From ba7bb2d6ad6e414dd567b2f4b959bd351de7cb75 Mon Sep 17 00:00:00 2001
From: Ed Page <eopage@gmail.com>
Date: Thu, 17 Aug 2023 14:51:05 -0500
Subject: [PATCH] perf(token): Speed up `take_until` under simd

This reverts commit 489aa2ace0ce77790cf8d2aa3fc0fcf8a8f7ccb2.

PR #86 "FindSubstring<&[u8]>: make use of `memchr::memmem`"

When testing under gitoxide, it turned out this slowed down
gitoxide-specific micro-benchmarks from ~170ns to ~240ns
---
 src/stream/mod.rs | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/stream/mod.rs b/src/stream/mod.rs
index fcacbd78..5f2152e2 100644
--- a/src/stream/mod.rs
+++ b/src/stream/mod.rs
@@ -2684,7 +2684,35 @@ fn memchr(token: u8, slice: &[u8]) -> Option<usize> {
 #[cfg(feature = "simd")]
 #[inline(always)]
 fn memmem(slice: &[u8], tag: &[u8]) -> Option<usize> {
-    memchr::memmem::find(slice, tag)
+    if tag.len() > slice.len() {
+        return None;
+    }
+
+    let (&substr_first, substr_rest) = match tag.split_first() {
+        Some(split) => split,
+        // an empty substring is found at position 0
+        // This matches the behavior of str.find("").
+        None => return Some(0),
+    };
+
+    if substr_rest.is_empty() {
+        return memchr::memchr(substr_first, slice);
+    }
+
+    let mut offset = 0;
+    let haystack = &slice[..slice.len() - substr_rest.len()];
+
+    while let Some(position) = memchr::memchr(substr_first, &haystack[offset..]) {
+        offset += position;
+        let next_offset = offset + 1;
+        if &slice[next_offset..][..substr_rest.len()] == substr_rest {
+            return Some(offset);
+        }
+
+        offset = next_offset;
+    }
+
+    None
 }
 
 #[cfg(not(feature = "simd"))]