#!/usr/bin/env python3 # Copyright 2010-2024 Google LLC # Licensed under the Apache License, Version 2.0 (the "License"); # you may not use this file except in compliance with the License. # You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # See the License for the specific language governing permissions and # limitations under the License. """Finding an optimal wedding seating chart. From Meghan L. Bellows and J. D. Luc Peterson "Finding an optimal seating chart for a wedding" http://www.improbable.com/news/2012/Optimal-seating-chart.pdf http://www.improbable.com/2012/02/12/finding-an-optimal-seating-chart-for-a-wedding Every year, millions of brides (not to mention their mothers, future mothers-in-law, and occasionally grooms) struggle with one of the most daunting tasks during the wedding-planning process: the seating chart. The guest responses are in, banquet hall is booked, menu choices have been made. You think the hard parts are over, but you have yet to embark upon the biggest headache of them all. In order to make this process easier, we present a mathematical formulation that models the seating chart problem. This model can be solved to find the optimal arrangement of guests at tables. At the very least, it can provide a starting point and hopefully minimize stress and arguments. Adapted from https://github.com/google/or-tools/blob/master/examples/csharp/wedding_optimal_chart.cs """ import time from typing import Sequence from absl import app from ortools.sat.python import cp_model class WeddingChartPrinter(cp_model.CpSolverSolutionCallback): """Print intermediate solutions.""" def __init__(self, seats, names, num_tables, num_guests): cp_model.CpSolverSolutionCallback.__init__(self) self.__solution_count = 0 self.__start_time = time.time() self.__seats = seats self.__names = names self.__num_tables = num_tables self.__num_guests = num_guests def on_solution_callback(self): current_time = time.time() objective = self.objective_value print( "Solution %i, time = %f s, objective = %i" % (self.__solution_count, current_time - self.__start_time, objective) ) self.__solution_count += 1 for t in range(self.__num_tables): print("Table %d: " % t) for g in range(self.__num_guests): if self.value(self.__seats[(t, g)]): print(" " + self.__names[g]) def num_solutions(self) -> int: return self.__solution_count def build_data(): """Build the data model.""" # Easy problem (from the paper) # num_tables = 2 # table_capacity = 10 # min_known_neighbors = 1 # Slightly harder problem (also from the paper) num_tables = 5 table_capacity = 4 min_known_neighbors = 1 # Connection matrix: who knows who, and how strong # is the relation connections = [ [1, 50, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [50, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 50, 1, 1, 1, 1, 10, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 50, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 50, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 50, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1, 1, 50, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 1, 1, 1, 1, 50, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [1, 1, 10, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 50, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 50, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], ] # Names of the guests. B: Bride side, G: Groom side names = [ "Deb (B)", "John (B)", "Martha (B)", "Travis (B)", "Allan (B)", "Lois (B)", "Jayne (B)", "Brad (B)", "Abby (B)", "Mary Helen (G)", "Lee (G)", "Annika (G)", "Carl (G)", "Colin (G)", "Shirley (G)", "DeAnn (G)", "Lori (G)", ] return num_tables, table_capacity, min_known_neighbors, connections, names def solve_with_discrete_model() -> None: """Discrete approach.""" num_tables, table_capacity, min_known_neighbors, connections, names = build_data() num_guests = len(connections) all_tables = range(num_tables) all_guests = range(num_guests) # Create the cp model. model = cp_model.CpModel() # # Decision variables # seats = {} for t in all_tables: for g in all_guests: seats[(t, g)] = model.new_bool_var("guest %i seats on table %i" % (g, t)) colocated = {} for g1 in range(num_guests - 1): for g2 in range(g1 + 1, num_guests): colocated[(g1, g2)] = model.new_bool_var( "guest %i seats with guest %i" % (g1, g2) ) same_table = {} for g1 in range(num_guests - 1): for g2 in range(g1 + 1, num_guests): for t in all_tables: same_table[(g1, g2, t)] = model.new_bool_var( "guest %i seats with guest %i on table %i" % (g1, g2, t) ) # Objective model.maximize( sum( connections[g1][g2] * colocated[g1, g2] for g1 in range(num_guests - 1) for g2 in range(g1 + 1, num_guests) if connections[g1][g2] > 0 ) ) # # Constraints # # Everybody seats at one table. for g in all_guests: model.add(sum(seats[(t, g)] for t in all_tables) == 1) # Tables have a max capacity. for t in all_tables: model.add(sum(seats[(t, g)] for g in all_guests) <= table_capacity) # Link colocated with seats for g1 in range(num_guests - 1): for g2 in range(g1 + 1, num_guests): for t in all_tables: # Link same_table and seats. model.add_bool_or( [ ~seats[(t, g1)], ~seats[(t, g2)], same_table[(g1, g2, t)], ] ) model.add_implication(same_table[(g1, g2, t)], seats[(t, g1)]) model.add_implication(same_table[(g1, g2, t)], seats[(t, g2)]) # Link colocated and same_table. model.add( sum(same_table[(g1, g2, t)] for t in all_tables) == colocated[(g1, g2)] ) # Min known neighbors rule. for g in all_guests: model.add( sum( same_table[(g, g2, t)] for g2 in range(g + 1, num_guests) for t in all_tables if connections[g][g2] > 0 ) + sum( same_table[(g1, g, t)] for g1 in range(g) for t in all_tables if connections[g1][g] > 0 ) >= min_known_neighbors ) # Symmetry breaking. First guest seats on the first table. model.add(seats[(0, 0)] == 1) ### Solve model. solver = cp_model.CpSolver() solution_printer = WeddingChartPrinter(seats, names, num_tables, num_guests) solver.solve(model, solution_printer) print("Statistics") print(" - conflicts : %i" % solver.num_conflicts) print(" - branches : %i" % solver.num_branches) print(" - wall time : %f s" % solver.wall_time) print(" - num solutions: %i" % solution_printer.num_solutions()) def main(argv: Sequence[str]) -> None: if len(argv) > 1: raise app.UsageError("Too many command-line arguments.") solve_with_discrete_model() if __name__ == "__main__": app.run(main)