Add exercise 10.2 (#24)

[rust-101.git] / src / part08.rs

diff --git a/src/part08.rs b/src/part08.rs
index 7a9c093d8aec4308f0c4463b5adf83e6821133c0..17beefa0625b30230fe7720af35139ce720ee045 100644 (file)
--- a/src/part08.rs
+++ b/src/part08.rs
@@ -4,33 +4,41 @@
 use std::{cmp,ops};
 use part05::BigInt;
 
 use std::{cmp,ops};
 use part05::BigInt;
 

-//@ As our next goal, let us implement addition for our `BigInt`. The main issue here will be dealing with the overflow.
-//@ First of all, we will have to detect when an overflow happens. This is stored in a so-called *carry* bit, and we have to carry this
-//@ information on to the next pair of digits we add. The core primitive of addition therefore is to add two digits *and* a
-//@ carry, and to return the sum digit and the next carry.
-
-// So, let us write a function to "add with carry", and give it the appropriate type. Notice Rust's native support for pairs.
+//@ As our next goal, let us implement addition for our `BigInt`. The main issue here will be
+//@ dealing with the overflow. First of all, we will have to detect when an overflow happens. This
+//@ is stored in a so-called *carry* bit, and we have to carry this information on to the next pair
+//@ of digits we add. The core primitive of addition therefore is to add two digits *and* a carry,
+//@ and to return the sum digit and the next 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) {
 fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) {

-    //@ Rust's stanza on integer overflows may be a bit surprising: In general, when we write `a + b`, an overflow is
-    //@ considered an *error*. If you compile your program in debug mode, Rust will actually check for that error and panic
-    //@ the program in case of overflows. For performance reasons, no such checks are currently inserted for release builds.
-    //@ The reason for this is that many serious security vulnerabilities have been caused by integer overflows, so just assuming
-    //@ "per default" that they are intended is dangerous. <br/>
-    //@ If you explicitly *do* want an overflow to happen, you can call the `wrapping_add`
-    //@ function (see [the documentation](http://doc.rust-lang.org/stable/std/primitive.u64.html#method.wrapping_add),
-    //@ there are similar functions for other arithmetic operations). There are also similar functions
-    //@ `checked_add` etc. to enforce the overflow check.
-    let sum = u64::wrapping_add(a, 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.
+    //@ Rust's stanza on integer overflows may be a bit surprising: In general, when we write `a +
+    //@ b`, an overflow is considered an *error*. If you compile your program in debug mode, Rust
+    //@ will actually check for that error and panic the program in case of overflows. For
+    //@ performance reasons, no such checks are currently inserted for release builds.
+    //@ The reason for this is that many serious security vulnerabilities have been caused by
+    //@ integer overflows, so just assuming "per default" that they are intended is dangerous.
+    //@ <br/>
+    //@ If you explicitly *do* want an overflow to happen, you can call the `wrapping_add` function
+    //@ (see the
+    //@ [documentation](https://doc.rust-lang.org/stable/std/primitive.u64.html#method.wrapping_add),
+    //@ there are similar functions for other arithmetic operations). There are also similar
+    //@ functions `checked_add` etc. to enforce the overflow check.
+    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 {
         // The addition did not overflow. <br/>
         // **Exercise 08.1**: Write the code to handle adding the carry in this case.
     if sum >= a {
         // The addition did not overflow. <br/>
         // **Exercise 08.1**: Write the code to handle adding the carry in this case.

-        unimplemented!()
+        let sum_total = sum.wrapping_add(if carry { 1 } else { 0 });/*@@*/
+        let had_overflow = sum_total < sum;                         /*@@*/
+        (sum_total, had_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.
     } 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.

-        (sum + if carry { 1 } else { 0 }, true)                                 /*@*/
+        (sum + if carry { 1 } else { 0 }, true)                     /*@*/

     }
 }
 
     }
 }
 


@@ -46,21 +54,27 @@ fn test_overflowing_add() {
 }
 
 // ## Associated Types
 }
 
 // ## Associated Types

-//@ Now we are equipped to write the addition function for `BigInt`. As you may have guessed, the `+` operator
-//@ is tied to a trait (`std::ops::Add`), which we are going to implement for `BigInt`.
+//@ Now we are equipped to write the addition function for `BigInt`. As you may have guessed, the
+//@ `+` operator is tied to a trait (`std::ops::Add`), which we are going to implement for
+//@ `BigInt`.

