Solution

rust/etl/src/lib.rs

@@ -1,5 +1,10 @@
 use std::collections::BTreeMap;
 
 pub fn transform(h: &BTreeMap<i32, Vec<char>>) -> BTreeMap<char, i32> {
-    unimplemented!("How will you transform the tree {:?}?", h)
+    h.into_iter()
+        .flat_map(|(value, letters)| letters.iter().map(move |l| (l.to_lowercase(), value)))
+        .fold(BTreeMap::new(), |mut acc, (letter, value)| {
+            acc.insert(letter.to_string().chars().nth(0).unwrap(), *value);
+            acc
+        })
 }

rust/etl/tests/etl.rs

@@ -12,7 +12,6 @@ fn test_transform_one_value() {
 }
 
 #[test]
-#[ignore]
 fn test_transform_more_values() {
     let input = input_from(&[(1, vec!['A', 'E', 'I', 'O', 'U'])]);
 
@@ -22,7 +21,6 @@ fn test_transform_more_values() {
 }
 
 #[test]
-#[ignore]
 fn test_more_keys() {
     let input = input_from(&[(1, vec!['A', 'E']), (2, vec!['D', 'G'])]);
 
@@ -32,7 +30,6 @@ fn test_more_keys() {
 }
 
 #[test]
-#[ignore]
 fn test_full_dataset() {
     let input = input_from(&[
         (1, vec!['A', 'E', 'I', 'O', 'U', 'L', 'N', 'R', 'S', 'T']),