Introduce peek_nth

[davidcornu]

I've been implementing Bob Nystrom's Crafting Interpreters in Rust as a way to learn the language and stumbled upon multipeek while looking for a way to peek more than one value ahead (as is required in order to tokenize number literals).

multipeek seemed like a great solution to this but I find it's tricky to use correctly. Because it makes peek stateful, you have to be careful to call reset_peek whenever handing control of the iterator to another part of the code or risk running into bugs when peek doesn't return what you expect.

To work around this I implemented peek_nth which allows you to peek more than one iteration into the future without relying on a cursor. I have a working version in my project but I figured I'd submit it here for discussion.

Example usage

let xs = vec![1,2,3];
let mut iter = peek_nth(xs.iter());

assert_eq!(iter.peek(), Some(&&1));
assert_eq!(iter.next(), Some(&1));

// The iterator does not advance even if we call `peek_nth` multiple times
assert_eq!(iter.peek_nth(0), Some(&&2));
assert_eq!(iter.peek_nth(1), Some(&&3));
assert_eq!(iter.next(), Some(&2));

// Calling `peek_nth` past the end of the iterator will return `None`
assert_eq!(iter.peek_nth(1), None);

Note - peek equivalent to peek_nth(0) and is included for convenience.

---

⚠️ I'm very new to Rust and took a lot of inspiration frommultipeek to implement this. I'm more than willing to rewrite/fix any mistakes I may have made 😅.

[davidcornu]

Ran my code against clippy and fixed all lints.

[bluss]

Thanks, looks very thoughtful, I'm sorry I haven't had time to look

[davidcornu]

No worries! Let me know if I can help in any way.

[davidcornu]

@jswrenn is this feature something that would be of interest? Happy to fix the conflicts and get tests passing again if that's the case. Otherwise feel free to close.

[jswrenn]

@davidcornu This is excellent. I'd be happy to merge it if you have time to fix up the conflicts and get the tests running.

[davidcornu]

@jswrenn will do

[davidcornu]

Done ✅

[davidcornu]

@jswrenn thanks for the comments! Definitely cleaned up the implementation.

[jswrenn]

Thanks for the contribution!

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push