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Discrete Optimization
A skyline heuristic for the 2D rectangular packing and strip packing problems
Lijun Wei
a
, Wee-Chong Oon
a
, Wenbin Zhu
b,c,
⇑
, Andrew Lim
a
a
Department of Management Sciences, City University of Hong Kong, Tat Chee Ave, Kowloon Tong, Hong Kong
b
Department of Computer Science, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
c
Department of Logistics and Maritime Studies, Faculty of Business, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
article info
Article history:
Received 20 January 2011
Accepted 14 June 2011
Available online 25 June 2011
Keywords:
Cutting and packing
Heuristics
Tabu search
abstract
In this paper, we propose a greedy heuristic for the 2D rectangular packing problem (2DRP) that repre-
sents packings using a skyline; the use of this heuristic in a simple tabu search approach outperforms the
best existing approach for the 2DRP on benchmark test cases. We then make use of this 2DRP approach as
a subroutine in an ‘‘iterative doubling’’ binary search on the height of the packing to solve the 2D rect-
angular strip packing problem (2DSP). This approach outperforms all existing approaches on standard
benchmark test cases for the 2DSP.
Ó2011 Elsevier B.V. All rights reserved.
1. Introduction
The 2D rectangular packing problem (2DRP) is a fundamental
problem in cutting and packing literature. We are given a set of n
rectangles with dimensions w
i
h
i
,i=1,...,n. The task is to
orthogonally pack the rectangles without overlap into a large rect-
angle of dimensions WH(which we call the sheet) such that the
total area of the packed rectangles is maximized. All dimensions
are assumed to be integral. We consider two variants of the prob-
lem: the fixed orientation variant does not allow the rotation of
rectangles, while the rotatable variant allows the rectangles to be
rotated by 90°.
Another important problem in cutting and packing, called the
2D strip packing problem (2DSP), is closely related to the 2DRP.
Given a set of nrectangles, the task is to pack all the rectangles into
a rectangular strip of width Wso as to minimize the height Hof the
packing. One possible algorithmic solution to the 2DSP is as fol-
lows: set a height Hand solve the 2DRP for a sheet of dimensions
WH. If all rectangles can be placed, then a 2DSP solution with
height Hhas been found, and the procedure can be repeated after
decreasing H; otherwise, increase H. This idea has previously been
proposed by Oliveira and Ferreira (1993) and Dowsland (1993).
There are two main contributions in this paper. Firstly, we pro-
pose a greedy 2DRP heuristic that places each rectangle according
to an evaluation function involving several components, all of
which are motivated by observations on the nature of 2D packing
problems. We show that by simply using this heuristic on a
number of sequences determined by a tabu search approach, we
are able to outperform the best existing approach to the 2DRP on
benchmark test instances. Secondly, we incorporate this 2DRP
solution as a subroutine in an ‘‘iterative doubling’’ binary search
on the height of the sheet to solve the 2DSP, which is more efficient
than the previously proposed approaches by Oliveira and Ferreira
(1993) and Dowsland (1993). This approach outperforms all exist-
ing approaches on the 2DSP.
The rest of the paper is organized as follows. In Section 2,we
provide a review of the relevant literature on the 2DRP and
2DSP. We present our greedy heuristic for the 2DRP in Section 3,
which is employed in the tabu search procedure given in Section
4. We then describe in Section 5how this procedure is used as a
subroutine in an iterative doubling binary search algorithm to
solve the 2DSP. Our computational experiments for both the
2DRP and 2DSP are reported in Section 6. Finally, we conclude
our article in Section 7with some closing remarks.
2. Literature review
There are many practical cutting and packing problems in
industry, giving rise to many variants. These variants can be classi-
fied according to several attributes. A non-exhaustive list includes
(1) by dimensionality: the rectangles to be packed are usually either
two-dimensional or three-dimensional (although the problem can
be generalized to higher dimensions). The 2D variants naturally
model the cutting of a sheet of stock into smaller parts, while the
3D variants reflect the packing of items into a container; (2) by
shape: the majority of existing work assumes the items to be
packed are rectangular. The study of 2D packing with non-rectan-
gular items is useful in applications like the cutting of bolts of cloth
0377-2217/$ - see front matter Ó2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ejor.2011.06.022
⇑
Corresponding author at: Department of Computer Science, Hong Kong
University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Tel.: +852 64067667; fax: +852 34420188.
E-mail address: i@zhuwb.com (W. Zhu).
European Journal of Operational Research 215 (2011) 337–346
Contents lists available at ScienceDirect
European Journal of Operational Research
journal homepage: www.elsevier.com/locate/ejor
into material for clothing; (3) by rotatability: the problem may al-
low or disallow the orthogonal rotation of items. Rotation may be
disallowed for applications where the sheet is patterned (such as
patterned cloth or wooden planks with grain patterns), or the posi-
tioning of advertisements on a newspaper page; (4) by guillotina-
bility: many applications require that the solution fulfills some
type of guillotine-cut constraint, where the resultant pattern must
be producible by a series of cuts parallel to the axes. Examples in-
clude the cutting of brittle glass sheets and the use of machinery
that performs only guillotine cuts; (5) by objective: the problem
can be generalized so that the objective is to maximize the total va-
lue of the placed rectangles, and the value of each rectangle does
not necessarily correspond to its area.
Our research focuses on the 2D case where all items are rectan-
gles and the objective is to maximize the area utilization. We do
not handle the guillotine-cut constraint, but we do consider rotat-
ability. Under the improved typology of cutting and packing prob-
lems by Wäscher et al. (2007), the 2DRP is a two-dimensional
rectangular single large object placement problem (2D-SLOPP),
while the 2DSP is a two-dimensional open dimension problem
(2D-ODP). We review only the relevant work of this type in the
remainder of this section.
Both the 2DRP and 2DSP are NP-hard in the strong sense.
Although some exact algorithms have been proposed for the
2DRP (Beasley, 1985b; Lesh et al., 2004) and the 2DSP (Hifi,
1998; Martello et al., 2003; Kenmochi et al., 2009), these problems
usually require the use of heuristics in practice to provide a good
solution in reasonable time.
