $[2]=n[1]$ . In
general, we move itemj in the capsule [1.. size $[i]$ ] by combination generation where size $[i]$ is thesize of the capsule. The capsule is activated by performing one step of combination generation,
when all lower items are to the left or to the right, that is, outside of it. Note that when we talkabout movement by swapping two items, we mean active movement of the lower item, notpassive movement of the higher item. Thus items never move actively. The absolute range of acapsule is obtained by adding base $[i]$ to it. The values of size $[i]$ and base $[i]$ for item are defined
as follows:
$i$

$i$

$[$

$a$

$k$

$i$

size $[i]=n[i]+\ldots+n[k],$ $i=1,$$\ldots$, k-lbase $[i]=$ the number of all lower items to the left of any

occurrence of item$i$

The computation ofbase[i] will be explained later. We maintain positions of items by the$q[|]$ ,combination generation algorithm. Suppose the positions of items changed from $(q[1],$..., $q[n[i]])$ to $(q[1], \ldots, q’ lj], \ldots, q[n[i]])$ in the capsule by the combination generation algorithm.
Then we see that an item moves Rom position $qD$ ] to position $q’[|]$ in the capsule of item in themain container array ”. In Algoritlun 4, those values ofand $q’[|]$ are retumed by the call to“combination-server” in global variables “from” and “to”. The values of “from” and “to” areadjusted by adding base $[i]$ to the positions in the range. Let $C(n[i], n)$ be the binomial coefficientof $n[i]$ -combinations out ofn items.
$i$

$i$

$\ldots,$

$i$

$a$

$i$

$q[\int]$

Example. Suppose the first item of 2 moved right from (1,1,1,3,2,2,3) to (1, 1, 1,3,3,2,2). Since $2s$
are moving right globally, the positions of $2s$ before and after the move are 5, $6)=3+(2,3)$ and
7, $6)=3+(4,3)$ , modified by adding the base 3 to 2-combinations, (2,3) and (4,3) out of4 items{1, , 4}. Thus 2 moves from position 5 to 7.
$($

$($

$\ldots$

In the following, the work “maintain combinations” includes ”retum the next combination”. For
readability, computation ofbase $[i]$ is omitted in the program, and explained in the text.

102
Algorithm 3. Multiset Permutation Generation
Lower Level
This seiver performs the following operations on demand
Combination-server
begin
Initialize $n[1]$ -combinations out of size[l] items
Initialize $n[2]$ -combinations out of size[2] items
$/*$

$*/$

Initialize $n[k- 1]$ -combinations out of size[k-l] items
Maintain $n[1]$ -combinations out of size[l] items
Maintain $n[2]$ -combinations out of size[2] items

Maintain $n[k- 1]$ -combinations out of size[k-l] items
endUpper Level
procedure move(x);begIn var $w$ ;$c[i];=c[i]+1$ ;Let the change of combination be from “from” to “to”;
from: $=ffom+$ base $[i]$ ; to: $\neg-0+$ base $[i]$ ;$w:=a[from];a[from]:=a[to];a[to]:=w;/*$ output

$*/$

end;

begin {main program}for $i:=1$ to k-l do call combination-server tomitialize $n[i]$ -combinations out of size $[i]$ items;

for$i:=1$

to$k$

do begin$a[i]:=i,$ $up[i]:=i;p[i]:=i;c[i]:=1;d[i]:=- 1$ ;end;repeat$i:=up[1];up[1];=1$ ;call combination-server for next combination for item ;if $i<k$ then move(i);if $c[i]=C(n[i], n)$ then begin checking a last child
Re-initialize combinations of item for reverse direction;
up $[i]:=up[i+1]$ ; up $[i]:=i+1$ ;$c[i]:=1$ ;$d[i];=- d[i]$ ;
$i$

$*/$

$/*$

$i$

end

until $i=n$ ;end.

