use crate::fmt;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedFused};
use crate::num::NonZeroUsize;
use crate::ops::{ControlFlow, Try};

/// An iterator that only accepts elements while `predicate` returns `true`.
///
/// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`take_while`]: Iterator::take_while
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct TakeWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}

impl<I, P> TakeWhile<I, P> {
    pub(in crate::iter) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
        TakeWhile { iter, flag: false, predicate }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile").field("iter", &self.iter).field("flag", &self.flag).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for TakeWhile<I, P>
where
    P: FnMut(&I::Item) -> bool,
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.flag {
            None
        } else {
            let x = self.iter.next()?;
            if (self.predicate)(&x) {
                Some(x)
            } else {
                self.flag = true;
                None
            }
        }
    }

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

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        fn check<'a, T, Acc, R: Try<Output = Acc>>(
            flag: &'a mut bool,
            p: &'a mut impl FnMut(&T) -> bool,
            mut fold: impl FnMut(Acc, T) -> R + 'a,
        ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
            move |acc, x| {
                if p(&x) {
                    ControlFlow::from_try(fold(acc, x))
                } else {
                    *flag = true;
                    ControlFlow::Break(try { acc })
                }
            }
        }

        if self.flag {
            try { init }
        } else {
            let flag = &mut self.flag;
            let p = &mut self.predicate;
            self.iter.try_fold(init, check(flag, p, fold)).into_try()
        }
    }

    impl_fold_via_try_fold! { fold -> try_fold }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for TakeWhile<I, P>
where
    I: FusedIterator,
    P: FnMut(&I::Item) -> bool,
{
}

#[unstable(issue = "none", feature = "trusted_fused")]
unsafe impl<I: TrustedFused, P> TrustedFused for TakeWhile<I, P> {}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<P, I> SourceIter for TakeWhile<I, P>
where
    I: SourceIter,
{
    type Source = I::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I: InPlaceIterable, F> InPlaceIterable for TakeWhile<I, F> {
    const EXPAND_BY: Option<NonZeroUsize> = I::EXPAND_BY;
    const MERGE_BY: Option<NonZeroUsize> = I::MERGE_BY;
}
