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//! The node arena: what the reconciler patches, and what `view` reads.
//!
//! Plain data with no libcosmic in it, so the edit half of the ABI is testable
//! without a window, a GPU or a display. libvidya and libjolttui keep the same
//! shape; what is different here is who reads it. There, the caller's thread
//! paints the tree; here iced's thread does, so the arena sits behind a mutex
//! and `view` works from a snapshot taken at the last commit.

use std::collections::BTreeMap;
use std::fmt::Write;

#[derive(Clone, Debug, PartialEq)]
pub enum Prop {
    Str(String),
    Num(f64),
    Bool(bool),
}

#[derive(Clone, Debug)]
pub struct Node {
    pub tag: String,
    pub props: BTreeMap<String, Prop>,
    pub children: Vec<i32>,
    pub parent: i32,
}

impl Node {
    pub fn str(&self, key: &str) -> &str {
        match self.props.get(key) {
            Some(Prop::Str(s)) => s,
            _ => "",
        }
    }

    pub fn num(&self, key: &str) -> Option<f64> {
        match self.props.get(key) {
            Some(Prop::Num(n)) => Some(*n),
            _ => None,
        }
    }

    pub fn bool(&self, key: &str) -> Option<bool> {
        match self.props.get(key) {
            Some(Prop::Bool(b)) => Some(*b),
            _ => None,
        }
    }

    /// The label a text-shaped widget shows: `label`, or `text` failing that.
    pub fn label(&self) -> &str {
        match self.str("label") {
            "" => self.str("text"),
            s => s,
        }
    }
}

/// `hbox` and `vbox` are one node here, as in libvidya; the tag only implies
/// an orientation, and the jolt side writes it as a prop.
fn canonical(tag: &str) -> &str {
    match tag {
        "hbox" | "vbox" => "box",
        "checkbox" => "checkbutton",
        other => other,
    }
}

#[derive(Clone, Debug, Default)]
pub struct Tree {
    /// Slot `i` holds handle `i + 1`; `None` is a freed handle waiting for reuse.
    slots: Vec<Option<Node>>,
    free: Vec<i32>,
    root: i32,
}

impl Tree {
    pub fn get(&self, id: i32) -> Option<&Node> {
        if id <= 0 {
            return None;
        }
        self.slots.get(id as usize - 1)?.as_ref()
    }

    fn get_mut(&mut self, id: i32) -> Option<&mut Node> {
        if id <= 0 {
            return None;
        }
        self.slots.get_mut(id as usize - 1)?.as_mut()
    }

    pub fn exists(&self, id: i32) -> bool {
        self.get(id).is_some()
    }

    pub fn root(&mut self) -> i32 {
        if !self.exists(self.root) {
            self.root = self.new_node("window");
        }
        self.root
    }

    /// The root without creating it, for a reader holding a snapshot.
    pub fn root_id(&self) -> i32 {
        self.root
    }

    pub fn new_node(&mut self, tag: &str) -> i32 {
        let node = Node {
            tag: canonical(tag).to_owned(),
            props: BTreeMap::new(),
            children: Vec::new(),
            parent: 0,
        };
        match self.free.pop() {
            Some(id) => {
                self.slots[id as usize - 1] = Some(node);
                id
            }
            None => {
                self.slots.push(Some(node));
                self.slots.len() as i32
            }
        }
    }

    /// Free `id` and everything under it, detaching it from its parent first.
    pub fn free(&mut self, id: i32) {
        let Some(node) = self.get(id) else { return };
        let parent = node.parent;
        if let Some(p) = self.get_mut(parent) {
            p.children.retain(|c| *c != id);
        }
        self.free_subtree(id);
    }

    fn free_subtree(&mut self, id: i32) {
        let Some(node) = self.slots.get_mut(id as usize - 1).and_then(Option::take) else {
            return;
        };
        for child in node.children {
            self.free_subtree(child);
        }
        if id == self.root {
            self.root = 0;
        }
        self.free.push(id);
    }

    pub fn set(&mut self, id: i32, key: &str, value: Prop) {
        if let Some(n) = self.get_mut(id) {
            n.props.insert(key.to_owned(), value);
        }
    }

    pub fn clear_props(&mut self, id: i32) {
        if let Some(n) = self.get_mut(id) {
            n.props.clear();
        }
    }

    fn detach(&mut self, child: i32) {
        let parent = match self.get(child) {
            Some(n) => n.parent,
            None => return,
        };
        if let Some(p) = self.get_mut(parent) {
            p.children.retain(|c| *c != child);
        }
        if let Some(c) = self.get_mut(child) {
            c.parent = 0;
        }
    }

    /// A node may not become its own ancestor; the reconciler never asks, but
    /// a cycle would make `view` recurse forever, so it is refused here.
    fn is_ancestor(&self, ancestor: i32, mut id: i32) -> bool {
        while id > 0 {
            if id == ancestor {
                return true;
            }
            id = self.get(id).map_or(0, |n| n.parent);
        }
        false
    }

    pub fn append(&mut self, parent: i32, child: i32) -> bool {
        if !self.exists(parent) || !self.exists(child) || self.is_ancestor(child, parent) {
            return false;
        }
        self.detach(child);
        self.get_mut(parent).unwrap().children.push(child);
        self.get_mut(child).unwrap().parent = parent;
        true
    }