Various heuristic algorithms based on different methodologies
have been presented for solving these problems. The bottom-left
(BL) method by Baker et al. (1980) places each rectangle sequen-
tially by pushing it downwards as much as possible, then leftwards
as much as possible. A genetic algorithm based on BL was later de-
vised by Liu and Teng (1999). The bottom-left fill (BLF) method by
Chazelle (1983) generalized the BL concept by placing each rectan-
gle in its bottom-leftmost position. Both BL and BLF placed the
rectangles in sequence according to decreasing area. Hopper and
Turton (2001) proposed executing the BL heuristic on a number
of different sequences and selecting the best result, an approach
subsequently called bottom-left-decreasing (BLD).
Instead of prioritizing bottom-left placement, another common
approach is to select both the rectangle and its position based on
some measure of best fit (i.e., a greedy heuristic). Burke et al.
(2004) suggested one such measure (BF) involving a number of pri-
ority rules; an efficient implementation for BF was later presented
by Imahori and Yagiura (2010) along with an analysis of its worst-
case approximation ratio. Wu et al. (2002) introduced another
measure called Less Flexibility First. More recently, Asßık and Özcan
(2009) presented a novel bidirectional best-fit heuristic.
Several metaheuristic approaches have also been tried. For the
2DRP, Huang et al. (2007) devised a backtracking procedure that
locates good ‘‘corner-occupying actions’’, which are rectangle
placements evaluated by the notion of caving degree. Gonçalves
(2007) later proposed a hybrid genetic algorithm based on random
keys. The Least Wasted First (LWF) strategy was utilized by Wei
et al. (2009) in a randomized random local search for this problem.
The best approach on benchmark test data to date is the hybrid
simulated annealing (HSA) approach by Leung et al. (2012). It uses
a greedy heuristic to find a locally good solution, and then per-
forms a simulated annealing procedure to perturb the solution in
order to search other neighborhoods.
For the 2DSP, Zhang et al. (2006) proposed a new heuristic
recursive algorithm, and then devised a simulated annealing algo-
rithm based on this strategy (Zhang et al., 2005). Bortfeldt (2006)
presented a genetic algorithm called SPGAL that works directly
on layouts rather than encoding the solutions. Burke et al. (2009)
examined first using the BF heuristic to pack some rectangles,
and then applying a simulated annealing metaheuristic for the
remaining rectangles. Leung and Zhang (2010) presented a simple
but fast heuristic algorithm based on a brick-laying strategy that
can solve large-scale problems effectively. For the fixed orientation
variant, the greedy randomized adaptive search procedure (GRASP)
by Alvarez-Valdes et al. (2008) had the best results at the time of
this writing.
3. Heuristic for the 2DRP
In this section, we present a new greedy heuristic for the 2DRP,
which we will subsequently use in our 2DSP approach. This deter-
ministic heuristic takes as input a sequence of rectangles, the
dimensions WHof the sheet, and a controlling parameter
max_spread.
3.1. Skyline representation of a packing pattern
We represent a current packing pattern by a rectilinear skyline,
which is the contour of a set of consecutive vertical bars. It can be
expressed as a sequence of khorizontal line segments (s
1
,s
2
,...,s
k
)
satisfying the following properties: (1) the y-coordinate of s
j
is
different from the y-coordinate of s
j+1
,j=1,...,k1; and (2) the
x-coordinate of the right endpoint of s
j
is the same as the x-coordi-
nate of the left endpoint of s
j+1
,j=1,...,k1. Fig. 1 gives an exam-
ple of a skyline, where each line segment s
j
is labeled as jat its left
endpoint. The initial empty packing pattern is represented by a sin-
gle line segment corresponding to the bottom of the sheet.
Our heuristic places rectangles one by one. Each rectangle is
placed with either its left bottom corner touching a left endpoint
or its right bottom corner touching a right endpoint of a line seg-
ment s
j
in the skyline. The left endpoint of a segment s
j
is a candi-
date position if and only if s
j1
is higher than s
j
; similarly, the right
endpoint of a segment s
j
is a candidate position if and only if s
j+1
is
higher than s
j
. Note that the left endpoint of s
1
and the right end-
point of s
k
are always candidates. The candidate positions of the
skyline in Fig. 1 are depicted by dots.
When a rectangle bis placed on a segment s
j
, the skyline is up-
dated. This is done in two steps. The first step instantiates a new
line segment corresponding to the top edge of band updates the
existing segments affected. There are two cases depending on
whether the width of bis greater than s
j
;Fig. 2 shows an example
of the two cases when a rectangle bis placed on the left endpoint of
s
j
, along with how the skyline is updated. Note that the shaded area
in Fig. 2(d) is considered wasted space because our heuristic will
never consider placing any rectangles into it.
In the second step, we check each line segment that is lower
than both its adjacent segments, which we call a locally lowest
2
3
5
1
4
Fig. 1. Example of skyline.
338 L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346
segment (for the first and last segments, we only compare them
with their one adjacent segment). If there are no unplaced rectan-
gles that can be placed on that segment, we raise it to the height of
its lower adjacent segment and merge them. This is repeated until
all line segments are considered. In Fig. 3(a), the rectangle bis
placed as shown and the first updating step has been performed.
If we find that no unplaced rectangles can be placed on the locally
lowest segment s
3
, then it is raised and merged with s
4
to form seg-
ment s
3
0
(Fig. 3(b)). Similarly, if bwas the only remaining rectangle
that could have been placed on s
5
, then s
5
will also be raised (and
merged with s
3
0
), as shown in Fig. 3(c). The shaded areas are simi-
larly considered wasted space.
3.2. Placement evaluation
Our heuristic considers all possible placements (i.e., a position-
rectangle pair), picks the best one and places the rectangle at that
position. This is repeated until no further rectangles can be placed
at any position (a failure) or all rectangles have been placed. We
first state our evaluation function, and then explain the motiva-
tions behind each of its components.
Consider a feasible placement (p,b), where pis a candidate po-
sition on line segment s
j
. When evaluating a placement, the algo-
rithm examines the resultant skyline after the first step of our
skyline updating procedure; this is done for reasons of time effi-
ciency. The evaluation function for a given feasible placement
(p,b) is then given by the following set of priority rules:
1. (spread constraint) The spread of a skyline is the difference
between the y-coordinates of its highest and lowest line seg-
ments. If the spread of the resultant skyline after placing the
rectangle at that position is greater than the input parameter
max_spread, then the placement is considered infeasible and
immediately rejected.
