module.rs

use std::cell::RefCell;

use boa_ast::scope::Scope;
use boa_gc::{Finalize, GcRefCell, Trace};

use crate::{JsResult, JsString, JsValue, error::PanicError, module::Module};

/// Type of accessor used to access an indirect binding.
#[derive(Debug, Clone)]
enum BindingAccessor {
    Identifier(JsString),
    Index(u32),
}

/// An indirect reference to a binding inside an environment.
#[derive(Clone, Debug, Trace, Finalize)]
struct IndirectBinding {
    module: Module,
    #[unsafe_ignore_trace]
    accessor: RefCell<BindingAccessor>,
}

/// The type of binding a [`ModuleEnvironment`] can contain.
#[derive(Clone, Debug, Trace, Finalize)]
enum BindingType {
    Direct(Option<JsValue>),
    Indirect(IndirectBinding),
}

/// A [**Module Environment Record**][spec].
///
/// Module environments allow referencing bindings inside other environments, in addition
/// to the usual declarative environment functionality.
///
///
/// [spec]: https://tc39.es/ecma262/#sec-module-environment-records
#[derive(Debug, Trace, Finalize)]
pub(crate) struct ModuleEnvironment {
    bindings: GcRefCell<Vec<BindingType>>,

    // Safety: Nothing in CompileTimeEnvironment needs tracing.
    #[unsafe_ignore_trace]
    compile: Scope,
}

impl ModuleEnvironment {
    /// Creates a new `LexicalEnvironment`.
    pub(crate) fn new(bindings: u32, compile: Scope) -> Self {
        Self {
            bindings: GcRefCell::new(vec![BindingType::Direct(None); bindings as usize]),
            compile,
        }
    }

    /// Gets the compile time environment of this module environment.
    pub(crate) const fn compile(&self) -> &Scope {
        &self.compile
    }

    /// Get the binding value from the environment by it's index.
    ///
    /// # Panics
    ///
    /// Panics if the binding value is out of range or not initialized.
    #[track_caller]
    pub(crate) fn get(&self, index: u32) -> Option<JsValue> {
        let bindings = self.bindings.borrow();

        match &bindings[index as usize] {
            BindingType::Direct(v) => v.clone(),
            BindingType::Indirect(IndirectBinding { module, accessor }) => {
                let env = module.environment()?;

                match &*accessor.clone().borrow() {
                    BindingAccessor::Identifier(name) => {
                        let index = env
                            .kind()
                            .as_module()
                            .expect("must be module environment")
                            .compile()
                            .get_binding(name)
                            .expect("linking must ensure the binding exists");

                        let value = env.get(index.binding_index())?;

                        *accessor.borrow_mut() = BindingAccessor::Index(index.binding_index());

                        Some(value)
                    }
                    BindingAccessor::Index(index) => env.get(*index),
                }
            }
        }
    }

    /// Sets the binding value from the environment by index.
    ///
    /// # Errors
    ///
    /// Returns a `TypeError` if the binding is an indirect reference to another environment.
    #[track_caller]
    pub(crate) fn set(&self, index: u32, value: JsValue) -> JsResult<()> {
        let mut bindings = self.bindings.borrow_mut();

        match &mut bindings[index as usize] {
            BindingType::Direct(v) => {
                *v = Some(value);
                Ok(())
            }
            BindingType::Indirect(_) => Err(PanicError::new(
                "cannot modify indirect references to other environments",
            )
            .into()),
        }
    }

    /// Creates an indirect binding reference to another environment binding.
    ///
    /// # Panics
    ///
    /// Panics if the binding value is out of range.
    #[track_caller]
    pub(crate) fn set_indirect(&self, index: u32, target_module: Module, target_binding: JsString) {
        let mut bindings = self.bindings.borrow_mut();

        bindings[index as usize] = BindingType::Indirect(IndirectBinding {
            module: target_module,
            accessor: RefCell::new(BindingAccessor::Identifier(target_binding)),
        });
    }
}