use syn::{
    visit_mut::{visit_item_mod_mut, VisitMut},
    Item, ItemForeignMod, ItemMod,
};

pub(super) fn merge_extern_blocks(item_mod: &mut ItemMod) {
    Visitor.visit_item_mod_mut(item_mod)
}

struct Visitor;

impl VisitMut for Visitor {
    fn visit_item_mod_mut(&mut self, item_mod: &mut ItemMod) {
        if let Some((_, ref mut items)) = item_mod.content {
            // Keep all the extern blocks in a different `Vec` for faster search.
            let mut extern_blocks = Vec::<ItemForeignMod>::new();

            for item in std::mem::take(items) {
                if let Item::ForeignMod(ItemForeignMod {
                    attrs,
                    abi,
                    brace_token,
                    items: extern_block_items,
                }) = item
                {
                    let mut exists = false;
                    for extern_block in &mut extern_blocks {
                        // Check if there is a extern block with the same ABI and
                        // attributes.
                        if extern_block.attrs == attrs &&
                            extern_block.abi == abi
                        {
                            // Merge the items of the two blocks.
                            extern_block
                                .items
                                .extend_from_slice(&extern_block_items);
                            exists = true;
                            break;
                        }
                    }
                    // If no existing extern block had the same ABI and attributes, store
                    // it.
                    if !exists {
                        extern_blocks.push(ItemForeignMod {
                            attrs,
                            abi,
                            brace_token,
                            items: extern_block_items,
                        });
                    }
                } else {
                    // If the item is not an extern block, we don't have to do anything and just
                    // push it back.
                    items.push(item);
                }
            }

            // Move all the extern blocks alongside the rest of the items.
            for extern_block in extern_blocks {
                items.push(Item::ForeignMod(extern_block));
            }
        }

        visit_item_mod_mut(self, item_mod)
    }
}