agb_eb_ext 0.25.3

use agb::display::GraphicsFrame;
use agb::display::tiled::BackgroundId;
use agb::fixnum::{Num, num};

/// Represents each step in a fade transition
///
/// Walk from [`Start`](Fade::Start) to [`End`](Fade::End) with [`next`](Fade::next)
/// (or backwards with [`prev`](Fade::prev)), passing the value to one of the `fade_*` functions each frame
#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
pub enum Fade {
    /// No fade applied
    #[default]
    Start,
    /// ~19%
    Step1,
    /// ~31%
    Step2,
    /// 50%
    Step3,
    /// ~69%
    Step4,
    /// ~81%
    Step5,
    /// Fully faded
    End,
}

impl Fade {
    /// The following step, or `None` at [`End`](Fade::End)
    pub const fn next(self) -> Option<Self> {
        match self {
            Fade::Start => Some(Fade::Step1),
            Fade::Step1 => Some(Fade::Step2),
            Fade::Step2 => Some(Fade::Step3),
            Fade::Step3 => Some(Fade::Step4),
            Fade::Step4 => Some(Fade::Step5),
            Fade::Step5 => Some(Fade::End),
            Fade::End => None,
        }
    }

    /// The preceding step, or `None` at [`Start`](Fade::Start)
    pub const fn prev(self) -> Option<Self> {
        match self {
            Fade::Start => None,
            Fade::Step1 => Some(Fade::Start),
            Fade::Step2 => Some(Fade::Step1),
            Fade::Step3 => Some(Fade::Step2),
            Fade::Step4 => Some(Fade::Step3),
            Fade::Step5 => Some(Fade::Step4),
            Fade::End => Some(Fade::Step5),
        }
    }

    /// `true` at [`End`](Fade::End)
    #[inline]
    pub const fn is_done(self) -> bool {
        matches!(self, Fade::End)
    }

    /// Blend amount for this step
    ///
    /// `FadeAmount` only has 4 fractional bits, so amounts are multiples of 1/16
    pub fn amount(self) -> FadeAmount {
        match self {
            Fade::Start => num!(0),
            Fade::Step1 => num!(0.1875),
            Fade::Step2 => num!(0.3125),
            Fade::Step3 => num!(0.5),
            Fade::Step4 => num!(0.6875),
            Fade::Step5 => num!(0.8125),
            Fade::End => num!(1),
        }
    }
}

impl From<Fade> for FadeAmount {
    fn from(value: Fade) -> Self {
        value.amount()
    }
}

/// Blend amount, `0` = no effect, `1` = fully black/white
pub type FadeAmount = Num<u8, 4>;

enum FadeKind {
    Darken,
    Brighten,
}

fn apply_fade(
    kind: FadeKind,
    amount: FadeAmount,
    bg_ids: &[BackgroundId],
    frame: &mut GraphicsFrame,
) {
    let mut effect = match kind {
        FadeKind::Darken => frame.blend().darken(amount),
        FadeKind::Brighten => frame.blend().brighten(amount),
    };
    effect.enable_object();
    for id in bg_ids {
        effect.enable_background(*id);
    }
}

/// Fade to fully black
///
/// Start with `Fade::Start`
pub fn fade_to_black<A: Into<FadeAmount>>(
    amount: A,
    bg_ids: &[BackgroundId],
    frame: &mut GraphicsFrame,
) {
    apply_fade(FadeKind::Darken, amount.into(), bg_ids, frame);
}

/// Fade from fully black
///
/// Start with `Fade::Start`
pub fn fade_from_black<A: Into<FadeAmount>>(
    amount: A,
    bg_ids: &[BackgroundId],
    frame: &mut GraphicsFrame,
) {
    apply_fade(FadeKind::Darken, num!(1) - amount.into(), bg_ids, frame);
}

/// Fade to fully white
///
/// Start with `Fade::Start`
pub fn fade_to_white<A: Into<FadeAmount>>(
    amount: A,
    bg_ids: &[BackgroundId],
    frame: &mut GraphicsFrame,
) {
    apply_fade(FadeKind::Brighten, amount.into(), bg_ids, frame);
}

/// Fade from fully white
///
/// Start with `Fade::Start`
pub fn fade_from_white<A: Into<FadeAmount>>(
    amount: A,
    bg_ids: &[BackgroundId],
    frame: &mut GraphicsFrame,
) {
    apply_fade(FadeKind::Brighten, num!(1) - amount.into(), bg_ids, frame);
}

/// Leave everything at normal brightness
pub fn reset_fade(bg_ids: &[BackgroundId], frame: &mut GraphicsFrame) {
    apply_fade(FadeKind::Brighten, num!(0), bg_ids, frame);
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test_case]
    fn steps_chain(_gba: &mut agb::Gba) {
        let mut fade = Fade::default();
        let mut count = 0;
        while let Some(next) = fade.next() {
            assert!(next > fade);
            assert_eq!(next.prev(), Some(fade));
            fade = next;
            count += 1;
        }
        assert!(fade.is_done());
        assert_eq!(count, 6);
        assert_eq!(Fade::End.amount(), num!(1));
        assert_eq!(Fade::Start.amount(), num!(0));
    }
}