use std::any::Any;
use std::ops::Deref;
use std::rc::Rc;

use mica_language::ast::DumpAst;
use mica_language::bytecode::{
   BuiltinDispatchTables, Chunk, DispatchTable, Environment, Function, FunctionKind,
   FunctionSignature, Opcode, Opr24,
};
use mica_language::codegen::CodeGenerator;
use mica_language::gc::{Gc, Memory};
use mica_language::lexer::Lexer;
use mica_language::parser::Parser;
use mica_language::value::{Closure, RawValue};
use mica_language::vm::{self, Globals};

use crate::{
   ffvariants, BuiltType, Error, Fiber, ForeignFunction, StandardLibrary, TryFromValue,
   TypeBuilder, UserData, Value,
};

/// The implementation of a raw foreign function.
pub use mica_language::bytecode::ForeignFunction as RawForeignFunction;
/// The kind of a raw function.
pub use mica_language::bytecode::FunctionKind as RawFunctionKind;

/// Options for debugging the language implementation.
#[derive(Debug, Clone, Copy, Default)]
pub struct DebugOptions {
   /// Set to `true` to print the AST to stdout after parsing.
   pub dump_ast: bool,
   /// Set to `true` to print the bytecode to stdout after successful compilation.
   pub dump_bytecode: bool,
}

/// **Start here!** An execution engine. Contains information about things like globals, registered
/// types, etc.
pub struct Engine {
   pub(crate) env: Environment,
   pub(crate) globals: Globals,
   // This field is needed to keep all builtin dispatch tables alive for longer than `gc`.
   pub(crate) gc: Memory,
   debug_options: DebugOptions,
}

impl Engine {
   /// Creates a new engine.
   pub fn new(stdlib: impl StandardLibrary) -> Self {
      Self::with_debug_options(stdlib, Default::default())
   }

   /// Creates a new engine with specific debug options.
   ///
   /// [`Engine::new`] creates an engine with [`Default`] debug options, and should generally be
   /// preferred unless you're debugging the language's internals.
   ///
   /// Constructing the engine can fail if the standard library defines way too many methods.
   pub fn with_debug_options(
      mut stdlib: impl StandardLibrary,
      debug_options: DebugOptions,
   ) -> Self {
      let mut gc = Memory::new();
      // This is a little bad because it allocates a bunch of empty dtables only to discard them.
      let mut env = Environment::new(BuiltinDispatchTables::empty());

      macro_rules! get_dtables {
         ($type_name:tt, $define:tt) => {{
            let tb = TypeBuilder::new($type_name);
            let tb = stdlib.$define(tb);
            // Unwrapping here is fine because the stdlib should never declare THAT many methods.
            tb.build(&mut env, &mut gc).unwrap()
         }};
      }
      // TODO: Expose _*_type in the global scope.
      let nil = get_dtables!("Nil", define_nil);
      let boolean = get_dtables!("Boolean", define_boolean);
      let number = get_dtables!("Number", define_number);
      let string = get_dtables!("String", define_string);
      let list = get_dtables!("List", define_list);
      let dict = get_dtables!("Dict", define_dict);
      env.builtin_dtables = BuiltinDispatchTables {
         nil: Gc::clone(&nil.instance_dtable),
         boolean: Gc::clone(&boolean.instance_dtable),
         number: Gc::clone(&number.instance_dtable),
         string: Gc::clone(&string.instance_dtable),
         function: Gc::new(DispatchTable::new_for_instance("Function")),
         list: Gc::clone(&list.instance_dtable),
         dict: Gc::clone(&dict.instance_dtable),
      };

      let mut engine = Self {
         env,
         globals: Globals::new(),
         gc,
         debug_options,
      };
      // Unwrapping here is fine because at this point we haven't got quite that many globals
      // registered to overflow an Opr24.
      engine.set_built_type(&nil).unwrap();
      engine.set_built_type(&boolean).unwrap();
      engine.set_built_type(&number).unwrap();
      engine.set_built_type(&string).unwrap();
      engine.set_built_type(&list).unwrap();

      engine
   }

   /// Compiles a script.
   ///
   /// # Errors
   ///  - [`Error::Compile`] - Syntax or semantic error
   pub fn compile(
      &mut self,
      filename: impl AsRef<str>,
      source: impl Into<String>,
   ) -> Result<Script, Error> {
      let module_name = Rc::from(filename.as_ref());
      let lexer = Lexer::new(Rc::clone(&module_name), source.into());
      let (ast, root_node) = Parser::new(lexer).parse()?;
      if self.debug_options.dump_ast {
         eprintln!("Mica - AST dump:");
         eprintln!("{:?}", DumpAst(&ast, root_node));
      }

      let main_chunk = CodeGenerator::new(module_name, &mut self.env).generate(&ast, root_node)?;
      if self.debug_options.dump_bytecode {
         eprintln!("Mica - global environment:");
         eprintln!("{:#?}", self.env);
         eprintln!("Mica - main chunk disassembly:");
         eprintln!("{:#?}", main_chunk);
      }