 //@ 
 //@ 

-//@ In general, addition need not be homogeneous: You could add things of different types, like vectors and points. So when implementing
-//@ `Add` for a type, one has to specify the type of the other operand. In this case, it will also be `BigInt` (and we could have left it
-//@ away, since that's the default).
+//@ In general, addition need not be homogeneous: You could add things of different types, like
+//@ vectors and points. So when implementing `Add` for a type, one has to specify the type of the
+//@ other operand. In this case, it will also be `BigInt` (and we could have left it away, since
+//@ that's the default).

 impl ops::Add<BigInt> for BigInt {
 impl ops::Add<BigInt> for BigInt {

-    //@ Besides static functions and methods, traits can contain *associated types*: This is a type chosen by every particular implementation
-    //@ of the trait. The methods of the trait can then refer to that type. In the case of addition, it is used to give the type of the result.
-    //@ (Also see the [documentation of `Add`](http://doc.rust-lang.org/stable/std/ops/trait.Add.html).)
+    //@ Besides static functions and methods, traits can contain *associated types*: This is a type
+    //@ chosen by every particular implementation of the trait. The methods of the trait can then
+    //@ refer to that type. In the case of addition, it is used to give the type of the result.
+    //@ (Also see the
+    //@[documentation of `Add`](https://doc.rust-lang.org/stable/std/ops/trait.Add.html).)

     //@ 
     //@ 

-    //@ In general, you can consider the two `BigInt` given above (in the `impl` line) *input* types of trait search: When
-    //@ `a + b` is invoked with `a` having type `T` and `b` having type `U`, Rust tries to find an implementation of `Add` for
-    //@ `T` where the right-hand type is `U`. The associated types, on the other hand, are *output* types: For every combination
-    //@ of input types, there's a particular result type chosen by the corresponding implementation of `Add`.
+    //@ In general, you can consider the two `BigInt` given above (in the `impl` line) *input*
+    //@ types of trait search: When `a + b` is invoked with `a` having type `T` and `b` having type
+    //@ `U`, Rust tries to find an implementation of `Add` for `T` where the right-hand type is
+    //@ `U`. The associated types, on the other hand, are *output* types: For every combination of
+    //@ input types, there's a particular result type chosen by the corresponding implementation of
+    //@ `Add`.

 
     // Here, we choose the result type to be again `BigInt`.
     type Output = BigInt;
 
     // Here, we choose the result type to be again `BigInt`.
     type Output = BigInt;


@@ -75,24 +89,30 @@ impl ops::Add<BigInt> for BigInt {
         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 };
         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.
-            //@ Notice how we can obtain names for the two components of the pair that `overflowing_add` returns.
+            // Compute next digit and carry. Then, store the digit for the result, and the carry
+            // for later.
+            //@ Notice how we can obtain names for the two components of the pair that
+            //@ `overflowing_add` returns.

             let (sum, new_carry) = overflowing_add(lhs_val, rhs_val, carry);    /*@*/
             result_vec.push(sum);                                               /*@*/
             carry = new_carry;                                                  /*@*/
         }
         // **Exercise 08.2**: Handle the final `carry`, and return the sum.
             let (sum, new_carry) = overflowing_add(lhs_val, rhs_val, carry);    /*@*/
             result_vec.push(sum);                                               /*@*/
             carry = new_carry;                                                  /*@*/
         }
         // **Exercise 08.2**: Handle the final `carry`, and return the sum.

-        unimplemented!()
+        if carry {                                                              /*@@*/
+            result_vec.push(1);                                                 /*@@*/
+        }                                                                       /*@@*/
+        BigInt { data: result_vec }                                             /*@@*/

     }
 }
 
     }
 }
 

-// ## Traits and borrowed types
-//@ If you inspect the addition function above closely, you will notice that it actually consumes ownership of both operands
-//@ to produce the result. This is, of course, in general not what we want. We'd rather like to be able to add two `&BigInt`.
+// ## Traits and reference types
+//@ If you inspect the addition function above closely, you will notice that it actually consumes
+//@ ownership of both operands to produce the result. This is, of course, in general not what we
+//@ want. We'd rather like to be able to add two `&BigInt`.

 
 

-// 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!
+// 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 reference type!

 impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
     type Output = BigInt;
     fn add(self, rhs: &'a BigInt) -> Self::Output {
 impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
     type Output = BigInt;
     fn add(self, rhs: &'a BigInt) -> Self::Output {


@@ -101,16 +121,19 @@ impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt {
     }
 }
 
     }
 }
 

-// **Exercise 08.4**: Implement the two missing combinations of arguments for `Add`. You should not have to duplicate the implementation.
+// **Exercise 08.4**: Implement the two missing combinations of arguments for `Add`. You should not
+// have to duplicate the implementation.

 
 // ## Modules
 
 // ## Modules

-//@ As you learned, tests can be written right in the middle of your development in Rust. However, it is
-//@ considered good style to bundle all tests together. This is particularly useful in cases where
-//@ you wrote utility functions for the tests, that no other code should use.
-
-// Rust calls a bunch of definitions that are grouped together a *module*. You can put the tests in a submodule as follows.
-//@ The `cfg` attribute controls whether this module is even compiled: If we added some functions that are useful for testing,
-//@ Rust would not bother compiling them when you just build your program for normal use. Other than that, tests work as usually.
+//@ As you learned, tests can be written right in the middle of your development in Rust. However,
+//@ it is considered good style to bundle all tests together. This is particularly useful in cases
+//@ where you wrote utility functions for the tests, that no other code should use.
+
+// Rust calls a bunch of definitions that are grouped together a *module*. You can put the tests in
+// a submodule as follows.
+//@ The `cfg` attribute controls whether this module is even compiled: If we added some functions
+//@ that are useful for testing, Rust would not bother compiling them when you just build your
+//@ program for normal use. Other than that, tests work as usually.

