xref: /third_party/rust/crates/peeking_take_while/README.md

# `peeking_take_while`

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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.

```rust
// Bring the `peeking_take_while` method for peekable iterators into
// scope.
use peeking_take_while::PeekableExt;

// 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.

    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));
```