      Ok(Script {
         engine: self,
         main_chunk,
      })
   }

   /// Compiles and starts running a script.
   ///
   /// This can be used as a shorthand if you don't intend to reuse the compiled bytecode.
   ///
   /// # Errors
   /// See [`compile`][`Self::compile`].
   pub fn start(
      &mut self,
      filename: impl AsRef<str>,
      source: impl Into<String>,
   ) -> Result<Fiber, Error> {
      let script = self.compile(filename, source)?;
      Ok(script.into_fiber())
   }

   /// Calls the provided function with the given arguments.
   ///
   /// # Errors
   ///
   /// - [`Error::Runtime`] - if a runtime error occurs - `function` isn't callable or an error is
   ///   raised during execution
   /// - [`Error::TooManyArguments`] - if more arguments than the implementation can support is
   ///   passed to the function
   pub fn call<T>(
      &mut self,
      function: Value,
      arguments: impl IntoIterator<Item = Value>,
   ) -> Result<T, Error>
   where
      T: TryFromValue,
   {
      let stack: Vec<_> =
         Some(function).into_iter().chain(arguments).map(|x| x.to_raw(&mut self.gc)).collect();
      // Having to construct a chunk here isn't the most clean, but it's the simplest way of making
      // the VM perform a function call. It reuses sanity checks such as ensuring `function`
      // can actually be called.
      let mut chunk = Chunk::new(Rc::from("(call)"));
      chunk.emit((
         Opcode::Call,
         // 1 has to be subtracted from the stack length there because the VM itself adds 1 to
         // count in the function argument.
         Opr24::try_from(stack.len() - 1).map_err(|_| Error::TooManyArguments)?,
      ));
      chunk.emit(Opcode::Halt);
      let chunk = Rc::new(chunk);
      let fiber = Fiber {
         engine: self,
         inner: vm::Fiber::new(chunk, stack),
      };
      fiber.trampoline()
   }

   /// Returns the unique ID of a method with a given name and arity.
   ///
   /// Note that there can only exist about 65 thousand unique method signatures. This is usually
   /// not a problem as method names often repeat. Also note that unlike functions, a method can
   /// only accept up to 256 arguments. Which, to be quite frankly honest, should be enough for
   /// anyone.
   ///
   /// # Errors
   ///
   /// - [`Error::TooManyMethods`] - raised when too many unique method signatures exist at once
   pub fn method_id(&mut self, signature: impl MethodSignature) -> Result<MethodId, Error> {
      signature.to_method_id(&mut self.env)
   }

   /// Calls a method on a receiver with the given arguments.
   ///
   /// Note that if you're calling a method often, it's cheaper to precompute the method signature
   /// into a [`MethodId`] by using the [`method_id`][`Self::method_id`] function, compared to
   /// passing a name+arity pair every single time.
   ///
   /// # Errors
   ///
   /// - [`Error::Runtime`] - if a runtime error occurs - `function` isn't callable or an error is
   ///   raised during execution
   /// - [`Error::TooManyArguments`] - if more arguments than the implementation can support is
   ///   passed to the function
   /// - [`Error::TooManyMethods`] - if too many methods with different signatures exist at the same
   ///   time
   /// - [`Error::ArgumentCount`] - if `arguments.count()` does not match the argument count of the
   ///   signature
   pub fn call_method<T>(
      &mut self,
      receiver: Value,
      signature: impl MethodSignature,
      arguments: impl IntoIterator<Item = Value>,
   ) -> Result<T, Error>
   where
      T: TryFromValue,
   {
      let method_id = signature.to_method_id(&mut self.env)?;
      // Unwrapping here is fine because `to_method_id` ensures that a method with a given ID
      // exists.
      let signature = self.env.get_function_signature(method_id.0).unwrap();
      let stack: Vec<_> =
         Some(receiver).into_iter().chain(arguments).map(|x| x.to_raw(&mut self.gc)).collect();
      let argument_count = u8::try_from(stack.len()).map_err(|_| Error::TooManyArguments)?;
      if Some(argument_count as u16) != signature.arity {
         return Err(Error::ArgumentCount {
            // Unwrapping here is fine because signatures of methods created by `to_method_id`
            // always have a static arity.
            expected: signature.arity.unwrap() as usize - 1,
            got: argument_count as usize - 1,
         });
      }
      let mut chunk = Chunk::new(Rc::from("(call)"));
      chunk.emit((
         Opcode::CallMethod,
         Opr24::pack((method_id.0, argument_count)),
      ));
      chunk.emit(Opcode::Halt);
      let chunk = Rc::new(chunk);
      let fiber = Fiber {
         engine: self,
         inner: vm::Fiber::new(chunk, stack),
      };
      fiber.trampoline()
   }

