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use std::rc::Rc;
use std::cell::Cell;
use value::RuntimeValue;
use Error;
use types::ValueType;
use susywasm::elements::{ValueType as EValueType};

/// Reference to a global variable (See [`GlobalInstance`] for details).
///
/// This reference has a reference-counting semantics.
///
/// [`GlobalInstance`]: struct.GlobalInstance.html
#[derive(Clone, Debug)]
pub struct GlobalRef(Rc<GlobalInstance>);

impl ::std::ops::Deref for GlobalRef {
	type Target = GlobalInstance;
	fn deref(&self) -> &GlobalInstance {
		&self.0
	}
}

/// Runtime representation of a global variable (or `global` for short).
///
/// Global contains a value of a specified type and flag which specifies whether this
/// global are mutable or immutable. Neither type of the value nor immutability can't be changed
/// after creation.
///
/// Attempt to change value of immutable global or to change type of
/// the value (e.g. assign [`I32`] value to a global that was created with [`I64`] type) will lead to an error.
///
/// [`I32`]: enum.RuntimeValue.html#variant.I32
/// [`I64`]: enum.RuntimeValue.html#variant.I64
#[derive(Debug)]
pub struct GlobalInstance {
	val: Cell<RuntimeValue>,
	mutable: bool,
}

impl GlobalInstance {
	/// Allocate a global variable instance.
	///
	/// Since it is possible to export only immutable globals,
	/// users likely want to set `mutable` to `false`.
	pub fn alloc(val: RuntimeValue, mutable: bool) -> GlobalRef {
		GlobalRef(Rc::new(GlobalInstance {
			val: Cell::new(val),
			mutable,
		}))
	}

	/// Change the value of this global variable.
	///
	/// # Errors
	///
	/// Returns `Err` if this global isn't mutable or if
	/// type of `val` doesn't match global's type.
	pub fn set(&self, val: RuntimeValue) -> Result<(), Error> {
		if !self.mutable {
			return Err(Error::Global("Attempt to change an immutable variable".into()));
		}
		if self.value_type() != val.value_type() {
			return Err(Error::Global("Attempt to change variable type".into()));
		}
		self.val.set(val);
		Ok(())
	}

	/// Get the value of this global variable.
	pub fn get(&self) -> RuntimeValue {
		self.val.get()
	}

	/// Returns if this global variable is mutable.
	///
	/// Note: Imported and/or exported globals are always immutable.
	pub fn is_mutable(&self) -> bool {
		self.mutable
	}

	/// Returns value type of this global variable.
	pub fn value_type(&self) -> ValueType {
		self.val.get().value_type()
	}

	pub(crate) fn elements_value_type(&self) -> EValueType {
		self.value_type().into_elements()
	}
}