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// Copyright 2014-2019 bluss and ndarray developers
//                     and Michał Krasnoborski (krdln)
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! A few iterator-related utilities and tools

use std::iter;

/// Iterate `iterable` with a running index.
///
/// `IntoIterator` enabled version of `.enumerate()`.
///
/// ```
/// use itertools::enumerate;
///
/// for (i, elt) in enumerate(&[1, 2, 3]) {
///     /* loop body */
/// }
/// ```
pub(crate) fn enumerate<I>(iterable: I) -> iter::Enumerate<I::IntoIter>
where
    I: IntoIterator,
{
    iterable.into_iter().enumerate()
}

/// Iterate `i` and `j` in lock step.
///
/// `IntoIterator` enabled version of `i.zip(j)`.
///
/// ```
/// use itertools::zip;
///
/// let data = [1, 2, 3, 4, 5];
/// for (a, b) in zip(&data, &data[1..]) {
///     /* loop body */
/// }
/// ```
pub(crate) fn zip<I, J>(i: I, j: J) -> iter::Zip<I::IntoIter, J::IntoIter>
where
    I: IntoIterator,
    J: IntoIterator,
{
    i.into_iter().zip(j)
}

/// Create an iterator running multiple iterators in lockstep.
///
/// The `izip!` iterator yields elements until any subiterator
/// returns `None`.
///
/// This is a version of the standard ``.zip()`` that's supporting more than
/// two iterators. The iterator element type is a tuple with one element
/// from each of the input iterators. Just like ``.zip()``, the iteration stops
/// when the shortest of the inputs reaches its end.
///
/// **Note:** The result of this macro is in the general case an iterator
/// composed of repeated `.zip()` and a `.map()`; it has an anonymous type.
/// The special cases of one and two arguments produce the equivalent of
/// `$a.into_iter()` and `$a.into_iter().zip($b)` respectively.
///
/// Prefer this macro `izip!()` over [`multizip`] for the performance benefits
/// of using the standard library `.zip()`.
///
/// [`multizip`]: fn.multizip.html
///
/// ```
/// #[macro_use] extern crate itertools;
/// # fn main() {
///
/// // iterate over three sequences side-by-side
/// let mut results = [0, 0, 0, 0];
/// let inputs = [3, 7, 9, 6];
///
/// for (r, index, input) in izip!(&mut results, 0..10, &inputs) {
///     *r = index * 10 + input;
/// }
///
/// assert_eq!(results, [0 + 3, 10 + 7, 29, 36]);
/// # }
/// ```
///
/// **Note:** To enable the macros in this crate, use the `#[macro_use]`
/// attribute when importing the crate:
///
/// ```
/// #[macro_use] extern crate itertools;
/// # fn main() { }
/// ```
macro_rules! izip {
    // @closure creates a tuple-flattening closure for .map() call. usage:
    // @closure partial_pattern => partial_tuple , rest , of , iterators
    // eg. izip!( @closure ((a, b), c) => (a, b, c) , dd , ee )
    ( @closure $p:pat => $tup:expr ) => {
        |$p| $tup
    };

    // The "b" identifier is a different identifier on each recursion level thanks to hygiene.
    ( @closure $p:pat => ( $($tup:tt)* ) , $_iter:expr $( , $tail:expr )* ) => {
        izip!(@closure ($p, b) => ( $($tup)*, b ) $( , $tail )*)
    };

    // unary
    ($first:expr $(,)*) => {
        IntoIterator::into_iter($first)
    };

    // binary
    ($first:expr, $second:expr $(,)*) => {
        izip!($first)
            .zip($second)
    };

    // n-ary where n > 2
    ( $first:expr $( , $rest:expr )* $(,)* ) => {
        izip!($first)
            $(
                .zip($rest)
            )*
            .map(
                izip!(@closure a => (a) $( , $rest )*)
            )
    };
}