Fig. 2. Updating skyline step 1.
b
1
2
5
6
4
3
(a) Before update
b
12
5
6
3'
(b) Neighboring waste
b
1
2
6
3'
(c) Non-neighboring waste
Fig. 3. Updating skyline step 2.
L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346 339
2. (only fit) If the rectangle bis the only rectangle remaining that
can fit onto p, then (p,b) has the highest priority.
3. (minimum local waste) If there are no such cases or more than
one such case, we select the placement that minimizes the
amount of wasted local space. This is calculated as the total vol-
ume of wasted space of the four types given in Fig. 4(a)–(d). Let
w_min and h_min be the minimum width and height, respec-
tively, of the remaining rectangles excluding b. The shaded area
in case (a) is always considered wasted space. The shaded areas
in cases (b) and (c) are considered wasted if the length of the
gap is less than w_min. Finally, the shaded area in case (d) is
wasted if the gap is less than h_min.
4. (maximum fitness number) If there is a tie, prefer the place-
ment with highest fitness number. The fitness number of a
placement is the number of sides of the rectangle that exactly
matches the segment it is touching in the skyline. The bottom
side of a rectangle is an exact match if its width is equal to
the length of s
j
. The left (resp. right) side of a rectangle is an
exact match if its height is equal to the y-coordinate of s
j1
(resp. s
j+1
) minus the y-coordinate of s
j
. Any side of a rectangle
that touches the left, right or bottom side of the sheet is not
considered an exact match unless it fills all the remaining space
on that side. However, when the top edge of a placed rectangle
touches the top of the sheet, this is conisidered an exact match.
The fitness number can be either 0, 1, 2, 3 or 4, as shown in
Fig. 5(a), where the number in each rectangle depicted is its fit-
ness number.
5. (earliest in sequence) If there is a tie, prefer the placement
involving the earliest rectangle in the input sequence. If the rect-
angle can be placed at multiple locations, prefer the location with
the smallest y-coordinate, then with the smallest x-coordinate.
For the rotatable variant, we try both orientations for each rect-
angle, and the better orientation determines the evaluation for the
placement. If both orientations have identical evaluations based on
the above five rules, we prefer the orientation corresponding to the
original input.
There are two major motivations for implementing a spread
constraint. Firstly, without this restriction our greedy heuristic
would tend to produce a tall tower of rectangles along the left side
until the sheet height His reached (see Fig. 5(b)). This is a possibly
undesirable phenomenon that can be controlled with the spread
constraint. Secondly, different values of max_spread causes our
heuristic to behave differently: a high max_spread value allows tall
towers, while a low max_spread value causes the heuristic to place
rectangles layer by layer. By trying a range of max_spread values,
we can generate more diverse packings, thereby exploring the
search space more thoroughly.
Recall that we only perform the first step of the skyline updat-
ing routine when performing our evaluation. This detects wasted
space in adjacent segments of the types shown in the example in
Fig. 4 but does not take other types of wasted space into account
6
1
2
3
4
5
b
(a) Wasted space below
6
1
2
3
4
5
gap
b
(b) Wasted space to the left
6
1
2
3
4
5
gap
b
(c) Wasted s
p
ace to the ri
g
ht
6
1
2
3
4
5
gap
b
(d) Wasted s
p
ace above
Fig. 4. Wasted local space.
20
32
0
1
4
(a) Fitness numbers
......
(b) Tall tower
Fig. 5. Fitness numbers and the tall tower phenomenon.
340 L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346
(like the example in Fig. 3(c)). The only fit criterion helps to ad-
dress this omission. It is based on the observation that if the only
rectangle bthat can be placed on a line segment s
j
is placed else-
where, then the area just above s
j
is wasted, which might be large.
Hence, it is likely that placing bon segment s
j
will reduce the over-
all amount of wasted space.
The key component of our evaluation function is the minimum
local waste priority rule. It is motivated by the natural assumption
that if the amount of wasted space is minimized at every stage of
the process, then the remaining unplaced rectangles will be more
likely to be placeable. Note that our local waste measure is only
an approximate measure of the actual amount of wasted space.
The maximum fitness number rule favors placements that re-
sult in a ‘‘smoother’’ skyline that contains fewer line segments,
which is more likely to allow the placement of larger rectangles
without producing wasted space.
Given multiple placements with least waste and highest fitness
number, the earliest in sequence criterion introduces a tie-break-
ing rule. This allows us to consider different ways of prioritizing
the placement of rectangles using different input sequences.
We provide some comments on the efficient implementation of
this heuristic in Section A in the online supplement.
4. Tabu search for 2DRP
For ease of discourse, we will use the notation 2DRP_Heuristic
(Seq, H, max_spread) to represent the use of the heuristic described
in the previous section on the rectangle sequence Seq on a sheet
with dimensions WHand the given max_spread value. It is con-
tained in a wrapper function 2DRPSolver that invokes the heuristic
several times with different inputs sequences that are generated
using a tabu search mechanism, as shown in Algorithm 1.
Algorithm 1: 2DRPSolver (H,iter)
1: for each sorting rule do
2: Seq sorted sequence of rectangles
3: for each maximum spread value ms do
4: if 2DRP_Heuristic (Seq,H, ms) places all rectangles then
5: return success
6: end if
7: for (i 1;i<iter;++i)do
8: Generate 10 non-tabu sequences {Seq
1
,...,Seq
10
}
from Seq by swapping two rectangles
9: Let Seq
x
be the sequence with highest area utilization
using 2DRP_Heuristic (Seq
x
,H,ms)
10: if 2DRP_Heuristic (Seq
x
,H,ms) places all rectangles
then
11: return success
12: end if
13: Let (b
i
,b
j
) be the rectangles swapped to produce
Seq
x
from Seq
14: Add (b
i
,b
j
) to the tabu list for the next 3niterations
15: Seq Seq
x
16: end for
17: end for
18: end for
19: return failure
We make use of the following six sorting rules to generate the
initial input sequence Seq for our 2DRP heuristic (line 1):
1. Sort by area in decreasing order
2. Sort by width in decreasing order
3. Sort by height in decreasing order
4. Sort by perimeter in decreasing order
5. Sort by maximum of width and height in decreasing order
6. Sort by length of diagonal + width + height (i.e.,perimeter of tri-
angular half) in decreasing order
Let mh be the height of the tallest rectangle. For each sequence
of rectangles, we try four maximum spread values given by the set
mh;mh þðHmhÞ
1
3
;mh þðHmhÞ
2
3
;H
, as given in line 3.
