metaheuristics for optimization problems in sports meta’08, october 2008 1/92 celso c. ribeiro...
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META’08, October 2008 1/92 Metaheuristics for optimization problems in sports
Celso C. Ribeiro
Joint work with S. Urrutia,A. Duarte, and A. Guedes
2nd International Conference on Metaheuristics and Nature Inspired Computing (META’08)
Applications of Metaheuristics to
Optimization Problems in Sports
Hammamet, October 2008
META’08, October 2008 2/92 Metaheuristics for optimization problems in sports
Summary• Optimization problems in sports– Motivation– Problems, applications, and solution
methods
• Applications of metaheuristics– Traveling tournament problem– Referee assignment– Carry-over effect minimization– Brazilian professional basketball
tournament
• Perspectives and concluding remarks
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Motivation• Sports competitions involve many
economic and logistic issues • Multiple decision makers: federations,
TV, teams, security authorities, ...• Conflicting objectives:– Maximize revenue (attractive games in
specific days)– Minimize costs (traveled distance)– Maximize athlete performance (time to rest)– Fairness (avoid playing all strong teams in a
row)– Avoid conflicts (teams with a history of
conflicts playing at the same place)
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Motivation• Professional sports:– Millions of fans– Multiple agents: organizers, media,
fans, players, security forces, ...– Big investments:
• Belgacom TV: €12 million per year for soccer broadcasting rights
• Baseball US: > US$ 500 millions• Basketball US: > US$ 600 millions
– Main problems: maximize revenues, optimize logistic, maximize fairness, minimize conflicts
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Motivation
• Amateur sports:– Thousands of athletes– Athletes pay for playing– Large number of simultaneous events– Amateur leagues do not involve as
much money as professional leagues but, on the other hand, amateur competitions abound
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Optimization problems in sports
• Examples:– Qualification/elimination problems– Tournament scheduling– Referee assignment– Tournament planning (teams? dates?
rules?)– League assignment (which teams in each
league?)– Carry-over minimization
...– Optimal strategies for curling!
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Qualification/elimination problems
• Team managers, players, fans and the press are often eager to know the chances of a team to be qualified for the playoffs of a given competition
How many points a team should make to:• … be sure of finishing among the p
teams in the first positions? (sufficient condition for play-offs qualification)
• … have a chance of finishing among the p teams in the first positions? (necessary condition for play-offs qualification)
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Qualification/elimination problems• Schwartz 1966: mathematical elimination
from play-offs in the Major League Baseball (MLB) solved with maximum flow algorithm
• Robinson 1991: IP models and further results for the play-offs elimination problem
• McCormick 2000: elimination from the p-th position is NP-complete.
• Bernholt et al. 2002: first place elimination is NP-complete under the {(3,0),(1,1)} soccer rule
• Adler et al. 2003: ILP models for MLB
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Qualification/elimination problems
• Ribeiro & Urrutia 2005: integer programming for qualification/elimination problems in the Brazilian soccer championship and the World Cup (FUTMAX)
• Cheng & Steffy 2006: integer programming for qualification/elimination problems in the National Hockey League.
