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use std::collections::HashMap;

use gc::Gc;

use context::Context;
use symbol::Symbol;
use util::{as_sym_list, is_shl};
use value::Value;

/// An error occurring from a call to
/// [`parse_module_header`](fn.parse_module_header.html).
#[derive(Clone, Debug, Eq, Finalize, Hash, PartialEq, Trace)]
pub enum ModuleHeaderError {
    /// An `import` form was invalid.
    BadImportForm,

    /// The `module` form was invalid.
    BadModuleForm,

    /// No `module` form was present.
    NoModuleForm,
}

/// Parses out the `module` and `import` forms from the given values, returning the body.
pub fn parse_module_header<C: 'static + Context>(
    mut values: Vec<Gc<Value<C>>>,
) -> Result<(Symbol, Vec<Symbol>, HashMap<Symbol, Vec<Symbol>>, Vec<Gc<Value<C>>>), ModuleHeaderError> {
    fn parse_header_form<C: 'static + Context>(
        import: bool,
        value: Gc<Value<C>>,
    ) -> Result<(Symbol, Vec<Symbol>), ModuleHeaderError> {
        if let Some(mut sym_list) = as_sym_list(value) {
            if sym_list.len() >= 2 {
                let head = sym_list.remove(0);
                if import && head != Symbol::from("import") {
                    Err(ModuleHeaderError::BadImportForm)
                } else if !import && head != Symbol::from("module") {
                    Err(ModuleHeaderError::BadModuleForm)
                } else {
                    let name = sym_list.remove(0);
                    Ok((name, sym_list))
                }
            } else {
                Err(if import {
                    ModuleHeaderError::BadImportForm
                } else {
                    ModuleHeaderError::BadModuleForm
                })
            }
        } else {
            Err(if import {
                ModuleHeaderError::BadImportForm
            } else {
                ModuleHeaderError::BadModuleForm
            })
        }
    }

    if values.len() == 0 {
        return Err(ModuleHeaderError::NoModuleForm);
    }

    let import_sym = Symbol::from("import");
    let (name, exports) = parse_header_form(false, values.remove(0))?;
    let end = values
        .iter()
        .cloned()
        .position(|x| !is_shl(import_sym, x))
        .unwrap_or_else(|| values.len());

    let imports = values
        .drain(..end)
        .map(|v| parse_header_form(true, v))
        .collect::<Result<_, _>>()?;
    Ok((name, exports, imports, values))
}