For each of these combinations of sequences and max_spread
values, we attempt to optimize the solution in terms of area utiliza-
tion using a tabu search mechanism for a number of iterations
determined by the input parameter iter (lines 7–16). The neighbor-
hood operator simply swaps two rectangles in the current se-
quence. We generate 10 such sequences that are not forbidden by
the tabu list, select the one that produces the solution with the high-
est area utilization, and insert the swap into the tabu list; for the
next 3niterations, where nis the number of rectangles in the prob-
lem instance, this swap is forbidden when generating sequences.
At any point in the process, if a packing pattern is found that
places all nrectangles, then the procedure halts with success. If this
does not occur, then 2DRPSolver reports a failure. In either case, the
best packing pattern found in terms of area utilization over the
course of the entire algorithm is recorded and returned as the
2DRP solution.
5. Iterative doubling binary search for 2DSP
Our solution for the 2DSP uses the 2DRPSolver procedure as a
subroutine in an iterative doubling binary search as given in Algo-
rithm 2. Let LB1 be the naive lower bound computed as the total
area of all rectangles divided by the width of the sheet, i.e.,
LB1=dtotal rectangle area/ We. For the fixed orientation variant,
we can further strengthen the lower bound. Observe that if there
are rectangles with width greater than W/2, then these rectangles
cannot be placed side by side on the sheet. Hence, the sum of the
heights of all such rectangles is also a valid lower bound, which
we denote by LB2. Also note that if there are rectangles of width
exactly W/2, then no packing can have a height less than half the
total height of these rectangles; we set LB3 to be this value. Fur-
thermore, such rectangles cannot be placed side by side with rect-
angles of width greater than W/2 on the sheet. Hence, LB2+dLB3e
is also a valid lower bound (this is a special case of the bound L
2
used by Martello et al. (2003) where
a
=W/2). We set our initial
lower bound LB to be LB1 for the rotatable variant, and
LB = max{LB1,LB2+dLB3e} for the fixed orientation variant.
Several other lower bound measures exist in literature. These
can be broadly classified into three categories. Our bounds are
based on geometric considerations like (Martello et al., 2003; Bort-
feldt, 2006; Iori et al., 2003; Alvarez-Valdes et al., 2009), while
other bounds can be derived by relaxing a mathematical model
(Scheithauer, 1999; Belov et al., 2008) or using dual feasible func-
tions (Fekete and Schepers, 2004; Boschetti and Montaletti, 2010;
Carlier et al., 2007; Clautiaux et al., 2007a,b,c, 2008; Alvarez-Valdes
et al., 2009). We refer the reader to Boschetti and Montaletti (2010)
for a thorough overview of existing bounds.
We set the upper bound UB to be LB 1.1. We then perform a bin-
ary search on the height of the sheet H. If a feasible solution is found,
we record the solution and update the upper bound UB to be the
height of the current solution. If a feasible solution cannot be found
in the given number of iterations, then proceed with the binary
search by setting the lower bound to be H+ 1. Note that the initial
upper bound of UB =LB 1.1 may be too optimistic, and no solution
can be found within the prescribed number of iterations with height
no larger than UB. If this is the case, then we increase the upper
bound once again by 10% and repeat the process (line 15).
L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346 341
After each binary search attempt, we double the number of iter-
ations of tabu search allowed by the 2DRPSolver subroutine. Since
our lower bound may be far from the optimal value, our binary
search must check several values of H, some of which may be infea-
sible. Given an infeasible value of H, we are unable to quickly
determine if it is feasible or infeasible. The rationale behind this
iterative doubling approach is based on the observation that our
heuristic can never find a feasible solution to infeasible instances,
so we wish to minimize the amount of effort spent on infeasible
values of H. Early in the process, we spend a small amount of com-
putational effort for each value of Hin the binary search to deter-
mine a good upper bound by quickly solving ‘‘easy’’ 2DRP
instances. Later in the process, when more computational effort
is required to find a better feasible solution, we increase the effort
by doubling it after each iteration. This iterative doubling tech-
nique allows us to apportion a larger amount of computation time
to the latter parts of the process to increase the probability of find-
ing good feasible solutions.
Algorithm 2: Iterative Doubling Binary Search for 2DSP
1: Compute lower bound LB
2: UB LB 1.1
3: iter 1;UBfound false
4: while time limit not exceeded and LB –UB do
5: tempLB LB
6: while tempLB <UB do
7: H b(tempLB +UB)/2c
8: if 2DRPSolver (H,iter) is successful then
9: Record solution in Sol
10: UB H;UBfound true
11: else
12: tempLB H+1
13: end if
14: end while
15: if UBfound ==false then UB UB 1.1
16: iter iter⁄2
17: end while
18: return Sol
6. Computational experiments
Our algorithms were implemented as sequential algorithms in
C++ and compiled by GCC 4.1.2, and no multi-threading was
explicitly utilized. It was executed on an Intel Xeon E5430 clocked
at 2.66 GHz (Quad Core) with 8 GB RAM running the CentOS 5 li-
nux operating system.
6.1. Benchmark test data
In order to evaluate the performance of our algorithms, we used
a wide range of benchmark test instances for 2D packing from the
literature. Some of the test sets were generated by cutting a large
sheet into several pieces to form the rectangles, so a perfect opti-
mal solution with 100% area utilization is known. For the remain-
ing test sets, the optimal solution is unknown.
The test sets with known perfect optimal solutions are as
follows:
The 21 instances in test set Cwere generated by Hopper and
Turton (2001). They are divided into seven categories, each cat-
egory containing three instances.
Hopper (2000) generated 70 instances divided into two sets.
The first set Tcorresponds to guillotine patterns, and the second
set Ncorresponds to non-guillotine patterns.
Babu and Babu (1999) proposed a single test instance with 50
input rectangles, which we label as Babu.
The test set Burke was generated by Burke et al. (2004) to test
their best fit algorithm.
The CX test set generated by Pinto and Oliveira (2005) contains
large instances, with the number of input rectangles ranging
from 50 to 15,000.
