knapsack: add benchmark against brute force

exercises/practice/knapsack/benches/benchmark.rs

@@ -0,0 +1,100 @@
+#![feature(test)]
+extern crate knapsack;
+extern crate test;
+
+use knapsack::Item;
+use std::cmp;
+use std::hint::black_box;
+use test::Bencher;
+
+#[bench]
+fn bench_fifteen(b: &mut Bencher) {
+    b.iter(|| knapsack::maximum_value(MAX_WEIGHT, black_box(&ITEMS)));
+}
+#[bench]
+fn bench_fifteen_brute_force(b: &mut Bencher) {
+    b.iter(|| maximum_value(MAX_WEIGHT, black_box(&ITEMS)));
+}
+
+/// Simple and inefficient brute force. Can it be beat?
+pub fn maximum_value(max_weight: u32, items: &[Item]) -> u32 {
+    if items.len() == 0 {
+        return 0;
+    }
+
+    // Put the item in the knapsack (if fits)
+    let mut value_taking = 0;
+    if items[0].weight <= max_weight {
+        value_taking = items[0].value + maximum_value(max_weight - items[0].weight, &items[1..]);
+    }
+
+    // Don't put the item in the knapsack
+    let value_not_taking = maximum_value(max_weight, &items[1..]);
+
+    // Return the value of the richest knapsack
+    cmp::max(value_taking, value_not_taking)
+}
+
+const MAX_WEIGHT: u32 = 750;
+const ITEMS: [Item; 15] = [
+    Item {
+        weight: 70,
+        value: 135,
+    },
+    Item {
+        weight: 73,
+        value: 139,
+    },
+    Item {
+        weight: 77,
+        value: 149,
+    },
+    Item {
+        weight: 80,
+        value: 150,
+    },
+    Item {
+        weight: 82,
+        value: 156,
+    },
+    Item {
+        weight: 87,
+        value: 163,
+    },
+    Item {
+        weight: 90,
+        value: 173,
+    },
+    Item {
+        weight: 94,
+        value: 184,
+    },
+    Item {
+        weight: 98,
+        value: 192,
+    },
+    Item {
+        weight: 106,
+        value: 201,
+    },
+    Item {
+        weight: 110,
+        value: 210,
+    },
+    Item {
+        weight: 113,
+        value: 214,
+    },
+    Item {
+        weight: 115,
+        value: 221,
+    },
+    Item {
+        weight: 118,
+        value: 229,
+    },
+    Item {
+        weight: 120,
+        value: 240,
+    },
+];