Initial commit

.gitignore

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+target/
+**/*.rs.bk
+Cargo.lock

.travis.yml

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+sudo: false
+language: rust
+
+rust:
+  - stable
+  - beta
+  - nightly
+
+cache: cargo
+
+env:
+  matrix:
+    - PROFILE=""
+    - PROFILE="--release"
+
+script:
+  - cargo build $PROFILE --verbose
+  - cargo test $PROFILE --verbose

Cargo.toml

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+[package]
+authors = ["Nick Fitzgerald <fitzgen@gmail.com>"]
+description = "Like `Iterator::take_while`, but calls the predicate on a peeked value. This allows you to use `Iterator::by_ref` and `Iterator::take_while` together, and still get the first value for which the `take_while` predicate returned false after dropping the `by_ref`."
+categories = ["development-tools::debugging", "development-tools::ffi"]
+keywords = ["iterator", "take_while", "peek", "by_ref"]
+license = "Apache-2.0/MIT"
+name = "peeking_take_while"
+readme = "./README.md"
+repository = "https://github.com/fitzgen/peeking_take_while"
+version = "0.1.0"
+
+[badges]
+[badges.travis-ci]
+repository = "fitzgen/peeking_take_while"

src/lib.rs

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+//! # `peeking_take_while`
+//!
+//! Provides the `peeking_take_while` iterator adaptor method.
+//!
+//! The `peeking_take_while` method is very similar to `take_while`, but behaves
+//! differently when used with a borrowed iterator (perhaps returned by
+//! `Iterator::by_ref`).
+//!
+//! `peeking_take_while` peeks at the next item in the iterator and runs the
+//! predicate on that peeked item. This avoids consuming the first item yielded
+//! by the underlying iterator for which the predicate returns `false`. On the
+//! other hand, `take_while` will consume that first item for which the
+//! predicate returns `false`, and it will be lost.
+//!
+//! ```
+//! extern crate peeking_take_while;
+//!
+//! // Bring the `peeking_take_while` method for peekable iterators into
+//! // scope.
+//! use peeking_take_while::PeekableExt;
+//!
+//! # fn main() {
+//! // Let's say we have two collections we want to iterate through: `xs` and
+//! // `ys`. We want to perform one operation on all the leading contiguous
+//! // elements that match some predicate, and a different thing with the rest of
+//! // the elements. With the `xs`, we will use the normal `take_while`. With the
+//! // `ys`, we will use `peeking_take_while`.
+//!
+//! let xs: Vec<u8> = (0..100).collect();
+//! let ys = xs.clone();
+//!
+//! let mut iter_xs = xs.into_iter();
+//! let mut iter_ys = ys.into_iter().peekable();
+//!
+//! {
+//!     // Let's do one thing with all the items that are less than 10.
+//! #   fn do_things_with<T>(_: T) {}
+//!
+//!     let xs_less_than_ten = iter_xs.by_ref().take_while(|x| *x < 10);
+//!     for x in xs_less_than_ten {
+//!         do_things_with(x);
+//!     }
+//!
+//!     let ys_less_than_ten = iter_ys.by_ref().peeking_take_while(|y| *y < 10);
+//!     for y in ys_less_than_ten {
+//!         do_things_with(y);
+//!     }
+//! }
+//!
+//! // And now we will do some other thing with the items that are greater than
+//! // or equal to 10.
+//!
+//! // ...except, when using plain old `take_while` we lost 10!
+//! assert_eq!(iter_xs.next(), Some(11));
+//!
+//! // However, when using `peeking_take_while` we did not! Great!
+//! assert_eq!(iter_ys.next(), Some(10));
+//! # }
+//! ```
+
+use std::iter::Peekable;
+
+/// The iterator returned by `peeking_take_while`.
+///
+/// See the [module documentation](./index.html) for details.
+pub struct PeekingTakeWhile<'a, I, P>
+    where I: 'a + Iterator
+{
+    iter: &'a mut Peekable<I>,
+    predicate: P,
+}
+
+impl<'a, I, P> Iterator for PeekingTakeWhile<'a, I, P>
+    where I: Iterator,
+          I::Item: ::std::fmt::Debug,
+          P: FnMut(&<I as Iterator>::Item) -> bool
+{
+    type Item = <I as Iterator>::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let predicate = &mut self.predicate;
+        if self.iter.peek().map_or(false, |x| {
+            println!("PeekingTakeWhile::next:");
+            println!("    x = {:?}", x);
+            let result = !(predicate)(x);
+            println!("    result = {:?}", result);
+            result
+        }) {
+            None
+        } else {
+            self.iter.next()
+        }
+    }
+}
+
+/// The `Iterator` extension trait that provides the `peeking_take_while`
+/// method.
+///
+/// See the [module documentation](./index.html) for details.
+pub trait PeekableExt<'a, I>: Iterator
+    where I: 'a + Iterator
+{
+    /// The `Iterator` extension trait that provides the `peeking_take_while`
+    /// method.
+    ///
+    /// See the [module documentation](./index.html) for details.
+    fn peeking_take_while<P>(&'a mut self, predicate: P) -> PeekingTakeWhile<'a, I, P>
+        where Self: Sized,
+              P: FnMut(&<Self as Iterator>::Item) -> bool;
+}
+
+impl<'a, I> PeekableExt<'a, I> for Peekable<I>
+    where I: 'a + Iterator
+{
+    fn peeking_take_while<P>(&'a mut self, predicate: P) -> PeekingTakeWhile<I, P>
+        where P: FnMut(&<Self as Iterator>::Item) -> bool
+    {
+        PeekingTakeWhile {
+            iter: self,
+            predicate: predicate,
+        }
+    }
+}