//! Iterators that are sources (produce elements from parameters,

//! not from another iterator).

#![allow(deprecated)]

use std::fmt;

use std::mem;

/// Creates a new unfold source with the specified closure as the "iterator

/// function" and an initial state to eventually pass to the closure

///

/// `unfold` is a general iterator builder: it has a mutable state value,

/// and a closure with access to the state that produces the next value.

///

/// This more or less equivalent to a regular struct with an [`Iterator`]

/// implementation, and is useful for one-off iterators.

///

/// ```

/// // an iterator that yields sequential Fibonacci numbers,

/// // and stops at the maximum representable value.

///

/// use itertools::unfold;

///

/// let mut fibonacci = unfold((1u32, 1u32), |(x1, x2)| {

///     // Attempt to get the next Fibonacci number

///     let next = x1.saturating_add(*x2);

///

///     // Shift left: ret <- x1 <- x2 <- next

///     let ret = *x1;

///     *x1 = *x2;

///     *x2 = next;

///

///     // If addition has saturated at the maximum, we are finished

///     if ret == *x1 && ret > 1 {

///         None

///     } else {

///         Some(ret)

///     }

/// });

///

/// itertools::assert_equal(fibonacci.by_ref().take(8),

///                         vec![1, 1, 2, 3, 5, 8, 13, 21]);

/// assert_eq!(fibonacci.last(), Some(2_971_215_073))

/// ```

#[deprecated(

    note = "Use [std::iter::from_fn](https://doc.rust-lang.org/std/iter/fn.from_fn.html) instead",

    since = "0.13.0"

)]

pub fn unfold<A, St, F>(initial_state: St, f: F) -> Unfold<St, F>

where

    F: FnMut(&mut St) -> Option<A>,

{

    Unfold {

        f,

        state: initial_state,

    }

}

impl<St, F> fmt::Debug for Unfold<St, F>

where

    St: fmt::Debug,

{

    debug_fmt_fields!(Unfold, state);

}

/// See [`unfold`](crate::unfold) for more information.

#[derive(Clone)]

#[must_use = "iterators are lazy and do nothing unless consumed"]

#[deprecated(

    note = "Use [std::iter::FromFn](https://doc.rust-lang.org/std/iter/struct.FromFn.html) instead",

    since = "0.13.0"

)]

pub struct Unfold<St, F> {

    f: F,

    /// Internal state that will be passed to the closure on the next iteration

    pub state: St,

}

impl<A, St, F> Iterator for Unfold<St, F>

where

    F: FnMut(&mut St) -> Option<A>,

{

    type Item = A;

    #[inline]

    fn next(&mut self) -> Option<Self::Item> {

        (self.f)(&mut self.state)

    }

}

/// An iterator that infinitely applies function to value and yields results.

///

/// This `struct` is created by the [`iterate()`](crate::iterate) function.

/// See its documentation for more.

#[derive(Clone)]

#[must_use = "iterators are lazy and do nothing unless consumed"]

pub struct Iterate<St, F> {

    state: St,

    f: F,

}

impl<St, F> fmt::Debug for Iterate<St, F>

where

    St: fmt::Debug,

{

    debug_fmt_fields!(Iterate, state);

}

impl<St, F> Iterator for Iterate<St, F>

where

    F: FnMut(&St) -> St,

{

    type Item = St;

    #[inline]

    fn next(&mut self) -> Option<Self::Item> {

        let next_state = (self.f)(&self.state);

        Some(mem::replace(&mut self.state, next_state))

    }

    #[inline]

    fn size_hint(&self) -> (usize, Option<usize>) {

        (usize::MAX, None)

    }

}

/// Creates a new iterator that infinitely applies function to value and yields results.

///

/// ```

/// use itertools::iterate;

///

/// itertools::assert_equal(iterate(1, |i| i % 3 + 1).take(5), vec![1, 2, 3, 1, 2]);

/// ```

///

/// **Panics** if compute the next value does.

///

/// ```should_panic

/// # use itertools::iterate;

/// let mut it = iterate(25u32, |x| x - 10).take_while(|&x| x > 10);

/// assert_eq!(it.next(), Some(25)); // `Iterate` holds 15.

/// assert_eq!(it.next(), Some(15)); // `Iterate` holds 5.

/// it.next(); // `5 - 10` overflows.

/// ```

///

/// You can alternatively use [`core::iter::successors`] as it better describes a finite iterator.

pub fn iterate<St, F>(initial_value: St, f: F) -> Iterate<St, F>

where

    F: FnMut(&St) -> St,

{

    Iterate {

        state: initial_value,

        f,

    }

}