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FUTMAX in the WWW• FUTMAX project• Results of the games automatically
collected from the web (multi-agents)• Models generated (four problems for each
team)• Problems solved with CPLEX 9.0• HTML file automatically built from the
results • Automatic publication in the web• FUTMAX is often able to prove that
statements made by the Press and administrators are not true
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ResultsFUTMAX can also be used to follow the situation of each team:
Possible points
Points for guaranteed qualification
Points for possible qualification
Points accumulated
FLUMINENSE
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Tournament scheduling• Timetabling is the major area of
applications: game scheduling is a difficult task, involving different types of constraints, logistic issues, multiple objectives, and several decision makers
• Round Robin schedules:– Every team plays each other a fixed
number of times– Every team plays once in each round– Single (SRR) or double (DRR) round robin
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Tournament scheduling• Problems:– Minimize distance (costs)– Minimize breaks (fairness and equilibrium,
every two rounds there is a game in the city)
– Balanced tournaments (even distribution of fields used by the teams: n teams, n/2 fields, SRR with n-1 games/team, 2 games/team in n/2-1 fields and 1 in the other)
– Minimize carry over effect (maximize fairness, polygon method)
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1-factorizations• Factor of a graph G=(V, E): subgraph
G’=(V,E’) with E’E• 1-factor: all nodes have degree equal to
1• Factorization of G=(V,E): set of edge-
disjoint factors G1=(V,E1), ..., Gp=(V,Ep), such that E1...Ep=E
• 1-factorization: factorization into 1-factors
• Oriented factorization: orientations assigned to edges
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Example: 1-factorization of K6
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Oriented 1-factorization of K6
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• SRR tournament:– Each node of Kn represents a team
– Each edge of Kn represents a game
– Each 1-factor of Kn represents a round
– Each ordered 1-factorization of Kn represents a feasible schedule for n teams
– Edge orientations define teams playing at home
– Dinitz, Garnick & McKay, “There are 526,915,620 nonisomorphic one-factorizations of K12” (1995)
1-factorizations
Open problem:How many schedules exist for a single round robin tournament with n teams?
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Distance minimization problems
• Whenever a team plays two consecutive games away, it travels directly from the facility of the first opponent to that of the second
• Maximum number of consecutive games away (or at home) is often constrained
• Minimize the total distance traveled (or the maximum distance traveled by any team)
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Distance minimization problems
• Methods:– Metaheuristics: simulated annealing,
iterated local search, hill climbing, tabu search, GRASP, genetic algorithms, ant colonies
– Integer programming– Constraint programming– IP/CP column generation– CP with local search
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Break minimization problems• There is a break whenever a team has
two consecutive home games (or two away games)
• Break minimization:– De Werra 1981: minimum number of
breaks is n-2• Every team must have a different home-away
pattern (they must play in some round)• Only two patterns without breaks:
– HAHAHAH...– AHAHAHA...
– Constructive algorithm to obtain schedules with exactly n-2 breaks
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Break minimization problems
• Break minimization is somehow opposed to distance minimization
• Urrutia & Ribeiro 2006: a special case of the Traveling Tournament Problem is equivalent to a break maximization problem
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Fixed timetables/venues• Given a fixed timetable, find a home-away
assignment minimizing breaks/distance:– Metaheuristics, constraint programming,
integer programming– Miyashiro & Matsui 2005: polynomial method
for break minimization if the minimal number of breaks is smaller than or equal to n
• Given a fixed venue assignment for each game, find a timetable minimizing breaks/distance:– Melo, Urrutia & Ribeiro 2007: integer
programming
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Decomposition methods• Nemhauser and Trick 1998:
1. Find home-away patterns2. Create an schedule for place holders
consistent with a subset of home-away patterns
3. Assign teams to place holders
• Order in which the above tasks are tackled may vary depending on the application
• Henz 2001: CP may work better than IP and complete enumeration for all the tasks
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Decomposition methods• Frequently used for scheduling real
tournaments:– Nemhauser & Trick 1998: Atlantic Coast
