From b18a91312e33f13e6b82f5a3385fc6a5d5381796 Mon Sep 17 00:00:00 2001
From: Maneren <49210777+Maneren@users.noreply.github.com>
Date: Wed, 23 Aug 2023 05:16:05 +0200
Subject: [PATCH] refactor `loss-of-precision` rule to remove the slow regex
 (#775)

This achieves around 2x speedup in the rule execution by using a manual
parser instead of a regex.
---
 .../src/rules/eslint/no_loss_of_precision.rs  | 87 ++++++++++++-------
 1 file changed, 56 insertions(+), 31 deletions(-)

diff --git a/crates/oxc_linter/src/rules/eslint/no_loss_of_precision.rs b/crates/oxc_linter/src/rules/eslint/no_loss_of_precision.rs
index 62da2400e..c30161e12 100644
--- a/crates/oxc_linter/src/rules/eslint/no_loss_of_precision.rs
+++ b/crates/oxc_linter/src/rules/eslint/no_loss_of_precision.rs
@@ -1,4 +1,3 @@
-use once_cell::sync::Lazy;
 use oxc_ast::ast::NumberLiteral;
 use oxc_ast::AstKind;
 use oxc_diagnostics::{
@@ -7,7 +6,6 @@ use oxc_diagnostics::{
 };
 use oxc_macros::declare_oxc_lint;
 use oxc_span::Span;
-use regex::Regex;
 use std::borrow::Cow;
 
 use crate::{context::LintContext, rule::Rule, AstNode};
@@ -77,35 +75,62 @@ impl PartialEq for ScientificNotation<'_> {
     }
 }
 
-static RE: Lazy<Regex> = Lazy::new(|| {
-    Regex::new(r"-?0*(?P<int>0|[1-9]\d*)?(?:\.(?P<frac>\d+))?(?:[eE](?P<exp>[+-]?\d+))?").unwrap()
-});
-
 impl<'a> RawNum<'a> {
     fn new(num: &str) -> Option<RawNum<'_>> {
-        if let Some(captures) = RE.captures(num) {
-            let int = captures.name("int").map_or("0", |m| m.as_str());
-            let frac = captures.name("frac").map_or("", |m| m.as_str());
-            let exp = captures.name("exp").map_or("0", |m| m.as_str());
+        // remove sign
+        let num = num.trim_start_matches(['+', '-']);
 
-            let exp = match exp.parse::<isize>() {
-                Ok(x) => x,
-                Err(_) => return None,
-            };
+        let (int, num_without_int) = {
+            // skip leading zeros
+            let num_without_zeros = num.trim_start_matches('0');
 
-            Some(RawNum { int, frac, exp })
-        } else {
-            None
-        }
+            // read the integer part and store the end index
+            let int_end = num_without_zeros
+                .chars()
+                .position(|ch| !ch.is_ascii_digit())
+                .unwrap_or(num_without_zeros.len());
+
+            // if no integer part was found, default to 0
+            let int = if int_end == 0 { "0" } else { &num_without_zeros[..int_end] };
+
+            // make a slice of the rest of the string
+            let num_without_int = &num_without_zeros[int_end..];
+
+            (int, num_without_int)
+        };
+
+        // if next char is a dot, parse the fractional part
+        let (frac, num_without_frac) =
+            num_without_int.strip_prefix('.').map_or(("", num_without_int), |num_without_dot| {
+                let frac_end = num_without_dot
+                    .chars()
+                    .position(|ch| !ch.is_ascii_digit())
+                    .unwrap_or(num_without_dot.len());
+
+                // slice the fractional part and the rest of the string
+                let frac = &num_without_dot[..frac_end];
+                let num_without_frac = &num_without_dot[frac_end..];
+
+                (frac, num_without_frac)
+            });
+
+        // if next char is an e, treat the rest as the exponent
+        let exp =
+            num_without_frac.strip_prefix(['e', 'E']).map_or("0", |num_without_e| num_without_e);
+
+        let Ok(exp) = exp.parse::<isize>() else { return None; };
+
+        Some(RawNum { int, frac, exp })
     }
 
     fn normalize(&mut self) -> ScientificNotation<'a> {
         let scientific = self.exp != 0;
         let precision = self.frac.len();
         if self.int.starts_with('0') {
+            let frac_zeros = self.frac.chars().take_while(|&ch| ch == '0').count();
             #[allow(clippy::cast_possible_wrap)]
-            let exp = self.exp - 1 - self.frac.chars().take_while(|&ch| ch == '0').count() as isize;
-            self.frac = self.frac.trim_start_matches('0');
+            let exp = self.exp - 1 - frac_zeros as isize;
+            self.frac = &self.frac[frac_zeros..];
 
             match self.frac.len() {
                 0 => ScientificNotation {
@@ -142,17 +167,17 @@ impl<'a> RawNum<'a> {
                     precision,
                 }
             } else {
-                ScientificNotation {
-                    int: &self.int[..1],
-                    frac: Cow::Owned(
+                let frac = if self.frac.is_empty() {
+                    Cow::Borrowed(&self.int[1..])
+                } else {
+                    Cow::Owned(
                         format!("{}{}", &self.int[1..], self.frac)
                             .trim_end_matches('0')
                             .to_string(),
-                    ),
-                    exp,
-                    scientific,
-                    precision,
-                }
+                    )
+                };
+
+                ScientificNotation { int: &self.int[..1], frac, exp, scientific, precision }
             }
         }
     }
@@ -184,17 +209,17 @@ impl NoLossOfPrecision {
 
     fn base_ten_loses_precision(node: &'_ NumberLiteral) -> bool {
         let raw = Self::get_raw(node);
-        let raw = if let Some(s) = Self::normalize(&raw) { s } else { return true };
+        let Some(raw) = Self::normalize(&raw) else { return true };
 
         if raw.frac.len() >= 100 {
             return true;
         }
         let stored = match (raw.scientific, raw.precision) {
             (true, _) => format!("{:.1$e}", node.value, raw.frac.len()),
-            (false, 0) => format!("{}", node.value),
+            (false, 0) => node.value.to_string(),
             (false, precision) => format!("{:.1$}", node.value, precision),
         };
-        let stored = if let Some(s) = Self::normalize(&stored) { s } else { return true };
+        let Some(stored) = Self::normalize(&stored) else { return true };
         raw != stored
     }