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
10use crate::ast::*;
11
12pub 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 match expr {
19 Expr::Choice(lhs, rhs) => match (*lhs, *rhs) {
20 (Expr::Seq(l1, r1), Expr::Seq(l2, r2)) => {
21 if l1 == l2 {
22 Expr::Seq(l1, Box::new(Expr::Choice(r1, r2)))
23 } else {
24 Expr::Choice(Box::new(Expr::Seq(l1, r1)), Box::new(Expr::Seq(l2, r2)))
25 }
26 }
27 // Converts `(rule ~ rest) | rule` to `rule ~ rest?`, avoiding trying to match `rule` twice.
28 // This is only done for atomic rules, because other rule types have implicit whitespaces.
29 // FIXME: "desugar" implicit whitespace rules before applying any optimizations
30 (Expr::Seq(l1, l2), r)
31 if matches!(ty, RuleType::Atomic | RuleType::CompoundAtomic) =>
32 {
33 if *l1 == r {
34 Expr::Seq(l1, Box::new(Expr::Opt(l2)))
35 } else {
36 Expr::Choice(Box::new(Expr::Seq(l1, l2)), Box::new(r))
37 }
38 }
39 // Converts `rule | (rule ~ rest)` to `rule` since `(rule ~ rest)`
40 // will never match if `rule` didn't.
41 (l, Expr::Seq(r1, r2)) => {
42 if l == *r1 {
43 l
44 } else {
45 Expr::Choice(Box::new(l), Box::new(Expr::Seq(r1, r2)))
46 }
47 }
48 (lhs, rhs) => Expr::Choice(Box::new(lhs), Box::new(rhs)),
49 },
50 expr => expr,
51 }
52 }),
53 }
54}
55