Re-initialization of combinations of item must be done in $O(1)$ time. This will be mentioned in
the next section The value of $C(n[i], n)$ is potentially large, and may not be contained in a single
precision variable. We can use the termination condition in the Algorithm 4 $(i=0)$ in the appendix,
and bring the effect to the calling site. As initialization for combinations of item takes $O(n[i])$time, Algorithm 3 takes $O(n)$ time at the beginning. As the size of an object of $n[i]$ -combinations
is $O(n[i])$ , the total space requirement is $O(n)$ . In the appendix we declare fixed-sized arrays forreadability. It is straightforward to allocate necessary space dynamically using the ”calloc”
hnction in C.
. Let theNow we explain how to compute base $[i]$ . Let $l=n[i]$ occurrences of item be$i_{m}(1\leqq
m\leqql)$be. We defme LEFT $[i]$number of items lower than occurring to the left of
$i$

$i$

$i$

$i_{1},$$i_{l}$$\ldots,$$i$$N(i_{m})$103
as follows: LEFT $[i]=N(i_{1})+\ldots+N(i_{l})$. Then base $[i]$ can be computed as base $[i]=$ LEFT $[i]/n[i]$ .
LEFT $[i]$ is initialized to $n[i]^{*}(n[1]+\ldots+n[i- 1])$. When item and itemj are swapped in “move”
where $i=a[from]$ and j $=a[to]$ , LEFT is updated by LEFT $[|]=LEFT[|]+$ (from-to).
$i$

$[|]$

Example. (1,1,3,2,2,3,4) changes to (1,1,3,3,2,2,4). Since from$=4$ and to $=6$ , LEFT[3] changes
from 6 to 4. This is because item 3 goes left over two of items 2. Note that LEFT[2] does notchange as LEFT is concemed with lower items.

6. Review of Takaoka’s algorithm for in-place combination generation.

We describe an 0(1) time algorithm for generating the set of combinations ofn elements out of
the set 1,. In the following array is the main container of combinations and is an
auxiliary array for book-keeping; it keeps track oftree traversal with some delay. We state a few
lemmas describing necessary properties for our multi-set permutation generation.
$S$

$\{$

$\ldots,$

”

$r\}$

$q$

$a$

”

Example The twisted lexico tree for combinations of4 elements out of 6 elements is given withthe contents of arrayand ” at the leaves. A white circle is for even parity and black for odd.
$a$

level

$0$

”

$q$

level 1

leve12

leve13

leve14 array a

array

1235

1235

1236

1236

1246

1246

1245

1245

1256

1265

1356

1365

1345

1345

1346

1346

1456

1546

2456

2546

2346

2346

2345

2345

2356

2365

3456

4365

1234

$q$

1234

Lemma 1. The set generated by the twisted lexico-tree generates combinations with one
change per combination. See [14] for a proof.
$S$

104
Now we describe implementation details. There are many nodes which have only one childdown to the leaf, causing straight lines. Traversal of these lines downwards will cause $O(n)$ time.
Having one child is caused by a node with the maximum possible label at the level. To control the
position to which we come down, we keep an array “down”. In the above example, we use “up”
to go from A to , and use “down” to go from to D. A snapshot of the movement is like (...,
F,A, $B,C_{2}D,E,$. As we see below, we cannot generate combinations in labels attached to thenodes of the twisted lexico-tree with a fixed number of changes. Let array a contain those labels.
Then we have a situation with
$B$

$C$

$\ldots)$$a=(a_{1}, \ldots,a_{i\cdot 1}, a_{i}, \ldots,a_{j}, \ldots,a_{n})$and $a’=(a_{1}, \ldots,a_{i- 1}, a_{i}+1, \ldots,a_{j}+1\ldots,a_{n})$,where $a_{k+1}=a_{k}+1$ for $i\leq k<j$ , or a symmetric case where,
where. Note that is the difference point and is the solution point. In the fust case,goes out of the combination andcomes into the combination like a revolving door
system. We keep the combinations in arrayand use arrayfor book-keeping. As we cannot”$i+1$changejplaces inin 0(1) time, we just change and , and change the contents ofcorrespondingly. Thus we do not implement line 11 of Algorithm 2 to prepare for the opposite
path to the next object. To keep track of the positions of these elements in , we use array”pos”.
Solution points are maintained in array “solve”. Since some $paMs$ of arrayare not maintained
to the corresponding labels in the tree, we use Boolean array mark” to show no maintenance.
When mark $[i]=mle$ , the proper value of a[i-l] is given by its child $a[i]$ . The main part of thealgorithm follows in which up $[i]$ , down $[i]$ , solve $[i]$ , and pos $[i]$ are initialized to for all . The
values of $d[i]$ are initialized to 1 and those of “mark” to false. Line-by-line explanation follows
the algorithm. If we change the termination condition at line 23 to false we can keep generating
combinations in forward order and reverse order alternately for ever. The proof of the next lemma
is omitted.
$a’=(a_{1}, \ldots,a_{i- 1}, a_{i^{-}}1, \ldots,a_{j}- 1, \ldots,a_{n})$$a_{k+1}=a_{k}- 1$

