Efficient Constructive procedures for the distributed blocking flow shop scheduling problem (presented at the 6th IESM Conference, October 2015, Seville, Spain) © I 4e2 2015 Ramon Companys, Imma Ribas Departament d’Organització d’Empreses. Universitat Politècnica de Catalunya, BarcelonaTech Barcelona, Spain Email: ramon.companys@upc.edu, imma.ribas@upc.edu Abstract— the distributed blocking flow shop scheduling problem (DBFSP) allows modeling of the scheduling process in companies with more than one factory, with productive systems configured as flow shop lines where the blocking constraint has to be considered. To the best of our knowledge, this variant of the distributed permutation flow shop scheduling problem has not been studied. In this paper, we propose some constructive heuristics that will solve the DBFSP and thus minimize the maximum completion time among the factories. The proposed procedures use two approaches that are totally different from those proposed for the distributed permutation flow shop scheduling problem (DPFSP). By taking the DPFSP procedures that we adapted to DBFSP and comparing them to the new approaches that were specifically designed for DBPFSP, we find that the latter perform considerably better. Keywords—distributed blocking flow shop; blocking flow shop; distributed permutation flow shop; constructive heuristics; I. INTRODUCTION Distributed manufacturing is a common situation for large enterprises that compete in a globalized market. Because of current globalization trends, production has shifted from single factory production to a multi-factory production network [1]. In this environment, the scheduling problems deal with the allocation of jobs to factories and the scheduling of jobs in each plant. Since the flow shop configuration is the most common processing layout, the flow shop scheduling problem has been studied greatly since the seminal paper of Johnson [2]. However, its extension to a multi-plant environment was first presented by Naderi and Ruiz [3], who referred to it as the Distributed Permutation Flow Shop Scheduling Problem (DPFSP). According to [3], the DPFSP is defined as follows. A set N of n jobs must be processed by a set G of F identical factories. Each factory has the same set M of m machines. The processing times of all the tasks of a given job do not change from factory to factory. The objective is to minimize the maximum makespan among factories. After the publication of [3], several authors proposed various heuristics to solve this problem ([4]–[13]), but the blocking constraint has been considered in none of them. The blocking flow shop scheduling problem allows many productive systems to be modeled when there are no buffers between consecutive machines. In general, it is useful for those systems that have a production line without a drag system that forces a job to be transferred between two consecutive stations at pre-established times. Some industrial examples can be found in the iron and steel industry [14]; in the treatment of industrial waste and the manufacture of metallic parts [15]; or in a robotic cell, where a job may block a machine while waiting for the robot to pick it up and move it to the next stage [16]. The blocking constraint tends to increase the completion time of jobs, because the processed job cannot leave the machine if the next machine is busy. Therefore, the heuristics designed to schedule jobs in this environment have to consider this fact in order to minimize the idle time of machines due to possible blockage. Therefore, the distributed blocking flow shop scheduling problem (DBFSSP) deals with the allocation and scheduling of jobs in a multi-factory production network with the blocking constraint present in the manufacturing system. It is interesting to study this problem in order to design specific procedures, since the adaptation of those designed for the DPFSP probably perform worse than procedures which consider its characteristics. In this paper we propose new constructive procedures built from two different approaches to solve the DBPFSP. The computational evaluation shows good performance of these simple procedures, which can be used either to obtain a fast solution to the problem or as the initial solution procedure in more sophisticated metaheuristics. II. CONSTRUCTIVE HEURISTICS As stated before, the DPFSP needs to deal with two related decisions: the allocation of jobs to factories and the sequence of jobs assigned to each plant. In this paper, we have compared 33 constructive heuristics. 