Wang and Valenzela (2001) provided data instances with known
perfect optimal solutions of height 100, but the dimensions of
the input rectangles are real numbers. The data is divided into
two types: the test set Nice involves similar rectangles while
the set Path use rectangles with vastly differing dimensions.
Each data set contains instances ranging from 25 to 1000 rectan-
gles. These instances are made integral by multiplying the origi-
nal data by 10 and rounding to the nearest integer. Although this
invalidates the original perfect optimal solutions, they serve as
close approximations and are treated as such.
The remaining test sets were not produced by cutting the sheet
into pieces and they do not have known perfect optimal solutions:
The ngcut test set was generated by Beasley (1985b)
The gcut test set was generated by Beasley (1985a)
The cgcut test set was generated by Christofides and Whitlock
(1977)
The beng test set was generated by Bengtsson (1982)
The bwmv test set was originally designed for the bin packing
problem. It consists of 500 instances divided into 10 classes,
each with 5 groups of 10 instances. Six of the classes were cre-
ated by Berkey and Wang (1987), and the remaining four by
Martello and Vigo (1998).
Bortfeldt and Gehring (2006) generated the AH test set, com-
prising 360 large instances involving 1000 rectangles.
The characteristics of the benchmark test data are summarized
in Section B in the online supplement.
6.2. Parameter tuning
There are only two parameters in our tabu search, namely the
number of sequences generated in each iteration (neighborhood
size) and the number of iterations where a swap is forbidden (tabu
tenure). We followed a common practice to select these values.
First, we randomly select a single instance from each test set as a
representative sample. We then tried various combinations of val-
ues for the neighborhood size and tabu tenure on this sample, and
pick the combination that performs best.
Preliminary experiments indicate that a neighborhood size be-
tween 5 and 15 and a tabu tenure of between 2nand 4n(where
nis the number of rectangles in the instance) produce the best re-
sults. We followed the 2
2
factorial design with center point to
determine the best values within these ranges following the prac-
tice of design of experiments. For the neighborhood size factor, we
tried the two levels {5,15}, and for the tabu tenure factor we tried
the two levels {2n,4n} with the center located at (10, 3n).
The results of these experiments show that the center point
(10,3n) is the setting that achieves the best performance. Hence,
all our subsequent experiments are based on a neighborhood size
of 10 and a tabu tenure of 3n.
6.3. Results for 2DRP
The current best results for the 2DRP are held by the hybrid
simulated annealing (HSA) approach by Leung et al. (2012).We
compared our 2DRPSolver (Algorithm 1), which is a simple tabu
search using our 2DRP heuristic, with HSA on the test sets used
342 L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346
by the authors in the original publication, which considered the
fixed orientation variant only. In our approach, we invoke the
2DRPSolver procedure several times starting with iter = 1, and then
double the value of iter each time until a time limit of 10 CPU sec-
onds is exceeded, whereupon we return the best result found so
far. The time limit given to the HSA approach is 60 CPU seconds,
and the results for both sets of experiments were achieved on com-
parable machine configurations (see Section C in the online
supplement).
The results are shown in Table 1. Both algorithms were exe-
cuted 10 times for each instance. The percentage of unutilized
space in the final solutions for the 10 executions was averaged
for each instance; the columns labeled loss
avg
give the average of
these values over all instances in the test set. The average amount
of time over all executions for the corresponding algorithm to ter-
minate is reported under column t(s). Note that both algorithms
will terminate before reaching their respective time limits once a
solution with all rectangles loaded is found. When (Leung et al.,
2012) reported the results of HSA on the test set Nice/Path, they
also included an additional 60 test instances obtained from the
authors of Wang and Valenzela (2001). Consequently, we also in-
cluded these additional instances for a fair comparison.
The better result is highlighted in bold. Our approach outper-
forms HSA for eight out of ten test sets (except cgcut and gcut).
These results indicate that our heuristic combined with a simple
tabu search can be considered the best existing 2DRP algorithm
over these benchmark test cases.
An inspection of the cgcut,gcut and ngcut test sets reveal that
these sets contain several ‘‘large’’ rectangles of width greater than
W/2, where Wis the width of the sheet, so no two such large rect-
angles can be placed side by side on the sheet. Our approach tends
to perform relatively poorly for these types of instances. Consider
the example in Fig. 6(a), where the two large rectangles aand b
have already been placed, and rectangles cand dare the only
remaining rectangles that can fit into the space beside aand b.
Our heuristic may first place con top of b(generating no local
waste), and then the only fit rule would place don segment s
3
,
resulting in a large amount of wasted space. Instead, placing both
cand din the area beside aand bis optimal (e.g., Fig. 6(b)). Note
that this issue mainly arises in the fixed orientation variant.
Instances with several large rectangles may not be common in
practice. Also, we can partially address this issue by reducing such
instances to equivalent problems with fewer large rectangles using
the method proposed by Boschetti and Montaletti (2010). Further
refinements of our heuristic to handle this special case is a possible
direction for future work.
6.4. Results for 2DSP
We first analyze the results for the rotatable variant, which al-
lows the rectangles to be rotated by 90°. We compare our iterative
doubling binary search (IDBS) approach with the best approaches
for the 2DSP in existing literature:
BF: a deterministic placement heuristic by Burke et al. (2004)
based on the best fit strategy.
BF + SA: a simulated annealing enhancement of the BF heuristic
by Burke et al. (2009).
BBF: a bi-directional best-fit heuristic by Asßık and Özcan (2009).
FH: an iterative greedy heuristic by Leung and Zhang (2010).
The computational environments for these approaches are sum-
marized in Section C in the online supplement.
In the existing work on the rotatable variant of 2DSP, the test
sets used all had known perfect optimal solutions. Consequently,
we also evaluate our IDBS approach on these sets of instances;
the results are given in Table 2. For each test instance, the relative
gap of the height of the produced solution sol from the optimal
solution opt is computed as (sol opt)/opt. The original authors
for BF, FH and BBF heuristics only executed their approaches once
for each instance; we report the average relative gaps for each set
of instances in the columns rg. The original authors executed the
BF + SA method 10 times for each instance, and each execution
was given a time limit of 60 CPU seconds. The values in the column
rg
avg
is computed by first finding the average relative gap over the
10 executions for each instance, and then taking the average over
the test set. Note that the amount of time required to execute
the BF heuristic is negligible. For FH and BBF, the columns t(s) re-
port the average time taken over all instances in the test set. A dash
(‘–’) indicates that the result was not reported in the corresponding
publication.
