+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);
+ }
+}
+
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