From dcb12abbae80e062e393fe39cfcd0b16faef15f8 Mon Sep 17 00:00:00 2001
From: Noah <33094578+coolreader18@users.noreply.github.com>
Date: Wed, 7 Oct 2020 13:32:12 -0500
Subject: [PATCH] Don't double loop over chunks

---
 vm/src/stdlib/json/machinery.rs | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/vm/src/stdlib/json/machinery.rs b/vm/src/stdlib/json/machinery.rs
index c5b8495d5..0e041f55c 100644
--- a/vm/src/stdlib/json/machinery.rs
+++ b/vm/src/stdlib/json/machinery.rs
@@ -123,16 +123,24 @@ impl StrOrChar<'_> {
         }
     }
 }
-pub fn scanstring(s: &str, end: usize, strict: bool) -> Result<(String, usize), DecodeError> {
-    let mut chunks: Vec<StrOrChar> = Vec::new();
+pub fn scanstring<'a>(
+    s: &'a str,
+    end: usize,
+    strict: bool,
+) -> Result<(String, usize), DecodeError> {
+    let mut chunks: Vec<StrOrChar<'a>> = Vec::new();
+    let mut output_len = 0usize;
+    let mut push_chunk = |chunk: StrOrChar<'a>| {
+        output_len += chunk.len();
+        chunks.push(chunk);
+    };
     let mut chunk_start = end;
     let mut chars = s.char_indices().enumerate().skip(end).peekable();
     while let Some((char_i, (i, c))) = chars.next() {
         match c {
             '"' => {
-                chunks.push(StrOrChar::Str(&s[chunk_start..i]));
-                let len = chunks.iter().map(|x| x.len()).sum();
-                let mut out = String::with_capacity(len);
+                push_chunk(StrOrChar::Str(&s[chunk_start..i]));
+                let mut out = String::with_capacity(output_len);
                 for x in chunks {
                     match x {
                         StrOrChar::Str(s) => out.push_str(s),
@@ -158,7 +166,7 @@ pub fn scanstring(s: &str, end: usize, strict: bool) -> Result<(String, usize),
                     't' => "\t",
                     'u' => {
                         let surrogate_err = || DecodeError::new("unpaired surrogate", char_i);
-                        chunks.push(StrOrChar::Str(&s[chunk_start..i]));
+                        push_chunk(StrOrChar::Str(&s[chunk_start..i]));
                         let mut uni = decode_unicode(&mut chars, char_i)?;
                         chunk_start = char_i + 6;
                         if (0xd800..=0xdbff).contains(&uni) {
@@ -184,7 +192,7 @@ pub fn scanstring(s: &str, end: usize, strict: bool) -> Result<(String, usize),
                                 }
                             }
                         }
-                        chunks.push(StrOrChar::Char(
+                        push_chunk(StrOrChar::Char(
                             std::char::from_u32(uni).ok_or_else(surrogate_err)?,
                         ));
                         continue;
@@ -196,9 +204,9 @@ pub fn scanstring(s: &str, end: usize, strict: bool) -> Result<(String, usize),
                         ))
                     }
                 };
-                chunks.push(StrOrChar::Str(&s[chunk_start..i]));
+                push_chunk(StrOrChar::Str(&s[chunk_start..i]));
                 chunk_start = i + 2;
-                chunks.push(StrOrChar::Str(esc));
+                push_chunk(StrOrChar::Str(esc));
             }
             '\x00'..='\x1f' if strict => {
                 return Err(DecodeError::new(