use crate::{Bounds, Identified, LayoutId, Pixels, Point, Result, ViewContext};
use derive_more::{Deref, DerefMut};
pub(crate) use smallvec::SmallVec;
use util::arc_cow::ArcCow;

pub trait Element: 'static {
    type State;
    type FrameState;

    fn element_id(&self) -> Option<ElementId> {
        None
    }

    fn layout(
        &mut self,
        state: &mut Self::State,
        cx: &mut ViewContext<Self::State>,
    ) -> Result<(LayoutId, Self::FrameState)>;

    fn paint(
        &mut self,
        bounds: Bounds<Pixels>,
        state: &mut Self::State,
        frame_state: &mut Self::FrameState,
        cx: &mut ViewContext<Self::State>,
    ) -> Result<()>;

    fn id(self, id: ElementId) -> Identified<Self>
    where
        Self: Sized,
    {
        Identified { element: self, id }
    }
}

pub trait StatefulElement: Element {
    fn element_id(&self) -> ElementId {
        Element::element_id(self).unwrap()
    }
}

#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub struct ElementId(ArcCow<'static, [u8]>);

#[derive(Deref, DerefMut, Default, Clone, Debug)]
pub(crate) struct GlobalElementId(SmallVec<[ElementId; 8]>);

pub trait ParentElement {
    type State;

    fn children_mut(&mut self) -> &mut SmallVec<[AnyElement<Self::State>; 2]>;

    fn child(mut self, child: impl IntoAnyElement<Self::State>) -> Self
    where
        Self: Sized,
    {
        self.children_mut().push(child.into_any());
        self
    }

    fn children(mut self, iter: impl IntoIterator<Item = impl IntoAnyElement<Self::State>>) -> Self
    where
        Self: Sized,
    {
        self.children_mut()
            .extend(iter.into_iter().map(|item| item.into_any()));
        self
    }

    // HACK: This is a temporary hack to get children working for the purposes
    // of building UI on top of the current version of gpui2.
    //
    // We'll (hopefully) be moving away from this in the future.
    fn children_any<I>(mut self, children: I) -> Self
    where
        I: IntoIterator<Item = AnyElement<Self::State>>,
        Self: Sized,
    {
        self.children_mut().extend(children.into_iter());
        self
    }

    // HACK: This is a temporary hack to get children working for the purposes
    // of building UI on top of the current version of gpui2.
    //
    // We'll (hopefully) be moving away from this in the future.
    fn child_any(mut self, children: AnyElement<Self::State>) -> Self
    where
        Self: Sized,
    {
        self.children_mut().push(children);
        self
    }
}

trait ElementObject<S> {
    fn layout(&mut self, state: &mut S, cx: &mut ViewContext<S>) -> Result<LayoutId>;
    fn paint(
        &mut self,
        state: &mut S,
        offset: Option<Point<Pixels>>,
        cx: &mut ViewContext<S>,
    ) -> Result<()>;
}

struct RenderedElement<E: Element> {
    element: E,
    phase: ElementRenderPhase<E::FrameState>,
}

#[derive(Default)]
enum ElementRenderPhase<S> {
    #[default]
    Rendered,
    LayoutRequested {
        layout_id: LayoutId,
        frame_state: S,
    },
    Painted {
        bounds: Bounds<Pixels>,
        frame_state: S,
    },
}

/// Internal struct that wraps an element to store Layout and FrameState after the element is rendered.
/// It's allocated as a trait object to erase the element type and wrapped in AnyElement<E::State> for
/// improved usability.
impl<E: Element> RenderedElement<E> {
    fn new(element: E) -> Self {
        RenderedElement {
            element,
            phase: ElementRenderPhase::Rendered,
        }
    }
}

impl<E: Element> ElementObject<E::State> for RenderedElement<E> {
    fn layout(&mut self, state: &mut E::State, cx: &mut ViewContext<E::State>) -> Result<LayoutId> {
        let (layout_id, frame_state) = self.element.layout(state, cx)?;
        self.phase = ElementRenderPhase::LayoutRequested {
            layout_id,
            frame_state,
        };
        Ok(layout_id)
    }

    fn paint(
        &mut self,
        state: &mut E::State,
        offset: Option<Point<Pixels>>,
        cx: &mut ViewContext<E::State>,
    ) -> Result<()> {
        self.phase = match std::mem::take(&mut self.phase) {
            ElementRenderPhase::Rendered => panic!("must call layout before paint"),

            ElementRenderPhase::LayoutRequested {
                layout_id,
                mut frame_state,
            } => {
                let mut bounds = cx.layout_bounds(layout_id)?.clone();
                offset.map(|offset| bounds.origin += offset);
                self.element.paint(bounds, state, &mut frame_state, cx)?;
                ElementRenderPhase::Painted {
                    bounds,
                    frame_state,
                }
            }

            ElementRenderPhase::Painted {
                bounds,
                mut frame_state,
            } => {
                self.element
                    .paint(bounds.clone(), state, &mut frame_state, cx)?;
                ElementRenderPhase::Painted {
                    bounds,
                    frame_state,
                }
            }
        };

        Ok(())
    }
}

pub struct AnyElement<S>(Box<dyn ElementObject<S>>);

impl<S> AnyElement<S> {
    pub fn layout(&mut self, state: &mut S, cx: &mut ViewContext<S>) -> Result<LayoutId> {
        self.0.layout(state, cx)
    }

    pub fn paint(
        &mut self,
        state: &mut S,
        offset: Option<Point<Pixels>>,
        cx: &mut ViewContext<S>,
    ) -> Result<()> {
        self.0.paint(state, offset, cx)
    }
}

pub trait IntoAnyElement<S> {
    fn into_any(self) -> AnyElement<S>;
}

impl<E: Element> IntoAnyElement<E::State> for E {
    fn into_any(self) -> AnyElement<E::State> {
        AnyElement(Box::new(RenderedElement::new(self)))
    }
}

impl<S> IntoAnyElement<S> for AnyElement<S> {
    fn into_any(self) -> AnyElement<S> {
        self
    }
}