For each instance, we ran our IDBS algorithm 10 times with a
time limit of 100 CPU seconds. The column rg
best
gives the relative
gap from optimal of the best solution found out of the 10 execu-
tions on each instance, averaged over the number of instances.
Similarly, rg
worst
reports the value for the worst soluions out of
the 10 executions on each instance. Finally, the column t
⁄
(s) gives
the average time required for IDBS to first find its final best solu-
tion for the test set.
The results show that IDBS outperforms all existing heuristics
for the rotatable variant of the 2DSP. For the C,Babu and Burke test
sets, IDBS is able to find the optimal solution in every execution.
For the remaining test sets, the average height of the solutions
found by IDBS is at most 1.31% away from optimal, which is the
best result in literature. Furthermore, even when the worst solu-
tions from 10 executions for each instance are taken, the average
gap to optimal is at most 1.43%. The detailed results for each test
set can be found in Section D in the online supplement.
The columns under the heading No Tabu give the results of our
approach when we remove the tabu search component (lines 7–16
in Algorithm 1), which reduces our algorithm to a simple binary
search on the height Hof the packing. For each height value, this
algorithm considers 4 different spread values for each of the 6 ini-
tial input sequences, for a total of 24 solutions. We see that aside
from the single Babu class instance, this algorithm performs signif-
icantly worse than IDBS for all test sets, which shows the impact of
our tabu search to find alternative input sequences.
For the fixed orientation variant, the GRASP approach by Alva-
rez-Valdes et al. (2008) is the current best algorithm. Hence, we
only compare our IDBS algorithm with this GRASP approach. Both
groups of test instances (with and without known perfect optimal
solutions) were employed. The results are summarized in Table 3.
The GRASP approach was executed 10 times for each instance
and was given a time limit of 60 CPU seconds per execution. We
set the time limit for our IDBS approach at 100 CPU seconds for
the test sets with known perfect optimal solutions, and 200 CPU
seconds for the other sets. All entries in the table are averaged over
Table 1
Results for 2DRP.
Test set HSA 2DRPSolver
loss
avg
t(s) loss
avg
t(s)
C 0.19 41.27 0.06 4.90
N 1.62 53.59 1.16 6.73
T 1.62 52.79 1.16 6.67
Burke 0.08 33.06 0.00 2.17
CX 0.36 26.50 0.33 4.68
cgcut 3.47 40.07 3.97 7.22
gcut 10.43 60.02 11.12 7.87
ngcut 14.86 25.02 14.02 3.64
beng 0.67 6.05 0.57 0.07
Nice/Path 0.65 60.05 0.61 10.75
L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346 343
all instances in the test sets; the columns have the same meanings
as in Table 2. The best results are shown in bold.
For the instances with known perfect optimal solutions, IDBS is
superior to GRASP. For many test sets, the average relative gap
from optimal achieved by GRASP is about twice that of IDBS. In
fact, for every test set, the average gap to optimal of the worst solu-
tions found from 10 executions of each instance for IDBS is equal or
superior to the average gap to optimal of the best solutions found
from 10 executions of each instance for GRASP. Note that IDBS was
able to find the optimal solution for the Babu instance and Burke
test set within 5 seconds.
For the remaining instances, since the optimal solutions are un-
known, the relative gap values are computed in terms of the naive
lower bound LB1. There are some where IDBS produces a better
solution than GRASP and others where GRASP is superior, and both
approaches found the optimal solutions for all instances in the
beng test set. On average, GRASP outperforms IDBS by a very small
amount (less than 0.3%) for these instances. Note that these in-
stances contain several large rectangles, which tend to cause our
heuristic to perform relatively poorly (as discussed in Section
6.3). Overall, IDBS outperforms GRASP for the entire set of test in-
stances considered.
Finally, the columns under the heading No Tabu verify that the
tabu search component of IDBS is similarly signifcant for the fixed
orientation variant.
We report the results for the bwmv test set twice. The first set of
values labeled bwmv is calculated in the same way as the other en-
tries in the table. The values for the test set labeled bwmv
⁄
were
calculated according to the method used by Alvarez-Valdes et al.
(2008). Recall that the bwmv test set is divided into 10 classes, each
class containing 5 groups. For each group, (Alvarez-Valdes et al.,
2008) calculated the group average as follows: take the total height
of all solutions for each instance averaged over 10 executions, sum
them up, and find the relative gap between this value and the total
of the lower bounds for all instances in the group, i.e.,
ðP
i2group
H
i
Þ=ðP
i2group
LB
i
Þ1 where H
i
is the average height of the
a
b
d
c
1
2
3
(a) Our heuristic
a
b
d
c
1
2
3
(b) Optimal solution
Fig. 6. Possible poor performance due to large rectangles.
Table 2
Results for rotatable variant.
Test set BF BF + SA FH BBF IDBS No tabu
rg rg
avg
rg t(s) rg t(s) rg
avg
rg
best
rg
worst
t
⁄
(s) rg
avg
t(s)
C 6.10 2.33 1.71 2.09 2.77 0.3 00 0 0.31 1.06 0.02
Babu 6.67 6.67 – – 6.67 0.4 000 0.02 0.00 0.00
Burke 4.05 1.78 1.41 0.78 2.24 13 00 0 0.41 0.81 0.04
N – – – – – – 0.93 0.80 1.03 19.11 4.23 0.04
T – – – – 3.90 211 1.01 0.80 1.20 19.48 4.09 0.05
CX – – 1.02 0.87 2.52 922 0.22 0.19 0.26 13.51 1.33 0.41
Nice 5.85 4.19 – – 5.75 9.80 1.06 0.92 1.27 44.93 2.87 1.73
Path 6.88 3.13 – – 5.95 8.8 1.31 1.17 1.43 48.87 2.72 3.61
Table 3
Results for fixed orientation variant.
