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use crate::{
    perf::Check,
    util::{alias_if_required, undefined, StmtLike},
};
use std::mem::replace;
use swc_common::DUMMY_SP;
use swc_ecma_ast::*;
use swc_ecma_transforms_macros::fast_path;
use swc_ecma_visit::{noop_fold_type, noop_visit_type, Fold, FoldWith, Node, Visit, VisitWith};

#[cfg(test)]
mod tests;

pub fn nullish_coalescing() -> impl Fold + 'static {
    NullishCoalescing::default()
}

#[derive(Debug, Default)]
struct NullishCoalescing {
    vars: Vec<VarDeclarator>,
}

impl NullishCoalescing {
    fn fold_stmt_like<T>(&mut self, stmts: Vec<T>) -> Vec<T>
    where
        T: FoldWith<Self> + StmtLike,
    {
        let mut buf = Vec::with_capacity(stmts.len() + 2);

        for stmt in stmts {
            let stmt = stmt.fold_with(self);

            if !self.vars.is_empty() {
                buf.push(T::from_stmt(Stmt::Decl(Decl::Var(VarDecl {
                    span: DUMMY_SP,
                    kind: VarDeclKind::Var,
                    decls: replace(&mut self.vars, Default::default()),
                    declare: false,
                }))));
            }

            buf.push(stmt);
        }

        buf
    }
}

#[fast_path(ShouldWork)]
impl Fold for NullishCoalescing {
    noop_fold_type!();

    fn fold_module_items(&mut self, n: Vec<ModuleItem>) -> Vec<ModuleItem> {
        self.fold_stmt_like(n)
    }

    fn fold_stmts(&mut self, n: Vec<Stmt>) -> Vec<Stmt> {
        self.fold_stmt_like(n)
    }

    fn fold_expr(&mut self, e: Expr) -> Expr {
        let e = e.fold_children_with(self);

        match e {
            Expr::Bin(BinExpr {
                span,
                left,
                op: op!("??"),
                right,
            }) => {
                //
                let (l, aliased) = alias_if_required(&left, "ref");

                if aliased {
                    self.vars.push(VarDeclarator {
                        span: DUMMY_SP,
                        name: Pat::Ident(l.clone()),
                        init: None,
                        definite: false,
                    });
                }

                let var_expr = if aliased {
                    Expr::Assign(AssignExpr {
                        span: DUMMY_SP,
                        op: op!("="),
                        left: PatOrExpr::Pat(Box::new(Pat::Ident(l.clone()))),
                        right: left,
                    })
                } else {
                    Expr::Ident(l.clone())
                };

                return Expr::Cond(CondExpr {
                    span,
                    test: Box::new(Expr::Bin(BinExpr {
                        span: DUMMY_SP,
                        left: Box::new(Expr::Bin(BinExpr {
                            span: DUMMY_SP,
                            left: Box::new(var_expr),
                            op: op!("!=="),
                            right: Box::new(Expr::Lit(Lit::Null(Null { span: DUMMY_SP }))),
                        })),
                        op: op!("&&"),
                        right: Box::new(Expr::Bin(BinExpr {
                            span: DUMMY_SP,
                            left: Box::new(Expr::Ident(l.clone())),
                            op: op!("!=="),
                            right: undefined(DUMMY_SP),
                        })),
                    })),
                    cons: Box::new(Expr::Ident(l.clone())),
                    alt: right,
                });
            }

            _ => {}
        }

        e
    }
}

#[derive(Default)]
struct ShouldWork {
    /// Found optional chaining?
    found: bool,
}

impl Visit for ShouldWork {
    noop_visit_type!();

    fn visit_bin_expr(&mut self, e: &BinExpr, _: &dyn Node) {
        if e.op == op!("??") {
            self.found = true;
        } else {
            e.visit_children_with(self)
        }
    }
}

impl Check for ShouldWork {
    fn should_handle(&self) -> bool {
        self.found
    }
}