iter.rs

// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// 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.

//! Composable external iterators
//!
//! # The `Iterator` trait
//!
//! This module defines Rust's core iteration trait. The `Iterator` trait has one
//! unimplemented method, `next`. All other methods are derived through default
//! methods to perform operations such as `zip`, `chain`, `enumerate`, and `fold`.
//!
//! The goal of this module is to unify iteration across all containers in Rust.
//! An iterator can be considered as a state machine which is used to track which
//! element will be yielded next.
//!
//! There are various extensions also defined in this module to assist with various
//! types of iteration, such as the `DoubleEndedIterator` for iterating in reverse,
//! the `FromIterator` trait for creating a container from an iterator, and much
//! more.
//!
//! ## Rust's `for` loop
//!
//! The special syntax used by rust's `for` loop is based around the `Iterator`
//! trait defined in this module. For loops can be viewed as a syntactical expansion
//! into a `loop`, for example, the `for` loop in this example is essentially
//! translated to the `loop` below.
//!
//! ```rust
//! let values = vec![1i, 2, 3];
//!
//! // "Syntactical sugar" taking advantage of an iterator
//! for &x in values.iter() {
//!     println!("{}", x);
//! }
//!
//! // Rough translation of the iteration without a `for` iterator.
//! let mut it = values.iter();
//! loop {
//!     match it.next() {
//!         Some(&x) => {
//!             println!("{}", x);
//!         }
//!         None => { break }
//!     }
//! }
//! ```
//!
//! This `for` loop syntax can be applied to any iterator over any type.

pub use self::MinMaxResult::*;

use clone::Clone;
use cmp;
use cmp::Ord;
use mem;
use num::{ToPrimitive, Int};
use ops::{Add, Deref};
use option::Option;
use option::Option::{Some, None};
use uint;

#[deprecated = "renamed to Extend"] pub use self::Extend as Extendable;

/// Conversion from an `Iterator`
#[unstable = "may be replaced by a more general conversion trait"]
pub trait FromIterator<A> {
    /// Build a container with elements from an external iterator.
    fn from_iter<T: Iterator<A>>(iterator: T) -> Self;
}

/// A type growable from an `Iterator` implementation
#[unstable = "just renamed as part of collections reform"]
pub trait Extend<A> {
    /// Extend a container with the elements yielded by an arbitrary iterator
    fn extend<T: Iterator<A>>(&mut self, iterator: T);
}

/// An interface for dealing with "external iterators". These types of iterators
/// can be resumed at any time as all state is stored internally as opposed to
/// being located on the call stack.
///
/// The Iterator protocol states that an iterator yields a (potentially-empty,
/// potentially-infinite) sequence of values, and returns `None` to signal that
/// it's finished. The Iterator protocol does not define behavior after `None`
/// is returned. A concrete Iterator implementation may choose to behave however
/// it wishes, either by returning `None` infinitely, or by doing something
/// else.
#[lang="iterator"]
#[unstable = "just split up for object safety"]
pub trait Iterator<A> {
    /// Advance the iterator and return the next value. Return `None` when the end is reached.
    fn next(&mut self) -> Option<A>;

    /// Returns a lower and upper bound on the remaining length of the iterator.
    ///
    /// An upper bound of `None` means either there is no known upper bound, or the upper bound
    /// does not fit within a `uint`.
    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) { (0, None) }
}

#[unstable = "new convention for extension traits"]
/// An extension trait providing numerous methods applicable to all iterators.
pub trait IteratorExt<A>: Iterator<A> {
    /// Chain this iterator with another, returning a new iterator which will
    /// finish iterating over the current iterator, and then it will iterate
    /// over the other specified iterator.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [0i];
    /// let b = [1i];
    /// let mut it = a.iter().chain(b.iter());
    /// assert_eq!(it.next().unwrap(), &0);
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn chain<U: Iterator<A>>(self, other: U) -> Chain<Self, U> {
        Chain{a: self, b: other, flag: false}
    }

    /// Creates an iterator which iterates over both this and the specified
    /// iterators simultaneously, yielding the two elements as pairs. When
    /// either iterator returns None, all further invocations of next() will
    /// return None.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [0i];
    /// let b = [1i];
    /// let mut it = a.iter().zip(b.iter());
    /// let (x0, x1) = (0i, 1i);
    /// assert_eq!(it.next().unwrap(), (&x0, &x1));
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn zip<B, U: Iterator<B>>(self, other: U) -> Zip<Self, U> {
        Zip{a: self, b: other}
    }

    /// Creates a new iterator which will apply the specified function to each
    /// element returned by the first, yielding the mapped element instead.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2];
    /// let mut it = a.iter().map(|&x| 2 * x);
    /// assert_eq!(it.next().unwrap(), 2);
    /// assert_eq!(it.next().unwrap(), 4);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn map<'r, B>(self, f: |A|: 'r -> B) -> Map<'r, A, B, Self> {
        Map{iter: self, f: f}
    }

    /// Creates an iterator which applies the predicate to each element returned
    /// by this iterator. Only elements which have the predicate evaluate to
    /// `true` will be yielded.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2];
    /// let mut it = a.iter().filter(|&x| *x > 1);
    /// assert_eq!(it.next().unwrap(), &2);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn filter<'r>(self, predicate: |&A|: 'r -> bool) -> Filter<'r, A, Self> {
        Filter{iter: self, predicate: predicate}
    }

    /// Creates an iterator which both filters and maps elements.
    /// If the specified function returns None, the element is skipped.
    /// Otherwise the option is unwrapped and the new value is yielded.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2];
    /// let mut it = a.iter().filter_map(|&x| if x > 1 {Some(2 * x)} else {None});
    /// assert_eq!(it.next().unwrap(), 4);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn filter_map<'r, B>(self, f: |A|: 'r -> Option<B>) -> FilterMap<'r, A, B, Self> {
        FilterMap { iter: self, f: f }
    }

    /// Creates an iterator which yields a pair of the value returned by this
    /// iterator plus the current index of iteration.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [100i, 200];
    /// let mut it = a.iter().enumerate();
    /// let (x100, x200) = (100i, 200i);
    /// assert_eq!(it.next().unwrap(), (0, &x100));
    /// assert_eq!(it.next().unwrap(), (1, &x200));
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn enumerate(self) -> Enumerate<Self> {
        Enumerate{iter: self, count: 0}
    }


    /// Creates an iterator that has a `.peek()` method
    /// that returns an optional reference to the next element.
    ///
    /// # Example
    ///
    /// ```rust
    /// let xs = [100i, 200, 300];
    /// let mut it = xs.iter().map(|x| *x).peekable();
    /// assert_eq!(*it.peek().unwrap(), 100);
    /// assert_eq!(it.next().unwrap(), 100);
    /// assert_eq!(it.next().unwrap(), 200);
    /// assert_eq!(*it.peek().unwrap(), 300);
    /// assert_eq!(*it.peek().unwrap(), 300);
    /// assert_eq!(it.next().unwrap(), 300);
    /// assert!(it.peek().is_none());
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn peekable(self) -> Peekable<A, Self> {
        Peekable{iter: self, peeked: None}
    }

    /// Creates an iterator which invokes the predicate on elements until it
    /// returns false. Once the predicate returns false, all further elements are
    /// yielded.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 2, 1];
    /// let mut it = a.iter().skip_while(|&a| *a < 3);
    /// assert_eq!(it.next().unwrap(), &3);
    /// assert_eq!(it.next().unwrap(), &2);
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn skip_while<'r>(self, predicate: |&A|: 'r -> bool) -> SkipWhile<'r, A, Self> {
        SkipWhile{iter: self, flag: false, predicate: predicate}
    }

    /// Creates an iterator which yields elements so long as the predicate
    /// returns true. After the predicate returns false for the first time, no
    /// further elements will be yielded.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 2, 1];
    /// let mut it = a.iter().take_while(|&a| *a < 3);
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert_eq!(it.next().unwrap(), &2);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures, may want to require peek"]
    fn take_while<'r>(self, predicate: |&A|: 'r -> bool) -> TakeWhile<'r, A, Self> {
        TakeWhile{iter: self, flag: false, predicate: predicate}
    }

