Cleanup `//absl/strings/internal/memutil.h`

`memmatch()` is only used in `string_view.cc`, so move it there.
The moving of `memmatch()` to `string_view.cc` decouples `string_view`
from `memutil`, which will allow us to move `string_view` into its
own target in a followup.

The only other function that is used is `memcasecmp()`, so delete
all other functions.

PiperOrigin-RevId: 547238386
Change-Id: Id6fad47dd24191c8e8f26dd923fffa1007c8db4a

1 files changed, 26 insertions, 4 deletions

diff --git a/absl/strings/string_view.cc b/absl/strings/string_view.cc
index e2261625..f20ff530 100644
--- a/absl/strings/string_view.cc
+++ b/absl/strings/string_view.cc
@@ -21,12 +21,35 @@
 #include <cstring>
 #include <ostream>
 
-#include "absl/strings/internal/memutil.h"
-
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
 namespace {
+
+// This is significantly faster for case-sensitive matches with very
+// few possible matches.
+const char* memmatch(const char* phaystack, size_t haylen, const char* pneedle,
+                     size_t neelen) {
+  if (0 == neelen) {
+    return phaystack;  // even if haylen is 0
+  }
+  if (haylen < neelen) return nullptr;
+
+  const char* match;
+  const char* hayend = phaystack + haylen - neelen + 1;
+  // A static cast is used here to work around the fact that memchr returns
+  // a void* on Posix-compliant systems and const void* on Windows.
+  while (
+      (match = static_cast<const char*>(memchr(
+           phaystack, pneedle[0], static_cast<size_t>(hayend - phaystack))))) {
+    if (memcmp(match, pneedle, neelen) == 0)
+      return match;
+    else
+      phaystack = match + 1;
+  }
+  return nullptr;
+}
+
 void WritePadding(std::ostream& o, size_t pad) {
   char fill_buf[32];
   memset(fill_buf, o.fill(), sizeof(fill_buf));
@@ -84,8 +107,7 @@ string_view::size_type string_view::find(string_view s,
     if (empty() && pos == 0 && s.empty()) return 0;
     return npos;
   }
-  const char* result =
-      strings_internal::memmatch(ptr_ + pos, length_ - pos, s.ptr_, s.length_);
+  const char* result = memmatch(ptr_ + pos, length_ - pos, s.ptr_, s.length_);
   return result ? static_cast<size_type>(result - ptr_) : npos;
 }