refactor(exercise/armstrong-numbers): optimize implementation to use custom iterator type

.pre-commit-config.yaml

@@ -1,7 +1,12 @@
 repos:
-- repo: https://github.com/doublify/pre-commit-rust
-  rev: v1.0
+# - repo: https://github.com/doublify/pre-commit-rust
+#   rev: v1.0
+#   hooks:
+#   - id: fmt
+#   - id: cargo-check
+#   - id: clippy
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v4.4.0
   hooks:
-  - id: fmt
-  - id: cargo-check
-  - id: clippy
+  - id: end-of-file-fixer
+  - id: trailing-whitespace

exercism/rust/armstrong-numbers/src/lib.rs

@@ -1,18 +1,48 @@
 pub fn is_armstrong_number(num: u32) -> bool {
-    let digits = digits(num);
-    let exponent = digits.len() as u32;
+    let digits = Digits::new(num);
+    let exponent = digits.num_digits;
 
     if exponent >= 10 {
+        // skip the calculation if the exponent is too large
         return false;
     }
 
-    num == digits.iter().map(|digit| digit.pow(exponent)).sum()
+    num == Digits::new(num).map(|digit| digit.pow(exponent)).sum()
 }
 
-fn digits(num: u32) -> Vec<u32> {
+struct Digits {
+    num: u32,
+    num_digits: u32,
+}
+
+impl Digits {
+    fn new(num: u32) -> Self {
+        Digits {
+            num,
+            num_digits: number_of_digits(num),
+        }
+    }
+}
+
+impl Iterator for Digits {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.num_digits == 0 {
+            None
+        } else {
+            let digit = self.num % 10;
+            self.num /= 10;
+            self.num_digits -= 1;
+            Some(digit)
+        }
+    }
+}
+
+fn number_of_digits(num: u32) -> u32 {
     if num < 10 {
-        vec![num]
+        1
     } else {
-        [vec![num % 10], digits(num / 10)].concat()
+        1 + number_of_digits(num / 10)
     }
 }