    /// Creates an iterator which skips the first `n` elements of this iterator,
    /// and then it yields all further items.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter().skip(3);
    /// assert_eq!(it.next().unwrap(), &4);
    /// assert_eq!(it.next().unwrap(), &5);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn skip(self, n: uint) -> Skip<Self> {
        Skip{iter: self, n: n}
    }

    /// Creates an iterator which yields the first `n` elements of this
    /// iterator, and then it will always return None.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter().take(3);
    /// assert_eq!(it.next().unwrap(), &1);
    /// assert_eq!(it.next().unwrap(), &2);
    /// assert_eq!(it.next().unwrap(), &3);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[stable]
    fn take(self, n: uint) -> Take<Self> {
        Take{iter: self, n: n}
    }

    /// Creates a new iterator which behaves in a similar fashion to fold.
    /// There is a state which is passed between each iteration and can be
    /// mutated as necessary. The yielded values from the closure are yielded
    /// from the Scan instance when not None.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter().scan(1, |fac, &x| {
    ///   *fac = *fac * x;
    ///   Some(*fac)
    /// });
    /// assert_eq!(it.next().unwrap(), 1);
    /// assert_eq!(it.next().unwrap(), 2);
    /// assert_eq!(it.next().unwrap(), 6);
    /// assert_eq!(it.next().unwrap(), 24);
    /// assert_eq!(it.next().unwrap(), 120);
    /// assert!(it.next().is_none());
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn scan<'r, St, B>(self, initial_state: St, f: |&mut St, A|: 'r -> Option<B>)
        -> Scan<'r, A, B, Self, St> {
        Scan{iter: self, f: f, state: initial_state}
    }

    /// Creates an iterator that maps each element to an iterator,
    /// and yields the elements of the produced iterators
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::count;
    ///
    /// let xs = [2u, 3];
    /// let ys = [0u, 1, 0, 1, 2];
    /// let mut it = xs.iter().flat_map(|&x| count(0u, 1).take(x));
    /// // Check that `it` has the same elements as `ys`
    /// let mut i = 0;
    /// for x in it {
    ///     assert_eq!(x, ys[i]);
    ///     i += 1;
    /// }
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn flat_map<'r, B, U: Iterator<B>>(self, f: |A|: 'r -> U)
        -> FlatMap<'r, A, Self, U> {
        FlatMap{iter: self, f: f, frontiter: None, backiter: None }
    }

    /// Creates an iterator that yields `None` forever after the underlying
    /// iterator yields `None`. Random-access iterator behavior is not
    /// affected, only single and double-ended iterator behavior.
    ///
    /// # Example
    ///
    /// ```rust
    /// fn process<U: Iterator<int>>(it: U) -> int {
    ///     let mut it = it.fuse();
    ///     let mut sum = 0;
    ///     for x in it {
    ///         if x > 5 {
    ///             break;
    ///         }
    ///         sum += x;
    ///     }
    ///     // did we exhaust the iterator?
    ///     if it.next().is_none() {
    ///         sum += 1000;
    ///     }
    ///     sum
    /// }
    /// let x = vec![1i,2,3,7,8,9];
    /// assert_eq!(process(x.into_iter()), 6);
    /// let x = vec![1i,2,3];
    /// assert_eq!(process(x.into_iter()), 1006);
    /// ```
    #[inline]
    #[stable]
    fn fuse(self) -> Fuse<Self> {
        Fuse{iter: self, done: false}
    }

    /// Creates an iterator that calls a function with a reference to each
    /// element before yielding it. This is often useful for debugging an
    /// iterator pipeline.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::AdditiveIterator;
    ///
    /// let xs = [1u, 4, 2, 3, 8, 9, 6];
    /// let sum = xs.iter()
    ///             .map(|&x| x)
    ///             .inspect(|&x| println!("filtering {}", x))
    ///             .filter(|&x| x % 2 == 0)
    ///             .inspect(|&x| println!("{} made it through", x))
    ///             .sum();
    /// println!("{}", sum);
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn inspect<'r>(self, f: |&A|: 'r) -> Inspect<'r, A, Self> {
        Inspect{iter: self, f: f}
    }

    /// Creates a wrapper around a mutable reference to the iterator.
    ///
    /// This is useful to allow applying iterator adaptors while still
    /// retaining ownership of the original iterator value.
    ///
    /// # Example
    ///
    /// ```rust
    /// let mut xs = range(0u, 10);
    /// // sum the first five values
    /// let partial_sum = xs.by_ref().take(5).fold(0, |a, b| a + b);
    /// assert!(partial_sum == 10);
    /// // xs.next() is now `5`
    /// assert!(xs.next() == Some(5));
    /// ```
    #[stable]
    fn by_ref<'r>(&'r mut self) -> ByRef<'r, Self> {
        ByRef{iter: self}
    }

    /// Loops through the entire iterator, collecting all of the elements into
    /// a container implementing `FromIterator`.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let b: Vec<int> = a.iter().map(|&x| x).collect();
    /// assert!(a.as_slice() == b.as_slice());
    /// ```
    #[inline]
    #[unstable = "waiting for general conversion traits, just changed to take self by value"]
    fn collect<B: FromIterator<A>>(self) -> B {
        FromIterator::from_iter(self)
    }

    /// Loops through `n` iterations, returning the `n`th element of the
    /// iterator.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter();
    /// assert!(it.nth(2).unwrap() == &3);
    /// assert!(it.nth(2) == None);
    /// ```
    #[inline]
    #[stable]
    fn nth(&mut self, mut n: uint) -> Option<A> {
        for x in *self {
            if n == 0 { return Some(x) }
            n -= 1;
        }
        None
    }

    /// Loops through the entire iterator, returning the last element of the
    /// iterator.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// assert!(a.iter().last().unwrap() == &5);
    /// ```
    #[inline]
    #[unstable = "just changed to take self by value"]
    fn last(mut self) -> Option<A> {
        let mut last = None;
        for x in self { last = Some(x); }
        last
    }

    /// Performs a fold operation over the entire iterator, returning the
    /// eventual state at the end of the iteration.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// assert!(a.iter().fold(0, |a, &b| a + b) == 15);
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures, just changed to take self by value"]
    fn fold<B>(mut self, init: B, f: |B, A| -> B) -> B {
        let mut accum = init;
        for x in self {
            accum = f(accum, x);
        }
        accum
    }

    /// Counts the number of elements in this iterator.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter();
    /// assert!(it.count() == 5);
    /// ```
    #[inline]
    #[unstable = "just changed to take self by value"]
    fn count(self) -> uint {
        self.fold(0, |cnt, _x| cnt + 1)
    }

    /// Tests whether the predicate holds true for all elements in the iterator.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// assert!(a.iter().all(|x| *x > 0));
    /// assert!(!a.iter().all(|x| *x > 2));
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures, just changed to take self by value"]
    fn all(mut self, f: |A| -> bool) -> bool {
        for x in self { if !f(x) { return false; } }
        true
    }

    /// Tests whether any element of an iterator satisfies the specified
    /// predicate.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter();
    /// assert!(it.any(|x| *x == 3));
    /// assert!(!it.any(|x| *x == 3));
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn any(&mut self, f: |A| -> bool) -> bool {
        for x in *self { if f(x) { return true; } }
        false
    }

    /// Returns the first element satisfying the specified predicate.
    ///
    /// Does not consume the iterator past the first found element.
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn find(&mut self, predicate: |&A| -> bool) -> Option<A> {
        for x in *self {
            if predicate(&x) { return Some(x) }
        }
        None
    }

    /// Return the index of the first element satisfying the specified predicate
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    fn position(&mut self, predicate: |A| -> bool) -> Option<uint> {
        let mut i = 0;
        for x in *self {
            if predicate(x) {
                return Some(i);
            }
            i += 1;
        }
        None
    }

