vendor/pest_meta/src/optimizer/factorizer.rs

   1 // pest. The Elegant Parser
   2 // Copyright (c) 2018 Dragoș Tiselice
   3 //
   4 // Licensed under the Apache License, Version 2.0
   5 // <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
   6 // license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. All files in the project carrying such notice may not be copied,
   8 // modified, or distributed except according to those terms.
   9
  10 use crate::ast::*;
  11
  12 pub fn factor(rule: Rule) -> Rule {
  13     let Rule { name, ty, expr } = rule;
  14     Rule {
  15         name,
  16         ty,
  17         expr: expr.map_top_down(|expr| {
  18             // TODO: Use box syntax when it gets stabilized.
  19             match expr {
  20                 Expr::Choice(lhs, rhs) => match (*lhs, *rhs) {
  21                     (Expr::Seq(l1, r1), Expr::Seq(l2, r2)) => {
  22                         if l1 == l2 {
  23                             Expr::Seq(l1, Box::new(Expr::Choice(r1, r2)))
  24                         } else {
  25                             Expr::Choice(Box::new(Expr::Seq(l1, r1)), Box::new(Expr::Seq(l2, r2)))
  26                         }
  27                     }
  28                     // Converts `(rule ~ rest) | rule` to `rule ~ rest?`, avoiding trying to match `rule` twice.
  29                     (Expr::Seq(l1, l2), r) => {
  30                         if *l1 == r {
  31                             Expr::Seq(l1, Box::new(Expr::Opt(l2)))
  32                         } else {
  33                             Expr::Choice(Box::new(Expr::Seq(l1, l2)), Box::new(r))
  34                         }
  35                     }
  36                     // Converts `rule | (rule ~ rest)` to `rule` since `(rule ~ rest)`
  37                     // will never match if `rule` didn't.
  38                     (l, Expr::Seq(r1, r2)) => {
  39                         if l == *r1 {
  40                             l
  41                         } else {
  42                             Expr::Choice(Box::new(l), Box::new(Expr::Seq(r1, r2)))
  43                         }
  44                     }
  45                     (lhs, rhs) => Expr::Choice(Box::new(lhs), Box::new(rhs)),
  46                 },
  47                 expr => expr,
  48             }
  49         }),
  50     }
  51 }