-// 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 {
+ // 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!()
}
}
+// `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));
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!()
+ }
+}
+
+// ## 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!()
}
}
-// [index](main.html) | [previous](part07.html) | [next](main.html)
+// **Exercise 08.4**: Implement the two missing combinations of arguments for `Add`. You should not have to duplicate the implementation.
+
+// ## 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 08.5**: 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`.
+
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