X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/29958c0fd33c5e714b52bed79a1832113c43b8d8..187609af5c0332a7fa027565425fcf0342db86f8:/workspace/src/part08.rs?ds=sidebyside

diff --git a/workspace/src/part08.rs b/workspace/src/part08.rs
index 90c35f6..5a5f792 100644
--- a/workspace/src/part08.rs
+++ b/workspace/src/part08.rs
@@ -1,21 +1,28 @@
-// Rust-101, Part 08: Associated Types, Modules (WIP)
-// ==================================================
+// Rust-101, Part 08: Associated Types, Modules
+// ============================================
 
-use std::cmp;
-use std::ops;
-use std::fmt;
+use std::{cmp,ops};
 use part05::BigInt;
 
-// Add with carry, returning the sum and the carry
+
+// 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);
-    if sum >= a { // first addition did not overflow
+    // 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 {
+        // The addition did not overflow. <br/>
+        // **Exercise 08.1**: Write the code to handle adding the carry in this case.
         unimplemented!()
-    } else { // first addition *did* overflow
+    } else {
+        // 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!()
     }
 }
 
+// `overflow_add` is a sufficiently intricate function that a test case is justified.
+// This should also help you to check your solution of the exercise.
 /*#[test]*/
 fn test_overflowing_add() {
     assert_eq!(overflowing_add(10, 100, false), (110, false));
@@ -25,12 +32,58 @@ fn test_overflowing_add() {
     assert_eq!(overflowing_add(1 << 63, (1 << 63) -1 , true), (0, true));
 }
 
-impl ops::Add for BigInt {
+// ## Associated Types
+impl ops::Add<BigInt> for BigInt {
+
+    // Here, we choose the result type to be again `BigInt`.
     type Output = BigInt;
+
+    // Now we can write the actual function performing the addition.
     fn add(self, rhs: BigInt) -> Self::Output {
-        let mut result_vec:Vec<u64> = Vec::with_capacity(cmp::max(self.data.len(), rhs.data.len()));
+        // We know that the result will be *at least* as long as the longer of the two operands,
+        // so we can create a vector with sufficient capacity to avoid expensive reallocations.
+        let max_len = cmp::max(self.data.len(), rhs.data.len());
+        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 {
+            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.
         unimplemented!()
     }
 }
 
-// [index](main.html) | [previous](part07.html) | [next](main.html)
+// ## Traits and borrowed 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. Notice that we can implement a trait for
+// a borrowed type!
+impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
+    type Output = BigInt;
+    fn add(self, rhs: &'a BigInt) -> Self::Output {
+        // **Exercise 08.3**: Implement this function.
+        unimplemented!()
+    }
+}
+
+// ## Modules
+
+// 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]
+    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 cases to this test.
+    }
+}
+
+// **Exercise 08.4**: 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`.
+