    /// Unparents and frees `child`, under `parent` only.
    pub fn remove(&mut self, parent: i32, child: i32) {
        if self.get(child).is_some_and(|n| n.parent == parent) {
            self.free(child);
        }
    }

    /// Move `child` after `sibling`; `sibling` 0 is the first position.
    pub fn insert_after(&mut self, parent: i32, child: i32, sibling: i32) -> bool {
        if !self.exists(parent) || !self.exists(child) || self.is_ancestor(child, parent) {
            return false;
        }
        if sibling != 0 && self.get(sibling).is_none_or(|n| n.parent != parent) {
            return false;
        }
        self.detach(child);
        let p = self.get_mut(parent).unwrap();
        let at = if sibling == 0 {
            0
        } else {
            p.children.iter().position(|c| *c == sibling).unwrap() + 1
        };
        p.children.insert(at, child);
        self.get_mut(child).unwrap().parent = parent;
        true
    }

    /// Put `new` where `old` was under `parent`, and free `old`.
    pub fn replace(&mut self, parent: i32, old: i32, new: i32) -> bool {
        if !self.exists(new) || self.get(old).is_none_or(|n| n.parent != parent) {
            return false;
        }
        if self.is_ancestor(new, parent) {
            return false;
        }
        self.detach(new);
        let p = self.get_mut(parent).unwrap();
        let at = p.children.iter().position(|c| *c == old).unwrap();
        p.children[at] = new;
        self.get_mut(new).unwrap().parent = parent;
        // `old` is no longer among the parent's children, so free only its subtree.
        self.get_mut(old).unwrap().parent = 0;
        self.free_subtree(old);
        true
    }

    /// The subtree at `id` as hiccup, one node to a line, props sorted — so two
    /// dumps of the same tree compare as text.
    pub fn dump(&self, id: i32) -> String {
        let mut out = String::new();
        self.dump_into(id, 0, &mut out);
        out
    }

    fn dump_into(&self, id: i32, depth: usize, out: &mut String) {
        let Some(n) = self.get(id) else { return };
        let _ = write!(out, "{}[:{} {{", "  ".repeat(depth), n.tag);
        for (i, (k, v)) in n.props.iter().enumerate() {
            if i > 0 {
                out.push(' ');
            }
            let _ = match v {
                Prop::Str(s) => write!(out, ":{k} {s:?}"),
                Prop::Num(x) => write!(out, ":{k} {x}"),
                Prop::Bool(b) => write!(out, ":{k} {b}"),
            };
        }
        out.push('}');
        for child in &n.children {
            out.push('\n');
            self.dump_into(*child, depth + 1, out);
        }
        out.push(']');
    }
}

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

    #[test]
    fn a_removed_subtree_frees_every_node_under_it() {
        let mut t = Tree::default();
        let root = t.root();
        let card = t.new_node("card");
        let label = t.new_node("label");
        assert!(t.append(root, card));
        assert!(t.append(card, label));
        t.remove(root, card);
        assert!(!t.exists(card));
        assert!(!t.exists(label));
        assert!(t.get(root).unwrap().children.is_empty());
    }

    #[test]
    fn insert_after_zero_moves_to_the_front() {
        let mut t = Tree::default();
        let root = t.root();
        let (a, b, c) = (
            t.new_node("label"),
            t.new_node("label"),
            t.new_node("label"),
        );
        for n in [a, b, c] {
            t.append(root, n);
        }
        assert!(t.insert_after(root, c, 0));
        assert_eq!(t.get(root).unwrap().children, vec![c, a, b]);
        assert!(t.insert_after(root, c, b));
        assert_eq!(t.get(root).unwrap().children, vec![a, b, c]);
    }

    #[test]
    fn replace_keeps_the_position_and_frees_the_old_node() {
        let mut t = Tree::default();
        let root = t.root();
        let (a, b, c) = (
            t.new_node("label"),
            t.new_node("button"),
            t.new_node("label"),
        );
        t.append(root, a);
        t.append(root, b);
        assert!(t.replace(root, a, c));
        assert_eq!(t.get(root).unwrap().children, vec![c, b]);
        assert!(!t.exists(a));
    }

    #[test]
    fn a_node_cannot_be_appended_under_itself() {
        let mut t = Tree::default();
        let root = t.root();
        let card = t.new_node("card");
        t.append(root, card);
        assert!(!t.append(card, root));
    }

    #[test]
    fn dump_reads_back_as_hiccup() {
        let mut t = Tree::default();
        let root = t.root();
        let row = t.new_node("hbox");
        t.set(row, "orientation", Prop::Str("horizontal".into()));
        let b = t.new_node("button");
        t.set(b, "label", Prop::Str("+ 1".into()));
        t.set(b, "kind", Prop::Str("primary".into()));
        t.append(root, row);
        t.append(row, b);
        assert_eq!(
            t.dump(root),
            "[:window {}\n  [:box {:orientation \"horizontal\"}\n    [:button {:kind \"primary\" :label \"+ 1\"}]]]"
        );
    }
}