    /// Return the element that gives the maximum value from the
    /// specified function.
    ///
    /// # Example
    ///
    /// ```rust
    /// use core::num::SignedInt;
    ///
    /// let xs = [-3i, 0, 1, 5, -10];
    /// assert_eq!(*xs.iter().max_by(|x| x.abs()).unwrap(), -10);
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures, just changed to take self by value"]
    fn max_by<B: Ord>(self, f: |&A| -> B) -> Option<A> {
        self.fold(None, |max: Option<(A, B)>, x| {
            let x_val = f(&x);
            match max {
                None             => Some((x, x_val)),
                Some((y, y_val)) => if x_val > y_val {
                    Some((x, x_val))
                } else {
                    Some((y, y_val))
                }
            }
        }).map(|(x, _)| x)
    }

    /// Return the element that gives the minimum value from the
    /// specified function.
    ///
    /// # Example
    ///
    /// ```rust
    /// use core::num::SignedInt;
    ///
    /// let xs = [-3i, 0, 1, 5, -10];
    /// assert_eq!(*xs.iter().min_by(|x| x.abs()).unwrap(), 0);
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures, just changed to take self by value"]
    fn min_by<B: Ord>(self, f: |&A| -> B) -> Option<A> {
        self.fold(None, |min: Option<(A, B)>, x| {
            let x_val = f(&x);
            match min {
                None             => Some((x, x_val)),
                Some((y, y_val)) => if x_val < y_val {
                    Some((x, x_val))
                } else {
                    Some((y, y_val))
                }
            }
        }).map(|(x, _)| x)
    }
}

#[unstable = "trait is unstable"]
impl<A, I> IteratorExt<A> for I where I: Iterator<A> {}

/// A range iterator able to yield elements from both ends
///
/// A `DoubleEndedIterator` can be thought of as a deque in that `next()` and `next_back()` exhaust
/// elements from the *same* range, and do not work independently of each other.
#[unstable = "recently split into two traits"]
pub trait DoubleEndedIterator<A>: Iterator<A> {
    /// Yield an element from the end of the range, returning `None` if the range is empty.
    fn next_back(&mut self) -> Option<A>;
}

/// Extension methods for double-ended iterators.
#[unstable = "new extension trait convention"]
pub trait DoubleEndedIteratorExt<A>: DoubleEndedIterator<A> {
    /// Change the direction of the iterator
    ///
    /// The flipped iterator swaps the ends on an iterator that can already
    /// be iterated from the front and from the back.
    ///
    ///
    /// If the iterator also implements RandomAccessIterator, the flipped
    /// iterator is also random access, with the indices starting at the back
    /// of the original iterator.
    ///
    /// Note: Random access with flipped indices still only applies to the first
    /// `uint::MAX` elements of the original iterator.
    #[inline]
    #[stable]
    fn rev(self) -> Rev<Self> {
        Rev{iter: self}
    }
}

#[unstable = "trait is unstable"]
impl<A, I> DoubleEndedIteratorExt<A> for I where I: DoubleEndedIterator<A> {}

/// A double-ended iterator yielding mutable references
#[experimental = "not widely used"]
pub trait MutableDoubleEndedIterator {
    // FIXME: #5898: should be called `reverse`
    /// Use an iterator to reverse a container in-place
    fn reverse_(&mut self);
}

#[experimental = "trait is experimental"]
impl<'a, A:'a, T: DoubleEndedIterator<&'a mut A>> MutableDoubleEndedIterator for T {
    // FIXME: #5898: should be called `reverse`
    /// Use an iterator to reverse a container in-place
    fn reverse_(&mut self) {
        loop {
            match (self.next(), self.next_back()) {
                (Some(x), Some(y)) => mem::swap(x, y),
                _ => break
            }
        }
    }
}


/// An object implementing random access indexing by `uint`
///
/// A `RandomAccessIterator` should be either infinite or a `DoubleEndedIterator`.
/// Calling `next()` or `next_back()` on a `RandomAccessIterator`
/// reduces the indexable range accordingly. That is, `it.idx(1)` will become `it.idx(0)`
/// after `it.next()` is called.
#[experimental = "not widely used, may be better decomposed into Index and ExactSizeIterator"]
pub trait RandomAccessIterator<A>: Iterator<A> {
    /// Return the number of indexable elements. At most `std::uint::MAX`
    /// elements are indexable, even if the iterator represents a longer range.
    fn indexable(&self) -> uint;

    /// Return an element at an index, or `None` if the index is out of bounds
    fn idx(&mut self, index: uint) -> Option<A>;
}

/// An iterator that knows its exact length
///
/// This trait is a helper for iterators like the vector iterator, so that
/// it can support double-ended enumeration.
///
/// `Iterator::size_hint` *must* return the exact size of the iterator.
/// Note that the size must fit in `uint`.
#[unstable = "could move DoubleEndedIterator bound onto rposition with method-level where clauses"]
pub trait ExactSizeIterator<A> : DoubleEndedIterator<A> {
    /// Return the index of the last element satisfying the specified predicate
    ///
    /// If no element matches, None is returned.
    #[inline]
    fn rposition(&mut self, predicate: |A| -> bool) -> Option<uint> {
        let len = self.len();
        for i in range(0, len).rev() {
            if predicate(self.next_back().expect("rposition: incorrect ExactSizeIterator")) {
                return Some(i);
            }
        }
        None
    }

    #[inline]
    /// Return the exact length of the iterator.
    fn len(&self) -> uint {
        let (lower, upper) = self.size_hint();
        // Note: This assertion is overly defensive, but it checks the invariant
        // guaranteed by the trait. If this trait were rust-internal,
        // we could use debug_assert!; assert_eq! will check all Rust user
        // implementations too.
        assert_eq!(upper, Some(lower));
        lower
    }
}

// All adaptors that preserve the size of the wrapped iterator are fine
// Adaptors that may overflow in `size_hint` are not, i.e. `Chain`.
#[unstable = "trait is unstable"]
impl<A, T: ExactSizeIterator<A>> ExactSizeIterator<(uint, A)> for Enumerate<T> {}
#[unstable = "trait is unstable"]
impl<'a, A, T: ExactSizeIterator<A>> ExactSizeIterator<A> for Inspect<'a, A, T> {}
#[unstable = "trait is unstable"]
impl<A, T: ExactSizeIterator<A>> ExactSizeIterator<A> for Rev<T> {}
#[unstable = "trait is unstable"]
impl<'a, A, B, T: ExactSizeIterator<A>> ExactSizeIterator<B> for Map<'a, A, B, T> {}
#[unstable = "trait is unstable"]
impl<A, B, T, U> ExactSizeIterator<(A, B)> for Zip<T, U>
    where T: ExactSizeIterator<A>, U: ExactSizeIterator<B> {}

/// An double-ended iterator with the direction inverted
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Rev<T> {
    iter: T
}

#[unstable = "trait is unstable"]
impl<A, T: DoubleEndedIterator<A>> Iterator<A> for Rev<T> {
    #[inline]
    fn next(&mut self) -> Option<A> { self.iter.next_back() }
    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
}

#[unstable = "trait is unstable"]
impl<A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A> for Rev<T> {
    #[inline]
    fn next_back(&mut self) -> Option<A> { self.iter.next() }
}

#[experimental = "trait is experimental"]
impl<A, T: DoubleEndedIterator<A> + RandomAccessIterator<A>> RandomAccessIterator<A>
    for Rev<T> {
    #[inline]
    fn indexable(&self) -> uint { self.iter.indexable() }
    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        let amt = self.indexable();
        self.iter.idx(amt - index - 1)
    }
}

/// A mutable reference to an iterator
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct ByRef<'a, T:'a> {
    iter: &'a mut T
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>+'a> Iterator<A> for ByRef<'a, T> {
    #[inline]
    fn next(&mut self) -> Option<A> { self.iter.next() }
    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
}

#[unstable = "trait is unstable"]
impl<'a, A, T: DoubleEndedIterator<A>+'a> DoubleEndedIterator<A> for ByRef<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<A> { self.iter.next_back() }
}