30 of them are formed by combining ten sequencing methods with three allocation rules (two of these rules came from the DPFSP literature and the third is a new approach presented in this paper). The remaining three heuristics take a different perspective in addressing the problem considered here. A. Proposed methods To the best of our knowledge, no paper dealing with the distributed blocking permutation flow shop has been published, but some ideas can be taken from the DPFSP. In particular, the constructive heuristics proposed in [3] consist of sequencing jobs according to an ordering rule before being assigned to a facility in accordance with an allocation rule. In this paper, we propose a new method for allocating the jobs after sequencing, as well as three new procedures that use a different approach to solve the problem. The new allocation method consists of dividing the job sequences into F (number of factories) fractions by assigning a similar load (ΣPi/F) to each plant. Finally, the sequence of jobs assigned to each plant is improved by an insertion procedure similar to that used in the second step of NEH [17]. This allocation method has been combined with the following ten sequencing rules. Some of them are used in the PFSP and some others were specially designed for the blocking PFSP: Shortest Processing Time (SPT), Largest Processing Time (LPT), Johnson rule [2], Palmer’s heuristic [18], CDS [19], NEH [17], Trapeziums (TR) [20], PF [21], PW [22] and HPF2[23]. The three last procedures were developed for the blocking flow shop problem. The resulting heuristics are named as the sequencing rule plus the number 3, following the notation used in [3]. Therefore, these heuristics are named SPT3, LPT3, Johnson3, Palmer3, CDS3, NEH3, TR3, PF3, PW3 and HPF23. In the TR rule, two indexes are calculated for each job (S1i and S2i), according to (3) and (4) respectively. Next, jobs are scheduled by applying the Johnson algorithm considering S1i as the processing time of job i in the first machine and S2i as the processing time of i in the second machine. m S1i (m j 1) p j ,i (3) j 1 S 2i m ( j 1) p j ,i (4) j 1 It is worth noting that ordering jobs in increasing order of S3i=S1i-S2i obtains the sequence given by Palmer’s heuristic. The HPF2 procedure is divided into two steps. The first step selects a job to be the first job in the sequence, which minimize the bicriteria index (R(i)). This index considers the contribution of the job to the completion time (minimum sum of its processing times, Pi) and the front delay generated. The selected job is the one whose R(i) value is smaller. m R (i ) 2 ( m j ) p j ,i j 1 m 1 m (1 ) p j ,i (5) j 1 The second step builds the remaining sequence to minimize the timeout of machines and the total flowtime, which is carried out with index ind1(i,k), where i denotes the job and k the position, calculated according to (6). Notice that the timeout can be due to idle time, blocking time or the sum of both. Therefore, the sequencing rule according to index ind1(i,k) is adequate when the blocking constraint is considered. ind 1 ( i ,k ) μ m ( c j , k 1( i) c j , k () p j ,i )) (1 μ) ( C i C [ k -1] ) (6) j 1 Instead of sequencing the jobs first and then the allocation of jobs to the plants, the new approach considers the jobs and factories together. Following this philosophy, we have implemented three methods, which are named RC1_1, RC1_m and RC2. In RC1_1 and RC1_m, the first job is selected for each plant according to index R(i), equation (5). Next, a factory is first selected in order to proceed with the other jobs. In RC1_1, the factory selected is the one which has the first machine available earlier, whereas the factory selected in RC1_m is the one which has the last machine available sooner. After the plant is selected, a job is chosen