use crate::error::SyntaxResult;
use crate::session::Session;
use crate::session::SessionHashMap;
use crate::session::SessionVec;
use crate::source::SourceRange;
use hashbrown::hash_map::Entry;
use std::cell::Ref;
use std::cell::RefCell;
use std::cell::RefMut;
use std::hash::Hash;

pub type Identifier<'a> = SourceRange<'a>;

// To attach additional custom state to a Symbol, use a HashMap. We prefer this instead of adding an extra generic state field on Symbol, as that would require propagating the generic type everywhere.
// Cloning means to cheaply clone the reference to this unique symbol, not create a duplicate symbol. This is useful for sharing a reference to a symbol, including uses in data structures like HashMap.
#[derive(Clone)]
pub struct Symbol<'a> {
  scope: Scope<'a>,
  // Index in the containing ScopeData's symbol_declaration_order Vec.
  ordinal_in_scope: usize,
}

// Equality means referring to the same unique symbol. Useful for HashMap.
impl<'a> PartialEq for Symbol<'a> {
  fn eq(&self, other: &Self) -> bool {
    core::ptr::eq(self.scope.0, other.scope.0) && self.ordinal_in_scope == other.ordinal_in_scope
  }
}

impl<'a> Eq for Symbol<'a> {}

impl<'a> Hash for Symbol<'a> {
  fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
    core::ptr::hash(self.scope.0, state);
    self.ordinal_in_scope.hash(state);
  }
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum ScopeType {
  Global,
  Module,
  // Closure with `this` (property initialisers have access to it) but not `arguments`.
  Class,
  // Functions with `this` and `arguments`.
  NonArrowFunction,
  // Functions with neither `this` nor `arguments`.
  // NOTE: Arrow function class properties are not on the prototype and simply have access to the class's `this` like other initialisers; it doesn't have a special `this` binding and inherits it like any other arrow function.
  ArrowFunction,
  Block,
}

impl ScopeType {
  pub fn is_closure(&self) -> bool {
    match self {
      ScopeType::Module => true,
      ScopeType::NonArrowFunction => true,
      ScopeType::ArrowFunction => true,
      _ => false,
    }
  }

  pub fn is_closure_or_global(&self) -> bool {
    match self {
      ScopeType::Global => true,
      t => t.is_closure(),
    }
  }
}

#[repr(u8)]
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum ScopeFlag {
  // Whether or not this scope has a `this` expression. Only applicable for Class and NonArrowFunction scopes.
  UsesThis,
  // Whether or not this scope has an `arguments` variable expression. Only applicable for NonArrowFunction scopes.
  UsesArguments,
}

struct ScopeData<'a> {
  symbols: SessionHashMap<'a, Identifier<'a>, Symbol<'a>>,
  // For deterministic outputs, and to give each Symbol an ID.
  symbol_declaration_order: SessionVec<'a, Identifier<'a>>,
  // Does not exist for top-level.
  parent: Option<Scope<'a>>,
  // Not used by the parser, but useful for some library consumers, as there's currently no other way to iterate all scopes.
  children: SessionVec<'a, Scope<'a>>,
  typ: ScopeType,
  flags: u64,
}

// Since Rust only supports newtypes with impls using structs, we cannot have our newtype as a reference, so we must use it like it's a reference despite being a struct i.e. cheaply copyable, take `self` instead of `&self`, use `Scope<'a>` instead of `&'a Scope<'a>`.
#[derive(Clone, Copy)]
pub struct Scope<'a>(&'a RefCell<ScopeData<'a>>);

impl<'a> Scope<'a> {
  fn get<'b>(self) -> Ref<'b, ScopeData<'a>> {
    self.0.borrow()
  }

  fn get_mut<'b>(self) -> RefMut<'b, ScopeData<'a>> {
    self.0.borrow_mut()
  }

  pub fn new(session: &'a Session, parent: Option<Scope<'a>>, typ: ScopeType) -> Scope<'a> {
    let scope = Scope(session.mem.alloc(RefCell::new(ScopeData {
      symbols: session.new_hashmap(),
      symbol_declaration_order: session.new_vec(),
      parent,
      children: session.new_vec(),
      typ,
      flags: 0,
    })));
    if let Some(parent) = parent {
      parent.get_mut().children.push(scope);
    };
    scope
  }

