follow up #19714; add memmem optimizations for find on Linux, Macos and BSDs (#20556)

* fixes tests

* add memmem optimization for find

* fixes

* ty[o

* fixes `"abc".find("") == 0 doesn't work on macOS

Co-authored-by: xflywind <43030857+xflywind@users.noreply.github.com>

lib/pure/strutils.nim

@@ -1936,6 +1936,14 @@ func find*(s: string, chars: set[char], start: Natural = 0, last = -1): int {.
     if s[i] in chars:
       return i
 
+when defined(linux):
+  proc memmem(haystack: pointer, haystacklen: csize_t,
+              needle: pointer, needlelen: csize_t): pointer {.importc, header: """#define _GNU_SOURCE
+#include <string.h>""".}
+elif defined(bsd) or (defined(macosx) and not defined(ios)):
+  proc memmem(haystack: pointer, haystacklen: csize_t,
+              needle: pointer, needlelen: csize_t): pointer {.importc, header: "#include <string.h>".}
+
 func find*(s, sub: string, start: Natural = 0, last = -1): int {.rtl,
     extern: "nsuFindStr".} =
   ## Searches for `sub` in `s` inside range `start..last` (both ends included).
@@ -1951,7 +1959,24 @@ func find*(s, sub: string, start: Natural = 0, last = -1): int {.rtl,
   if sub.len > s.len - start: return -1
   if sub.len == 1: return find(s, sub[0], start, last)
 
-  result = find(initSkipTable(sub), s, sub, start, last)
+  template useSkipTable =
+    result = find(initSkipTable(sub), s, sub, start, last)
+
+  when nimvm:
+    useSkipTable()
+  else:
+    when declared(memmem):
+      let subLen = sub.len
+      if last < 0 and start < s.len and subLen != 0:
+        let found = memmem(s[start].unsafeAddr, csize_t(s.len - start), sub.cstring, csize_t(subLen))
+        result = if not found.isNil:
+            cast[ByteAddress](found) -% cast[ByteAddress](s.cstring)
+          else:
+            -1
+      else:
+        useSkipTable()
+    else:
+      useSkipTable()
 
 func rfind*(s: string, sub: char, start: Natural = 0, last = -1): int {.rtl,
     extern: "nsuRFindChar".} =