[rust] Grains

1 files changed, 23 insertions, 0 deletions

diff --git a/grains/src/lib.rs b/grains/src/lib.rs
new file mode 100644
index 0000000..05449be
--- /dev/null
+++ b/grains/src/lib.rs
@@ -0,0 +1,23 @@
+pub fn square(s: u32) -> u64 {
+    assert!(s > 0 && s < 65,"Square must be between 1 and 64");
+    2_u64.pow(s-1)
+}
+
+/// This version uses the already defined `square(s)` function.
+///
+/// There is also a faster way to compute this according to
+/// [Wolfram|Alpha](https://www.wolframalpha.com/input/?i=sum_0%5En+2%5En)
+///
+/// 2_u64.pow(65) - 1 == 18_446_744_073_709_551_615
+///
+/// this solution however doesn't work because
+///
+/// 2_u64.pow(65) == 18_446_744_073_709_551_616
+///
+/// which overflows since
+///
+/// std::u64::MAX == 18_446_744_073_709_551_615
+///
+pub fn total() -> u64 {
+    (1..65).map(square).sum() // std::u64::MAX
+}