Test set GRASP IDBS No Tabu
rg
avg
rg
best
rg
avg
rg
best
rg
worst
t
⁄
(s) rg
avg
t(s)
C 0.98 0.85 0.14 0.04 0.20 10.46 1.34 0.03
Babu 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.00
Burke 0.91 0.91 0.00 0.00 0.00 4.73 1.73 0.32
N 2.41 2.31 1.29 0.87 1.94 28.38 5.27 0.05
T 2.27 2.16 1.33 1.04 1.79 21.55 5.67 0.05
CX 1.02 0.98 0.43 0.40 0.45 17.96 1.93 0.70
Nice 3.55 3.33 1.96 1.80 2.12 42.54 3.07 1.14
Path 2.75 2.61 2.03 1.91 2.15 48.72 3.68 2.58
beng 0.00 0.00 0.00 0.00 0.00 0.04 0.51 0.00
cgcut 2.37 2.37 2.42 2.42 2.42 37.18 4.60 0.01
gcut 5.43 5.42 5.63 5.55 5.71 56.76 9.02 0.04
ngcut 1.34 1.34 1.60 1.60 1.60 0.19 3.58 0.00
bwmv – – 3.01 2.86 3.13 39.11 5.43 0.06
bwmv
⁄
1.80 1.73 1.86 1.71 1.98 39.11 4.20 0.06
AH – – 0.87 0.77 0.95 104.06 1.36 13.06
344 L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346
solutions over 10 executions for instance iin the group. The class
average is the average of the group averages, and the reported va-
lue is the average of the class averages over the 10 classes. This
method of computing group average is non-standard, since in-
stances with a large lower bound may have a greater effect on this
value than instances with a small lower bound. Furthermore, the
method used by the authors to compute their lower bounds LB
i
is different from ours. Therefore, our values for bwmv
⁄
were com-
puted using their reported lower bound values.
The detailed results for all test instances for the fixed orienta-
tion variant are located in Section D in the online supplement.
The graphs in Fig. 7 give an indication of the convergence
behavior of our IDBS approach. For each test instance, we executed
IDBS for 1000 CPU seconds and plotted the average relative gap
achieved by IDBS for each test set as the computation time
increases; note that the x-axis values increase exponentially. As ex-
pected, the convergence behavior varies depending on the test set,
but for most test sets the solution found after 8 CPU seconds is not
much worse that the solution found after 1000 CPU seconds.
Hence, our IDBS approach exhibits rapid convergence for most of
these test sets. A visual inspection of Fig. 7(a) and (b) suggests that
a computation time limit of about 100 CPU seconds is sufficient to
achieve a high-quality solution for the test sets with known perfect
optimal solutions, and Fig. 7(c) suggests a computation time limit
of about 200 CPU seconds for the remaining test sets. Although
improvements continue when more time beyond these limits is
spent in most cases, they are achieved at a diminishing rate.
7. Conclusions
In this paper, we present a greedy heuristic for solving the 2DRP
involving several priority rules that are motivated by observations
on the nature of 2D packing problems. A tabu search procedure
that employs this 2DRP heuristic outperforms the best existing ap-
proach for this problem on benchmark test instances. We then use
this tabu search procedure as a subroutine in an iterative doubling
binary search (IDBS) on the height of the sheet to solve the 2DSP.
The resultant approach outperforms all existing approaches for
the 2DSP on both the fixed and rotatable variants on a large set
of benchmark test cases.
Our greedy 2DRP heuristic is based on the skyline representa-
tion of packing patterns. While this representation has the advan-
tage of efficient computation, it also eliminates some positions that
can contain rectangles. For example, in Fig. 4(a) there may be rect-
angles that can be placed on segment s
4
or s
5
, but these positions
are not preserved by the skyline representation. It may be possible
to produce a better heuristic using a more complete representation
of packing patterns, taking into account a possible tradeoff in terms
of computational efficiency.
To the best of our knowledge, the use of iterative doubling in a
binary search as realized in our IDBS approach has never been pre-
viously proposed. This idea may also be applicable to binary search
algorithms on other problems to improve efficiency.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.ejor.2011.06.022. Detailed
solution files can be downloaded from www.computational-logis-
tics.org/orlib/2dsp/index.html.
References
Alvarez-Valdes, R., Parreño, F., Tamarit, J.M., 2008. Reactive grasp for the strip-
packing problem. Computers & Operations Research 35, 1065–1083.
Alvarez-Valdes, R., Parreño, F., Tamarit, J., 2009. A branch and bound algorithm for
the strip packing problem. OR Spectrum 31, 431–459.
Asßık, O., Özcan, E., 2009. Bidirectional best-fit heuristic for orthogonal rectangular
strip packing. Annals of Operations Research 172, 405–427.
Babu, A.R., Babu, N.R., 1999. Effective nesting of rectangular parts in multiple
rectangular sheets using genetic and heuristic algorithms. International Journal
of Production Research 37, 1625–1643.
Baker, B.S., Coffman, E.G., Rivest, R.L., 1980. Orthogonal packings in two dimensions.
SIAM Journal on Computing 9, 846–855.
Beasley, J.E., 1985a. Algorithms for unconstrained two-dimensional guillotine
cutting. The Journal of the Operational Research Society 36, 297–306.
Beasley, J.E., 1985b. An exact two-dimensional non-guillotine cutting tree search
procedure. Operations Research 33, 49–64.
Belov, G., Scheithauer, G., Mukhacheva, E.A., 2008. One-dimensional heuristics
adapted for two-dimensional rectangular strip packing. Journal of the
Operational Research Society 59, 823–832.
Bengtsson, B.E., 1982. Packing rectangular piecesa heuristic approach. The
Computer Journal 25, 253–257.
Berkey, J.O., Wang, P.Y., 1987. Two-dimensional finite bin-packing algorithms. The
Journal of the Operational Research Society 38, 423–429.
Fig. 7. IDBS convergence.
L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346 345
Bortfeldt, A., 2006. A genetic algorithm for the two-dimensional strip packing
problem with rectangular pieces. European Journal of Operational Research
172, 814–837.
Bortfeldt, A., Gehring, H., 2006. New large benchmark instances for the two-
dimensional strip packing problem with rectangular pieces. In: Proceedings of
the 39th Annual Hawaii International Conference on System Sciences
(HICSS’06). IEEE, p. p. 30b.
Boschetti, M.A., Montaletti, L., 2010. An exact algorithm for the two-dimensional
strip-packing problem. Operations Research 58, 1774–1791.
Burke, E.K., Kendall, G., Whitwell, G., 2004. A new placement heuristic for the
orthogonal stock-cutting problem. Operations Research 52, 655–671.