according to index ind2(f,i), calculated as in (7), where f is one of the factories and f is the sequence of jobs already sequenced in plant f. Observe that ind2 differs from ind1 in the second term. In ind2, this term contemplates the workload of each job, whereas ind1 considers the contribution of each job to the total flow time. m ind 2 (i, k ) ( (c j , k 1 ( f i ) c j , k ( f ) p j ,i )) (1 ).Pi (7) j 1 In RC2 the first job assigned to each plant is also selected with index R(i), but each remaining job and plant are selected at the same time in order to minimize index ind3(f,i,kf), calculated as in (8). m ind 3( f , i, k f ) ( (c j ,k f 1 ( f i) c j ,k f ( f ) p j ,i )) (1 ).(cm,k f 1 ( f i) c0 ) , (8) j 1 where c0 is the current minimum makespan of any plant. Observe that the second term measures the difference between the partial makespan (completion time obtained with the jobs already sequenced in this plant) and the minimum partial makespan obtained in any of the plants. By trying to minimize this term, the workload of the plants tends to be similar. The last step in the three presented algorithms improves the sequence of each plant by using an insertion procedure, similar to the one used in NEH. Notice that HPF2, RC1_1, RC1_m and RC2 heuristics have two parameters (λ and µ) that must be determined adequately. Their calibration is addressed in the next section. III. EXPERIMENTAL EVALUATION In this section we calibrate and evaluate the presented procedures against other constructive procedures proposed in the literature for the DPFSP. These procedures consist of combining the two allocation methods proposed in [3] with six sequencing rules: (Shortest Processing Time (SPT), Largest Processing Time (LPT), Johnson rule [2], Palmer’s heuristic [18], CDS [19] and NEH [17]). The two allocation methods are: (1) Assign job j to the factory with the lowest current Cmax, not including job j. (2) Assign job j to the factory which completes it at the earliest time. These heuristics are identified by the name of the sequencing rule plus 1 or 2, depending on the allocation rule used. We have added the sequencing rules TR, PF, PW and HPF2 to these groups of heuristicsº. 0.70 6.87 6.77 6.98 6.78 6.91 7.06 7.24 0.75 6.99 6.80 7.03 6.94 7.03 7.21 7.31 As stated before, HPF2, RC1_1, RC1_m and RC2 have two parameters that must be calibrated. Next, we explain how this is carried out. A. Calibration of heuristics Parameters λ and µ of each heuristic were selected by measuring the performance of the algorithm, which itself was done by combining several λ and µ values. The values of λ and µ ranged from 0 to 1 in increments of 0.05. Therefore, we tested 21 values for each parameter. For this test, we used 600 randomly generated instances. 100 instances were grouped into 20 sets of size n x m, 5 instances per set, where n= {25, 50, 100, 200, 400} and m = {5, 10, 15, 20}. All these 100 instances were considered with a different number of factories. We had F={2,3,4,5,6,7}, which gave us 600 instances in total. The performance was measured by the Relative Percentage Deviation (RPD) from the best solution (minimum makespan), which was obtained during the experiment using all combinations of values. Therefore, RPD is calculated as in (9): RPD C max k Best k 100 , Best k (9) TABLE II. F TABLE I. ARPD FOR HPF23 FOR EACH COMBINATION OF AND FOR HPF23 Values of parameters 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.45 7.01 6.91 7.17 6.77 6.89 7.21 7.27 0.50 7.02 6.84 7.02 6.72 6.92 7.25 7.31 0.55 6.95 6.86 7.05 6.73 6.87 7.20 7.26 0.60 6.91 6.84 7.01 6.71 6.86 7.14 7.24 0.65 6.97 6.89 7.02 6.78 6.82 7.07 7.23 Value of Parameters Min ARPD 2 4.99 0.60 0.70 3 6.06 0.50 0.70 4 6.71 0.60 0.75 5 7.52 0.45 0.80 6 8.09 0.70 0.95 7 8.10 0.15 0.70 Hence, we had 6 matrices for each procedure. Next, in order to select a unique value that is independent of the number of plants (F), we calculated the ARPD for each combination of λ and µ values in the previous 6 matrices. The selected values of λ and µ came from those that lead to the minimum ARPD. Table III shows the selected values of λ and µ for each procedure. where Cmax,k is the makespan obtained in