Implement grains

Follows the standard test suite, which is currently waiting for merge.

exercism/problem-specifications#420

I did not include the test of square -1 since that would require the
function signature to be `i32`.

Placement is at the end of the "Introduction" section. It introduces a
couple of concepts that may be new,

- declaring number types
- panic & should_panic

I thought it would be nice to get some exposure to `panic` before
introducing `result` in the Hamming exercise.

Author: whit0694

config.json

@@ -11,6 +11,7 @@
     "bob",
     "beer-song",
     "difference-of-squares",
+    "grains",
     "hamming",
     "pascals-triangle",
     "scrabble-score",

exercises/grains/.gitignore

@@ -0,0 +1,7 @@
+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock

exercises/grains/Cargo.toml

@@ -0,0 +1,3 @@
+[package]
+name = "grains"
+version = "0.0.0"

exercises/grains/example.rs

@@ -0,0 +1,11 @@
+pub fn square(s: u32) -> u64 {
+    if s == 0 || s > 64 {
+        panic!("Square must be between 1 and 64");
+    }
+
+    2u64.pow(s - 1)
+}
+
+pub fn total() -> u64 {
+    (1..65).fold(0, |acc, s| acc + square(s))
+}

exercises/grains/tests/grains.rs

@@ -0,0 +1,54 @@
+extern crate grains;
+
+#[test]
+fn square_one() {
+    assert_eq!(grains::square(1), 1);
+}
+
+#[test]
+fn square_two() {
+    assert_eq!(grains::square(2), 2);
+}
+
+#[test]
+fn square_three() {
+    assert_eq!(grains::square(3), 4);
+}
+
+#[test]
+fn square_four() {
+    assert_eq!(grains::square(4), 8);
+}
+
+#[test]
+fn square_sixteen() {
+    assert_eq!(grains::square(16), 32_768);
+}
+
+#[test]
+fn square_thirty_two() {
+    assert_eq!(grains::square(32), 2_147_483_648);
+}
+
+#[test]
+fn square_sixty_four() {
+    assert_eq!(grains::square(64), 9_223_372_036_854_775_808);
+}
+
+#[test]
+#[should_panic(expected = "Square must be between 1 and 64")]
+fn square_zero_panics() {
+    grains::square(0);
+}
+
+#[test]
+#[should_panic(expected = "Square must be between 1 and 64")]
+fn square_sixty_five_panics() {
+    grains::square(65);
+}
+
+#[test]
+fn total_sums_all_squares() {
+    assert_eq!(grains::total(), 18_446_744_073_709_551_615);
+    grains::total();
+}

problems.md

@@ -23,6 +23,7 @@ raindrops |  case (or `format`). Mutable string
 bob |  chars, string functions
 beer-song |  case, string concatenation, vector (optional), loop
 difference-of-squares |  fold & map
+grains | math, panic
 
 ## Getting Rusty