Conference (basketball)– Bartsch et al. 2006: Austrian and
German soccer– Della Croce & Oliveri 2006: Italian soccer– Ribeiro & Urrutia 2006: Brazilian soccer– Durán, Noronha, Ribeiro, Sourys,
Weintraub 2006: Chilean soccer
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Applications of metaheuristics
Traveling Tournament Problem (TTP) and its mirrored version (mTTP)
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Formulation
• Traveling Tournament Problem (TTP)– n (even) teams take part in a
tournament– Each team has its own stadium at its
home city– Distances between the stadiums are
known– A team playing two consecutive away
games goes directly from one city to the other, without returning to its home city
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Formulation– Double round-robin tournament:
• 2(n-1) rounds, each with n/2 games• Each team plays against every other team
twice, one at home and the other away
– No team can play more than three games in a home stand (home games) or in a road trip (away games)
• Goal: minimize the distance traveled by all teams, to reduce traveling costs and to give more time to the players to rest and practice
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Formulation
• Mirrored Traveling Tournament Problem (mTTP):– All teams face each other once in the first
phase (n-1 rounds)– In the second phase (n-1 rounds), teams
play each other again in the same order, following an inverted home-away pattern
– Games in the second phase determined by those in the first
• Set of feasible solutions to the MTTP is a subset of those to the TTPRibeiro and Urrutia (PATAT 2004, EJOR 2007)
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• Three steps:1. Schedule games using abstract teams:
polygon method defines the structure of the tournament
2. Assign real teams to abstract teams: greedy heuristic to QAP (number of travels between stadiums of the abstract teams x distances between the stadiums of the real teams)
3. Select stadium for each game (home/away pattern) in the first phase (mirrored tournament):1. Build a feasible assignment of stadiums, starting
from a random assignment in the first round2. Improve this assignment, using a simple local
search algorithm based on home-away swaps
Constructive heuristic
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Constructive heuristic
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Example: “polygon method” for n=6
1st round
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Constructive heuristic
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Example: “polygon method” for n=6
2nd round
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Simple neighborhoods
• Home-away swap (HAS): modify the stadium of a game
• Team swap (TS): exchange the sequence of opponents of a pair of teams over all rounds
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Partial round swap (PRS)
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Partial round swap (PRS)
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Ejection chain: game rotation (GR)
• Neigborhood “game rotation” (GR) (ejection chain):– Enforce a game to be played at some
round: add a new edge to a given 1-factor of the current 1-factorization (schedule)
– Use an ejection chain to recover a 1-factorization
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Ejection chain: game rotation (GR)
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Neighborhoods• Only moves in neighborhoods PRS and GR
may change the structure of the initial schedule
• However, PRS moves not always exist, due to the structure of the solutions built by polygon method e.g. for n = 6, 8, 12, 14, 16, 20, 24
• PRS moves may appear after an ejection chain move is made
• Ejection chain moves may find solutions that are not reachable through other neighborhoods: escape from local optima
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GRASP+ILS heuristic• Hybrid improvement heuristic for the
MTTP:– Combination of GRASP and ILS– Initial solutions: randomized version of
the constructive heuristic– Local search with first improving move:
use TS, HAS, PRS and HAS cyclically in this order, until a local optimum for all neighborhoods is found
– Perturbation: random move in GR neighborhood
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GRASP+ILS heuristicwhile .not.StoppingCriterion
S GenerateRandomizedInitialSolution() S LocalSearch(S)repeat
S’ Perturbation(S,history)S’ LocalSearch(S’)S AceptanceCriterion(S,S’,history)S* UpdateBestSolution(S,S*)
until ReinitializationCriterionend
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• Constructive heuristic is very fast and effective
• GRASP+ILS is very fast and finds very good solutions, even better than the best known for the corresponding (less constrained) not necessarily mirrored instances
• Effectiveness of the ejection chains• Theoretical complexity still open• Lower bounds:– Independent lower bound: Easton et al. 2001– MNTLB (improvement over ILB): Urrutia et al.