Burke, E.K., Kendall, G., Whitwell, G., 2009. A simulated annealing enhancement of
the best-fit heuristic for the orthogonal stock-cutting problem. INFORMS
Journal on Computing 21, 505–516.
Carlier, J., Clautiaux, F., Moukrim, A., 2007. New reduction procedures and lower
bounds for the two-dimensional bin packing problem with fixed orientation.
Computers & Operations Research 34, 2223–2250.
Chazelle, 1983. The bottom-left bin-packing heuristic: an efficient implementation.
IEEE Transactions on Computers C-32, 697–707.
Christofides, N., Whitlock, C., 1977. An algorithm for two-dimensional cutting
problems. Operations Research 25, 30–44.
Clautiaux, F., Carlier, J., Moukrim, A., 2007a. A new exact method for the two-
dimensional bin-packing problem with fixed orientation. Operations Research
Letters 35, 357–364.
Clautiaux, F., Carlier, J., Moukrim, A., 2007b. A new exact method for the two-
dimensional orthogonal packing problem. European Journal of Operational
Research 183, 1196–1211.
Clautiaux, F., Jouglet, A., Elhayek, J., 2007c. A new lower bound for the non-oriented
two-dimensional bin-packing problem. Operations Research Letters 35, 365–
373.
Clautiaux, F., Jouglet, A., Carlier, J., Moukrim, A., 2008. A new constraint
programming approach for the orthogonal packing problem. Computers &
Operations Research 35, 944–959.
Dowsland, K., 1993. Some experiments with simulated annealing techniques for
packing problems. European Journal of Operational Research 68, 389–399.
Fekete, S.P., Schepers, J., 2004. A general framework for bounds for higher-
dimensional orthogonal packing problems. Mathematical Methods of
Operations Research 60, 311–329.
Gonçalves, J.F., 2007. A hybrid genetic algorithm-heuristic for a two-dimensional
orthogonal packing problem. European Journal of Operational Research 183,
1212–1229.
Hifi, M., 1998. Exact algorithms for the guillotine strip cutting/packing problem.
Computers & Operations Research 25, 925–940.
Hopper, E., 2000. Two-dimensional packing utilising evolutionary algorithms and
other meta-heuristic methods. Ph.D. thesis University of Wales, Cardiff School
of Engineering.
Hopper, E., Turton, B.C.H., 2001. An empirical investigation of meta-heuristic and
heuristic algorithms for a 2D packing problem. European Journal of Operational
Research 128, 34–57.
Huang, W., Chen, D., Xu, R., 2007. A new heuristic algorithm for rectangle packing.
Computers & Operations Research 34, 3270–3280.
Imahori, S., Yagiura, M., 2010. The best-fit heuristic for the rectangular strip packing
problem: An efficient implementation and the worst-case approximation ratio.
Computers & Operations Research 37, 325–333.
Iori, M., Martello, S., Monaci, M., 2003. Metaheuristic algorithms for the strip
packing problem. In: Pardalos, P.M., Korotkikh, V. (Eds.), Optimization and
Industry: New Frontiers, Applied Optimization, vol. 78. Springer, pp. 159–179
(Chapter 7).
Kenmochi, M., Imamichi, T., Nonobe, K., Yagiura, M., Nagamochi, H., 2009. Exact
algorithms for the two-dimensional strip packing problem with and without
rotations. European Journal of Operational Research 198, 73–83.
Lesh, N., Marks, J., McMahon, A., Mitzenmacher, M., 2004. Exhaustive approaches to
2d rectangular perfect packings. Information Processing Letters 90, 7–14.
Leung, S.C.H., Zhang, D., 2010. A new heuristic approach for the stock-cutting
problems. In: Engineering and Technology K: Business and Economic Sciences
2:2. World Academy of Science, pp. 121–126.
Leung, S.C.H., Zhang, D., Zhou, C., Wu, T., 2012. A hybrid simulated annealing
metaheuristic algorithm for the two-dimensional knapsack packing problem.
Computers & Operations Research 39, 64–73 [Special Issue on Knapsack
Problems and Applications.].
Liu, D., Teng, H., 1999. An improved bl-algorithm for genetic algorithm of the
orthogonal packing of rectangles. European Journal of Operational Research
112, 413–420.
Martello, S., Vigo, D., 1998. Exact solution of the two-dimensional finite bin packing
problem. Management Science 44, 388–399.
Martello, S., Monaci, M., Vigo, D., 2003. An exact approach to the strip-packing
problem. INFORMS Journal on Computing 15, 310–319.
Oliveira, J.E., Ferreira, J.S., 1993. Algorithms for nesting problems. In: Applied
simulated annealing. LNEMS, vol. 396. Springer, pp. 255–274.
Pinto, E., Oliveira, J.F., 2005. Algorithm based on graphs for the non-guillotinable
two-dimensional packing problem. In: Second ESICUP Meeting. Southampton,
United Kingdom.
Scheithauer, G., 1999. Lp-based bounds for the container and multi-container
loading problem. International Transactions in Operational Research 6, 199–
213.
Wang, P.Y., Valenzela, C.L., 2001. Data set generation for rectangular placement
problems. European Journal of Operational Research 134, 378–391.
Wäscher, G., Haußner, H., Schumann, H., 2007. An improved typology of cutting and
packing problems. European Journal of Operational Research 183, 1109–1130.
Wei, L., Zhang, D., Chen, Q., 2009. A least wasted first heuristic algorithm for the
rectangular packing problem. Computers & Operations Research 36, 1608–
1614.
Wu, Y.-L., Huang, W., Lau, S.-c., Wong, C.K., Young, G.H., 2002. An effective quasi-
human based heuristic for solving the rectangle packing problem. European
Journal of Operational Research 141, 341–358.
Zhang, D., Liu, Y., Chen, S., Xie, X., 2005. A meta-heuristic algorithm for the strip
rectangular packing problem. In: Wang, L., Chen, K., Ong (Eds.), Advances in
Natural Computation chapter 157, Lecture Notes in Computer Science, vol.
3612. Springer Berlin/Heidelberg, Berlin, Heidelberg, pp. 1235–1241.
Zhang, D., Kang, Y., Deng, A., 2006. A new heuristic recursive algorithm for the strip
rectangular packing problem. Computers & Operations Research 33, 2209–
2217.
346 L. Wei et al. / European Journal of Operational Research 215 (2011) 337–346