$i$

”

$j$

”

$a_{j}+1$$a_{i}$

”

”

”

$a$

$q$

$a$

$a_{i}$”$q$$a_{j}$

$q$

$a$

”

$i$

$i$

Lemma 2. The generation of combinations in the binary vector form by the above twisted lexicotree does not change the relative order of $0’ s$ . Items 1 move from the left end and finish at theright end for one mn in the binary vector. From the second mn on, the movement altemates in
direction.
To continue to generate combinations in reverse order, we can simply perform $i;=down[i]$ ,and repeat from line 3. In the source list in the Appendix, we use Algorithm 4 as “combinationserver”, where all variables are indexed by item. Thus simple variables become one-dimensional
arrays and one-dimensional arrays become two-dimensional arrays. Re-initialization in Algorithm
3 can be implemented by performing $i[I]=down[I][i[I]]$ for item I. Also “checking a last child”
can be implemented by testing $i[I]=0$ ”. In the program, Algorithm 3 and Algorithm 4communicate through global variables, which make the algorithm stmcmre less transparent, but
efficiency is gained. When Algorithm 4 is used as ”combination-server”, line 2 and 23 will be
removed.
, up, pos, sol, solve, mark used}
Algorithm 4. In-place algorithm for combinations {arrays$d[i]$
to 1, and other arrays to for all $i;d[0];=0$ ;1. $d[n+1]:=- 1$ ; up $[0]:=0;i:=n$ ; initialize
2. repeat
This is to output
3. output(q);4. if $mark[i]=true$ then begin a[i-l]: $=a[i]- 1;mark[i]:=false$ end;
Moving ffom an even node to an odd node
5. if $d[i+1]<0$ then begin$q[pos[a[i]]]:=a[i]+d[i]$ ; pos $[a[i]+d[i]]:=pos[a[i]]$6.
7. end else begin
value increasing
Moving from odd to even, with
8. if $d[i]>0$ then$a,$ $q,$

$d$

$i$

$/*$

$q^{*}/$

$*/$

$/*$

$/*$

$a$

”

$*/$

105
9.
10.
11.
12.
13.
14.
15.
16.
17.

$q[pos[a[i]]]:=a[solve[i]]+d[i]$ ; pos $[a[solve[i]]+d[i]]:=pos[a[i]]$end else If $d[i]<0$ then begin
Moving from odd to even, withq[pos[a[solve[i]]]]: $=a[i]+d[i]$ ; pos $[a[i]+d[i]];_{-)}7P[a[solve[i]]]$end;$/\star$$a$”value decreasing$*/$$a[i]:=a[i]+d[i]$ ;if $d[i+1]>0$ then [solve $[i]$ ] $;=a[solve[i]]+d[i]$ ;$up[i];=i$ ;if $(d[i]>0)$ and $(a[i]=r- n+i)$ or $(d[i]<0)$ and $(a[i]=a[i- 1]+1)$ then begin
up $[i]:=up[i- 1]$ ; up $[i- 1]:=i- 1$ ;18.
down[up[i]]: ;19.
if $d[i]<0$ then begin solve[[up[i]]: $=i;mark[i]:=true$ end;
$d[i]:=- d[i]$ ;20.
21.
if $(d[i]<0)$ or $(i=n)$ then $i:=up[i]$ else $i;=down[i]$22. end else $i;=down[i]$23. until $i=0$ ; {This termination condition is replaced by false for an infinite loop}
$a$

$=i$

24. output(q).

Line 4: Update the parent when maintenance is not done.
Lines 5-12: Update the contents of .
Lines 5-7: Note that $d[;+1]$ was altered in a previous step. Thus we regard parity
as evendespite $d[i+1]<0$ . Move from to with $i=1$ for example in the figure.
Lines 8-10: Siinilarly move from to A with $i=2$ for example.
Lines 10-12: Similarly move from to with $i=2$ for example.
Line 13: Take the next child.
Line 14: Change the value of a at the solution point.
Line 15: Originate the destination for descendants to come up to. This value will possibly
propagate down in line 17.
Line 16: Check if the node is a last child.
Line 17: Compute the correct value of up”.
Line 18: Let the ancestor know where to come down next.
Line 19: Ifparity is odd, update the solution point for ancestor by . This may further be updatedby larger . Also signal that the parent a[i-l] is not updated for the next object. If up $[i]arrow- 1,$
has been given a proper value of $a[i]- 1$ , but signaling $mark[i]=tme$ will not cause any harm.
Line 20: Change the parity at .
Line 21: Go up ifyou hit a last child with even parity or at a leaf Otherwise go down.
Line 22: Go down ifyou do not hit a last child.
Line 23: Go out of iteration if you come to the root, or go back to line 2 for repeated generation.
$q$$(a_{i})$

