-// Rust-101, Part 06: Copy
-// =======================
+// Rust-101, Part 06: Copy, Lifetimes
+// ==================================
+// We continue to work on our `BigInt`, so we start by importing what we already established.
use part05::BigInt;
// With `BigInt` being about numbers, we should be able to write a version of `vec_min`
// that computes the minimum of a list of `BigInt`. We start by writing `min` for
// `BigInt`. Now our assumption of having no trailing zeros comes in handy!
impl BigInt {
- fn min(self, other: Self) -> Self {
- // Just to be sure, we first check that both operands actually satisfy our invariant.
- // `debug_assert!` is a macro that checks that its argument (must be of type `bool`)
- // is `true`, and panics otherwise. It gets removed in release builds, which you do with
- // `cargo build --release`.
+ fn min_try1(self, other: Self) -> Self {
+ // Just to be sure, we first check that both operands actually satisfy our invariant. `debug_assert!` is a
+ // macro that checks that its argument (must be of type `bool`) is `true`, and panics otherwise. It gets
+ // removed in release builds, which you do with `cargo build --release`.
debug_assert!(self.test_invariant() && other.test_invariant());
// If the lengths of the two numbers differ, we already know which is larger.
if self.data.len() < other.data.len() {
} else if self.data.len() > other.data.len() {
other
} else {
- // **Exercise 05.1**: Fill in this code.
- panic!("Not yet implemented.");
+ // **Exercise 06.1**: Fill in this code.
+ unimplemented!()
}
}
}
for e in v {
min = Some(match min {
None => e.clone(),
- Some(n) => e.clone().min(n)
+ Some(n) => e.clone().min_try1(n)
});
}
min
// underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since
// we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own
// the copy that was created, and hence we can store it in `min`.<br/>
-// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that soon.
+// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that in the next part.
// ## `Copy` types
// But before we go there, I should answer the second question I brought up above: Why did our old `vec_min` work?
// stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to
// point to the same underlying buffer. We don't have two vectors, there's no duplication.
//
-// Rust calls types that can be freely duplicated `Copy` types. `Copy` is another trait, and it
-// is implemented for types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing
-// `trait Minimum : Copy { ...`? This tells Rust that every type that implements `Minimum` must also
-// implement `Copy`, and that's why the compiler accepted our generic `vec_min` in part 02.
-// `Copy` is the first *marker trait* that we encounter: It does not provide any methods, but
-// makes a promise about the behavior of the type - in this case, being duplicable.
+// Rust calls types that can be freely duplicated `Copy` types. `Copy` is another trait, and it is implemented for
+// types like `i32` and `bool`. Remember how we defined the trait `Minimum` by writing `trait Minimum : Copy { ...`?
+// This tells Rust that every type that implements `Minimum` must also implement `Copy`, and that's why the compiler
+// accepted our generic `vec_min` in part 02. `Copy` is the first *marker trait* that we encounter: It does not provide
+// any methods, but makes a promise about the behavior of the type - in this case, being duplicable.
// If you try to implement `Copy` for `BigInt`, you will notice that Rust
// does not let you do that. A type can only be `Copy` if all its elements
// `Clone`. This makes the cost explicit.
// ## Lifetimes
-// To fix the performance problems of `vec_min`, we need ti avoid using `clone()`. We'd like
+// To fix the performance problems of `vec_min`, we need to avoid using `clone()`. We'd like
// the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
// This is demonstrated by the function `head` that borrows the first element of a vector if it is non-empty.
// Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
// where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
-// [index](main.html) | [previous](part05.html) | [next](main.html)
+// [index](main.html) | [previous](part05.html) | [next](part07.html)
-use part05::BigInt;
+// Rust-101, Part 07: Operator Overloading, Tests, Output
+// ======================================================
+pub use part05::BigInt;
+
+// With our new knowledge on Lifetimes, we are now able to write down the desired type
+// of `min`: We want the function to take two borrows *of the same lifetime*, and then
+// return a borrow of that lifetime. If the two input lifetimes would be different, we
+// would not know which lifetime to use for the result.
pub trait Minimum {
- /// Return the smaller of the two
fn min<'a>(&'a self, other: &'a Self) -> &'a Self;
}
-/// Return a pointer to the minimal value of `v`.
+// Now we can implement a generic function `vec_min` that works on above trait.
