Add test: N-queens problem.

queens_4x4: 8 guesses.
queens_5x5: 21 guesses.
queens_6x6: 66 guesses.
queens_7x7: 179 guesses.
queens_8x8: 662 guesses.

tests/queens.rs

@@ -0,0 +1,93 @@
+//! N-queens problem.
+//!
+//! https://en.wikipedia.org/wiki/Eight_queens_puzzle
+
+extern crate puzzle_solver;
+
+use puzzle_solver::{Constraint,Puzzle,PuzzleSearch,Solution,Val,VarToken};
+
+struct NoDiagonal {
+    vars: Vec<VarToken>,
+}
+
+impl Constraint for NoDiagonal {
+    fn on_assigned(&self, search: &mut PuzzleSearch, var: VarToken, val: Val)
+            -> bool {
+        let y1 = self.vars.iter().position(|&v| v == var).expect("unreachable");
+        for (y2, &var2) in self.vars.iter().enumerate() {
+            if !search.is_assigned(var2) {
+                let x1 = val;
+                let dy = (y1 as Val) - (y2 as Val);
+                search.remove_candidate(var2, x1 - dy);
+                search.remove_candidate(var2, x1 + dy);
+            }
+        }
+
+        true
+    }
+}
+
+fn make_queens(n: usize) -> (Puzzle, Vec<VarToken>) {
+    let mut sys = Puzzle::new();
+    let pos: Vec<Val> = (0..n as Val).collect();
+    let vars = sys.new_vars_with_candidates_1d(n, &pos);
+
+    sys.all_different(&vars);
+    sys.add_constraint(Box::new(NoDiagonal{ vars: vars.clone() }));
+    (sys, vars)
+}
+
+fn print_queens(dict: &Solution, vars: &Vec<VarToken>) {
+    let n = vars.len() as Val;
+    for &var in vars.iter() {
+        for i in 0..n {
+            print!(" {}", if i == dict[var] { "Q" } else { "." });
+        }
+        println!();
+    }
+}
+
+#[test]
+fn queens_4x4() {
+    let (mut sys, vars) = make_queens(4);
+    let dict = sys.solve_all();
+    assert_eq!(dict.len(), 2);
+    print_queens(&dict[0], &vars);
+    println!("queens_4x4: {} guesses", sys.num_guesses());
+}
+
+#[test]
+fn queens_5x5() {
+    let (mut sys, vars) = make_queens(5);
+    let dict = sys.solve_all();
+    assert_eq!(dict.len(), 10);
+    print_queens(&dict[0], &vars);
+    println!("queens_5x5: {} guesses", sys.num_guesses());
+}
+
+#[test]
+fn queens_6x6() {
+    let (mut sys, vars) = make_queens(6);
+    let dict = sys.solve_all();
+    assert_eq!(dict.len(), 4);
+    print_queens(&dict[0], &vars);
+    println!("queens_6x6: {} guesses", sys.num_guesses());
+}
+
+#[test]
+fn queens_7x7() {
+    let (mut sys, vars) = make_queens(7);
+    let dict = sys.solve_all();
+    assert_eq!(dict.len(), 40);
+    print_queens(&dict[0], &vars);
+    println!("queens_7x7: {} guesses", sys.num_guesses());
+}
+
+#[test]
+fn queens_8x8() {
+    let (mut sys, vars) = make_queens(8);
+    let dict = sys.solve_all();
+    assert_eq!(dict.len(), 92);
+    print_queens(&dict[0], &vars);
+    println!("queens_8x8: {} guesses", sys.num_guesses());
+}