X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/c25f3400060ea1a02f8fa9de69c39fd7b020e8a5..53b9ab56f7b712e6710fd873d456a0fe1f13b785:/workspace/src/part08.rs

diff --git a/workspace/src/part08.rs b/workspace/src/part08.rs
index 7e2ba3b..5ddcb33 100644
--- a/workspace/src/part08.rs
+++ b/workspace/src/part08.rs
@@ -7,7 +7,7 @@ use part05::BigInt;
 
 // So, let us write a function to "add with carry", and give it the appropriate type. Notice Rust's native support for pairs.
 fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) {
-    let sum = u64::wrapping_add(a, b);
+    let sum = a.wrapping_add(b);
     // If an overflow happened, then the sum will be smaller than *both* summands. Without an overflow, of course, it will be
     // at least as large as both of them. So, let's just pick one and check.
     if sum >= a {
@@ -15,7 +15,7 @@ fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) {
         // **Exercise 08.1**: Write the code to handle adding the carry in this case.
         unimplemented!()
     } else {
-        // The addition *did* overflow. It is impossible for the addition of the carry
+        // Otherwise, the addition *did* overflow. It is impossible for the addition of the carry
         // to overflow again, as we are just adding 0 or 1.
         unimplemented!()
     }
@@ -46,9 +46,9 @@ impl ops::Add<BigInt> for BigInt {
         let mut result_vec:Vec<u64> = Vec::with_capacity(max_len);
         let mut carry = false; /* the current carry bit */
         for i in 0..max_len {
-            // Compute next digit and carry. Store the digit for the result, and the carry for later.
             let lhs_val = if i < self.data.len() { self.data[i] } else { 0 };
             let rhs_val = if i < rhs.data.len() { rhs.data[i] } else { 0 };
+            // Compute next digit and carry. Then, store the digit for the result, and the carry for later.
             unimplemented!()
         }
         // **Exercise 08.2**: Handle the final `carry`, and return the sum.
@@ -56,10 +56,11 @@ impl ops::Add<BigInt> for BigInt {
     }
 }
 
-// ## Traits and borrowed types
+// ## Traits and reference types
 
 // Writing this out becomes a bit tedious, because trait implementations (unlike functions) require full explicit annotation
-// of lifetimes. Make sure you understand exactly what the following definition says.
+// of lifetimes. Make sure you understand exactly what the following definition says. Notice that we can implement a trait for
+// a reference type!
 impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
     type Output = BigInt;
     fn add(self, rhs: &'a BigInt) -> Self::Output {
@@ -68,25 +69,25 @@ impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
     }
 }
 
-// ## Modules
+// **Exercise 08.4**: Implement the two missing combinations of arguments for `Add`. You should not have to duplicate the implementation.
 
-// Rust calls a bunch of definitions that are grouped together a *module*. You can put definitions in a submodule as follows.
-mod my_mod {
-    type MyType = i32;
-    fn my_fun() -> MyType { 42 }
-}
+// ## Modules
 
-// For the purpose of testing, one typically introduces a module called `tests` and tells the compiler
-// (by means of the `cfg` attribute) to only compile this module for tests.
+// Rust calls a bunch of definitions that are grouped together a *module*. You can put the tests in a submodule as follows.
 #[cfg(test)]
 mod tests {
-    #[test]
+    use part05::BigInt;
+
+    /*#[test]*/
     fn test_add() {
         let b1 = BigInt::new(1 << 32);
         let b2 = BigInt::from_vec(vec![0, 1]);
 
         assert_eq!(&b1 + &b2, BigInt::from_vec(vec![1 << 32, 1]));
-        // **Exercise 08.4**: Add some more testcases.
+        // **Exercise 08.5**: Add some more cases to this test.
     }
 }
 
+// **Exercise 08.6**: Write a subtraction function, and testcases for it. Decide for yourself how you want to handle negative results.
+// For example, you may want to return an `Option`, to panic, or to return `0`.
+