Add unit tests to Safe FFI exercise (#588)

Cargo.lock

@@ -248,6 +248,7 @@ version = "0.1.0"
 dependencies = [
  "reqwest",
  "scraper",
+ "tempfile",
  "thiserror",
 ]
 

src/exercises/Cargo.toml

@@ -44,3 +44,6 @@ path = "concurrency/link-checker.rs"
 reqwest = { version = "0.11.12", features = ["blocking"] }
 scraper = "0.15.0"
 thiserror = "1.0.37"
+
+[dev-dependencies]
+tempfile = "3.5.0"

src/exercises/day-3/safe-ffi-wrapper.rs

@@ -123,3 +123,42 @@ fn main() -> Result<(), String> {
     Ok(())
 }
 // ANCHOR_END: main
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::error::Error;
+
+    #[test]
+    fn test_nonexisting_directory() {
+        let iter = DirectoryIterator::new("no-such-directory");
+        assert!(iter.is_err());
+    }
+
+    #[test]
+    fn test_empty_directory() -> Result<(), Box<dyn Error>> {
+        let tmp = tempfile::TempDir::new()?;
+        let iter = DirectoryIterator::new(
+            tmp.path().to_str().ok_or("Non UTF-8 character in path")?,
+        )?;
+        let mut entries = iter.collect::<Vec<_>>();
+        entries.sort();
+        assert_eq!(entries, &[".", ".."]);
+        Ok(())
+    }
+
+    #[test]
+    fn test_nonempty_directory() -> Result<(), Box<dyn Error>> {
+        let tmp = tempfile::TempDir::new()?;
+        std::fs::write(tmp.path().join("foo.txt"), "The Foo Diaries\n")?;
+        std::fs::write(tmp.path().join("bar.png"), "<PNG>\n")?;
+        std::fs::write(tmp.path().join("crab.rs"), "//! Crab\n")?;
+        let iter = DirectoryIterator::new(
+            tmp.path().to_str().ok_or("Non UTF-8 character in path")?,
+        )?;
+        let mut entries = iter.collect::<Vec<_>>();
+        entries.sort();
+        assert_eq!(entries, &[".", "..", "bar.png", "crab.rs", "foo.txt"]);
+        Ok(())
+    }
+}