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exercises/02_basic_calculator/08_overflow/src/lib.rs

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+// Customize the `dev` profile to wrap around on overflow.
+// Check Cargo's documentation to find out the right syntax:
+// https://doc.rust-lang.org/cargo/reference/profiles.html
+//
+// For reasons that we'll explain later, the customization needs to be done in the `Cargo.toml`
+// at the root of the repository, not in the `Cargo.toml` of the exercise.
+
+pub fn factorial(n: u32) -> u32 {
+    let mut result = 1;
+    for i in 1..=n {
+        result *= i;
+    }
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::factorial;
+
+    #[test]
+    fn twentieth() {
+        // 20! is 2432902008176640000, which is too large to fit in a u32
+        // With the default dev profile, this will panic when you run `cargo test`
+        // We want it to wrap around instead
+        assert_eq!(factorial(20), 2_192_834_560);
+        //                           ☝️
+        // A large number literal using underscores to improve readability!
+    }
+
+    #[test]
+    fn first() {
+        assert_eq!(factorial(0), 1);
+    }
+
+    #[test]
+    fn second() {
+        assert_eq!(factorial(1), 1);
+    }
+
+    #[test]
+    fn third() {
+        assert_eq!(factorial(2), 2);
+    }
+
+    #[test]
+    fn fifth() {
+        assert_eq!(factorial(5), 120);
+    }
+}