/// A trait for iterators over elements which can be added together
#[experimental = "needs to be re-evaluated as part of numerics reform"]
pub trait AdditiveIterator<A> {
    /// Iterates over the entire iterator, summing up all the elements
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::AdditiveIterator;
    ///
    /// let a = [1i, 2, 3, 4, 5];
    /// let mut it = a.iter().map(|&x| x);
    /// assert!(it.sum() == 15);
    /// ```
    fn sum(self) -> A;
}

macro_rules! impl_additive {
    ($A:ty, $init:expr) => {
        #[experimental = "trait is experimental"]
        impl<T: Iterator<$A>> AdditiveIterator<$A> for T {
            #[inline]
            fn sum(self) -> $A {
                self.fold($init, |acc, x| acc + x)
            }
        }
    };
}
impl_additive!(i8,   0)
impl_additive!(i16,  0)
impl_additive!(i32,  0)
impl_additive!(i64,  0)
impl_additive!(int,  0)
impl_additive!(u8,   0)
impl_additive!(u16,  0)
impl_additive!(u32,  0)
impl_additive!(u64,  0)
impl_additive!(uint, 0)
impl_additive!(f32,  0.0)
impl_additive!(f64,  0.0)

/// A trait for iterators over elements which can be multiplied together.
#[experimental = "needs to be re-evaluated as part of numerics reform"]
pub trait MultiplicativeIterator<A> {
    /// Iterates over the entire iterator, multiplying all the elements
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::{count, MultiplicativeIterator};
    ///
    /// fn factorial(n: uint) -> uint {
    ///     count(1u, 1).take_while(|&i| i <= n).product()
    /// }
    /// assert!(factorial(0) == 1);
    /// assert!(factorial(1) == 1);
    /// assert!(factorial(5) == 120);
    /// ```
    fn product(self) -> A;
}

macro_rules! impl_multiplicative {
    ($A:ty, $init:expr) => {
        #[experimental = "trait is experimental"]
        impl<T: Iterator<$A>> MultiplicativeIterator<$A> for T {
            #[inline]
            fn product(self) -> $A {
                self.fold($init, |acc, x| acc * x)
            }
        }
    };
}
impl_multiplicative!(i8,   1)
impl_multiplicative!(i16,  1)
impl_multiplicative!(i32,  1)
impl_multiplicative!(i64,  1)
impl_multiplicative!(int,  1)
impl_multiplicative!(u8,   1)
impl_multiplicative!(u16,  1)
impl_multiplicative!(u32,  1)
impl_multiplicative!(u64,  1)
impl_multiplicative!(uint, 1)
impl_multiplicative!(f32,  1.0)
impl_multiplicative!(f64,  1.0)

/// A trait for iterators over elements which can be compared to one another.
#[unstable = "recently renamed for new extension trait conventions"]
pub trait IteratorOrdExt<A> {
    /// Consumes the entire iterator to return the maximum element.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// assert!(a.iter().max().unwrap() == &5);
    /// ```
    fn max(self) -> Option<A>;

    /// Consumes the entire iterator to return the minimum element.
    ///
    /// # Example
    ///
    /// ```rust
    /// let a = [1i, 2, 3, 4, 5];
    /// assert!(a.iter().min().unwrap() == &1);
    /// ```
    fn min(self) -> Option<A>;

    /// `min_max` finds the minimum and maximum elements in the iterator.
    ///
    /// The return type `MinMaxResult` is an enum of three variants:
    ///
    /// - `NoElements` if the iterator is empty.
    /// - `OneElement(x)` if the iterator has exactly one element.
    /// - `MinMax(x, y)` is returned otherwise, where `x <= y`. Two
    ///    values are equal if and only if there is more than one
    ///    element in the iterator and all elements are equal.
    ///
    /// On an iterator of length `n`, `min_max` does `1.5 * n` comparisons,
    /// and so is faster than calling `min` and `max` separately which does `2 * n` comparisons.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::{NoElements, OneElement, MinMax};
    ///
    /// let v: [int, ..0] = [];
    /// assert_eq!(v.iter().min_max(), NoElements);
    ///
    /// let v = [1i];
    /// assert!(v.iter().min_max() == OneElement(&1));
    ///
    /// let v = [1i, 2, 3, 4, 5];
    /// assert!(v.iter().min_max() == MinMax(&1, &5));
    ///
    /// let v = [1i, 2, 3, 4, 5, 6];
    /// assert!(v.iter().min_max() == MinMax(&1, &6));
    ///
    /// let v = [1i, 1, 1, 1];
    /// assert!(v.iter().min_max() == MinMax(&1, &1));
    /// ```
    fn min_max(self) -> MinMaxResult<A>;
}

#[unstable = "trait is unstable"]
impl<A: Ord, T: Iterator<A>> IteratorOrdExt<A> for T {
    #[inline]
    fn max(self) -> Option<A> {
        self.fold(None, |max, x| {
            match max {
                None    => Some(x),
                Some(y) => Some(cmp::max(x, y))
            }
        })
    }

    #[inline]
    fn min(self) -> Option<A> {
        self.fold(None, |min, x| {
            match min {
                None    => Some(x),
                Some(y) => Some(cmp::min(x, y))
            }
        })
    }

    fn min_max(mut self) -> MinMaxResult<A> {
        let (mut min, mut max) = match self.next() {
            None => return NoElements,
            Some(x) => {
                match self.next() {
                    None => return OneElement(x),
                    Some(y) => if x < y {(x, y)} else {(y,x)}
                }
            }
        };

        loop {
            // `first` and `second` are the two next elements we want to look at.
            // We first compare `first` and `second` (#1). The smaller one is then compared to
            // current minimum (#2). The larger one is compared to current maximum (#3). This
            // way we do 3 comparisons for 2 elements.
            let first = match self.next() {
                None => break,
                Some(x) => x
            };
            let second = match self.next() {
                None => {
                    if first < min {
                        min = first;
                    } else if first > max {
                        max = first;
                    }
                    break;
                }
                Some(x) => x
            };
            if first < second {
                if first < min {min = first;}
                if max < second {max = second;}
            } else {
                if second < min {min = second;}
                if max < first {max = first;}
            }
        }

        MinMax(min, max)
    }
}

/// `MinMaxResult` is an enum returned by `min_max`. See `IteratorOrdExt::min_max` for more detail.
#[deriving(Clone, PartialEq, Show)]
#[unstable = "waiting on namespaced enum conventions"]
pub enum MinMaxResult<T> {
    /// Empty iterator
    NoElements,

    /// Iterator with one element, so the minimum and maximum are the same
    OneElement(T),

    /// More than one element in the iterator, the first element is not larger than the second
    MinMax(T, T)
}

#[stable]
impl<T: Clone> MinMaxResult<T> {
    /// `into_option` creates an `Option` of type `(T,T)`. The returned `Option` has variant
    /// `None` if and only if the `MinMaxResult` has variant `NoElements`. Otherwise variant
    /// `Some(x,y)` is returned where `x <= y`. If `MinMaxResult` has variant `OneElement(x)`,
    /// performing this operation will make one clone of `x`.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::{NoElements, OneElement, MinMax, MinMaxResult};
    ///
    /// let r: MinMaxResult<int> = NoElements;
    /// assert_eq!(r.into_option(), None)
    ///
    /// let r = OneElement(1i);
    /// assert_eq!(r.into_option(), Some((1,1)));
    ///
    /// let r = MinMax(1i,2i);
    /// assert_eq!(r.into_option(), Some((1,2)));
    /// ```
    pub fn into_option(self) -> Option<(T,T)> {
        match self {
            NoElements => None,
            OneElement(x) => Some((x.clone(), x)),
            MinMax(x, y) => Some((x, y))
        }
    }
}

/// A trait for iterators that contain cloneable elements
#[unstable = "recently renamed for extension trait conventions"]
pub trait IteratorCloneExt<A> {
    /// Creates an iterator that clones the elements it yields. Useful for converting an
    /// Iterator<&T> to an Iterator<T>.
    fn cloned(self) -> Cloned<Self>;
}

#[unstable = "trait is unstable"]
impl<A: Clone, D: Deref<A>, I: Iterator<D>> IteratorCloneExt<A> for I {
    fn cloned(self) -> Cloned<I> {
        Cloned { it: self }
    }
}

/// An iterator that clones the elements of an underlying iterator
pub struct Cloned<I> {
    it: I,
}

impl<A: Clone, D: Deref<A>, I: Iterator<D>> Iterator<A> for Cloned<I> {
    fn next(&mut self) -> Option<A> {
        self.it.next().cloned()
    }

    fn size_hint(&self) -> (uint, Option<uint>) {
        self.it.size_hint()
    }
}

impl<A: Clone, D: Deref<A>, I: DoubleEndedIterator<D>>
        DoubleEndedIterator<A> for Cloned<I> {
    fn next_back(&mut self) -> Option<A> {
        self.it.next_back().cloned()
    }
}

