From: Ralf Jung <post@ralfj.de>
Date: Mon, 6 Jul 2015 15:53:42 +0000 (+0200)
Subject: part 09: iterator for BigInt
X-Git-Url: https://git.ralfj.de/rust-101.git/commitdiff_plain/66c777f23e032d445e29b1b40faa8e1799b4930c?ds=sidebyside;hp=-c

part 09: iterator for BigInt
---

66c777f23e032d445e29b1b40faa8e1799b4930c
diff --git a/src/part09.rs b/src/part09.rs
index 0045c7a..482b293 100644
--- a/src/part09.rs
+++ b/src/part09.rs
@@ -1,4 +1,64 @@
 // Rust-101, Part 09: Iterators (WIP)
 // ==================================
 
+use part05::BigInt;
+
+// In the following, we will look into the iterator mechanism of Rust and make our `BigInt` compatible
+// with the `for` loops. Of course, this is all about implementing particular traits again. In particular,
+// an iterator is something that implements the `Iterator` trait. As you can see in [the documentation](http://doc.rust-lang.org/beta/std/iter/trait.Iterator.html),
+// this trait mandates a single function `next` returning an `Option<Self::Item>`, where `Item` is an
+// associated type chosen by the implementation. (There are many more methods provided for `Iterator`,
+// but they all have default implementations, so we don't have to worry about them right now).
+// 
+// For the case of `BigInt`, we want our iterator to iterate over the digits in normal, notational order: The most-significant
+// digit comes first. So, we have to write down some type, and implement `Iterator` for it such that `next` returns the digits
+// one-by-one. Clearly, the iterator must somehow be able to access the number it iterates over, and it must store its current
+// location. However, it cannot *own* the `BigInt`, because then the number would be gone after iteration! That'd certainly be bad.
+// The only alternative is for the iterator to *borrow* the number.
+
+// In writing this down, we again have to be explicit about the lifetime of the borrow: We can't just have an
+// `Iter`, we must have an `Iter<'a>` that borrowed the number for lifetime `'a`. <br/>
+// `usize` here is the type of unsigned, pointer-sized numbers. It is typically the type of "lengths of things",
+// in particular, it is the type of the length of a `Vec` and hence the right type to store an offset into the vector of digits.
+struct Iter<'a> {
+    num: &'a BigInt,
+    idx: usize, // the index of the last number that was returned
+}
+
+// Now we are equipped to implement `Iterator` for `Iter`.
+impl<'a> Iterator for Iter<'a> {
+    // We choose the type of things that we iterate over to be the type of digits, i.e., `u64`.
+    type Item = u64;
+
+    fn next(&mut self) -> Option<u64> {
+        // First, check whether there's any more digits to return.
+        if self.idx == 0 {
+            // We already returned all the digits.
+            None                                                    /*@*/
+        } else {
+            // Decrement, and return next digit.
+            self.idx = self.idx - 1;                                /*@*/
+            Some(self.num.data[self.idx])                           /*@*/
+        }
+    }
+}
+
+// All we need now is a function that creates such an iterator for a given `BigInt`.
+impl BigInt {
+    // Notice that when we write the type of `iter`, we don't actually have to give the lifetime parameter of `Iter`. Just as it is
+    // the case with functions returning borrowed data, you can elide the lifetime. The rules for adding the lifetimes are exactly the
+    // same. (See the last section of [part 06](part06.html).)
+    fn iter(&self) -> Iter {
+        Iter { num: self, idx: self.data.len() }                    /*@*/
+    }
+}
+
+// We are finally ready to iterate! Remember to edit `main.rs` to run this function.
+pub fn main() {
+    let b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
+    for digit in b.iter() {
+        println!("{}", digit);
+    }
+}
+
 //@ [index](main.html) | [previous](part08.html) | [next](main.html)
diff --git a/workspace/src/part09.rs b/workspace/src/part09.rs
index 8e82f03..d6b8bba 100644
--- a/workspace/src/part09.rs
+++ b/workspace/src/part09.rs
@@ -1,3 +1,62 @@
 // Rust-101, Part 09: Iterators (WIP)
 // ==================================
 
+use part05::BigInt;
+
+// In the following, we will look into the iterator mechanism of Rust and make our `BigInt` compatible
+// with the `for` loops. Of course, this is all about implementing particular traits again. In particular,
+// an iterator is something that implements the `Iterator` trait. As you can see in [the documentation](http://doc.rust-lang.org/beta/std/iter/trait.Iterator.html),
+// this trait mandates a single function `next` returning an `Option<Self::Item>`, where `Item` is an
+// associated type chosen by the implementation. (There are many more methods provided for `Iterator`,
+// but they all have default implementations, so we don't have to worry about them right now).
+// 
+// For the case of `BigInt`, we want our iterator to iterate over the digits in normal, notational order: The most-significant
+// digit comes first. So, we have to write down some type, and implement `Iterator` for it such that `next` returns the digits
+// one-by-one. Clearly, the iterator must somehow be able to access the number it iterates over, and it must store its current
+// location. However, it cannot *own* the `BigInt`, because then the number would be gone after iteration! That'd certainly be bad.
+// The only alternative is for the iterator to *borrow* the number.
+
+// In writing this down, we again have to be explicit about the lifetime of the borrow: We can't just have an
+// `Iter`, we must have an `Iter<'a>` that borrowed the number for lifetime `'a`. <br/>
+// `usize` here is the type of unsigned, pointer-sized numbers. It is typically the type of "lengths of things",
+// in particular, it is the type of the length of a `Vec` and hence the right type to store an offset into the vector of digits.
+struct Iter<'a> {
+    num: &'a BigInt,
+    idx: usize, // the index of the last number that was returned
+}
+
+// Now we are equipped to implement `Iterator` for `Iter`.
+impl<'a> Iterator for Iter<'a> {
+    // We choose the type of things that we iterate over to be the type of digits, i.e., `u64`.
+    type Item = u64;
+
+    fn next(&mut self) -> Option<u64> {
+        // First, check whether there's any more digits to return.
+        if self.idx == 0 {
+            // We already returned all the digits.
+            unimplemented!()
+        } else {
+            // Decrement, and return next digit.
+            unimplemented!()
+        }
+    }
+}
+
+// All we need now is a function that creates such an iterator for a given `BigInt`.
+impl BigInt {
+    // Notice that when we write the type of `iter`, we don't actually have to give the lifetime parameter of `Iter`. Just as it is
+    // the case with functions returning borrowed data, you can elide the lifetime. The rules for adding the lifetimes are exactly the
+    // same. (See the last section of [part 06](part06.html).)
+    fn iter(&self) -> Iter {
+        unimplemented!()
+    }
+}
+
+// We are finally ready to iterate! Remember to edit `main.rs` to run this function.
+pub fn main() {
+    let b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
+    for digit in b.iter() {
+        println!("{}", digit);
+    }
+}
+