  pub fn parent(self) -> Option<Scope<'a>> {
    self.get().parent
  }

  pub fn typ(self) -> ScopeType {
    self.get().typ
  }

  pub fn create_child_scope(self, session: &'a Session, typ: ScopeType) -> Scope<'a> {
    Scope::new(session, Some(self), typ)
  }

  pub fn find_self_or_ancestor<F: Fn(ScopeType) -> bool>(self, pred: F) -> Option<Scope<'a>> {
    let cur = self.get();
    if pred(cur.typ) {
      Some(self)
    } else if let Some(parent) = cur.parent {
      parent.find_self_or_ancestor(pred)
    } else {
      None
    }
  }

  pub fn has_flag(&self, flag: ScopeFlag) -> bool {
    (self.get().flags & (1 << (flag as u8))) != 0
  }

  pub fn set_flag(&mut self, flag: ScopeFlag) {
    self.get_mut().flags |= 1 << (flag as u8);
  }

  pub fn add_symbol(self, identifier: Identifier<'a>) -> SyntaxResult<'a, ()> {
    // We must get before we borrow as mut, even if we won't use it.
    let ordinal_in_scope = self.get().symbol_declaration_order.len();
    let mut as_mut = self.get_mut();
    match as_mut.symbols.entry(identifier.clone()) {
      Entry::Occupied(_) => {
        // Do not replace existing entry, as it has associated index in symbol_declaration_order.
        // TODO Investigate raising an error; however, many production codebases redeclare `var`.
      }
      Entry::Vacant(e) => {
        e.insert(Symbol {
          scope: self,
          ordinal_in_scope,
        });
        as_mut.symbol_declaration_order.push(identifier.clone());
      }
    };
    Ok(())
  }

  pub fn add_block_symbol(self, identifier: Identifier<'a>) -> SyntaxResult<'a, ()> {
    if self.get().typ != ScopeType::Global {
      self.add_symbol(identifier)?;
    };
    Ok(())
  }

  pub fn get_symbol(self, identifier: Identifier<'a>) -> Option<Symbol<'a>> {
    self.get().symbols.get(&identifier).cloned()
  }

  pub fn find_symbol(self, identifier: Identifier<'a>) -> Option<Symbol<'a>> {
    match self.get().symbols.get(&identifier) {
      Some(symbol) => Some(symbol.clone()),
      None => match self.get().parent {
        Some(parent) => parent.find_symbol(identifier),
        None => None,
      },
    }
  }

  pub fn find_symbol_up_to_nearest_scope_of_type<'b>(
    self,
    identifier: Identifier<'a>,
    scope_type: ScopeType,
  ) -> Option<Symbol<'a>> {
    let mut scope = self;
    loop {
      let scope_data = scope.get();
      if let Some(symbol) = scope_data.symbols.get(&identifier) {
        return Some(symbol.clone());
      };
      if scope_data.typ == scope_type {
        break;
      };
      if let Some(parent) = scope_data.parent {
        scope = parent;
        continue;
      };
      break;
    }
    None
  }

  pub fn symbol_count(self) -> usize {
    self.get().symbols.len()
  }

  // Returning an iterator within a Ref is difficult (see // https://stackoverflow.com/a/33542412/6249022), so we just return the Vec (immutable) instead.
  pub fn symbol_names<'b>(self) -> Ref<'b, SessionVec<'a, SourceRange<'a>>> {
    Ref::map(self.get(), |scope| &scope.symbol_declaration_order)
  }

  // Same return value rationale as `symbol_names`.
  pub fn children<'b>(self) -> Ref<'b, SessionVec<'a, Scope<'a>>> {
    Ref::map(self.get(), |scope| &scope.children)
  }
}

// Equality means referring to the same unique scope. Useful for HashMap.
impl<'a> PartialEq for Scope<'a> {
  fn eq(&self, other: &Self) -> bool {
    core::ptr::eq(self.0, other.0)
  }
}

impl<'a> Eq for Scope<'a> {}

impl<'a> Hash for Scope<'a> {
  fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
    core::ptr::hash(self.0, state);
  }
}