#[unstable = "trait is unstable"]
impl<A: Clone, D: Deref<A>, I: ExactSizeIterator<D>> ExactSizeIterator<A> for Cloned<I> {}

#[unstable = "recently renamed for extension trait conventions"]
/// An extension trait for cloneable iterators.
pub trait CloneIteratorExt {
    /// Repeats an iterator endlessly
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::iter::{CloneIteratorExt, count};
    ///
    /// let a = count(1i,1i).take(1);
    /// let mut cy = a.cycle();
    /// assert_eq!(cy.next(), Some(1));
    /// assert_eq!(cy.next(), Some(1));
    /// ```
    #[stable]
    fn cycle(self) -> Cycle<Self>;
}

impl<A, I> CloneIteratorExt for I where I: Iterator<A> + Clone {
    #[inline]
    fn cycle(self) -> Cycle<I> {
        Cycle{orig: self.clone(), iter: self}
    }
}

/// An iterator that repeats endlessly
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Cycle<T> {
    orig: T,
    iter: T,
}

#[unstable = "trait is unstable"]
impl<A, T: Clone + Iterator<A>> Iterator<A> for Cycle<T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        match self.iter.next() {
            None => { self.iter = self.orig.clone(); self.iter.next() }
            y => y
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        // the cycle iterator is either empty or infinite
        match self.orig.size_hint() {
            sz @ (0, Some(0)) => sz,
            (0, _) => (0, None),
            _ => (uint::MAX, None)
        }
    }
}

#[experimental = "trait is experimental"]
impl<A, T: Clone + RandomAccessIterator<A>> RandomAccessIterator<A> for Cycle<T> {
    #[inline]
    fn indexable(&self) -> uint {
        if self.orig.indexable() > 0 {
            uint::MAX
        } else {
            0
        }
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        let liter = self.iter.indexable();
        let lorig = self.orig.indexable();
        if lorig == 0 {
            None
        } else if index < liter {
            self.iter.idx(index)
        } else {
            self.orig.idx((index - liter) % lorig)
        }
    }
}

/// An iterator which strings two iterators together
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Chain<T, U> {
    a: T,
    b: U,
    flag: bool,
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>, U: Iterator<A>> Iterator<A> for Chain<T, U> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.flag {
            self.b.next()
        } else {
            match self.a.next() {
                Some(x) => return Some(x),
                _ => ()
            }
            self.flag = true;
            self.b.next()
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (a_lower, a_upper) = self.a.size_hint();
        let (b_lower, b_upper) = self.b.size_hint();

        let lower = a_lower.saturating_add(b_lower);

        let upper = match (a_upper, b_upper) {
            (Some(x), Some(y)) => x.checked_add(y),
            _ => None
        };

        (lower, upper)
    }
}

#[unstable = "trait is unstable"]
impl<A, T: DoubleEndedIterator<A>, U: DoubleEndedIterator<A>> DoubleEndedIterator<A>
for Chain<T, U> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        match self.b.next_back() {
            Some(x) => Some(x),
            None => self.a.next_back()
        }
    }
}

#[experimental = "trait is experimental"]
impl<A, T: RandomAccessIterator<A>, U: RandomAccessIterator<A>> RandomAccessIterator<A>
for Chain<T, U> {
    #[inline]
    fn indexable(&self) -> uint {
        let (a, b) = (self.a.indexable(), self.b.indexable());
        a.saturating_add(b)
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        let len = self.a.indexable();
        if index < len {
            self.a.idx(index)
        } else {
            self.b.idx(index - len)
        }
    }
}

/// An iterator which iterates two other iterators simultaneously
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Zip<T, U> {
    a: T,
    b: U
}

#[unstable = "trait is unstable"]
impl<A, B, T: Iterator<A>, U: Iterator<B>> Iterator<(A, B)> for Zip<T, U> {
    #[inline]
    fn next(&mut self) -> Option<(A, B)> {
        match self.a.next() {
            None => None,
            Some(x) => match self.b.next() {
                None => None,
                Some(y) => Some((x, y))
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (a_lower, a_upper) = self.a.size_hint();
        let (b_lower, b_upper) = self.b.size_hint();

        let lower = cmp::min(a_lower, b_lower);

        let upper = match (a_upper, b_upper) {
            (Some(x), Some(y)) => Some(cmp::min(x,y)),
            (Some(x), None) => Some(x),
            (None, Some(y)) => Some(y),
            (None, None) => None
        };

        (lower, upper)
    }
}

#[unstable = "trait is unstable"]
impl<A, B, T: ExactSizeIterator<A>, U: ExactSizeIterator<B>> DoubleEndedIterator<(A, B)>
for Zip<T, U> {
    #[inline]
    fn next_back(&mut self) -> Option<(A, B)> {
        let a_sz = self.a.len();
        let b_sz = self.b.len();
        if a_sz != b_sz {
            // Adjust a, b to equal length
            if a_sz > b_sz {
                for _ in range(0, a_sz - b_sz) { self.a.next_back(); }
            } else {
                for _ in range(0, b_sz - a_sz) { self.b.next_back(); }
            }
        }
        match (self.a.next_back(), self.b.next_back()) {
            (Some(x), Some(y)) => Some((x, y)),
            (None, None) => None,
            _ => unreachable!(),
        }
    }
}

#[experimental = "trait is experimental"]
impl<A, B, T: RandomAccessIterator<A>, U: RandomAccessIterator<B>>
RandomAccessIterator<(A, B)> for Zip<T, U> {
    #[inline]
    fn indexable(&self) -> uint {
        cmp::min(self.a.indexable(), self.b.indexable())
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<(A, B)> {
        match self.a.idx(index) {
            None => None,
            Some(x) => match self.b.idx(index) {
                None => None,
                Some(y) => Some((x, y))
            }
        }
    }
}

/// An iterator which maps the values of `iter` with `f`
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Map<'a, A, B, T> {
    iter: T,
    f: |A|: 'a -> B
}

impl<'a, A, B, T> Map<'a, A, B, T> {
    #[inline]
    fn do_map(&mut self, elt: Option<A>) -> Option<B> {
        match elt {
            Some(a) => Some((self.f)(a)),
            _ => None
        }
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, B, T: Iterator<A>> Iterator<B> for Map<'a, A, B, T> {
    #[inline]
    fn next(&mut self) -> Option<B> {
        let next = self.iter.next();
        self.do_map(next)
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        self.iter.size_hint()
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, B, T: DoubleEndedIterator<A>> DoubleEndedIterator<B> for Map<'a, A, B, T> {
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        let next = self.iter.next_back();
        self.do_map(next)
    }
}

#[experimental = "trait is experimental"]
impl<'a, A, B, T: RandomAccessIterator<A>> RandomAccessIterator<B> for Map<'a, A, B, T> {
    #[inline]
    fn indexable(&self) -> uint {
        self.iter.indexable()
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<B> {
        let elt = self.iter.idx(index);
        self.do_map(elt)
    }
}

/// An iterator which filters the elements of `iter` with `predicate`
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Filter<'a, A, T> {
    iter: T,
    predicate: |&A|: 'a -> bool
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>> Iterator<A> for Filter<'a, A, T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        for x in self.iter {
            if (self.predicate)(&x) {
                return Some(x);
            } else {
                continue
            }
        }
        None
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A> for Filter<'a, A, T> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        for x in self.iter.by_ref().rev() {
            if (self.predicate)(&x) {
                return Some(x);
            }
        }
        None
    }
}

/// An iterator which uses `f` to both filter and map elements from `iter`
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct FilterMap<'a, A, B, T> {
    iter: T,
    f: |A|: 'a -> Option<B>
}

