//! A document: a tree of nodes under one root, loose nodes beside it, and
//! cross-links between any of them.

use std::collections::BTreeMap;

use thiserror::Error;

use super::geometry::CanvasPos;
use super::ids::{DocUid, NodeId};
use super::link::{Link, LinkKey};
use super::look::LineLook;
use super::node::{Content, Node};
use super::portal::{Backlink, Portal};
use super::text::Title;

/// Where a node sits in its document's tree.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Place {
    /// The document's main topic. There is exactly one, and its title is the
    /// document's name.
    Root,
    /// A top-level idea beside the root's tree, not attached to anything yet.
    Loose,
    /// Attached under another node.
    Child {
        /// The node above this one.
        parent: NodeId,
        /// How the line from the parent is drawn.
        branch: LineLook,
    },
}

/// A node, with where it sits in the tree.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(super) struct Entry {
    pub(super) node: Node,
    pub(super) place: Place,
    /// Its children, in order.
    pub(super) children: Vec<NodeId>,
}

/// One brainstorm: a tree of nodes under a single root, loose nodes beside
/// it, cross-links between any two nodes, and the backlinks of portals in
/// other documents that lead here.
///
/// The fields are private and every change goes through
/// [`Document::edit`](Document::edit), so these rules always hold:
/// - there is exactly one root, and it can't be removed or given a parent;
/// - a node has at most one parent, and there are no cycles;
/// - a link joins two different nodes that both exist, at most one link per
///   pair.
#[derive(Debug, Clone)]
pub struct Document {
    uid: DocUid,
    root: NodeId,
    pub(super) entries: BTreeMap<NodeId, Entry>,
    /// The loose nodes, in order.
    pub(super) loose: Vec<NodeId>,
    pub(super) links: BTreeMap<LinkKey, Link>,
    backlinks: Vec<Backlink>,
    /// The number the next new node gets. It only ever grows, so ids aren't
    /// handed out twice, not even after an undo.
    pub(super) next_id: u32,
}

impl PartialEq for Document {
    /// Two documents are equal when they hold the same things. `next_id` is
    /// left out: it only says which number comes next.
    fn eq(&self, other: &Document) -> bool {
        self.uid == other.uid
            && self.root == other.root
            && self.entries == other.entries
            && self.loose == other.loose
            && self.links == other.links
            && self.backlinks == other.backlinks
    }
}

impl Eq for Document {}

/// A node as read from a file: the node, plus where the file says it goes.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct NodeRecord {
    pub id: NodeId,
    pub parent: Option<NodeId>,
    /// How the line from the parent is drawn (ignored without a parent).
    pub branch: LineLook,
    pub node: Node,
}

/// Why the nodes and links read from a file don't make a valid document.
#[derive(Debug, Clone, PartialEq, Eq, Error)]
pub enum StructureError {
    #[error("node {0} appears twice")]
    DuplicateNode(NodeId),
    #[error("the root (node {0}) isn't in the file")]
    MissingRoot(NodeId),
    #[error("the root (node {0}) can't have a parent")]
    RootWithParent(NodeId),
    #[error("node {child} has parent {parent}, which isn't in the file")]
    UnknownParent { child: NodeId, parent: NodeId },
    #[error("node {0} is inside its own branch")]
    Cycle(NodeId),
    #[error("node ids must be smaller than {}", u32::MAX)]
    IdTooLarge,
    #[error("a link points at node {0}, which isn't in the file")]
    UnknownLinkEnd(NodeId),
    #[error("nodes {0} and {1} are linked twice")]
    DuplicateLink(NodeId, NodeId),
}

impl Document {
    /// A new document with just its root, at the middle of the canvas.
    pub fn new(title: Title) -> Document {
        Document::with_uid(title, DocUid::random())
    }

    /// A new document with just its root, and a uid chosen by the caller.
    pub fn with_uid(title: Title, uid: DocUid) -> Document {
        let root = NodeId::new(1);
        let entry =
            Entry { node: Node::idea(title, CanvasPos::default()), place: Place::Root, children: Vec::new() };
        Document {
            uid,
            root,
            entries: BTreeMap::from([(root, entry)]),
            loose: Vec::new(),
            links: BTreeMap::new(),
            backlinks: Vec::new(),
            next_id: 2,
        }
    }

