feat: add read_file_part() helper

closes #37

README.md

@@ -51,6 +51,9 @@ Individual solutions live in the `./src/bin/` directory as separate binaries. _I
 
 Every [solution](https://github.com/fspoettel/advent-of-code-rust/blob/main/src/template/commands/scaffold.rs#L9-L35) has _tests_ referencing its _example_ file in `./data/examples`. Use these tests to develop and debug your solutions against the example input.
 
+> [!TIP]
+> If a day has different example inputs for both parts, you can use the `read_file_part()` helper in your tests instead of `read_file()`. For example, if this applies to day 1, you can create a second example file `01-2.txt` and invoke the helper like `let result = part_two(&advent_of_code::template::read_file_part("examples", DAY, 2));` to read it in `test_part_two`.
+
 > [!TIP]
 > when editing a solution, `rust-analyzer` will display buttons for running / debugging unit tests above the unit test blocks.
 

src/template/mod.rs

@@ -19,6 +19,15 @@ pub fn read_file(folder: &str, day: Day) -> String {
     f.expect("could not open input file")
 }
 
+/// Helper function that reads a text file to string, appending a part suffix. E.g. like `01-2.txt`.
+#[must_use]
+pub fn read_file_part(folder: &str, day: Day, part: u8) -> String {
+    let cwd = env::current_dir().unwrap();
+    let filepath = cwd.join("data").join(folder).join(format!("{day}-{part}.txt"));
+    let f = fs::read_to_string(filepath);
+    f.expect("could not open input file")
+}
+
 /// Creates the constant `DAY` and sets up the input and runner for each part.
 #[macro_export]
 macro_rules! solution {