#[unstable = "trait is unstable"]
impl<'a, A, B, T: Iterator<A>> Iterator<B> for FilterMap<'a, A, B, T> {
    #[inline]
    fn next(&mut self) -> Option<B> {
        for x in self.iter {
            match (self.f)(x) {
                Some(y) => return Some(y),
                None => ()
            }
        }
        None
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, B, T: DoubleEndedIterator<A>> DoubleEndedIterator<B>
for FilterMap<'a, A, B, T> {
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        for x in self.iter.by_ref().rev() {
            match (self.f)(x) {
                Some(y) => return Some(y),
                None => ()
            }
        }
        None
    }
}

/// An iterator which yields the current count and the element during iteration
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Enumerate<T> {
    iter: T,
    count: uint
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>> Iterator<(uint, A)> for Enumerate<T> {
    #[inline]
    fn next(&mut self) -> Option<(uint, A)> {
        match self.iter.next() {
            Some(a) => {
                let ret = Some((self.count, a));
                self.count += 1;
                ret
            }
            _ => None
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        self.iter.size_hint()
    }
}

#[unstable = "trait is unstable"]
impl<A, T: ExactSizeIterator<A>> DoubleEndedIterator<(uint, A)> for Enumerate<T> {
    #[inline]
    fn next_back(&mut self) -> Option<(uint, A)> {
        match self.iter.next_back() {
            Some(a) => {
                let len = self.iter.len();
                Some((self.count + len, a))
            }
            _ => None
        }
    }
}

#[experimental = "trait is experimental"]
impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<(uint, A)> for Enumerate<T> {
    #[inline]
    fn indexable(&self) -> uint {
        self.iter.indexable()
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<(uint, A)> {
        match self.iter.idx(index) {
            Some(a) => Some((self.count + index, a)),
            _ => None,
        }
    }
}

/// An iterator with a `peek()` that returns an optional reference to the next element.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Peekable<A, T> {
    iter: T,
    peeked: Option<A>,
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>> Iterator<A> for Peekable<A, T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.peeked.is_some() { self.peeked.take() }
        else { self.iter.next() }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (lo, hi) = self.iter.size_hint();
        if self.peeked.is_some() {
            let lo = lo.saturating_add(1);
            let hi = match hi {
                Some(x) => x.checked_add(1),
                None => None
            };
            (lo, hi)
        } else {
            (lo, hi)
        }
    }
}

#[stable]
impl<'a, A, T: Iterator<A>> Peekable<A, T> {
    /// Return a reference to the next element of the iterator with out advancing it,
    /// or None if the iterator is exhausted.
    #[inline]
    pub fn peek(&'a mut self) -> Option<&'a A> {
        if self.peeked.is_none() {
            self.peeked = self.iter.next();
        }
        match self.peeked {
            Some(ref value) => Some(value),
            None => None,
        }
    }

    /// Check whether peekable iterator is empty or not.
    #[inline]
    pub fn is_empty(&mut self) -> bool {
        self.peek().is_none()
    }
}

/// An iterator which rejects elements while `predicate` is true
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct SkipWhile<'a, A, T> {
    iter: T,
    flag: bool,
    predicate: |&A|: 'a -> bool
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>> Iterator<A> for SkipWhile<'a, A, T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        for x in self.iter {
            if self.flag || !(self.predicate)(&x) {
                self.flag = true;
                return Some(x);
            }
        }
        None
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }
}

/// An iterator which only accepts elements while `predicate` is true
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct TakeWhile<'a, A, T> {
    iter: T,
    flag: bool,
    predicate: |&A|: 'a -> bool
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>> Iterator<A> for TakeWhile<'a, A, T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.flag {
            None
        } else {
            match self.iter.next() {
                Some(x) => {
                    if (self.predicate)(&x) {
                        Some(x)
                    } else {
                        self.flag = true;
                        None
                    }
                }
                None => None
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }
}

/// An iterator which skips over `n` elements of `iter`.
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Skip<T> {
    iter: T,
    n: uint
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>> Iterator<A> for Skip<T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        let mut next = self.iter.next();
        if self.n == 0 {
            next
        } else {
            let mut n = self.n;
            while n > 0 {
                n -= 1;
                match next {
                    Some(_) => {
                        next = self.iter.next();
                        continue
                    }
                    None => {
                        self.n = 0;
                        return None
                    }
                }
            }
            self.n = 0;
            next
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (lower, upper) = self.iter.size_hint();

        let lower = lower.saturating_sub(self.n);

        let upper = match upper {
            Some(x) => Some(x.saturating_sub(self.n)),
            None => None
        };

        (lower, upper)
    }
}

#[experimental = "trait is experimental"]
impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Skip<T> {
    #[inline]
    fn indexable(&self) -> uint {
        self.iter.indexable().saturating_sub(self.n)
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        if index >= self.indexable() {
            None
        } else {
            self.iter.idx(index + self.n)
        }
    }
}

/// An iterator which only iterates over the first `n` iterations of `iter`.
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Take<T> {
    iter: T,
    n: uint
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>> Iterator<A> for Take<T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.n != 0 {
            self.n -= 1;
            self.iter.next()
        } else {
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (lower, upper) = self.iter.size_hint();

        let lower = cmp::min(lower, self.n);

        let upper = match upper {
            Some(x) if x < self.n => Some(x),
            _ => Some(self.n)
        };

        (lower, upper)
    }
}

#[experimental = "trait is experimental"]
impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Take<T> {
    #[inline]
    fn indexable(&self) -> uint {
        cmp::min(self.iter.indexable(), self.n)
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        if index >= self.n {
            None
        } else {
            self.iter.idx(index)
        }
    }
}


/// An iterator to maintain state while iterating another iterator
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[unstable = "waiting for unboxed closures"]
pub struct Scan<'a, A, B, T, St> {
    iter: T,
    f: |&mut St, A|: 'a -> Option<B>,

    /// The current internal state to be passed to the closure next.
    pub state: St,
}

#[unstable = "trait is unstable"]
impl<'a, A, B, T: Iterator<A>, St> Iterator<B> for Scan<'a, A, B, T, St> {
    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().and_then(|a| (self.f)(&mut self.state, a))
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the scan function
    }
}

/// An iterator that maps each element to an iterator,
/// and yields the elements of the produced iterators
///
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[unstable = "waiting for unboxed closures"]
pub struct FlatMap<'a, A, T, U> {
    iter: T,
    f: |A|: 'a -> U,
    frontiter: Option<U>,
    backiter: Option<U>,
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>, B, U: Iterator<B>> Iterator<B> for FlatMap<'a, A, T, U> {
    #[inline]
    fn next(&mut self) -> Option<B> {
        loop {
            for inner in self.frontiter.iter_mut() {
                for x in *inner {
                    return Some(x)
                }
            }
            match self.iter.next().map(|x| (self.f)(x)) {
                None => return self.backiter.as_mut().and_then(|it| it.next()),
                next => self.frontiter = next,
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
        let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
        let lo = flo.saturating_add(blo);
        match (self.iter.size_hint(), fhi, bhi) {
            ((0, Some(0)), Some(a), Some(b)) => (lo, a.checked_add(b)),
            _ => (lo, None)
        }
    }
}

#[unstable = "trait is unstable"]
impl<'a,
     A, T: DoubleEndedIterator<A>,
     B, U: DoubleEndedIterator<B>> DoubleEndedIterator<B>
     for FlatMap<'a, A, T, U> {
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        loop {
            for inner in self.backiter.iter_mut() {
                match inner.next_back() {
                    None => (),
                    y => return y
                }
            }
            match self.iter.next_back().map(|x| (self.f)(x)) {
                None => return self.frontiter.as_mut().and_then(|it| it.next_back()),
                next => self.backiter = next,
            }
        }
    }
}

/// An iterator that yields `None` forever after the underlying iterator
/// yields `None` once.
#[deriving(Clone)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[stable]
pub struct Fuse<T> {
    iter: T,
    done: bool
}

#[unstable = "trait is unstable"]
impl<A, T: Iterator<A>> Iterator<A> for Fuse<T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.done {
            None
        } else {
            match self.iter.next() {
                None => {
                    self.done = true;
                    None
                }
                x => x
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        if self.done {
            (0, Some(0))
        } else {
            self.iter.size_hint()
        }
    }
}

#[unstable = "trait is unstable"]
impl<A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A> for Fuse<T> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        if self.done {
            None
        } else {
            match self.iter.next_back() {
                None => {
                    self.done = true;
                    None
                }
                x => x
            }
        }
    }
}