+// The code is pretty much straight-forward, and Rust checks that all the
+// lifetimes actually work out.
pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
- let mut min = None;
+ let mut min: Option<&T> = None;
for e in v {
min = Some(match min {
None => e,
- Some(n) => e.min(n)
+ Some(n) => n.min(e)
});
}
min
}
-// Notice that `Option<&T>` is technically (leaving the borrowing story aside) a pointer to a `T`,
-// that could optionally be invalid. In other words, it's just like a pointer in C(++) or Java
-// that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget to check
-// the pointer for validity, avoiding the safety issues of C(++). At the same time, when we
+// Notice that the return type `Option<&T>` is technically (leaving the borrowing story aside) a
+// pointer to a `T`, that could optionally be invalid. In other words, it's just like a pointer in
+// C(++) or Java that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget
+// to check the pointer for validity, avoiding the safety issues of C(++). At the same time, when we
// have a borrow like `v` above that's not an `Option`, we *know* that is has to be a valid
-// pointer, so we don't even need to do a `NULL`-check.<br/>
+// pointer, so we don't even need to do the `NULL`-check that Java does all the time.<br/>
// Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be
// `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented
// as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like
// a pointer in C(++), if you look at what happens during execution - but it's much safer to use.
+// For our `vec_min` to be usable with `BigInt`, we need to provide an implementation of
+// `minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1.
impl Minimum for BigInt {
fn min<'a>(&'a self, other: &'a Self) -> &'a Self {
+ unimplemented!()
+ }
+}
+
+// ## Operator Overloading
+// How can we know that our `min` function actually does what we want it to do? One possibility
+// here is to do *testing*. Rust comes with nice build-in support for both unit tests and integration
+// tests. However, before we go there, we need to have a way of checking whether the results are
+// correct. In other words, we need to define how to test equality of `BigInt`. Being able to
+// test equality is a property of a type, that - you guessed it - Rust expresses as a trait:
+// `PartialEq`. Once a type implements that trait, one can use the `==` operator on it.
+
+// Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing zeros.
+// The `inline` attribute tells Rust that we will typically want this function to be inlined.
+impl PartialEq for BigInt {
+ #[inline]
+ fn eq(&self, other: &BigInt) -> bool {
debug_assert!(self.test_invariant() && other.test_invariant());
- if self.data.len() < other.data.len() {
- self
- } else if self.data.len() > other.data.len() {
- other
- } else {
- // compare back-to-front, i.e., most significant digit first
- let mut idx = self.data.len()-1;
- while idx > 0 {
- if self.data[idx] < other.data[idx] {
- return self;
- } else if self.data[idx] > other.data[idx] {
- return other;
- }
- else {
- idx = idx-1;
- }
- }
- // the two are equal
- return self;
- }
+ self.data == other.data
+ }
+}
+// Since implementing `PartialEq` is a fairly mechanical business, you can let Rust automate this
+// by adding the attribute `derive(PartialEq)` to the type definition. In case you wonder about
+// the "partial", I suggest you check out the documentation of [`PartialEq`](http://doc.rust-lang.org/std/cmp/trait.PartialEq.html)
+// and [`Eq`](http://doc.rust-lang.org/std/cmp/trait.Eq.html). Again, `Eq` can be automatically derived.
+
+// Now we can compare `BigInt`s! Speaking in C++ terms, we just overloaded the `==` operator
+// for `BigInt`. Rust does not have function overloading (i.e., it will not dispatch to different
+// functions depending on the type of the argument). Instead, one typically finds (or defines) a
+// trait that catches the core characteristic common to all the overloads, and writes a single
+// function that's generic in the trait. For example, instead of overloading a function for all
+// the ways a string can be represented, one write a generic functions over [ToString](http://doc.rust-lang.org/std/string/trait.ToString.html).
+// Usually, there is a trait like this that fits the purpose - and if there is, this has the great
+// advantage that any type *you* write, that can convert to a string, just has to implement
+// that trait to be immediately usable with all the functions out there that generalize over `ToString`.
+// Compare that to C++ or Java, where the only chance to add a new overloading variant is to
+// edit the class of the receiver.
+fn compare_big_ints() {
+ let b1 = BigInt::new(13);
+ let b2 = BigInt::new(37);
+ println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2);
+}
+
+// ## Testing
+// With our equality test written, we are now ready to write out first testcase. It doesn't get much
+// simpler: You just write a function (with no arguments or return value), and give it the `test` attribute.
+// `assert!` is like `debug_assert!`, but does not get compiled away in a release build.
+#[test]
+fn test_min() {
+ let b1 = BigInt::new(1);
+ let b2 = BigInt::new(42);
+ let b3 = BigInt::from_vec(vec![0, 1]);
+
+ assert!(*b1.min(&b2) == b1);
+ assert!(*b3.min(&b2) == b2);
+}
+// Now run `cargo test` to execute the test. If you implemented `min` correctly, it should all work!