2007– Benders decomposition: Trick & Rasmussen
2007
Concluding remarks
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Applications of metaheuristics
Referee Assignment Problem (RAP)
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Motivation• Regional amateur leagues in the
US (baseball, basketball, soccer): hundreds of games every weekend in different divisions
• In a single league in California there are up to 500 soccer games in a weekend, to be refereed by hundreds of certified referees
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Motivation• MOSA (Monmouth & Ocean Counties Soccer
Association) League (NJ): boys & girls, ages 8-18, six divisions per age/gender group, six teams per division: 396 games every Sunday (US$ 40 per referee; U$ 20 per linesman, two linesmen)
• Problem: assign referees to gamesDuarte, Ribeiro & Urrutia (PATAT 2006, LNCS 2007)
• Referee assignment involves many constraints and multiple objectives
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Referee assignment
• Possible constraints:– Different number of referees may be
necessary for each game– Games require referees with different
levels of certification: higher division games require referees with higher skills
– A referee cannot be assigned to a game where he/she is a player
– Timetabling conflicts and traveling times
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Referee assignment• Possible constraints (cont.):– Referee groups: cliques of referees that
request to be assigned to the same games (relatives, car pools)• Hard links• Soft links
– Number of games a referee is willing to referee
– Traveling constraints– Referees that can officiate games only at
a certain location or period of the day
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Referee assignment
• Possible objectives:– Difference between the target number
of games a referee is willing to referee and the number of games he/she is assigned to
– Traveling/idle time between consecutive games
– Number of inter-facility travels– Number of games assigned outside
his/her preferred time-slots or facilities– Number of violated soft links
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Problem statement• Games are already scheduled (facility
– time slot)• Each game has a number of refereeing
positions to be assigned to referees• Each refereeing position to be filled by
a referee is called a refereeing slot
• S = {s1, s2,..., sn}: refereeing slots to be filled by referees
• R = {r1, r2,..., rm}: referees
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Problem statement• pi: skill level of referee ri • qj: minimum skill level a referee must
have to be assigned to refereeing slot sj
• Mi: maximum number of games referee ri can officiate
• Ti: target number of games referee ri is willing to officiate
• Each referee may choose a set of time slots where he/she is not available to officiate
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Problem statement
• Problem: assign a referee to each refereeing slot
• Constraints:– Referees officiate in a single facility on the same
day– Minimum skill level requirements– Maximum number of games– Timetabling conflicts and availability
• Objective: minimize the sum over all referees of the absolute value of the difference between the target and the actual number of games assigned to each referee (0-1 integer linear programming model)
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Solution approach
• Three-phase heuristic approach 1. Greedy constructive heuristic2. ILS-based repair heuristic to make the
initial solution feasible (if necessary): minimization of the number of violations
3. ILS-based procedure to improve a feasible solution
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Solution approachAlgorithm RefereeAssignmentHeuristic (MaxIter)1. S* BuildGreedyRandomizedSolution ();2. If not isFeasible (S*) then3. S* RepairHeuristic (S*, MaxIter);4. If isFeasible (S*) then5. S* ImprovementHeuristic (S*);6. Else “infeasible”7.Return S*
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Numerical results• Randomly generated instances following
patterns similar to real-life applications• Instances with up to 500 games and
1,000 referees– Different number of facilities– Different patterns of the target number of
games
• Five different instances for each configuration
• MaxIter = 1,000
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Numerical results
• For each instance, average time and average objective value over ten runs
• Codes implemented in C• Results on a 2.0 GHz Pentium IV
processor with 256 Mbytes• Initial solutions:– greedy constructive heuristic– random assignments (to test the repair
heuristic)
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Numerical results
Instance
Construction Repair Improvement
time
(s)value feas. time (s) value feas. time (s) value
I1 0.02 1286.20 10 — — — 32.34 619.60
I2 0.02 1360.00 5 0.47 1338.00 10 31.81 623.40
I3 0.02 1269.00 2 0.60 1247.00 10 33.87 621.60
I4 0.03 — — 1.14 1303.20 10 30.28 627.20
I5 0.03 1302.00 3 1.40 12591.14
10 33.73 654.00
Table 1: Instances with 500 games, 750 referees, and 65 facilities
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Numerical resultsInstan
cepattern
Greedy Random
const.
(s)repair
(s)feas. repair
(s)feas.