// Allow RandomAccessIterators to be fused without affecting random-access behavior
#[experimental = "trait is experimental"]
impl<A, T: RandomAccessIterator<A>> RandomAccessIterator<A> for Fuse<T> {
    #[inline]
    fn indexable(&self) -> uint {
        self.iter.indexable()
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        self.iter.idx(index)
    }
}

#[experimental = "seems marginal"]
impl<T> Fuse<T> {
    /// Resets the fuse such that the next call to .next() or .next_back() will
    /// call the underlying iterator again even if it previously returned None.
    #[inline]
    pub fn reset_fuse(&mut self) {
        self.done = false
    }
}

/// An iterator that calls a function with a reference to each
/// element before yielding it.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
#[unstable = "waiting for unboxed closures"]
pub struct Inspect<'a, A, T> {
    iter: T,
    f: |&A|: 'a
}

impl<'a, A, T> Inspect<'a, A, T> {
    #[inline]
    fn do_inspect(&mut self, elt: Option<A>) -> Option<A> {
        match elt {
            Some(ref a) => (self.f)(a),
            None => ()
        }

        elt
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, T: Iterator<A>> Iterator<A> for Inspect<'a, A, T> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        let next = self.iter.next();
        self.do_inspect(next)
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        self.iter.size_hint()
    }
}

#[unstable = "trait is unstable"]
impl<'a, A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A>
for Inspect<'a, A, T> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        let next = self.iter.next_back();
        self.do_inspect(next)
    }
}

#[experimental = "trait is experimental"]
impl<'a, A, T: RandomAccessIterator<A>> RandomAccessIterator<A>
for Inspect<'a, A, T> {
    #[inline]
    fn indexable(&self) -> uint {
        self.iter.indexable()
    }

    #[inline]
    fn idx(&mut self, index: uint) -> Option<A> {
        let element = self.iter.idx(index);
        self.do_inspect(element)
    }
}

/// An iterator which passes mutable state to a closure and yields the result.
///
/// # Example: The Fibonacci Sequence
///
/// An iterator that yields sequential Fibonacci numbers, and stops on overflow.
///
/// ```rust
/// use std::iter::Unfold;
/// use std::num::Int; // For `.checked_add()`
///
/// // This iterator will yield up to the last Fibonacci number before the max value of `u32`.
/// // You can simply change `u32` to `u64` in this line if you want higher values than that.
/// let mut fibonacci = Unfold::new((Some(0u32), Some(1u32)), |&(ref mut x2, ref mut x1)| {
///     // Attempt to get the next Fibonacci number
///     // `x1` will be `None` if previously overflowed.
///     let next = match (*x2, *x1) {
///         (Some(x2), Some(x1)) => x2.checked_add(x1),
///         _ => None,
///     };
///
///     // Shift left: ret <- x2 <- x1 <- next
///     let ret = *x2;
///     *x2 = *x1;
///     *x1 = next;
///
///     ret
/// });
///
/// for i in fibonacci {
///     println!("{}", i);
/// }
/// ```
#[experimental]
pub struct Unfold<'a, A, St> {
    f: |&mut St|: 'a -> Option<A>,
    /// Internal state that will be passed to the closure on the next iteration
    pub state: St,
}

#[experimental]
impl<'a, A, St> Unfold<'a, A, St> {
    /// Creates a new iterator with the specified closure as the "iterator
    /// function" and an initial state to eventually pass to the closure
    #[inline]
    pub fn new<'a>(initial_state: St, f: |&mut St|: 'a -> Option<A>)
               -> Unfold<'a, A, St> {
        Unfold {
            f: f,
            state: initial_state
        }
    }
}

#[experimental]
impl<'a, A, St> Iterator<A> for Unfold<'a, A, St> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        (self.f)(&mut self.state)
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        // no possible known bounds at this point
        (0, None)
    }
}

/// An infinite iterator starting at `start` and advancing by `step` with each
/// iteration
#[deriving(Clone)]
#[unstable = "may be renamed"]
pub struct Counter<A> {
    /// The current state the counter is at (next value to be yielded)
    state: A,
    /// The amount that this iterator is stepping by
    step: A,
}

/// Creates a new counter with the specified start/step
#[inline]
#[unstable = "may be renamed"]
pub fn count<A>(start: A, step: A) -> Counter<A> {
    Counter{state: start, step: step}
}

#[unstable = "trait is unstable"]
impl<A: Add<A, A> + Clone> Iterator<A> for Counter<A> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        let result = self.state.clone();
        self.state = self.state + self.step;
        Some(result)
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        (uint::MAX, None) // Too bad we can't specify an infinite lower bound
    }
}

/// An iterator over the range [start, stop)
#[deriving(Clone)]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub struct Range<A> {
    state: A,
    stop: A,
    one: A,
}

/// Returns an iterator over the given range [start, stop) (that is, starting
/// at start (inclusive), and ending at stop (exclusive)).
///
/// # Example
///
/// ```rust
/// let array = [0, 1, 2, 3, 4];
///
/// for i in range(0, 5u) {
///     println!("{}", i);
///     assert_eq!(i,  array[i]);
/// }
/// ```
#[inline]
pub fn range<A: Int>(start: A, stop: A) -> Range<A> {
    Range {
        state: start,
        stop: stop,
        one: Int::one(),
    }
}

// FIXME: #10414: Unfortunate type bound
#[unstable = "trait is unstable"]
impl<A: Int + ToPrimitive> Iterator<A> for Range<A> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if self.state < self.stop {
            let result = self.state.clone();
            self.state = self.state + self.one;
            Some(result)
        } else {
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        // This first checks if the elements are representable as i64. If they aren't, try u64 (to
        // handle cases like range(huge, huger)). We don't use uint/int because the difference of
        // the i64/u64 might lie within their range.
        let bound = match self.state.to_i64() {
            Some(a) => {
                let sz = self.stop.to_i64().map(|b| b.checked_sub(a));
                match sz {
                    Some(Some(bound)) => bound.to_uint(),
                    _ => None,
                }
            },
            None => match self.state.to_u64() {
                Some(a) => {
                    let sz = self.stop.to_u64().map(|b| b.checked_sub(a));
                    match sz {
                        Some(Some(bound)) => bound.to_uint(),
                        _ => None
                    }
                },
                None => None
            }
        };

        match bound {
            Some(b) => (b, Some(b)),
            // Standard fallback for unbounded/unrepresentable bounds
            None => (0, None)
        }
    }
}

/// `Int` is required to ensure the range will be the same regardless of
/// the direction it is consumed.
#[unstable = "trait is unstable"]
impl<A: Int + ToPrimitive> DoubleEndedIterator<A> for Range<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        if self.stop > self.state {
            self.stop = self.stop - self.one;
            Some(self.stop.clone())
        } else {
            None
        }
    }
}

/// An iterator over the range [start, stop]
#[deriving(Clone)]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub struct RangeInclusive<A> {
    range: Range<A>,
    done: bool,
}

/// Return an iterator over the range [start, stop]
#[inline]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub fn range_inclusive<A: Int>(start: A, stop: A) -> RangeInclusive<A> {
    RangeInclusive {
        range: range(start, stop),
        done: false,
    }
}

#[unstable = "trait is unstable"]
impl<A: Int + ToPrimitive> Iterator<A> for RangeInclusive<A> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        match self.range.next() {
            Some(x) => Some(x),
            None => {
                if !self.done && self.range.state == self.range.stop {
                    self.done = true;
                    Some(self.range.stop.clone())
                } else {
                    None
                }
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        let (lo, hi) = self.range.size_hint();
        if self.done {
            (lo, hi)
        } else {
            let lo = lo.saturating_add(1);
            let hi = match hi {
                Some(x) => x.checked_add(1),
                None => None
            };
            (lo, hi)
        }
    }
}

#[unstable = "trait is unstable"]
impl<A: Int + ToPrimitive> DoubleEndedIterator<A> for RangeInclusive<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        if self.range.stop > self.range.state {
            let result = self.range.stop.clone();
            self.range.stop = self.range.stop - self.range.one;
            Some(result)
        } else if !self.done && self.range.state == self.range.stop {
            self.done = true;
            Some(self.range.stop.clone())
        } else {
            None
        }
    }
}