   /// Returns the unique global ID for the global with the given name, or an error if there
   /// are too many globals in scope.
   ///
   /// The maximum amount of globals is about 16 million, so you shouldn't worry too much about
   /// hitting that limit unless you're stress-testing the VM or accepting untrusted input as
   /// globals.
   ///
   /// # Errors
   ///  - [`Error::TooManyGlobals`] - Too many globals with unique names were created
   pub fn global_id(&mut self, name: impl GlobalName) -> Result<GlobalId, Error> {
      name.to_global_id(&mut self.env)
   }

   /// Sets a global variable that'll be available to scripts executed by the engine.
   ///
   /// The `id` parameter can be either an `&str` or a prefetched [`global_id`][`Self::global_id`].
   ///
   /// # Errors
   ///  - [`Error::TooManyGlobals`] - Too many globals with unique names were created
   pub fn set<G, T>(&mut self, id: G, value: T) -> Result<(), Error>
   where
      G: GlobalName,
      T: Into<Value>,
   {
      let id = id.to_global_id(&mut self.env)?;
      self.globals.set(id.0, value.into().to_raw(&mut self.gc));
      Ok(())
   }

   /// Returns the value of a global variable, or `nil` if it's not set.
   ///
   /// The `id` parameter can be either an `&str` or a prefetched [`global_id`][`Self::global_id`].
   ///
   /// # Errors
   ///  - [`Error::TooManyGlobals`] - Too many globals with unique names were created
   ///  - [`Error::TypeMismatch`] - The type of the value is not convertible to `T`
   pub fn get<G, T>(&self, id: G) -> Result<T, Error>
   where
      G: OptionalGlobalName,
      T: TryFromValue,
   {
      if let Some(id) = id.try_to_global_id(&self.env) {
         T::try_from_value(&Value::from_raw(self.globals.get(id.0)))
      } else {
         T::try_from_value(&Value::from_raw(RawValue::from(())))
      }
   }

   /// Declares a "raw" function in the global scope. Raw functions do not perform any type checks
   /// by default and accept a variable number of arguments.
   ///
   /// You should generally prefer [`add_function`][`Self::add_function`] instead of this.
   ///
   /// Note that this cannot accept [`GlobalId`]s, because a name is required to create the function
   /// and global IDs have their name erased.
   ///
   /// `parameter_count` should reflect the parameter count of the function. Pass `None` if the
   /// function accepts a variable number of arguments. Note that because this function omits type
   /// checks you may receive a different amount of arguments than specified.
   ///
   /// # Errors
   ///  - [`Error::TooManyGlobals`] - Too many globals with unique names were created
   ///  - [`Error::TooManyFunctions`] - Too many functions were registered into the engine
   pub fn add_raw_function(
      &mut self,
      name: &str,
      parameter_count: impl Into<Option<u16>>,
      f: FunctionKind,
   ) -> Result<(), Error> {
      let global_id = name.to_global_id(&mut self.env)?;
      let function_id = self
         .env
         .create_function(Function {
            name: Rc::from(name),
            parameter_count: parameter_count.into(), // doesn't matter for non-methods
            kind: f,
         })
         .map_err(|_| Error::TooManyFunctions)?;
      let function = RawValue::from(self.gc.allocate(Closure {
         function_id,
         captures: Vec::new(),
      }));
      self.globals.set(global_id.0, function);
      Ok(())
   }

   /// Declares a function in the global scope.
   ///
   /// # Errors
   /// See [`add_raw_function`][`Self::add_raw_function`].
   pub fn add_function<F, V>(&mut self, name: &str, f: F) -> Result<(), Error>
   where
      V: ffvariants::Bare,
      F: ForeignFunction<V>,
   {
      self.add_raw_function(
         name,
         F::parameter_count(),
         FunctionKind::Foreign(f.into_raw_foreign_function()),
      )
   }