$B$

$C$

$F$

$D$

$E$

$i$

$i$

$a[iarrow 1]$”$i$

7. Concluding Remarks
We showed how to design a loop-less algoritlun for multiset permutation generation based on a
two-level approach. To avoid complications, we used an abstract algorithm, called “combination
server”, which delivers combinations of various sizes one by one at the request of the upper
algorithm of Johnson-Trotter. If we implement our algorithm completely in a procedural language,
we can use two-dimensional arrays, another dimension corresponding to which kind of items are
moving. The source code in Appendix is based on this approach by two dimensional arrays. Notethat with this approach, memory requirement can be $O(n)$ , where is the total size of each
permutation. Precisely speaking, we would need to maintain eight arrays of size $n[i]$ for each item
. Those arrays need to be maintained by pointers for dynamic memory allocation, although themain container ofpermutations remains to be a fixed array of size . In the upper structure of thealgorithm we need eight arrays of size where is the number of distinct items. In [10], 20
$n$

$i$

$n$

$k$

$k$

106
arrays of various sizes are used and in [17] an array of size and nine arrays of size are used. It
the algorithm or reduce memory requirement.
remains to be seen whether we can mrther
It is hoped that the multi-level approach can be extended to the $O(1)$ time generation of othercomplicated combinatorial objects.
$n$

$k$

$Simpl1\mathfrak{h}^{r}$

References[1] Bitner, J.R., G. Ehrlich and E.M. Reingold, “Effcient Generation of Binary Reflected Gray
Code and its Applications,” CACM 19 (1976) pp. 517-521.
[2] Canfield, E. R., and S. G. Williamson, A loop-free algorithm for generating the linear

extensions of a poset, Order 12 (1995) 57-75[3] Chase, P. J., Combinatorial generation and graylex ordering, Congressus Numerantium, 69(1989) 215-242[4] Eades, P and McKay, B, An algorithm for generating subsets of fixed size with a strong
minimal change property, Info. Proc. Lett., 19 (1984) 131-133.
[5] Ehrlich, G., Loopless algorithms for generating permutations, combinations, and othercombinatorial configurations, JACM, 20 (1973) 500-513.
[6] Heap, B.R., “Pennutations by Interchanges,” Computer Joumal 6 (1963) pp. 293-294[7] Johnson, S.M., “Generation of Permutations by Transpositions,” Math. Comp. 15(1963)282-

285.

[8] Joichi, J.T,, D.E. White, and S.G. Williamson, Combinatorial Gray codes, SIAM Jour.
Comput., 9 (1980) 130-141.
[9] Korsh, J and S. Lipschutz, Generating Multiset Permutations in Constant Time,” Jour.
Algorithms, 25 (1997) pp. 321-335.
[10] Korsh, J and LaFollette, P. S., Loopless array generation of multiset permutations, The
Computer Joumal 47 (5) $(2004)612- 621$
[11] Lehmer, D.H., “The Machine Tools of Combinatorics,” in Applied Combinatorial
Mathematics (E.F. Beckenbach Ed.), Wiley, New York (1964) pp. 5-31.
[12] Mikawa, K and Takaoka, T, ”Generation of Parenthesis Strings by transpositions,” Proc. The
Computing: The Australasian Theory Symposium (CATS ‘97) (1997) 51-58.
[13] Ruskey, F. and Savage, C., A Gray code for combinations of a multi-set, European Jour.
combinatorics, 68 (1996) 1-8.
[14] Savage, C., A survey of combinatorial Gray codes, SIAM Review, 39 (1997) 605-629.
[15] Takaoka, T, $O(1)$ Time Algorithms for Combinatorial Generation by Tree Traversal,
Computer Journa142, 5 (1999) 400-408[16] Takaoka, T., An $O(1)$ Time Algorithm for Generating Multiset Permutations, ISAAC 1999,