/// An iterator over the range [start, stop) by `step`. It handles overflow by stopping.
#[deriving(Clone)]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub struct RangeStep<A> {
    state: A,
    stop: A,
    step: A,
    rev: bool,
}

/// Return an iterator over the range [start, stop) by `step`. It handles overflow by stopping.
#[inline]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub fn range_step<A: Int>(start: A, stop: A, step: A) -> RangeStep<A> {
    let rev = step < Int::zero();
    RangeStep{state: start, stop: stop, step: step, rev: rev}
}

#[unstable = "trait is unstable"]
impl<A: Int> Iterator<A> for RangeStep<A> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if (self.rev && self.state > self.stop) || (!self.rev && self.state < self.stop) {
            let result = self.state;
            match self.state.checked_add(self.step) {
                Some(x) => self.state = x,
                None => self.state = self.stop.clone()
            }
            Some(result)
        } else {
            None
        }
    }
}

/// An iterator over the range [start, stop] by `step`. It handles overflow by stopping.
#[deriving(Clone)]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub struct RangeStepInclusive<A> {
    state: A,
    stop: A,
    step: A,
    rev: bool,
    done: bool,
}

/// Return an iterator over the range [start, stop] by `step`. It handles overflow by stopping.
#[inline]
#[unstable = "may be refactored due to numerics reform or ops reform"]
pub fn range_step_inclusive<A: Int>(start: A, stop: A, step: A) -> RangeStepInclusive<A> {
    let rev = step < Int::zero();
    RangeStepInclusive {
        state: start,
        stop: stop,
        step: step,
        rev: rev,
        done: false,
    }
}

#[unstable = "trait is unstable"]
impl<A: Int> Iterator<A> for RangeStepInclusive<A> {
    #[inline]
    fn next(&mut self) -> Option<A> {
        if !self.done && ((self.rev && self.state >= self.stop) ||
                          (!self.rev && self.state <= self.stop)) {
            let result = self.state;
            match self.state.checked_add(self.step) {
                Some(x) => self.state = x,
                None => self.done = true
            }
            Some(result)
        } else {
            None
        }
    }
}

/// An iterator that repeats an element endlessly
#[deriving(Clone)]
#[stable]
pub struct Repeat<A> {
    element: A
}

impl<A: Clone> Repeat<A> {
    /// Create a new `Repeat` that endlessly repeats the element `elt`.
    #[inline]
    #[deprecated = "use iter::repeat instead"]
    pub fn new(elt: A) -> Repeat<A> {
        Repeat{element: elt}
    }
}

#[unstable = "trait is unstable"]
impl<A: Clone> Iterator<A> for Repeat<A> {
    #[inline]
    fn next(&mut self) -> Option<A> { self.idx(0) }
    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) { (uint::MAX, None) }
}

#[unstable = "trait is unstable"]
impl<A: Clone> DoubleEndedIterator<A> for Repeat<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> { self.idx(0) }
}

#[experimental = "trait is experimental"]
impl<A: Clone> RandomAccessIterator<A> for Repeat<A> {
    #[inline]
    fn indexable(&self) -> uint { uint::MAX }
    #[inline]
    fn idx(&mut self, _: uint) -> Option<A> { Some(self.element.clone()) }
}

type IterateState<'a, T> = (|T|: 'a -> T, Option<T>, bool);

/// An iterator that repeatedly applies a given function, starting
/// from a given seed value.
#[experimental]
pub type Iterate<'a, T> = Unfold<'a, T, IterateState<'a, T>>;

/// Create a new iterator that produces an infinite sequence of
/// repeated applications of the given function `f`.
#[experimental]
pub fn iterate<'a, T: Clone>(seed: T, f: |T|: 'a -> T) -> Iterate<'a, T> {
    Unfold::new((f, Some(seed), true), |st| {
        let &(ref mut f, ref mut val, ref mut first) = st;
        if *first {
            *first = false;
        } else {
            match val.take() {
                Some(x) => {
                    *val = Some((*f)(x))
                }
                None => {}
            }
        }
        val.clone()
    })
}

/// Create a new iterator that endlessly repeats the element `elt`.
#[inline]
#[stable]
pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
    Repeat{element: elt}
}

/// Functions for lexicographical ordering of sequences.
///
/// Lexicographical ordering through `<`, `<=`, `>=`, `>` requires
/// that the elements implement both `PartialEq` and `PartialOrd`.
///
/// If two sequences are equal up until the point where one ends,
/// the shorter sequence compares less.
#[experimental = "likely to be removed after cmp reform"]
pub mod order {
    use cmp;
    use cmp::{Eq, Ord, PartialOrd, PartialEq};
    use cmp::Ordering::{Equal, Less, Greater};
    use option::Option;
    use option::Option::{Some, None};
    use super::Iterator;

    /// Compare `a` and `b` for equality using `Eq`
    pub fn equals<A: Eq, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> bool {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return true,
                (None, _) | (_, None) => return false,
                (Some(x), Some(y)) => if x != y { return false },
            }
        }
    }

    /// Order `a` and `b` lexicographically using `Ord`
    pub fn cmp<A: Ord, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> cmp::Ordering {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return Equal,
                (None, _   ) => return Less,
                (_   , None) => return Greater,
                (Some(x), Some(y)) => match x.cmp(&y) {
                    Equal => (),
                    non_eq => return non_eq,
                },
            }
        }
    }

    /// Order `a` and `b` lexicographically using `PartialOrd`
    pub fn partial_cmp<A: PartialOrd, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S)
            -> Option<cmp::Ordering> {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return Some(Equal),
                (None, _   ) => return Some(Less),
                (_   , None) => return Some(Greater),
                (Some(x), Some(y)) => match x.partial_cmp(&y) {
                    Some(Equal) => (),
                    non_eq => return non_eq,
                },
            }
        }
    }

    /// Compare `a` and `b` for equality (Using partial equality, `PartialEq`)
    pub fn eq<A, B, L, R>(mut a: L, mut b: R) -> bool where
        A: PartialEq<B>,
        L: Iterator<A>,
        R: Iterator<B>,
    {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return true,
                (None, _) | (_, None) => return false,
                (Some(x), Some(y)) => if !x.eq(&y) { return false },
            }
        }
    }

    /// Compare `a` and `b` for nonequality (Using partial equality, `PartialEq`)
    pub fn ne<A, B, L, R>(mut a: L, mut b: R) -> bool where
        A: PartialEq<B>,
        L: Iterator<A>,
        R: Iterator<B>,
    {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return false,
                (None, _) | (_, None) => return true,
                (Some(x), Some(y)) => if x.ne(&y) { return true },
            }
        }
    }

    /// Return `a` < `b` lexicographically (Using partial order, `PartialOrd`)
    pub fn lt<A: PartialOrd, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> bool {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return false,
                (None, _   ) => return true,
                (_   , None) => return false,
                (Some(x), Some(y)) => if x.ne(&y) { return x.lt(&y) },
            }
        }
    }

    /// Return `a` <= `b` lexicographically (Using partial order, `PartialOrd`)
    pub fn le<A: PartialOrd, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> bool {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return true,
                (None, _   ) => return true,
                (_   , None) => return false,
                (Some(x), Some(y)) => if x.ne(&y) { return x.le(&y) },
            }
        }
    }

    /// Return `a` > `b` lexicographically (Using partial order, `PartialOrd`)
    pub fn gt<A: PartialOrd, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> bool {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return false,
                (None, _   ) => return false,
                (_   , None) => return true,
                (Some(x), Some(y)) => if x.ne(&y) { return x.gt(&y) },
            }
        }
    }

    /// Return `a` >= `b` lexicographically (Using partial order, `PartialOrd`)
    pub fn ge<A: PartialOrd, T: Iterator<A>, S: Iterator<A>>(mut a: T, mut b: S) -> bool {
        loop {
            match (a.next(), b.next()) {
                (None, None) => return true,
                (None, _   ) => return false,
                (_   , None) => return true,
                (Some(x), Some(y)) => if x.ne(&y) { return x.ge(&y) },
            }
        }
    }
}