   /// Declares a type in the global scope.
   ///
   /// # Errors
   ///  - [`Error::TooManyGlobals`] - Too many globals with unique names were created
   ///  - [`Error::TooManyFunctions`] - Too many functions were registered into the engine
   ///  - [`Error::TooManyMethods`] - Too many unique method signatures were created
   pub fn add_type<T>(&mut self, builder: TypeBuilder<T>) -> Result<(), Error>
   where
      T: Any + UserData,
   {
      let built = builder.build(&mut self.env, &mut self.gc)?;
      self.set_built_type(&built)?;
      Ok(())
   }

   pub(crate) fn set_built_type<T>(&mut self, typ: &BuiltType<T>) -> Result<(), Error>
   where
      T: Any,
   {
      let value = typ.make_type(&mut self.gc);
      self.set(typ.type_name.deref(), value)
   }
}

/// A script pre-compiled into bytecode.
pub struct Script<'e> {
   engine: &'e mut Engine,
   main_chunk: Rc<Chunk>,
}

impl<'e> Script<'e> {
   /// Starts running a script in a new fiber.
   pub fn start(&mut self) -> Fiber {
      Fiber {
         engine: self.engine,
         inner: vm::Fiber::new(Rc::clone(&self.main_chunk), Vec::new()),
      }
   }

   /// Starts running a script in a new fiber, consuming the script.
   pub fn into_fiber(self) -> Fiber<'e> {
      Fiber {
         engine: self.engine,
         inner: vm::Fiber::new(Rc::clone(&self.main_chunk), Vec::new()),
      }
   }
}

/// An ID unique to an engine, identifying a global variable.
///
/// Note that these IDs are not portable across different engine instances.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[repr(transparent)]
pub struct GlobalId(Opr24);

mod global_id {
   use crate::GlobalId;

   pub trait Sealed {}
   impl Sealed for GlobalId {}
   impl Sealed for &str {}
}

/// A trait for names convertible to global IDs.
pub trait GlobalName: global_id::Sealed {
   #[doc(hidden)]
   fn to_global_id(&self, env: &mut Environment) -> Result<GlobalId, Error>;
}

impl GlobalName for GlobalId {
   fn to_global_id(&self, _: &mut Environment) -> Result<GlobalId, Error> {
      Ok(*self)
   }
}

impl GlobalName for &str {
   fn to_global_id(&self, env: &mut Environment) -> Result<GlobalId, Error> {
      Ok(if let Some(slot) = env.get_global(*self) {
         GlobalId(slot)
      } else {
         env.create_global(*self).map(GlobalId).map_err(|_| Error::TooManyGlobals)?
      })
   }
}

/// A trait for names convertible to global IDs.
pub trait OptionalGlobalName {
   #[doc(hidden)]
   fn try_to_global_id(&self, env: &Environment) -> Option<GlobalId>;
}

impl OptionalGlobalName for GlobalId {
   fn try_to_global_id(&self, _: &Environment) -> Option<GlobalId> {
      Some(*self)
   }
}

impl OptionalGlobalName for &str {
   fn try_to_global_id(&self, env: &Environment) -> Option<GlobalId> {
      env.get_global(*self).map(GlobalId)
   }
}

mod method_id {
   use crate::MethodId;

   pub trait Sealed {}

   impl Sealed for MethodId {}
   impl Sealed for (&str, u8) {}
}

/// An ID unique to an engine, identifying a method signature.
///
/// Note that these IDs are not portable across different engine instances.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[repr(transparent)]
pub struct MethodId(u16);

/// Implemented by every type that can be used as a method signature.
///
/// See [`Engine::call_method`].
pub trait MethodSignature: method_id::Sealed {
   #[doc(hidden)]
   fn to_method_id(&self, env: &mut Environment) -> Result<MethodId, Error>;
}

impl MethodSignature for MethodId {
   fn to_method_id(&self, _: &mut Environment) -> Result<MethodId, Error> {
      Ok(*self)
   }
}

/// Tuples of string slices and `u8`s are a user-friendly representation of method signatures.
/// For instance, `("cat", 1)` represents the method `cat/1`.
impl MethodSignature for (&str, u8) {
   fn to_method_id(&self, env: &mut Environment) -> Result<MethodId, Error> {
      env.get_method_index(&FunctionSignature {
         name: Rc::from(self.0),
         arity: Some(self.1 as u16 + 1),
      })
      .map(MethodId)
      .map_err(|_| Error::TooManyMethods)
   }
}