use std::hash::{BuildHasherDefault, Hash, Hasher};

use hashbrown::hash_map::RawEntryMut;
use rustc_hash::FxHasher;

use crate::{
    cow_mut::CowMut,
    green::{GreenElement, GreenNode, GreenToken, SyntaxKind},
    NodeOrToken, SmolStr,
};

type HashMap<K, V> = hashbrown::HashMap<K, V, BuildHasherDefault<FxHasher>>;

#[derive(Default, Debug)]
pub struct NodeCache {
    nodes: HashMap<GreenNode, ()>,
    tokens: HashMap<GreenToken, ()>,
}

impl NodeCache {
    fn node(
        &mut self,
        kind: SyntaxKind,
        children: &mut Vec<(u64, GreenElement)>,
        first_child: usize,
    ) -> (u64, GreenNode) {
        let build_node = move |children: &mut Vec<(u64, GreenElement)>| {
            GreenNode::new(kind, children.drain(first_child..).map(|(_, it)| it))
        };
        let children_ref = &children[first_child..];
        if children_ref.len() > 3 {
            let node = build_node(children);
            return (0, node);
        }

        // Manually compute the hash to avoid repeatedly hashing subtrees.
        let hash = {
            let mut h = FxHasher::default();
            kind.hash(&mut h);
            for &(hash, _) in children_ref {
                if hash == 0 {
                    let node = build_node(children);
                    return (0, node);
                }
                hash.hash(&mut h);
            }
            h.finish()
        };

        // Green nodes are fully immutable, so it's ok to deduplicate them.
        // This is the same optimization that Roslyn does
        // https://github.com/KirillOsenkov/Bliki/wiki/Roslyn-Immutable-Trees
        //
        // For example, all `#[inline]` in this file share the same green node!
        // For `libsyntax/parse/parser.rs`, measurements show that deduping saves
        // 17% of the memory for green nodes!
        let entry = self.nodes.raw_entry_mut().from_hash(hash, |node| {
            node.kind() == kind
                && node.children().len() == children_ref.len()
                && node.children().eq(children_ref.iter().map(|(_, it)| it.as_ref()))
        });

        let node = match entry {
            RawEntryMut::Occupied(entry) => {
                drop(children.drain(first_child..));
                entry.key().clone()
            }
            RawEntryMut::Vacant(entry) => {
                let node = build_node(children);
                entry.insert_hashed_nocheck(hash, node.clone(), ());
                node
            }
        };

        (hash, node)
    }

    fn token(&mut self, kind: SyntaxKind, text: SmolStr) -> (u64, GreenToken) {
        let hash = {
            let mut h = FxHasher::default();
            kind.hash(&mut h);
            text.as_str().hash(&mut h);
            h.finish()
        };
        let entry = self
            .tokens
            .raw_entry_mut()
            .from_hash(hash, |token| token.kind() == kind && token.text() == &text);

        let token = match entry {
            RawEntryMut::Occupied(entry) => entry.key().clone(),
            RawEntryMut::Vacant(entry) => {
                let token = GreenToken::new(kind, text);
                entry.insert_hashed_nocheck(hash, token.clone(), ());
                token
            }
        };
        (hash, token)
    }
}

/// A checkpoint for maybe wrapping a node. See `GreenNodeBuilder::checkpoint` for details.
#[derive(Clone, Copy, Debug)]
pub struct Checkpoint(usize);

/// A builder for a green tree.
#[derive(Default, Debug)]
pub struct GreenNodeBuilder<'cache> {
    cache: CowMut<'cache, NodeCache>,
    parents: Vec<(SyntaxKind, usize)>,
    children: Vec<(u64, GreenElement)>,
}

impl GreenNodeBuilder<'_> {
    /// Creates new builder.
    pub fn new() -> GreenNodeBuilder<'static> {
        GreenNodeBuilder::default()
    }

    /// Reusing `NodeCache` between different `GreenNodeBuilder`s saves memory.
    /// It allows to structurally share underlying trees.
    pub fn with_cache(cache: &mut NodeCache) -> GreenNodeBuilder<'_> {
        GreenNodeBuilder {
            cache: CowMut::Borrowed(cache),
            parents: Vec::new(),
            children: Vec::new(),
        }
    }

    /// Adds new token to the current branch.
    #[inline]
    pub fn token(&mut self, kind: SyntaxKind, text: SmolStr) {
        let (hash, token) = self.cache.token(kind, text);
        self.children.push((hash, token.into()));
    }

    /// Start new node and make it current.
    #[inline]
    pub fn start_node(&mut self, kind: SyntaxKind) {
        let len = self.children.len();
        self.parents.push((kind, len));
    }

    /// Finish current branch and restore previous
    /// branch as current.
    #[inline]
    pub fn finish_node(&mut self) {
        let (kind, first_child) = self.parents.pop().unwrap();
        let (hash, node) = self.cache.node(kind, &mut self.children, first_child);
        self.children.push((hash, node.into()));
    }

    /// Prepare for maybe wrapping the next node.
    /// The way wrapping works is that you first of all get a checkpoint,
    /// then you place all tokens you want to wrap, and then *maybe* call
    /// `start_node_at`.
    /// Example:
    /// ```rust
    /// # use rowan::{GreenNodeBuilder, SyntaxKind};
    /// # const PLUS: SyntaxKind = SyntaxKind(0);
    /// # const OPERATION: SyntaxKind = SyntaxKind(1);
    /// # struct Parser;
    /// # impl Parser {
    /// #     fn peek(&self) -> Option<SyntaxKind> { None }
    /// #     fn parse_expr(&mut self) {}
    /// # }
    /// # let mut builder = GreenNodeBuilder::new();
    /// # let mut parser = Parser;
    /// let checkpoint = builder.checkpoint();
    /// parser.parse_expr();
    /// if parser.peek() == Some(PLUS) {
    ///   // 1 + 2 = Add(1, 2)
    ///   builder.start_node_at(checkpoint, OPERATION);
    ///   parser.parse_expr();
    ///   builder.finish_node();
    /// }
    /// ```
    #[inline]
    pub fn checkpoint(&self) -> Checkpoint {
        Checkpoint(self.children.len())
    }

    /// Wrap the previous branch marked by `checkpoint` in a new branch and
    /// make it current.
    #[inline]
    pub fn start_node_at(&mut self, checkpoint: Checkpoint, kind: SyntaxKind) {
        let Checkpoint(checkpoint) = checkpoint;
        assert!(
            checkpoint <= self.children.len(),
            "checkpoint no longer valid, was finish_node called early?"
        );

        if let Some(&(_, first_child)) = self.parents.last() {
            assert!(
                checkpoint >= first_child,
                "checkpoint no longer valid, was an unmatched start_node_at called?"
            );
        }

        self.parents.push((kind, checkpoint));
    }

    /// Complete tree building. Make sure that
    /// `start_node_at` and `finish_node` calls
    /// are paired!
    #[inline]
    pub fn finish(mut self) -> GreenNode {
        assert_eq!(self.children.len(), 1);
        match self.children.pop().unwrap().1 {
            NodeOrToken::Node(node) => node,
            NodeOrToken::Token(_) => panic!(),
        }
    }
}