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    // pest. The Elegant Parser
    // Copyright (c) 2018 Dragoș Tiselice
    //
    // Licensed under the Apache License, Version 2.0
    // <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
    // license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. All files in the project carrying such notice may not be copied,
    // modified, or distributed except according to those terms.

    use crate::ast::*;

    pub fn factor(rule: Rule) -> Rule {
        let Rule { name, ty, expr } = rule;
        Rule {
            name,
            ty,
            expr: expr.map_top_down(|expr| {
                // TODO: Use box syntax when it gets stabilized.
                match expr {
                    Expr::Choice(lhs, rhs) => match (*lhs, *rhs) {
                        (Expr::Seq(l1, r1), Expr::Seq(l2, r2)) => {
                            if l1 == l2 {
                                Expr::Seq(l1, Box::new(Expr::Choice(r1, r2)))
                            } else {
                                Expr::Choice(Box::new(Expr::Seq(l1, r1)), Box::new(Expr::Seq(l2, r2)))
                            }
                        }
                        // Converts `(rule ~ rest) | rule` to `rule ~ rest?`, avoiding trying to match `rule` twice.
                        (Expr::Seq(l1, l2), r) => {
                            if *l1 == r {
                                Expr::Seq(l1, Box::new(Expr::Opt(l2)))
                            } else {
                                Expr::Choice(Box::new(Expr::Seq(l1, l2)), Box::new(r))
                            }
                        }
                        // Converts `rule | (rule ~ rest)` to `rule` since `(rule ~ rest)`
                        // will never match if `rule` didn't.
                        (l, Expr::Seq(r1, r2)) => {
                            if l == *r1 {
                                l
                            } else {
                                Expr::Choice(Box::new(l), Box::new(Expr::Seq(r1, r2)))
                            }
                        }
                        (lhs, rhs) => Expr::Choice(Box::new(lhs), Box::new(rhs)),
                    },
                    expr => expr,
                }
            }),
        }
    }