I1 P00.03 11.27 10 79.80 9
I2 P00.03 6.69 10 80.80 10
I3 P00.03 11.33 10 86.20 8
I4 P00.03 4.61 10 30.60 10
I5 P00.03 3.39 10 29.10 10
I1 P10.03 2.75 10 33.50 10
I2 P10.02 19.29 10 134.60 2
I3 P10.03 14.77 10 135.10 8
I4 P10.03 1.22 10 38.00 10
I5 P10.03 2.69 10 32.90 10
Table 4: Greedy vs. randomly generated initial solutions
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Improvements and extensions• Greedy constructive heuristic:– First, assign each referee to a number of
refereeing slots as close as possible to his/her target number of games
– Second, if there are still unassigned slots, assign more games to each referee
• Improvement heuristic:– After each perturbation, instead of applying
a local search for both facilities involved in this perturbation, solve a MIP model associated with the subproblem considering all refereeing slots and referees corresponding to these facilities (“MIP it!”)
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Numerical results
Figure 3: 500 games, 750 referees, 85 facilities, pattern P0 (target = 529)
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Bi-criteria problem (biRAP)
• Same constraints as in the single objective version
• Objectives:1. minimize the sum over all referees of the
absolute value of the difference between the target and the actual number of games assigned to each referee
2. minimize the sum over all referees of the total idle time between consecutive games
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Bi-criteria problem (biRAP)• Formulation: bi-criteria set partitioning
problem• Variables: possible “routes” for each
referee (stops correspond to refereeing positions)
• Each “route” has at most 4 to 5 stops: number of variables is limited
• Each refereeing position has to be filled by exactly one qualified refere
• Each referee must perform exactly one “route”
• Goal: find the set of (potentially) efficient solutions
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Solution approach
• Exact approach: dichotomic method
50 games and 100 referees
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Solution approach
• Heuristic approach:– Perform three-phase ILS-based heuristic for a
fixed number of search directions– Each search direction represents a set of
weights associated with each objective– Directions are chosen as in the dichotomic
method– All new potentially efficient solutions found
during the search are progressively stored– Former potentially efficient solutions are
discarded during the search (quadtree is used)– Perform a post-optimization path-relinking
procedure
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Numerical results
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Numerical results
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Numerical results
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Conclusions
• New optimization problem in sports• Effective heuristics:
construction, repair, improvement, path relinking
• Quick procedures to build good initial solutions
• Bicriteria approach finds good approximations of the Pareto frontier
• Other constraints and criteria may be considered
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Applications of metaheuristics
Carry-over minimization problem
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Carry-over effects
• Team B receives a carry-over effect (COE) due to team A if there is a team X that plays A in round r and B in round r+1
1 2 3 4 5 6 7A H C D E F G BB C D E F G H AC B A F H E D GD E B A G H C FE D G B A C F HF G H C B A E DG F E H D B A CH A F G C D B E
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Carry-over effects
• Team B receives a carry-over effect (COE) due to team A if there is a team X that plays A in round r and B in round r+1
1 2 3 4 5 6 7A H C D E F G BB C D E F G H AC B A F H E D GD E B A G H C FE D G B A C F HF G H C B A E DG F E H D B A CH A F G C D B E
Team A receives
COE due to B
Team G receives
COE due to D
Team A receives
COE due to E
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Carry-over effects matrix
• SRRT and carry-over effects matrix (COEM)
A B C D E F G HA 0 0 3 0 1 2 1 0B 5 0 0 0 1 0 0 1C 0 1 0 3 0 3 0 0D 0 2 0 0 2 0 3 0E 1 1 0 2 0 2 0 1F 0 0 0 0 2 0 3 2G 0 3 1 0 0 0 0 3H 1 0 3 2 1 0 0 0
