Add test: xkcd knapsack problem.

xkcd_knapsack: 60 guesses.

tests/xkcd.rs

@@ -0,0 +1,50 @@
+//! xkcd Knapsack Problem.
+//!
+//! https://en.wikipedia.org/wiki/Knapsack_problem
+//! https://xkcd.com/287/
+
+extern crate num_rational;
+extern crate num_traits;
+extern crate puzzle_solver;
+
+use num_rational::Ratio;
+use num_traits::ToPrimitive;
+use puzzle_solver::{LinExpr,Puzzle,Val};
+
+#[test]
+fn xkcd_knapsack() {
+    let menu = [
+        (Ratio::new(2_15, 100), "Mixed Fruit"),
+        (Ratio::new(2_75, 100), "French Fries"),
+        (Ratio::new(3_35, 100), "Side Salad"),
+        (Ratio::new(3_55, 100), "Hot Wings"),
+        (Ratio::new(4_20, 100), "Mozzarella Sticks"),
+        (Ratio::new(5_80, 100), "Sampler Plate") ];
+
+    let mut sys = Puzzle::new();
+    let mut vars = Vec::with_capacity(menu.len());
+    let total = Ratio::new(15_05, 100);
+
+    for &(cost, _) in menu.iter() {
+        let num = (total / cost).floor().to_integer();
+        let var = sys.new_var_with_candidates(&(0..(num + 1)).collect::<Vec<Val>>());
+        vars.push(var)
+    }
+
+    sys.equals(total, vars.iter().zip(menu.iter()).fold(LinExpr::from(0),
+            |sum, (&var, &(cost, _))| sum + var * cost));
+
+    let solutions = sys.solve_all();
+    assert_eq!(solutions.len(), 2);
+
+    for dict in solutions.iter() {
+        println!("");
+        for (&var, &(cost, string)) in vars.iter().zip(menu.iter()) {
+            let numer = cost.numer().to_f32().unwrap();
+            let denom = cost.denom().to_f32().unwrap();
+            println!(" {} x {:.2} {}", dict[var], numer / denom, string);
+        }
+    }
+
+    println!("xkcd_knapsack: {} guesses", sys.num_guesses());
+}