add luhn's algorithm (issue #312)

Cargo.lock

@@ -37,6 +37,10 @@ version = "0.2.139"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "201de327520df007757c1f0adce6e827fe8562fbc28bfd9c15571c66ca1f5f79"
 
+[[package]]
+name = "luhn"
+version = "0.1.0"
+
 [[package]]
 name = "once_cell"
 version = "1.17.1"

Cargo.toml

@@ -3,6 +3,7 @@ members = [
   "balance",
   "empty-zeros",
   "fraction-math",
+  "luhn",
   "parens",
   "repeated-groups",
   "scrabble",

luhn/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "luhn"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

luhn/src/main.rs

@@ -0,0 +1,24 @@
+fn main() {
+    println!("{}", validate(vec![1, 2, 3, 4, 5, 6, 7, 8, 9]));
+}
+
+fn validate(card: Vec<u8>) -> bool {
+    let mut card = card;
+    let given_check = card.pop().unwrap();
+
+    let mut check = 0;
+    for (i, n) in card.iter().rev().enumerate() {
+        check += match i & 1 == 0 {
+            true => {
+                if *n > 4 {
+                    (*n * 2) - 9
+                } else {
+                    *n * 2
+                }
+            }
+            false => *n,
+        };
+    }
+
+    10 - (check % 10) == given_check
+}