1 2 3 4 5 6 7A H C D E F G BB C D E F G H AC B A F H E D GD E B A G H C FE D G B A C F HF G H C B A E DG F E H D B A CH A F G C D B E
RRT COE matrix
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Carry-over effects matrix
• RRT and carry-over effects matrix (COEM)
A B C D E F G HA 0 0 3 0 1 2 1 0B 5 0 0 0 1 0 0 1C 0 1 0 3 0 3 0 0D 0 2 0 0 2 0 3 0E 1 1 0 2 0 2 0 1F 0 0 0 0 2 0 3 2G 0 3 1 0 0 0 0 3H 1 0 3 2 1 0 0 0
1 2 3 4 5 6 7A H C D E F G BB C D E F G H AC B A F H E D GD E B A G H C FE D G B A C F HF G H C B A E DG F E H D B A CH A F G C D B E
RRT COE Matrix
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Carry-over effects valueA B C D E F G H
A 0 0 3 0 1 2 1 0B 5 0 0 0 1 0 0 1C 0 1 0 3 0 3 0 0D 0 2 0 0 2 0 3 0E 1 1 0 2 0 2 0 1F 0 0 0 0 2 0 3 2G 0 3 1 0 0 0 0 3H 1 0 3 2 1 0 0 0
COE matrix
COEMDG = 3
COEMFH = 2
H
Ai
H
AjijCOEMCOEV 2)(
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Carry-over effects valueA B C D E F G H
A 0 0 3 0 1 2 1 0B 5 0 0 0 1 0 0 1C 0 1 0 3 0 3 0 0D 0 2 0 0 2 0 3 0E 1 1 0 2 0 2 0 1F 0 0 0 0 2 0 3 2G 0 3 1 0 0 0 0 3H 1 0 3 2 1 0 0 0
COE Matrix
H
Ai
H
AjijCOEMCOEV 2)(
Minimize!!!
COEMDG = 3
COEMFH = 2
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Example• Karate-Do competitions• Groups playing round-robin tournaments– Pan-american Karate-Do championship– Brazilian classification for World Karate-Do
championship
• Open weight categories– Physically strong contestants may fight
weak ones– One should avoid that a competitor benefits
from fighting (physically) tired opponents coming from matches against strong athletes
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Problem statement• Find a schedule with minimum COEV– RRT distributing the carry-over effects
as evenly as possible among the teams
• Best solution approaches to date in literature:– Random generation of 1-factors
permutations– Constraint Programming– Combinatorial designs
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Solution approach
• Multi-start + ILS heuristic• Solutions represented by 1-
factorizations– Canonical factorizations – Binary 1-factorizations
• Constructive algorithms– Rearragment of the 1-factors of a
solution (TSP-like greedy algorithms)• Nearest neighbor• Arbitrary insertion
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Solution approach
• Local search– Rearrangement of the 1-factors of the
solution (TSP-like procedures)– Partial Round Swap (PRS)
• Pertubations– Ejection chain: Game Rotation (GR)
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Multi-start + ILS heuristic
• Multi-start phase: generation of 10,000 solutions– 50% based on canonical 1-factorizations– 50% based on binary 1-factorizations
(whenever possible)– Constructive methods applied to the 1-
factors of the 1-factorizations– Local search
• Best solution of the multi-start phase is the input for the ILS algorithm
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Multi-start + ILS heuristicFor try = 1 to 10000 Do
S ← Initial_Solution();S ← Local_Search(S);S* ← Update_Best_Solution(S, S*);
End-For;S ← S*;While Not Stopping-Criterion Do
S' ← Pertubation(S);S' ← Local_Search(S’);S ← Acceptance_Criterion(S, S');S* ← Update_Best_Solution(S, S*);
End-While;
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Results
• Literature: instances with up to 20 teams
Teams Best results Our results4 12 126 60 608 56 56
10 108 10812 176 16014 234 25416 240 24018 340 40020 380 486
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Future research
• Weighted COEV minimization problem
• Weighted COEV min-max problem
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Applications of metaheuristics
Scheduling the Brazilian basketball tournament
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Perspectives and concluding remarks
• Optimization in sports is a field of increasing interest
• Sports management and scheduling are very attractive areas for applications of Operations Research
• Many interesting applications, often reviewed by the media
• Several problems with interesting theoretical structure
• Even small instances are hard to solve (e.g., TTP for n=10)
• Quick construction procedures to build good initial (feasible) solutions are a must
• Repair procedures• Successful applications of metaheuristics
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