instance k and Bestk is the minimum Cmax obtained in this instance by any combination of values. For each procedure and number of factories, we built a matrix with the average RPD (ARPD) value that was obtained through each combination of λ and µ. As an example, table I shows a piece of the table built for the calibration of HPF23 when F=4 plants. Notice that the minimum APRD is obtained when λ=0.6 and = 0.75. A similar table was built for all numbers of plants. Table II shows the λ and µ values that lead to the minimum ARPD for each plant. AND VALUES FOR EACH NUMBER OF PLANTS TABLE III. Procedures λ AND µ VALUES FOR EACH PROCEDURE Values of parameters RC1_1 1 0.95 RC1_m 0.85 1 RC2 0.75 0.05 HPF2 0.55 0.70 B. Computational evaluation We compared the 33 implemented constructive procedures in order to analyze their behavior and to recommend one of them, if possible, for solving the distributed blocking flow shop problem. Among the compared heuristics, 12 of them were presented in [3] (SPT1, LPT1, Johnson1, Palmer1, CDS1, NEH1, SPT2, LPT2, Johnson2, Palmer2, CDS2, NEH2). We added to them TR1, PF1, PW1, HPF21, TR2, PF2, PW2 and HPF22. The remaining 13 procedures are those presented in this paper: SPT3, LPT3, Johnson3, Palmer3, CDS3, NEH3, TR3, PF3, PW3, HPF23, RC_1, RC_m and RC2. The algorithms were coded in the same language (QB64) and tested on the same computer, a 3 GHz Intel Core 2 Duo E8400 CPU with 2 GB of RAM. The heuristic performance was measured by the Relative Percentage Deviation (RPD), using the best solution (minimum makespan) chosen from among the solutions that were obtained by all of the implemented procedures. JOHN2 10.75 8.45 6.79 5.80 5.16 4.67 6.94 PAL2 14.15 11.79 9.73 8.62 7.75 7.35 9.90 CDS2 9.05 7.73 6.63 5.65 5.15 4.99 6.53 In order to do a comprehensive analysis of the results, we separated the heuristics into groups. Three groups use the same allocation methods, and one group uses RC1_1, RC1_m and RC2. We first compared the procedures in each group, and then we selected from each group the two best algorithms in terms of minimum overall ARPD, in order to compare them with the two best heuristics from the other groups. NEH2 7.96 8.37 8.35 7.88 7.87 7.54 8.00 TR2 8.82 6.29 4.91 3.70 2.77 2.43 4.82 PF2 16.29 18.11 19.11 18.80 18.96 18.68 18.33 PW2 15.90 17.87 18.78 18.67 18.65 18.37 18.04 HYB2 16.68 18.67 18.85 18.89 18.85 18.69 18.44 Table IV shows the ARPD calculated for each procedure and the number of factories. As can be seen, the best performing heuristic of this group is the one that uses the TR sequencing rule with an overall ARPD of 7.45 (number in bold). Notice that NEH1 is better for F=2, but TR1 shows the best performance for the remaining number of plants. The second best performing method is CDS1, with an overall ARPD of 9.35. TABLE IV. ARPD VALUES PER HEURISTIC AND NUMBER OF FACTORIES IN GROUP 1 Heuristics Number of factories 2 3 4 5 6 7 All SPT1 18.78 19.35 19.33 19.53 20.20 19.78 19.49 LPT1 18.35 18.49 18.33 17.93 18.08 17.79 18.16 JOHN1 11.95 10.31 PAL1 15.35 13.68 12.57 12.55 11.93 11.21 12.88 CDS1 10.09 NEH1 9.58 9.45 9.15 8.82 8.95 8.47 9.22 8.34 9.24 7.31 6.65 6.35 5.97 9.37 TR1 10.07 PF1 16.65 19.23 19.75 20.68 21.54 21.86 19.95 PW1 16.21 19.33 20.43 21.05 21.88 22.50 20.23 HYB1 16.79 19.19 20.34 20.84 21.60 22.12 20.15 7.45 The results of the heuristics belonging to the second group are very similar to those from the first group. Table V shows the ARPD of each heuristic, grouped by number of factories F. Observe that the best performing procedure is TR2, except for the case of two factories where NEH2 performs better. The second best heuristic in this group is also CDS2. Notice that the allocation rule 2 leads to better performance than allocation rule 1, just as it does in DPFSP. TABLE V. ARPD VALUES PER HEURISTIC AND NUMBER OF FACTORIES IN GROUP 2 Heuristics Number of factories 2 3 4 5 6 TABLE VI. ARPD VALUES PER HEURISTIC AND NUMBER OF FACTORIES IN GROUP 3 Heuristics 9.56 8.82 10.04 10.63 11.00 11.33 11.61 10.57 8.38 In the third group, Table VI shows the ARPD for each heuristic and number of factories. Here, the best heuristics ranking has changed. Remember that in this case the sequence of jobs is divided into F parts, and each of these parts is assigned to one factory. This situation is totally different from that which used the two allocation methods in the previous two groups. This explains the good performance of the three sequencing rules proposed for the blocking flow shop problem, as compared to the others proposed for the DPFSP. These methods sequence the jobs in order to minimize the idle time of machines, and this order is maintained in the segment assigned to each factory. 