    /// Builds a document from what a file holds, checking every rule.
    /// Siblings keep the order they have in `records`.
    pub fn from_records(
        uid: DocUid,
        root: NodeId,
        records: Vec<NodeRecord>,
        links: Vec<Link>,
        backlinks: Vec<Backlink>,
    ) -> Result<Document, StructureError> {
        let mut entries = BTreeMap::new();
        let mut order = Vec::with_capacity(records.len());
        for record in records {
            if record.id.get() == u32::MAX {
                return Err(StructureError::IdTooLarge);
            }
            let place = match record.parent {
                Some(_) if record.id == root => return Err(StructureError::RootWithParent(root)),
                Some(parent) => Place::Child { parent, branch: record.branch },
                None if record.id == root => Place::Root,
                None => Place::Loose,
            };
            order.push((record.id, record.parent));
            let entry = Entry { node: record.node, place, children: Vec::new() };
            if entries.insert(record.id, entry).is_some() {
                return Err(StructureError::DuplicateNode(record.id));
            }
        }
        if !entries.contains_key(&root) {
            return Err(StructureError::MissingRoot(root));
        }

        let mut loose = Vec::new();
        for (id, parent) in order {
            match parent {
                Some(parent) => match entries.get_mut(&parent) {
                    Some(entry) => entry.children.push(id),
                    None => return Err(StructureError::UnknownParent { child: id, parent }),
                },
                None if id != root => loose.push(id),
                None => {}
            }
        }

        // Climbing from any node must reach the top level. A node that keeps
        // climbing for longer than there are nodes is going round in a cycle.
        for &id in entries.keys() {
            let mut current = id;
            for steps in 0.. {
                match entries.get(&current).map(|entry| &entry.place) {
                    Some(Place::Child { parent, .. }) => current = *parent,
                    _ => break,
                }
                if steps > entries.len() {
                    return Err(StructureError::Cycle(id));
                }
            }
        }

        let mut link_map = BTreeMap::new();
        for link in links {
            for end in [link.start(), link.end()] {
                if !entries.contains_key(&end) {
                    return Err(StructureError::UnknownLinkEnd(end));
                }
            }
            let key = link.key();
            if link_map.insert(key, link).is_some() {
                let (a, b) = key.nodes();
                return Err(StructureError::DuplicateLink(a, b));
            }
        }

        // Ids are below u32::MAX (checked above), so this can't overflow.
        let next_id = entries.keys().next_back().map_or(1, |id| id.get() + 1);
        Ok(Document { uid, root, entries, loose, links: link_map, backlinks, next_id })
    }

    /// The document's uid, which stays the same when its file is renamed.
    pub fn uid(&self) -> DocUid {
        self.uid
    }

    /// The root: the document's main topic.
    pub fn root(&self) -> NodeId {
        self.root
    }

    /// The document's name: its root's title.
    pub fn title(&self) -> &Title {
        // The root can't be removed (see `Edit::remove_branch`), so it's
        // always there.
        self.entries[&self.root].node.title()
    }

    /// How many nodes the document has, the root included.
    pub fn node_count(&self) -> usize {
        self.entries.len()
    }

    /// Whether the document has a node with this id.
    pub fn contains(&self, id: NodeId) -> bool {
        self.entries.contains_key(&id)
    }

    /// A node, if it exists.
    pub fn node(&self, id: NodeId) -> Option<&Node> {
        self.entries.get(&id).map(|entry| &entry.node)
    }

    /// Where a node sits in the tree.
    pub fn place(&self, id: NodeId) -> Option<&Place> {
        self.entries.get(&id).map(|entry| &entry.place)
    }

    /// A node's parent (none for the root and loose nodes).
    pub fn parent(&self, id: NodeId) -> Option<NodeId> {
        match self.place(id)? {
            Place::Child { parent, .. } => Some(*parent),
            Place::Root | Place::Loose => None,
        }
    }

    /// A node's children, in order (empty for an unknown id).
    pub fn children(&self, id: NodeId) -> &[NodeId] {
        self.entries.get(&id).map_or(&[], |entry| &entry.children)
    }

    /// The loose nodes, in order.
    pub fn loose(&self) -> &[NodeId] {
        &self.loose
    }

    /// The top level: the root, then the loose nodes.
    pub fn top_level(&self) -> impl Iterator<Item = NodeId> + '_ {
        std::iter::once(self.root).chain(self.loose.iter().copied())
    }