+
+// ## Formatting
+// There is also a macro `assert_eq!` that's specialized to test for equality, and that prints the two
+// values (left and right) if they differ. To be able to do that, the macro needs to know how to format
+// the value for printing. This means that we - guess what? - have to implement an appropriate trait.
+// Rust knows about two ways of formatting a value: `Display` is for pretty-printing something in a way
+// that users can understand, while `Debug` is meant to show the internal state of data and targeted at
+// the programmer. The latter is what we want for `assert_eq!`, so let's get started.
+
+// Al formating is handled by [`std::fmt`](http://doc.rust-lang.org/std/fmt/index.html). I won't explain
+// all the details, and refer you to the documentation instead.
+use std::fmt;
+
+// In the case of `BigInt`, we'd like to just output our internal `data` array, so we
+// simply call the formating function of `Vec<u64>`.
+impl fmt::Debug for BigInt {
+ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+ self.data.fmt(f)
}
}
+// `Debug` implementations can be automatically generated using the `derive(Debug)` attribute.
+
+// Now we are ready to use `assert_eq!` to test `vec_min`. While we are at it, let's also follow the usual
+// Rust style of putting tests into a *submodule*, to avoid polluting the namespace. The attribute `cfg(test)`
+// at the submodule means that it will only be compiled when building the tests.
+#[cfg(test)]
+mod tests {
+ use super::*;
+
+ #[test]
+ fn test_vec_min() {
+ let b1 = BigInt::new(1);
+ let b2 = BigInt::new(42);
+ let b3 = BigInt::from_vec(vec![0, 1]);
+
+ let v1 = vec![b2.clone(), b1.clone(), b3.clone()];
+ let v2 = vec![b2.clone(), b3.clone()];
+ assert_eq!(vec_min(&v1), Some(&b1));
+ assert_eq!(vec_min(&v2), Some(&b2));
+ }
+}
+
+// **Exercise 07.1**: Add some more testcases. In particular, make sure you test the behavior of
+// `vec_min` on an empty vector. Also add tests for `BigInt::from_vec` (in particular, removing
+// trailing zeros) and the functions you wrote for exercise 05.1. Finally, break one of your
+// functions in a subtle way and watch the test fail.
+//
+// **Exercise 07.2**: Go back to your good ol' `SomethingOrNothing`, and implement `Display` for it. (This will,
+// of course, need a `Display` bound on `T`.) Then you should be able to use them with `println!` just like you do with numbers.
+
+// [index](main.html) | [previous](part06.html) | [next](main.html)
+// Rust-101, Part 08: Associated Types, Modules
+// ============================================
+
use std::cmp;
use std::ops;
use std::fmt;
use part05::BigInt;
-impl PartialEq for BigInt {
- fn eq(&self, other: &BigInt) -> bool {
- debug_assert!(self.test_invariant() && other.test_invariant());
- self.data == other.data
- }
-}
-
-fn call_eq() {
- let b1 = BigInt::new(13);
- let b2 = BigInt::new(37);
- println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2);
-}
-
-
-impl fmt::Debug for BigInt {
- fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
- self.data.fmt(f)
- }
-}
-
-
-
-impl BigInt {
- pub fn inc(&mut self, mut by: u64) {
- panic!("Not yet implemented.");
- }
-}
-
-
-#[test]
-fn test_inc() {
- let mut b = BigInt::new(1337);
- b.inc(1337);
- assert!(b == BigInt::new(1337 + 1337));
-
- b = BigInt::new(0);
- assert_eq!(b, BigInt::from_vec(vec![0]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![1 << 63]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![0, 1]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![1 << 63, 1]));
- b.inc(1 << 63);
- assert_eq!(b, BigInt::from_vec(vec![0, 2]));
-}
-
-
// Add with carry, returning the sum and the carry
fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) {
let sum = u64::wrapping_add(a, b);
- if sum >= a {
- panic!("First addition did not overflow. Not implemented.");
- } else {
- panic!("First addition *did* overflow. Not implemented.");
+ if sum >= a { // first addition did not overflow
+ unimplemented!()
+ } else { // first addition *did* overflow
+ unimplemented!()
}
}
type Output = BigInt;
fn add(self, rhs: BigInt) -> Self::Output {
let mut result_vec:Vec<u64> = Vec::with_capacity(cmp::max(self.data.len(), rhs.data.len()));
- panic!("Not yet implemented.");
+ unimplemented!()
}
}
+
+// [index](main.html) | [previous](part07.html) | [next](main.html)
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