Add convenience functions for equality.

This function takes a LHS and RHS only to give the illusion of an
equality.  It simply subtracts one from the other because our
equations are all in the form: coef_i var_i + constant == 0.

src/constraint/equality.rs

@@ -73,7 +73,6 @@ impl Constraint for Equality {
 #[cfg(test)]
 mod tests {
     use ::Puzzle;
-    use super::Equality;
 
     #[test]
     fn test_contradiction() {
@@ -81,7 +80,7 @@ mod tests {
         let v0 = puzzle.new_var_with_candidates(&[3]);
         let v1 = puzzle.new_var_with_candidates(&[0,1]);
 
-        puzzle.add_constraint(Box::new(Equality::new(v0 + 2 * v1 - 4)));
+        puzzle.equals(v0 + 2 * v1, 4);
 
         let search = puzzle.step();
         assert!(search.is_none());
@@ -93,7 +92,7 @@ mod tests {
         let v0 = puzzle.new_var_with_candidates(&[1]);
         let v1 = puzzle.new_var_with_candidates(&[1,2,3]);
 
-        puzzle.add_constraint(Box::new(Equality::new(v0 + v1 - 4)));
+        puzzle.equals(v0 + v1, 4);
 
         let search = puzzle.step().expect("contradiction");
         assert_eq!(search[v0], 1);

src/puzzle.rs

@@ -8,7 +8,7 @@ use std::ops::Index;
 use std::rc::Rc;
 use bit_set::BitSet;
 
-use ::{Constraint,Solution,Val,VarToken};
+use ::{Constraint,LinExpr,Solution,Val,VarToken};
 use constraint;
 
 /// A collection of candidates.
@@ -314,6 +314,23 @@ impl Puzzle {
         self.add_constraint(Box::new(constraint::AllDifferent::new(vars)));
     }
 
+    /// Add an Equality constraint.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut magic_square = puzzle_solver::Puzzle::new();
+    /// let vars = magic_square.new_vars_with_candidates_2d(3, 3,
+    ///         &[1,2,3,4,5,6,7,8,9]);
+    ///
+    /// magic_square.equals(vars[0][0] + vars[0][1] + vars[0][2], 15);
+    /// ```
+    pub fn equals<L,R>(&mut self, lhs: L, rhs: R)
+            where L: Into<LinExpr>, R: Into<LinExpr> {
+        self.add_constraint(Box::new(constraint::Equality::new(
+                lhs.into() - rhs.into())));
+    }
+
     /// Find any solution to the given puzzle.
     ///
     /// # Examples