LNCS 1741 $(1999)237- 246$ .
[17] Takaoka, T. and Violich, S., Fusing Loopless Algorithms for Combinatorial Generation,
International Joumal of Foundations of Computer Science, Vo118, no 2 $(2007)263- 293$ .
[18] Vajnovszki, V., On the loopless generation of binary tree sequences, Info. Proc. Lett., 68(1998) $113- 117$[19] Vajnovszki, V., A loopless algorithm for generating the permutations of a multiset,
Theoretical Computer Science, 307 (2) $(2003)415A31$[20] Walsh, T.R., Generation of well-formed parenthesis strings in constant worst-case time, Jour.
Algorithms, 29, (1998) 165-173[21] Wilf, H.S., Combinatorial Algorithms: An Update, SIAM, Philadelphia, 1989.
[22] Zerling, D, Generating binary trees by rotations, JACM, 32, (1985) 694-701

107
Appendix. Program Source List in$C$

For the readability this program is based on two-dimensional arrays for local variables for
combination generation. Available at http://WWW.cosc.canterbury.ac.nz/tad.takaoka/muitiperm.c//up to 10 tems. up to multipl icity of 20//array Ms the main container of multiset permutati ons

nt ttint 1, K.

.

A[101, $P[10]$ $D[10]$ , UP [10]1
int , count; nt countk, init, COUNT, rom, to;
int $M[40]$ , base [401, imit [40]; $/*$ array “ 1 imit” is “size”
nt a[10][20], $q[10][20]$ , up $[10][201$ solve $[10][20]$$d[10][20]$ , pos [10] [20], mark [10] [201, down [10] [20],$i$$N[10]$ ,

$R$

$f$

$i$

$|$

.

’

$|$

.

in the text $*/$$i[10]$ ;nt truefalse
init-baseO {int
$|I$

$=1$

,

$i$

base $[1]=0$ ;

$=0$

.

,
$k$

;

//base $[i]$ is LEFT $[i]$ in the text
for $(k=2;k\langle=K;k++)$ base [kl $=base[k-1]+N[k-1]$ ,$//|$ imit $[k]$ si ze of the world forfor $(k=1;k\langle=K;k++)|$ imit $[k]=R$-base $[k]$ ;$(k=1:k\langle=K;k++)$forbase $[k]–base[k]*N[k]://$ base $[k]$ is multipl ied by $N[k]$

$k$

$\}$

OUTO {int
for

$i$

;

$(i=1;i\langle=R;i++)$pri ntf (”base$\prime\prime$

pri ntf $(”*d$ ”, $M[i])$ ;);for $(i=1, i\langle=K;i++)$ printf $(”*d$ ”, base $[i])$ ;pri ntf (”COUNT %d n”, COUNT) ;$=$$\backslash$$\}$;from$=from+base[1]/N[I];to=to+base[1]/N[1]$ ;MOVEO {int$w$$//no$

base [ [from]] $=base[M[from]]+from-to$ ;$COUNT++$ ;
OUTO;
$M$

$main(\{int]k,$
$j$

$(”$

$i\langle 2$$*$$////3321122$$[2]=6-2=4$;: number of kinds of tems ”);
intf lnput
scanf (”%d’’, &K); getchar ; //no of di st nct tems
printf (” lnput multipl ici ties of each kind of tems ”):
for $(1=1;|\langle=K;|++)$ scanf ( %d , &N [1]);
$pr$

. of

//3 311222////
base[2] $=2*3=6$$w\vec{-}M[from]:M[from]=M[to]:M[to]--w$ ,

base

$K$

$()$

$\acute\acute$

$i$

$i$

$\acute\acute$getchar :; for $(1=1;|\langle=K:1++)R=R+N[1];//$ Total size$0$

$R_{-}^{-}0$$//M$

is the main contai ner of permutati ons.

$tt=clock()$ ;$init=1$ ,for countk ; $countk\langle=K;countk++)$ {for $(1=$ init $;|\langle=init+N[countk]-1;|++)$ {
$($

$=1$

$M[1]=countk$ ;

$\}$

1

init$=init+N$ [countk] ;pri ntf $(”M=”)$ ;$M$

is initial ized to $(1_{I}1,1,2,2,3,3\ldots)$ etc.