    /// The node after `id` among its siblings. For the root, that's the first
    /// loose node: the top level counts as one row of siblings.
    pub fn next_sibling(&self, id: NodeId) -> Option<NodeId> {
        let (siblings, index) = self.siblings(id)?;
        siblings.get(index + 1).copied()
    }

    /// The node before `id` among its siblings.
    pub fn previous_sibling(&self, id: NodeId) -> Option<NodeId> {
        let (siblings, index) = self.siblings(id)?;
        siblings.get(index.checked_sub(1)?).copied()
    }

    /// The row of siblings `id` is in, and its index there.
    fn siblings(&self, id: NodeId) -> Option<(Vec<NodeId>, usize)> {
        let siblings: Vec<NodeId> = match self.place(id)? {
            Place::Child { parent, .. } => self.children(*parent).to_vec(),
            Place::Root | Place::Loose => self.top_level().collect(),
        };
        let index = siblings.iter().position(|&sibling| sibling == id)?;
        Some((siblings, index))
    }

    /// The nodes above `id`: its parent, grandparent and so on up to the top.
    pub fn ancestors(&self, id: NodeId) -> impl Iterator<Item = NodeId> + '_ {
        std::iter::successors(self.parent(id), |&node| self.parent(node))
    }

    /// Whether `id` is somewhere below `branch` (not `branch` itself).
    pub fn is_inside(&self, id: NodeId, branch: NodeId) -> bool {
        self.ancestors(id).any(|ancestor| ancestor == branch)
    }

    /// How deep a node is: 0 at the top level, 1 for their children…
    pub fn depth(&self, id: NodeId) -> usize {
        self.ancestors(id).count()
    }

    /// Every node below `id`, in reading order (each node before its
    /// children).
    pub fn descendants(&self, id: NodeId) -> Vec<NodeId> {
        let mut found = Vec::new();
        let mut to_visit: Vec<NodeId> = self.children(id).iter().rev().copied().collect();
        while let Some(node) = to_visit.pop() {
            found.push(node);
            to_visit.extend(self.children(node).iter().rev());
        }
        found
    }

    /// `id` and every node below it, in reading order (empty for an unknown
    /// id).
    pub fn branch(&self, id: NodeId) -> Vec<NodeId> {
        if !self.contains(id) {
            return Vec::new();
        }
        let mut branch = vec![id];
        branch.extend(self.descendants(id));
        branch
    }

    /// Every node in reading order: the root's branch, then each loose node's
    /// branch. Files list their nodes in this order.
    pub fn outline(&self) -> Vec<NodeId> {
        self.top_level().flat_map(|top| self.branch(top)).collect()
    }

    /// Every cross-link, in the order of their keys.
    pub fn links(&self) -> impl Iterator<Item = &Link> {
        self.links.values()
    }

    /// The link between two nodes, if there is one.
    pub fn link(&self, key: LinkKey) -> Option<&Link> {
        self.links.get(&key)
    }

    /// The cross-links touching `id`, whichever way they point.
    pub fn links_of(&self, id: NodeId) -> impl Iterator<Item = &Link> + '_ {
        self.links.values().filter(move |link| link.key().touches(id))
    }

    /// The portals in other documents that lead here.
    pub fn backlinks(&self) -> &[Backlink] {
        &self.backlinks
    }

    /// Replaces the backlinks. They're bookkeeping, kept in step with other
    /// documents when they're saved, so this isn't an undoable edit.
    pub fn set_backlinks(&mut self, backlinks: Vec<Backlink>) {
        self.backlinks = backlinks;
    }

    /// The portals in this document, with the nodes they sit on.
    pub fn portals(&self) -> impl Iterator<Item = (NodeId, &Portal)> + '_ {
        self.entries.iter().filter_map(|(id, entry)| match &entry.node.content {
            Content::Portal(portal) => Some((*id, portal)),
            Content::Idea(_) => None,
        })
    }
}

