//! A column that remembers where it put its rows.
//!
//! iced can scroll a list to an offset and to nothing else: `snap_to`,
//! `scroll_to` and `scroll_by` all take a number of points, and there is no
//! "show me this child" anywhere in the toolkit. So a client that wants to be
//! taken to one row of a hundred has to answer the question itself — how far
//! down the content is that row? — and the only thing that knows is the layout.
//!
//! This is that, in the one place the answer is free. `Rows` wraps the column
//! inside a scroll area and does nothing to it but read the layout it already
//! computed, writing down where each row landed. A jump is then a lookup and
//! the `scroll_to` that already exists.
//!
//! It is one widget per scroll area, always, whether or not anything is asking
//! to be scrolled to. That is the whole of its safety: iced matches widgets by
//! where they sit, so a wrapper that came and went as rows became interesting
//! would leave the tree a different shape than the state kept for it. The
//! `State` below is why it can be told apart from the column it wraps at all —
//! iced tells two widgets apart by the type of their state, and two stateless
//! ones are the same widget as far as it knows.

use std::collections::HashMap;
use std::sync::{Arc, Mutex};

use cosmic::iced::advanced::widget::{Operation, Tree, tree};
use cosmic::iced::advanced::{Clipboard, Layout, Shell, Widget, layout, mouse, overlay, renderer};
// `cosmic::Element`, not iced's: the two differ in their theme, and this
// widget lives in a cosmic tree.
use cosmic::Element;
use cosmic::iced::{Event, Length, Rectangle, Size, Vector};

/// Where each row of one scroll area was last laid out, in points from the top
/// of the content, beside how tall it is.
///
/// Shared with whoever wants to scroll: the widget writes during layout, the
/// app reads when a jump asks for a row. Late answers are the point — a row
/// only just mounted is measured on the frame it appears, and the jump that
/// wanted it can ask again on the next one rather than having missed its one
/// chance.
#[derive(Clone, Default)]
pub struct Placements(Arc<Mutex<HashMap<i32, (f32, f32)>>>);

impl Placements {
    pub fn new() -> Self {
        Self::default()
    }

    /// Where row `key` sits, if it has been laid out since the last time the
    /// list it is in was.
    pub fn get(&self, key: i32) -> Option<(f32, f32)> {
        self.0.lock().ok()?.get(&key).copied()
    }

    /// How many rows have a place written down. For the log alone.
    pub fn len(&self) -> usize {
        self.0.lock().map(|held| held.len()).unwrap_or(0)
    }

    fn write(&self, rows: HashMap<i32, (f32, f32)>) {
        if let Ok(mut held) = self.0.lock() {
            *held = rows;
        }
    }
}

/// The trivial state that gives this widget a type of its own.
///
/// It holds nothing. What it is for is the tag: a wrapper with no state is
/// indistinguishable from a plain column to iced's diffing, which reuses the
/// tree it kept for one as the tree for the other — and the mismatch is not
/// noticed until something deep inside reads a child that was never there.
struct State;

pub struct Rows<'a, Message> {
    inner: Element<'a, Message>,
    keys: Vec<i32>,
    placements: Placements,
}

impl<'a, Message> Rows<'a, Message> {
    /// `keys` are the rows of `inner`, in the order they were given to it.
    pub fn new(
        inner: impl Into<Element<'a, Message>>,
        keys: Vec<i32>,
        placements: Placements,
    ) -> Self {
        Self {
            inner: inner.into(),
            keys,
            placements,
        }
    }
}

impl<Message> Widget<Message, cosmic::Theme, cosmic::Renderer> for Rows<'_, Message> {
    fn tag(&self) -> tree::Tag {
        tree::Tag::of::<State>()
    }

    fn state(&self) -> tree::State {
        tree::State::new(State)
    }

    fn children(&self) -> Vec<Tree> {
        vec![Tree::new(&self.inner)]
    }

    fn diff(&mut self, tree: &mut Tree) {
        tree.diff_children(std::slice::from_mut(&mut self.inner));
    }

    fn size(&self) -> Size<Length> {
        self.inner.as_widget().size()
    }

    fn layout(
        &mut self,
        tree: &mut Tree,
        renderer: &cosmic::Renderer,
        limits: &layout::Limits,
    ) -> layout::Node {
        let node = self
            .inner
            .as_widget_mut()
            .layout(&mut tree.children[0], renderer, limits);

        // The column's own children, in the order its keys were given. Their
        // bounds are already relative to the content's top, which is the
        // coordinate `scroll_to` is asking for.
        //
        // Written whole rather than merged: a row that has left the list has
        // no place any more, and an old answer for it would send a jump to
        // wherever it used to be.
        let mut rows = HashMap::with_capacity(self.keys.len());
        for (key, child) in self.keys.iter().zip(node.children()) {
            let bounds = child.bounds();
            rows.insert(*key, (bounds.y, bounds.height));
        }
        self.placements.write(rows);

        let size = node.size();
        layout::Node::with_children(size, vec![node])
    }

    fn operate(
        &mut self,
        tree: &mut Tree,
        layout: Layout<'_>,
        renderer: &cosmic::Renderer,
        operation: &mut dyn Operation,
    ) {
        operation.traverse(&mut |operation| {
            self.inner.as_widget_mut().operate(
                &mut tree.children[0],
                inner_layout(layout),
                renderer,
                operation,
            );
        });
    }

    fn update(
        &mut self,
        tree: &mut Tree,
        event: &Event,
        layout: Layout<'_>,
        cursor: mouse::Cursor,
        renderer: &cosmic::Renderer,
        clipboard: &mut dyn Clipboard,
        shell: &mut Shell<'_, Message>,
        viewport: &Rectangle,
    ) {
        self.inner.as_widget_mut().update(
            &mut tree.children[0],
            event,
            inner_layout(layout),
            cursor,
            renderer,
            clipboard,
            shell,
            viewport,
        );
    }

    fn mouse_interaction(
        &self,
        tree: &Tree,
        layout: Layout<'_>,
        cursor: mouse::Cursor,
        viewport: &Rectangle,
        renderer: &cosmic::Renderer,
    ) -> mouse::Interaction {
        self.inner.as_widget().mouse_interaction(
            &tree.children[0],
            inner_layout(layout),
            cursor,
            viewport,
            renderer,
        )
    }

    fn draw(
        &self,
        tree: &Tree,
        renderer: &mut cosmic::Renderer,
        theme: &cosmic::Theme,
        style: &renderer::Style,
        layout: Layout<'_>,
        cursor: mouse::Cursor,
        viewport: &Rectangle,
    ) {
        self.inner.as_widget().draw(
            &tree.children[0],
            renderer,
            theme,
            style,
            inner_layout(layout),
            cursor,
            viewport,
        );
    }

    fn overlay<'b>(
        &'b mut self,
        tree: &'b mut Tree,
        layout: Layout<'b>,
        renderer: &cosmic::Renderer,
        viewport: &Rectangle,
        translation: Vector,
    ) -> Option<overlay::Element<'b, Message, cosmic::Theme, cosmic::Renderer>> {
        self.inner.as_widget_mut().overlay(
            &mut tree.children[0],
            inner_layout(layout),
            renderer,
            viewport,
            translation,
        )
    }
}

/// The one child this widget's layout node has.
fn inner_layout(layout: Layout<'_>) -> Layout<'_> {
    layout
        .children()
        .next()
        .expect("a Rows layout holds exactly one child")
}

impl<'a, Message: 'a> From<Rows<'a, Message>> for Element<'a, Message> {
    fn from(rows: Rows<'a, Message>) -> Self {
        Element::new(rows)
    }
}