108
for $(1=1;1\langle=R;1++)$ printf $(”\%d ", M[1])$ ; printf $(”*n”)$ ;init-base ;$()$for $(1=1 ; 1 \langle=K-1;|++)$ {combi-init (1, $N[1]$ ,mit [1]);}upper structure $***/$
for $(1=1;1$ く $=K; I++)\{A[1]=1 ; D[|]=1 ; P[1]=1;UP[1]=1;\}$
COUNT ; OUT ;
do {
$|$

$/***Johnson$ -Trotter
$=1$

$0$

$1=UP[1]$ ;UP $[1]=1$ ;comb (1, $N[1]$ , I imit [1]);ifMOVE (1);$i$$(|\langle K)$$if(i[11==0)\{$$i[1]=down[1][i[1]]$ ;UP[I] $=UP[1+1]$ ;$D[|]=-D[|]$ ;UP $[1+1]=1+1$ ;$\}$wh le $(1 !=K)$ ;pri ntf $(”ti$ me %d n”, clock $()-tt)$ ;
$i$

$\}$

$\backslash$$\}$lnitial izati on for combi $***/$
combi $arrow init$ (int , int , int ){ int ;
for $(i=1;i\langle=n:i++)\{a[t][i]=i,$ $q[t][1]=i;d[tl[i]=1,$$/***$

$t$

$n$

$r$

$\{$

$i$

$\}$

for $(i=0;i\langle=n;i++)\{up[t][i]=i$ ; down $[t][i]=i$ ;pos $[t][i]=i$ ; solve $[t][i]=1$ ;$\}$1$d[t][n+1]=-1$ ; up $[t][0]=0;i[t]=n$ ;$\}$Combi nation generati on server $***/$
combi (int , int , int r) $\{//t$ for item
1 nt 1 ;$/***$$t$$n$$ii=|’[t]$ ,if mark $[t][ii]=true)$
a $[t][ii-1]=a[t][i$ il-l;
mark $[t][ii]=false$ ;if $(d[t][ii+1]\langle 0)$ {from$=q[t]$ pos $[t]$ [a $[t][ii]]]$ :$q[t]$ pos $[t]$ [a $[t][ii]]]$
$($

$[$

$\}$

$[$

$[$

$=a[t][|i]+d[t][ii]$ ;$to=q[t]$ [pos tl [a $[t][ii]]]$ :pos $[t]$ [a $[tl [ii]+d[t][ii]]=pos[t]$ $[a [t][ii]]$
$[$

$\}$

else

:

if $(d[t][ii]\rangle 0)$ {$from=q[t]$ [pos $[t]$ [a $[t][ii]11$ ;[tl pos $[t]$ [a $[t][ii]]]=a[t]$ [solve $[t][iil]+d$
$q$

$[$

pos $[t]$ [a $[t][ii]]]$ ;
$[t]$
pos
a $[t]$ [solve $[t][ii]]+d[t][ii]]=$$to=q[t]$

$[$

$[$

pos $[t]$ [a tl $[ii]]$
else if$[$$\}$$(ii\rangle 0)\{$$from=q[t]$ [pos $[t]$ [a $[t]$ [solve $[t][ii]]]$ ];

$[$

tl

$[i$

il;

109
$q[t]$ [pos[solve $[t][i$ ]]] $]=a[t][i|]+d[t][ii]$ ;$to=q[t]$ [pos $[t]$ [a $[t]$ [solve $[t][ii]]]$ ];$[t][a\underline{\lceil}t]$pos [ $tl$ $[a [t][ii]+d[t][ii]]=pos[t]$ [a $[t]$ [solve [tl $[ii]]$ ];$\}$a $[t][ii]=a$ [tl $[ii]+d[t][ii]$ ;if $(d[t][ii+1]\rangle 0)$ a $[t]$ [solve $[t][|’|’]$ ] $=a[t]$ [solve $[t][ii]$ ] $+d[t][ii]$ ;up $[t][ii]=ii$ ;if $(((d[t][ii]\rangle 0) \ \ (a [t][ii]==r-n+ii))||$$((d[t][ii] \langle 0) \ \ (a [t][ii]==a[tl[iiarrow 1]+1)))$

up $[t][ii]=$
up $[t][ii-1]$ ; up [tl[ii-l] $=ii-1$ ,down $[t]$ $[up [t][ii]]=ii$ ,if $(d[t][i ] \langle 0)$ {solve $[t]$ $[up [t][ii]]=$ ii ;mark $[t][ii]=true:\}$$d[t][ii]=-d[t][ii]$ :

$\}$

$\}$

if $((d[t][ii]\langle 0)||(ii-=n))$$i[t]=up[t][ii]$ ; elsei $[t]=down[t][ii]$ ,else $|r[t]=down[tl[ii]$ ,{