From: Skgland <bennet.blessmann+github@googlemail.com>
Date: Sun, 26 Apr 2026 11:43:04 +0000 (+0200)
Subject: optimize put_back_char
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=af54fb2aba46af538b4564045ba40ba7b854d1d3;p=scryer-prolog.git

optimize put_back_char

Always encode the char directly into the buffer.
Only shift the buffer content if there isn't enough room in the front.
---

diff --git a/src/parser/char_reader.rs b/src/parser/char_reader.rs
index 69d592e1..1ad6971e 100644
--- a/src/parser/char_reader.rs
+++ b/src/parser/char_reader.rs
@@ -235,16 +235,17 @@ impl<R: Read> CharRead for CharReader<R> {
     #[inline(always)]
     fn put_back_char(&mut self, c: char) {
         let c_len = c.len_utf8();
+        if c_len <= self.pos {
+            self.pos -= c_len;
+        } else {
+            self.buf.insert_from_slice(
+                0,
+                &[0u8; 4/* char::MAX_LEN_UTF8 once msrv reached 1.93 */][..c_len - self.pos],
+            );
+            self.pos = 0;
+        }
 
-        let mut shifted_slice = SmallVec::<[u8; 32]>::new();
-        shifted_slice.extend_from_slice(&self.buf[self.pos..]);
-
-        self.buf.clear();
-        self.buf.resize(c_len, 0);
-        c.encode_utf8(&mut self.buf[..c_len]);
-
-        self.buf.extend_from_slice(&shifted_slice);
-        self.pos = 0;
+        c.encode_utf8(&mut self.buf[self.pos..]);
     }
 
     #[inline(always)]