7 Average SPT2 17.86 17.51 17.30 16.94 17.17 17.00 17.30 LPT2 17.03 15.83 14.96 14.15 13.78 12.85 14.77 Number of factories 2 3 4 5 6 7 All SPT3 3.97 5.27 5.27 5.75 6.92 7.00 5.70 LPT3 4.28 4.91 5.46 5.53 6.53 6.33 5.51 JOHN3 5.52 7.10 8.41 8.57 9.02 9.86 8.08 PAL3 6.19 7.43 8.32 8.51 8.98 9.11 8.09 CDS3 5.83 7.22 8.23 8.50 9.05 9.01 7.97 NEH3 3.35 4.28 4.87 5.50 5.61 6.24 4.97 TRAP3 6.41 7.81 8.79 9.46 10.09 10.08 8.77 PF3 2.05 2.22 2.98 3.17 4.05 4.05 3.09 PW3 3.11 3.59 4.16 4.96 5.33 5.67 4.47 HYB3 1.72 2.13 2.71 3.19 4.13 4.37 3.04 It is worth noting that the overall ARPD obtained with these methods is smaller than that obtained by the best procedures using allocation rule 2. Observe that, in this group, PF3 and HPF23 have similar performance. The overall ARPD is 3.09 and 3.04, respectively. Notice that HPF23 is slightly better for 2, 3 and 4 plants; whereas PF3 is slightly better when the number of plants increases. Finally, the ARPD of the last group of heuristics is shown in Table VII. As can be seen, heuristics RC1_1 and RC1_m perform better than RC2. In particular, RC1_m is the one with smaller ARPD. This means that, during the allocation process of jobs, it is better to select the factory which has the last machine available sooner. TABLE VII. ARPD VALUES PER HEURISTIC AND NUMBER OF FACTORIES IN GROUP 4 Heuristics Number of factories 2 3 4 5 6 7 All RC1_1 2.24 3.73 4.46 5.21 5.76 6.58 4.66 RC1_m 2.27 2.89 3.66 4.49 5.78 6.05 4.19 RC2 4.17 4.81 5.34 5.35 7.14 7.58 5.73 for the heuristic factor (Figure 1), which is the most significant. As can be seen, there are no statistically significant differences between PF3 and HPF23, because their confidence intervals overlap; nor are there significant differences between RC1_m and RC1_1. Fig. 1.Interval Plot of compared heuristics at 95% confidence Interval Plot of Heuristics 95% CI for the Mean 10 9 8 7 ARPD Next, in order to compare the best heuristics of each group, we summarize in Table VIII the ARPD values of the two best methods from each group. From these results, one can see that HPF23 is the heuristic which performs better if 2, 3, and 4 factories are considered, whereas PF3 is better with 5 factories. When the number of factories increases (to 6 and 7 factories), TR2 is the best one. 6 5 4 TABLE VIII. ARPD VALUES OF THE BEST HEURISTICS OF EACH GROUP 3 2 Heuristics RC1 _1 Number of factories 2 3 4 5 6 7 All TR1 10.1 8.38 7.31 6.65 6.35 5.97 7.45 CDS1 10.1 9.58 9.15 8.95 9.22 9.24 9.37 TR2 8.82 6.29 4.91 3.7 2.77 2.43 4.82 CDS2 9.05 7.73 6.63 5.65 5.15 4.99 6.53 PF3 2.05 2.22 2.98 3.17 4.05 4.05 3.09 HPF23 1.72 2.13 2.71 3.19 4.13 4.37 3.04 RC1_1 2.24 3.73 4.46 5.21 5.76 6.58 4.66 RC1_m 2.27 2.89 3.66 4.49 5.78 6.05 4.19 RC1 _m PF3 HPF23 CDS2 TR2 CDS1 TR1 A more detailed analysis has been carried out by pairwise comparison between heuristics using the ARPD as the response variable. Figure 2 shows the obtained results. The means that do not share a letter are significantly different. Fig.2 Tukey Pairwise Comparison Grouping Information Using the Tukey Method and 95% Confidence In order to see if the differences observed in the previous table are significant, we carried out a multifactorial ANOVA on the results of these heuristics and all instances. The hypotheses of ANOVA were checked and satisfied. The response variable is the RPD, and the factors are the algorithm, n, m and F. Table IX shows the ANOVA results, where it can be seen that all factors are significant (p-value=0.00). TABLE IX. MULTIFACTORIAL ANOVA TABLE FOR HEURISTICS, n, m AND F Source Analysis of Variance DF Adj SS Heuristics 7 n 4 1036 m 2 F 5 Adj MS 24898 3556.87 F-Value P-Value 241.44 0.000 