Add exercise traits6 practicing extension traits

solutions/15_traits/traits6.rs

@@ -0,0 +1,53 @@
+trait Digits {
+    fn digits(&self) -> Vec<usize>;
+}
+
+impl Digits for usize {
+    fn digits(&self) -> Vec<usize> {
+        let mut res = Vec::new();
+        let mut temp = *self;
+        while temp != 0 {
+            res.push(temp % 10);
+            temp /= 10;
+        }
+        res.reverse();
+        res
+    }
+}
+
+impl Digits for &str {
+    fn digits(&self) -> Vec<usize> {
+        self.chars()
+            .map(|c| c.to_digit(10).unwrap() as usize)
+            .collect()
+    }
+}
+
+fn calculate_digit_sum(number: impl Digits) -> usize {
+    number.digits().into_iter().sum()
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn integer() {
+        assert_eq!(calculate_digit_sum(0), 1);
+        assert_eq!(calculate_digit_sum(1), 1);
+        assert_eq!(calculate_digit_sum(1235), 11);
+        assert_eq!(calculate_digit_sum(789), 24);
+    }
+
+    #[test]
+    fn string() {
+        assert_eq!(calculate_digit_sum(""), 1);
+        assert_eq!(calculate_digit_sum("1"), 1);
+        assert_eq!(calculate_digit_sum("1235"), 11);
+        assert_eq!(calculate_digit_sum("789"), 24);
+    }
+}