#[cfg(test)]
impl Document {
    /// Panics if one of the document's rules is broken. Tests call this
    /// after edits.
    pub(crate) fn assert_rules(&self) {
        let roots: Vec<_> =
            self.entries.iter().filter(|(_, e)| e.place == Place::Root).map(|(id, _)| *id).collect();
        assert_eq!(roots, vec![self.root], "exactly one root");

        let loose: Vec<_> =
            self.entries.iter().filter(|(_, e)| e.place == Place::Loose).map(|(id, _)| *id).collect();
        let mut listed = self.loose.clone();
        listed.sort();
        assert_eq!(listed, loose, "the loose list matches the loose nodes");

        for (id, entry) in &self.entries {
            if let Place::Child { parent, .. } = &entry.place {
                let siblings = self.children(*parent);
                assert_eq!(
                    siblings.iter().filter(|&&s| s == *id).count(),
                    1,
                    "{id} is listed once by its parent"
                );
            }
            for child in &entry.children {
                assert_eq!(self.parent(*child), Some(*id), "{child} knows its parent {id}");
            }
            assert!(id.get() < self.next_id, "{id} is below next_id");
        }

        let mut outline = self.outline();
        assert_eq!(outline.len(), self.entries.len(), "every node is reachable once (no cycles)");
        outline.sort();
        outline.dedup();
        assert_eq!(outline.len(), self.entries.len(), "no node is reached twice");

        for (key, link) in &self.links {
            assert_eq!(*key, link.key());
            assert!(self.contains(link.start()) && self.contains(link.end()), "link ends exist");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::model::test_support::{garden, idea, title};

    fn record(id: u32, parent: Option<u32>, name: &str) -> NodeRecord {
        NodeRecord {
            id: NodeId::new(id),
            parent: parent.map(NodeId::new),
            branch: LineLook::default(),
            node: idea(name),
        }
    }

    fn build(records: Vec<NodeRecord>) -> Result<Document, StructureError> {
        Document::from_records(
            DocUid::parse("0000000000000001").unwrap(),
            NodeId::new(1),
            records,
            vec![],
            vec![],
        )
    }

    #[test]
    fn records_become_a_tree_in_file_order() {
        let doc = build(vec![
            record(1, None, "root"),
            record(3, Some(1), "b"),
            record(2, Some(1), "a"),
            record(9, None, "loose"),
            record(4, Some(3), "b1"),
        ])
        .unwrap();
        doc.assert_rules();
        let ids = |raw: &[u32]| raw.iter().map(|&n| NodeId::new(n)).collect::<Vec<_>>();
        assert_eq!(doc.children(NodeId::new(1)), ids(&[3, 2]).as_slice());
        assert_eq!(doc.loose(), ids(&[9]).as_slice());
        assert_eq!(doc.outline(), ids(&[1, 3, 4, 2, 9]));
        assert_eq!(doc.title().as_str(), "root");
    }

    #[test]
    fn broken_records_are_refused() {
        use StructureError as E;
        let n = NodeId::new;
        assert_eq!(build(vec![record(2, None, "x")]), Err(E::MissingRoot(n(1))));
        assert_eq!(build(vec![record(1, Some(2), "r"), record(2, None, "x")]), Err(E::RootWithParent(n(1))));
        assert_eq!(build(vec![record(1, None, "r"), record(1, None, "again")]), Err(E::DuplicateNode(n(1))));
        assert_eq!(
            build(vec![record(1, None, "r"), record(2, Some(5), "x")]),
            Err(E::UnknownParent { child: n(2), parent: n(5) })
        );
        assert!(matches!(
            build(vec![record(1, None, "r"), record(2, Some(3), "x"), record(3, Some(2), "y")]),
            Err(E::Cycle(_))
        ));
    }

    #[test]
    fn siblings_ancestors_and_branches() {
        let (doc, g) = garden();
        assert_eq!(doc.next_sibling(g.soil), Some(g.beds));
        assert_eq!(doc.previous_sibling(g.soil), None);
        assert_eq!(
            doc.next_sibling(g.root),
            Some(g.compost),
            "the root's next sibling is the first loose node"
        );
        assert_eq!(doc.ancestors(g.cedar).collect::<Vec<_>>(), vec![g.beds, g.root]);
        assert!(doc.is_inside(g.cedar, g.root) && !doc.is_inside(g.root, g.cedar));
        assert_eq!(doc.depth(g.cedar), 2);
        assert_eq!(doc.branch(g.beds), vec![g.beds, g.cedar]);
        assert_eq!(doc.outline(), vec![g.root, g.soil, g.beds, g.cedar, g.watering, g.compost]);
        assert_eq!(Document::new(title("x")).node_count(), 1);
    }
}