README.md

N-Queens - 92 Solutions In Prolog¶

This code solves the N-Queens problem in Prolog using the CLP(FD): Constraint Logic Programming over Finite Domain Library

nqueens() creates a 2D array datastructure, representing the board coordinates of each queen
applyConstraints() recursively iterates through each queen on the board
checkConstraints() applies the constraints: no two queens on same row/column/diagonal; and recurses through the list of remaining queens
optimizeQueens() hardcodes each queen to live in a named row, this greatly reduces the computational complexity of the problem
- Note: it is not possible to pass Index + 1 into a prolog function, instead it must be declared and solved first as its own variable: NextIndex is Index + 1
print_board() + print_line() render the ASCII graphics in a functional manner
all_nqueens() uses findall() to solve nqueens() whilst repeatedly adding previous solutions as future constraints
print_nqueens_all(N) is the main display/execution function

brew install swi-prolog  # swipl
brew install gnu-prolog  # gprolog

swipl -f nqueens.prolog -t 'print_nqueens_all(8)' > nqueens.txt