feat(rust): add counting duplicates

rust/tests/2_counting_duplicates.rs

@@ -0,0 +1,32 @@
+use std::collections::HashMap;
+
+fn count_duplicates(text: &str) -> u32 {
+    text.to_lowercase()
+        .chars()
+        .fold(HashMap::<char, u32>::new(), |mut hm, c| {
+            hm.insert(c, hm.get(&c).copied().unwrap_or_default() + 1);
+            hm
+        })
+        .values()
+        .filter(|&&count| count > 1)
+        .count() as u32
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_abcde() {
+        assert_eq!(count_duplicates("abcde"), 0);
+    }
+
+    #[test]
+    fn test_abcdea() {
+        assert_eq!(count_duplicates("abcdea"), 1);
+    }
+    #[test]
+    fn test_indivisibility() {
+        assert_eq!(count_duplicates("indivisibility"), 1);
+    }
+}