Templater: Show entire prefix even if it's long + refactor

This refactors the unique prefix functionality and extracts a function
that will be reused by the styled prefix highlighting.

This also includes a minor change in functionality: if the unique prefix
of an id does not fit into the space provided, it is still printed.

Currently, this would happen if the prefix is more than 12 characters
long, which takes a huge repository. However, I'm hoping that in the
future we'll make this number customizeable.

src/templater.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::cmp::{max, min};
 use std::io;
 
 use itertools::Itertools;
@@ -494,7 +495,7 @@ impl CommitOrChangeId<'_> {
     }
 
     pub fn short_prefix_and_brackets(&self) -> String {
-        highlight_shortest_prefix(self, 12)
+        highlight_shortest_prefix_brackets(self, 12)
     }
 }
 
@@ -504,18 +505,63 @@ impl Template<()> for CommitOrChangeId<'_> {
     }
 }
 
-fn highlight_shortest_prefix(id: &CommitOrChangeId, total_len: usize) -> String {
+/// This function supports short `total_len` by ensuring that the entire
-    let prefix_len = id.repo.shortest_unique_id_prefix_len(id.as_bytes());
+/// unique prefix is always printed
-    let mut hex = id.hex();
+fn extract_entire_prefix_and_trimmed_tail(
-    if prefix_len < total_len - 2 {
+    s: &str,
-        format!(
+    prefix_len: usize,
-            "{}[{}]",
+    total_len: usize,
-            &hex[0..prefix_len],
+) -> (&str, &str) {
-            &hex[prefix_len..total_len - 2]
+    let prefix_len = min(prefix_len, s.len());
+    let total_len = max(prefix_len, min(total_len, s.len()));
+    (&s[0..prefix_len], &s[prefix_len..total_len])
+}
+
+#[cfg(test)]
+mod tests {
+    use super::extract_entire_prefix_and_trimmed_tail;
+
+    #[test]
+    fn test_prefix() {
+        let s = "0123456789";
+        insta::assert_debug_snapshot!(extract_entire_prefix_and_trimmed_tail(s, 2, 5), @r###"
+        (
+            "01",
+            "234",
         )
+        "###);
+        insta::assert_debug_snapshot!(extract_entire_prefix_and_trimmed_tail(s, 2, 11), @r###"
+        (
+            "01",
+            "23456789",
+        )
+        "###);
+        insta::assert_debug_snapshot!(extract_entire_prefix_and_trimmed_tail(s, 11, 2), @r###"
+        (
+            "0123456789",
+            "",
+        )
+        "###);
+        insta::assert_debug_snapshot!(extract_entire_prefix_and_trimmed_tail(s, 11, 11), @r###"
+        (
+            "0123456789",
+            "",
+        )
+        "###);
+    }
+}
+
+fn highlight_shortest_prefix_brackets(id: &CommitOrChangeId, total_len: usize) -> String {
+    let hex = id.hex();
+    let (prefix, rest) = extract_entire_prefix_and_trimmed_tail(
+        &hex,
+        id.repo.shortest_unique_id_prefix_len(id.as_bytes()),
+        total_len - 2,
+    );
+    if rest.is_empty() {
+        prefix.to_string()
     } else {
-        hex.truncate(total_len);
+        format!("{prefix}[{rest}]")
-        hex
     }
 }