Add exercise 10.2 (#24)

[rust-101.git] / src / part07.rs

// Rust-101, Part 07: Operator Overloading, Tests, Formatting
// ==========================================================

pub use part05::BigInt;

// With our new knowledge of lifetimes, we are now able to write down the desired type of `min`:
//@ We want the function to take two references *with the same lifetime*, and then
//@ return a reference with that lifetime. If the two input lifetimes would be different, we
//@ would not know which lifetime to use for the result.
pub trait Minimum {
    fn min<'a>(&'a self, other: &'a Self) -> &'a Self;
}

//@ 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. Observe that we don't have to make any copies!
pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
    let mut min: Option<&T> = None;
    for e in v {
        min = Some(match min {
            None => e,
            Some(n) => n.min(e)
        });
    }
    min
}
//@ 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(++). <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.

// **Exercise 07.1**: For our `vec_min` to be usable with `BigInt`, you will have to provide an
// implementation of `Minimum`. You should be able to pretty much copy the code you wrote for
// exercise 06.1. You should *not* make any copies of `BigInt`!
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 built-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 of function calls 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`.

//@ Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing
//@ zeros. We simply re-use the equality test on vectors, which compares all the elements
//@ individually.
//@ 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());
        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`](https://doc.rust-lang.org/std/cmp/trait.PartialEq.html) and
//@ [`Eq`](https://doc.rust-lang.org/std/cmp/trait.Eq.html). `Eq` can be automatically derived as
//@ well.

// Now we can compare `BigInt`s. Rust treats `PartialEq` special in that it is wired to the operator
// `==`:
//@ That operator can now be used on our numbers! 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 writes a generic functions over
//@ [ToString](https://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.
//@ 
//@ Why can we also use `!=`, even though we just overloaded `==`? The answer lies in what's called
//@ a *default implementation*. If you check out the documentation of `PartialEq` I linked above,
//@ you will see that the trait actually provides two methods: `eq` to test equality, and `ne` to
//@ test inequality. As you may have guessed, `!=` is wired to `ne`. The trait *definition* also
//@ provides a default implementation of `ne` to be the negation of `eq`. Hence you can just
//@ provide `eq`, and `!=` will work fine. Or, if you have a more efficient way of deciding
//@ inequality, you can provide `ne` for your type yourself.
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 our 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.

// All formating is handled by [`std::fmt`](https://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`.
/*#[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). 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, and get rid of the inherent functions to print
// `SomethingOrNothing<i32>` and `SomethingOrNothing<f32>`.

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