258.88 17.57 0.000 2789 1394.31 94.65 0.000 3.34 0.000 246 49.24 To analyze the differences between heuristics, we built the corresponding mean plot with the confidence interval at 95% Heuristics CDS1 TR1 CDS2 TR2 RC1_1 RC1_m PF3 HPF23 N 720 720 720 720 720 720 720 720 Mean Grouping 9,77715 A 7,85901 B 6,93755 C 5,22055 D 5,06572 D E 4,59480 E 3,48889 F 3,44187 F As can be observed in Figure 2, PF3 and HPF23 have the same performance, as well as RC1_1 and RC1_m. Both groups can be distinguished easily in Figure 1, but we can also see from this test that TR2 and RC1_m have similar performance. A second analysis that is necessary for evaluating heuristic efficiency is the CPU time required for reaching the solution. Table X shows the average CPU time in milliseconds for each procedure and number of factories. The algorithms that consume the least are those that use allocation methods 1 and 2. Next is the RC1 group. Finally, the algorithms that consume the most are those that use allocation method 3. Remember that, in this method, the segment of the original sequence is first assigned to each plant, and then the insertion procedures are applied to improve the sequence. This helps to obtain a high quality solution, but the required CPU time increases considerably. Hence, from the overall ARPD point of view, HPF23 or PF3 are the best heuristics for solving the DBFSP. However, RC1_m and TR2 cannot be discarded as initial metaheuristic solution procedures, because they obtain quite good solutions with less CPU time. TABLE X. AVERAGE CPU TIME, IN MILLISECONDS, FOR EACH HEURISTIC AND NUMBER OF FACTORIES Heuristics Number of factories 2 3 4 5 6 7 All TR1 2.6 2.6 2.5 3 3 3 2.7 CDS1 7.2 7.2 7.2 7.2 7.2 7.2 7.2 TR2 3 3 2.9 2.9 2.9 2.9 2.9 CDS2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 PF3 226 103 59.6 39.3 28.3 22.4 79.6 HPF23 238 114 70.1 49.8 38.9 32.5 90.5 RC1_1 10.2 10.1 10.5 44.8 34.2 27.1 22.8 RC1_m 10.1 10.1 10.2 44.9 34.2 27.4 22.8 IV. Another direction for future research is to develop efficient metaheuristics to solve this problem. Such a method would make use of these constructive procedures to obtain the initial solution, since the use of good initial solution procedures normally lead to improved metaheuristic performance. ACKNOWLEDGMENT Ramon Companys and Imma Ribas are partially supported by the Spanish Ministry of Science and Innovation, under the project RESULT - Realistic Extended Scheduling Using Light Techniques, with reference DPI2012-36243-C02-01. REFERENCES [1] J. Behnamian and S. M. T. Fatemi Ghomi, “A survey of multifactory scheduling,” J. Intell. Manuf., no. October 2012, pp. 1–19, 2014. [2] S. M. Johnson, “Optimal two-and three-stage production schedules with set up times included,” Nav. Res. Logist. Q., vol. 1, pp. 61–68, 1954. [3] B. Naderi and R. Ruiz, “The distributed permutation flowshop scheduling problem,” Comput. Oper. Res., vol. 37, no. 4, pp. 754– 768, Apr. 2010. [4] J. Gao and R. Chen, “A hybrid genetic algorithm for the distributed permutation flowshop scheduling problem,” Mar. 2012. [5] J. Gao, R. Chen, and Y. Liu, “A Knowledge-based Genetic Algorithm for Permutation Flowshop Scheduling Problems with Multiple Factories,” Int. J. Adv. Comput. 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One typical complication for this problem is that the blocking constraint tends to increase the completion time of jobs if the job being processed cannot leave the machine, which results from the next machine being unavailable. Therefore, it is interesting to analyze not only the performance of the adapted heuristics proposed by the DPFSP, but also to design heuristics that are specific to this problem and that allow us to address it adequately. The proposed methods use an approach that is totally different from those proposed for the DPFSP. After taking the DPFSP procedures that we adapted to DBFSP and comparing them to the approaches that were specifically designed for DBPFSP, we find that HPF23 and PF3 (which consider the specificity of the DBFSP) performed much better. However, the time required is considerably greater than for the other methods. 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