 #[cfg(test)]
 mod tests {
     use part05::BigInt;
 #[cfg(test)]
 mod tests {
     use part05::BigInt;


@@ -124,27 +147,36 @@ mod tests {
         // **Exercise 08.5**: Add some more cases to this test.
     }
 }
         // **Exercise 08.5**: Add some more cases to this test.
     }
 }

-//@ As already mentioned, outside of the module, only those items declared public with `pub` may be used. Submodules can access
-//@ everything defined in their parents. Modules themselves are also hidden from the outside per default, and can be made public
-//@ with `pub`. When you use an identifier (or, more general, a *path* like `mod1::submod::name`), it is interpreted as being
-//@ relative to the current module. So, for example, to access `overflowing_add` from within `my_mod`, you would have to give a more
-//@ explicit path by writing `super::overflowing_add`, which tells Rust to look in the parent module. 
+//@ As already mentioned, outside of the module, only those items declared public with `pub` may be
+//@ used. Submodules can access everything defined in their parents. Modules themselves are also
+//@ hidden from the outside per default, and can be made public with `pub`. When you use an
+//@ identifier (or, more general, a *path* like `mod1::submod::name`), it is interpreted as being
+//@ relative to the current module. So, for example, to access `overflowing_add` from within
+//@ `my_mod`, you would have to give a more explicit path by writing `super::overflowing_add`,
+//@ which tells Rust to look in the parent module.

 //@ 
 //@ 

-//@ You can make names from other modules available locally with `use`. Per default, `use` works globally, so e.g.
-//@ `use std;` imports the *global* name `std`. By adding `super::` or `self::` to the beginning of the path, you make it relative
-//@ to the parent or current module, respectively. (You can also explicitly construct an absolute path by starting it with `::`,
-//@ e.g., `::std::cmp::min`). You can say `pub use path;` to simultaneously *import* names and make them publicly available to others.
-//@ Finally, you can import all public items of a module at once with `use module::*;`.
+//@ You can make names from other modules available locally with `use`. Per default, `use` works
+//@ globally, so e.g. `use std;` imports the *global* name `std`. By adding `super::` or `self::`
+//@ to the beginning of the path, you make it relative to the parent or current module,
+//@ respectively. (You can also explicitly construct an absolute path by starting it with `::`,
+//@ e.g., `::std::cmp::min`). You can say `pub use path;` to simultaneously *import* names and make
+//@ them publicly available to others. Finally, you can import all public items of a module at once
+//@ with `use module::*;`.

 //@ 
 //@ 

-//@ Modules can be put into separate files with the syntax `mod name;`. To explain this, let me take a small detour through
-//@ the Rust compilation process. Cargo starts by invoking`rustc` on the file `src/lib.rs` or `src/main.rs`, depending on whether
-//@ you compile an application or a library. When `rustc` encounters a `mod name;`, it looks for the files `name.rs` and
-//@ `name/mod.rs` and goes on compiling there. (It is an error for both of them to exist.) You can think of the contents of the
-//@ file being embedded at this place. However, only the file where compilation started, and files `name/mod.rs` can load modules
-//@ from other files. This ensures that the directory structure mirrors the structure of the modules, with `mod.rs`, `lib.rs`
-//@ and `main.rs` representing a directory or crate itself (similar to, e.g., `__init__.py` in Python).
-
-// **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`.
-
-//@ [index](main.html) | [previous](part07.html) | [next](part09.html)
+//@ Modules can be put into separate files with the syntax `mod name;`. To explain this, let me
+//@ take a small detour through the Rust compilation process. Cargo starts by invoking`rustc` on
+//@ the file `src/lib.rs` or `src/main.rs`, depending on whether you compile an application or a
+//@ library. When `rustc` encounters a `mod name;`, it looks for the files `name.rs` and
+//@ `name/mod.rs` and goes on compiling there. (It is an error for both of them to exist.)
+//@ You can think of the contents of the file being embedded at this place. However, only the file
+//@ where compilation started, and files `name/mod.rs` can load modules from other files. This
+//@ ensures that the directory structure mirrors the structure of the modules, with `mod.rs`,
+//@ `lib.rs` and `main.rs` representing a directory or crate itself (similar to, e.g.,
+//@ `__init__.py` in Python).
+
+// **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`.
+
+//@ [index](main.html) | [previous](part07.html) | [raw source](workspace/src/part08.rs) |
+//@ [next](part09.html)