use std::cell::Cell;
use std::fmt;

/// Format all iterator elements lazily, separated by `sep`.
///
/// The format value can only be formatted once, after that the iterator is
/// exhausted.
///
/// See [`.format_with()`](crate::Itertools::format_with) for more information.
pub struct FormatWith<'a, I, F> {
    sep: &'a str,
    /// FormatWith uses interior mutability because Display::fmt takes &self.
    inner: Cell<Option<(I, F)>>,
}

/// Format all iterator elements lazily, separated by `sep`.
///
/// The format value can only be formatted once, after that the iterator is
/// exhausted.
///
/// See [`.format()`](crate::Itertools::format)
/// for more information.
pub struct Format<'a, I> {
    sep: &'a str,
    /// Format uses interior mutability because Display::fmt takes &self.
    inner: Cell<Option<I>>,
}

pub fn new_format<I, F>(iter: I, separator: &str, f: F) -> FormatWith<'_, I, F>
where
    I: Iterator,
    F: FnMut(I::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result,
{
    FormatWith {
        sep: separator,
        inner: Cell::new(Some((iter, f))),
    }
}

pub fn new_format_default<I>(iter: I, separator: &str) -> Format<'_, I>
where
    I: Iterator,
{
    Format {
        sep: separator,
        inner: Cell::new(Some(iter)),
    }
}

impl<'a, I, F> fmt::Display for FormatWith<'a, I, F>
where
    I: Iterator,
    F: FnMut(I::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let (mut iter, mut format) = match self.inner.take() {
            Some(t) => t,
            None => panic!("FormatWith: was already formatted once"),
        };

        if let Some(fst) = iter.next() {
            format(fst, &mut |disp: &dyn fmt::Display| disp.fmt(f))?;
            iter.try_for_each(|elt| {
                if !self.sep.is_empty() {
                    f.write_str(self.sep)?;
                }
                format(elt, &mut |disp: &dyn fmt::Display| disp.fmt(f))
            })?;
        }
        Ok(())
    }
}

impl<'a, I> Format<'a, I>
where
    I: Iterator,
{
    fn format(
        &self,
        f: &mut fmt::Formatter,
        cb: fn(&I::Item, &mut fmt::Formatter) -> fmt::Result,
    ) -> fmt::Result {
        let mut iter = match self.inner.take() {
            Some(t) => t,
            None => panic!("Format: was already formatted once"),
        };

        if let Some(fst) = iter.next() {
            cb(&fst, f)?;
            iter.try_for_each(|elt| {
                if !self.sep.is_empty() {
                    f.write_str(self.sep)?;
                }
                cb(&elt, f)
            })?;
        }
        Ok(())
    }
}

macro_rules! impl_format {
    ($($fmt_trait:ident)*) => {
        $(
            impl<'a, I> fmt::$fmt_trait for Format<'a, I>
                where I: Iterator,
                      I::Item: fmt::$fmt_trait,
            {
                fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                    self.format(f, fmt::$fmt_trait::fmt)
                }
            }
        )*
    }
}

impl_format! {Display Debug UpperExp LowerExp UpperHex LowerHex Octal Binary Pointer}

impl<'a, I, F> Clone for FormatWith<'a, I, F>
where
    (I, F): Clone,
{
    fn clone(&self) -> Self {
        struct PutBackOnDrop<'r, 'a, I, F> {
            into: &'r FormatWith<'a, I, F>,
            inner: Option<(I, F)>,
        }
        // This ensures we preserve the state of the original `FormatWith` if `Clone` panics
        impl<'r, 'a, I, F> Drop for PutBackOnDrop<'r, 'a, I, F> {
            fn drop(&mut self) {
                self.into.inner.set(self.inner.take())
            }
        }
        let pbod = PutBackOnDrop {
            inner: self.inner.take(),
            into: self,
        };
        Self {
            inner: Cell::new(pbod.inner.clone()),
            sep: self.sep,
        }
    }
}

impl<'a, I> Clone for Format<'a, I>
where
    I: Clone,
{
    fn clone(&self) -> Self {
        struct PutBackOnDrop<'r, 'a, I> {
            into: &'r Format<'a, I>,
            inner: Option<I>,
        }
        // This ensures we preserve the state of the original `FormatWith` if `Clone` panics
        impl<'r, 'a, I> Drop for PutBackOnDrop<'r, 'a, I> {
            fn drop(&mut self) {
                self.into.inner.set(self.inner.take())
            }
        }
        let pbod = PutBackOnDrop {
            inner: self.inner.take(),
            into: self,
        };
        Self {
            inner: Cell::new(pbod.inner.clone()),
            sep: self.sep,
        }
    }
}