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//! This crate provides a way to create iterators on the fly.
//!
//! It currently only consists of the `iter_vals` macro to archive that.

/// Creates an iterator for all the given values.
///
/// # Examples
///
/// You can use it to return an iterator over literals:
///
/// ```
/// use iter_vals::iter_vals;
///
/// #[derive(Debug, PartialEq)]
/// enum Media {
///     Book,
///     Newspaper,
///     TV,
///     PC,
/// }
///
/// use Media::*;
///
/// impl Media {
///     fn digital() -> impl Iterator<Item = Media> {
///         iter_vals!(TV, PC)
///     }
/// 
///     fn non_digital() -> impl Iterator<Item = Media> {
///         iter_vals!(Book, Newspaper)
///     }
/// }
///
/// let mut digital = Media::digital();
/// assert_eq!(digital.next(), Some(TV));
/// assert_eq!(digital.next(), Some(PC));
/// assert_eq!(digital.next(), None);
///
/// let mut non_digital = Media::non_digital();
/// assert_eq!(non_digital.next(), Some(Book));
/// assert_eq!(non_digital.next(), Some(Newspaper));
/// assert_eq!(non_digital.next(), None);
/// ```
///
/// You can conditionally include values.
/// This can be useful to return a variable number of values without having to allocate:
///
/// ```
/// use iter_vals::iter_vals;
///
/// fn next_numbers(start: i32, include_first_number: bool) -> impl Iterator<Item = i32> {
///     iter_vals!(
///         [..= include_first_number; start + 1],
///         (start + 2),
///         (start + 3)
///     )
/// }
///
/// let mut next_nums = next_numbers(5, true);
/// assert_eq!(next_nums.next(), Some(6));
/// assert_eq!(next_nums.next(), Some(7));
/// assert_eq!(next_nums.next(), Some(8));
/// assert_eq!(next_nums.next(), None);
///
/// let mut next_nums = next_numbers(5, false);
/// assert_eq!(next_nums.next(), Some(7));
/// assert_eq!(next_nums.next(), Some(8));
/// assert_eq!(next_nums.next(), None);
/// ```
///
/// You can expand other iterators inside the iterator you return.
/// This can be especially useful, when dealing with `Option`s:
///
/// ```
/// use iter_vals::iter_vals;
///
/// fn make_iter(num1: i32, num2: Option<i32>, num3: i32) -> impl Iterator<Item = i32> {
///     iter_vals!(
///         num1,
///         [.. num2],
///         num3
///     )
/// }
///
/// let mut nums = make_iter(1, Some(2), 3);
/// assert_eq!(nums.next(), Some(1));
/// assert_eq!(nums.next(), Some(2));
/// assert_eq!(nums.next(), Some(3));
/// assert_eq!(nums.next(), None);
///
/// let mut nums = make_iter(1, None, 3);
/// assert_eq!(nums.next(), Some(1));
/// assert_eq!(nums.next(), Some(3));
/// assert_eq!(nums.next(), None);
///
/// let mut nums = iter_vals!(1, [.. vec![2, 2]], 3);
/// assert_eq!(nums.next(), Some(1));
/// assert_eq!(nums.next(), Some(2));
/// assert_eq!(nums.next(), Some(2));
/// assert_eq!(nums.next(), Some(3));
/// assert_eq!(nums.next(), None);
/// ```
///
/// # Note
///
/// If you want to return computed values, you currently have to put them in parenthesis for that
/// to work, because of a limitation in the macro system.
///
/// This will **not** work:
///
/// ```no_compile
/// use iter_vals::iter_vals;
///
/// let nums: Vec<_> = iter_vals!(1 + 1, 2 + 2, 3 + 3).collect();
/// assert_eq!(nums, vec![2, 4, 6]);
/// ```
///
/// But this will:
///
/// ```
/// use iter_vals::iter_vals;
///
/// let nums: Vec<_> = iter_vals!((1 + 1), (2 + 2), (3 + 3)).collect();
/// assert_eq!(nums, vec![2, 4, 6]);
/// ```
#[macro_export]
macro_rules! iter_vals {
    () => {
        core::iter::empty()
    };
    ([..= $cond:expr ; $val:expr]) => {
        if $cond {
            Some($val)
        } else {
            None
        }.into_iter()
    };
    ([.. $val:expr]) => {
        $val.into_iter()
    };
    ($val:expr) => {
        core::iter::once($val)
    };
    ($first_val:tt, $($other_vals:tt),*) => {
        iter_vals!($first_val)
            $(
                .chain(iter_vals!($other_vals))
            )*
    };
}

#[cfg(test)]
mod tests {
    #[test]
    fn basic_vals() {
        assert_eq!(iter_vals!(1, 2, 3).collect::<Vec<_>>(), vec![1, 2, 3]);
        assert_eq!(iter_vals!("this", "is", "a", "test").collect::<Vec<_>>(), vec!["this", "is", "a", "test"]);
    }

    #[test]
    fn empty() {
        assert_eq!(iter_vals!().collect::<Vec<i32>>(), vec![]);
    }

    #[test]
    fn conditional() {
        assert_eq!(iter_vals!([..=  1 == 1; 5], [..=  1 == 2; 10]).collect::<Vec<_>>(), vec![5]);
    }

    #[test]
    fn expansion() {
        assert_eq!(iter_vals!([.. vec![1, 2, 3]], [.. Some(4)]).collect::<Vec<_>>(), vec![1, 2, 3, 4]);
    }

    #[test]
    fn mixed() {
        assert_eq!(iter_vals!(
                1,
                [..= 2 % 2 == 1; 2],
                3,
                [..= 4 % 2 == 0; 4],
                [.. vec![5, 6, 7]]
            ).collect::<Vec<_>>(),
            vec![1, 3, 4, 5, 6, 7]
        );
    }
}