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# $Id: queens5.zpl,v 1.5 2009/09/13 16:15:53 bzfkocht Exp $
#
# This is a formulation of the n queens problem using binary variables.
# variables. Since the number of queens is maximized, the size of the
# board can be set arbitrarily.
#
# set packing formulation
#
param columns := 8;
set I := { 1 .. columns };
set IxI := I * I;
var x[IxI] binary;
maximize queens: sum <i,j> in IxI : x[i,j];
# one in a row
subto row: forall <i> in I do
sum <i,j> in IxI : x[i,j] <= 1;
# one in a column
subto col: forall <j> in I do
sum <i,j> in IxI : x[i,j] <= 1;
# rowwise down
subto drd: forall <i> in I do
sum <m,n> in IxI with m - i == n - 1: x[m,n] <= 1;
# rowwise up
subto dru: forall <i> in I do
sum <m,n> in IxI with m - i == 1 - n: x[m,n] <= 1;
# colwise down
subto dcd: forall <j> in I do
sum <m,n> in IxI with m - 1 == n - j: x[m,n] <= 1;
# colwise up
subto dcu: forall <j> in I do
sum <m,n> in IxI with columns - m == n - j: x[m,n] <= 1;
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