fix #19500; remove find optimization [backport: 1.6] (#19714)

* remove find optimization

close #19500

* save find to std

* add simple tests

* Apply suggestions from code review

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

Co-authored-by: sandytypical <43030857+xflywind@users.noreply.github.com>
Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>
(cherry picked from commit 65c2518d5c)

lib/pure/strutils.nim

@@ -1810,6 +1810,15 @@ func initSkipTable*(a: var SkipTable, sub: string) {.rtl,
   for i in 0 ..< m - 1:
     a[sub[i]] = m - 1 - i
 
+func initSkipTable*(sub: string): SkipTable {.noinit, rtl,
+    extern: "nsuInitNewSkipTable".} =
+  ## Returns a new table initialized for `sub`.
+  ##
+  ## See also:
+  ## * `initSkipTable func<#initSkipTable,SkipTable,string>`_
+  ## * `find func<#find,SkipTable,string,string,Natural,int>`_
+  initSkipTable(result, sub)
+
 func find*(a: SkipTable, s, sub: string, start: Natural = 0, last = 0): int {.
     rtl, extern: "nsuFindStrA".} =
   ## Searches for `sub` in `s` inside range `start..last` using preprocessed
@@ -1842,9 +1851,6 @@ func find*(a: SkipTable, s, sub: string, start: Natural = 0, last = 0): int {.
 when not (defined(js) or defined(nimdoc) or defined(nimscript)):
   func c_memchr(cstr: pointer, c: char, n: csize_t): pointer {.
                 importc: "memchr", header: "<string.h>".}
-  func c_strstr(haystack, needle: cstring): cstring {.
-    importc: "strstr", header: "<string.h>".}
-
   const hasCStringBuiltin = true
 else:
   const hasCStringBuiltin = false
@@ -1909,28 +1915,7 @@ func find*(s, sub: string, start: Natural = 0, last = 0): int {.rtl,
   if sub.len > s.len - start: return -1
   if sub.len == 1: return find(s, sub[0], start, last)
 
-  template useSkipTable {.dirty.} =
-    var a {.noinit.}: SkipTable
-    initSkipTable(a, sub)
-    result = find(a, s, sub, start, last)
-
-  when not hasCStringBuiltin:
-    useSkipTable()
-  else:
-    when nimvm:
-      useSkipTable()
-    else:
-      when hasCStringBuiltin:
-        if last == 0 and s.len > start:
-          let found = c_strstr(s[start].unsafeAddr, sub)
-          if not found.isNil:
-            result = cast[ByteAddress](found) -% cast[ByteAddress](s.cstring)
-          else:
-            result = -1
-        else:
-          useSkipTable()
-      else:
-        useSkipTable()
+  result = find(initSkipTable(sub), s, sub, start, last)
 
 func rfind*(s: string, sub: char, start: Natural = 0, last = -1): int {.rtl,
     extern: "nsuRFindChar".} =

lib/std/private/strimpl.nim

@@ -74,3 +74,40 @@ template endsWithImpl*[T: string | cstring](s, suffix: T) =
 
 func cmpNimIdentifier*[T: string | cstring](a, b: T): int =
   cmpIgnoreStyleImpl(a, b, true)
+
+func c_memchr(cstr: pointer, c: char, n: csize_t): pointer {.
+              importc: "memchr", header: "<string.h>".}
+func c_strstr(haystack, needle: cstring): cstring {.
+  importc: "strstr", header: "<string.h>".}
+
+
+func find*(s: cstring, sub: char, start: Natural = 0, last = 0): int =
+  ## Searches for `sub` in `s` inside the range `start..last` (both ends included).
+  ## If `last` is unspecified, it defaults to `s.high` (the last element).
+  ##
+  ## Searching is case-sensitive. If `sub` is not in `s`, -1 is returned.
+  ## Otherwise the index returned is relative to `s[0]`, not `start`.
+  ## Use `s[start..last].rfind` for a `start`-origin index.
+  let last = if last == 0: s.high else: last
+  let L = last-start+1
+  if L > 0:
+    let found = c_memchr(s[start].unsafeAddr, sub, cast[csize_t](L))
+    if not found.isNil:
+      return cast[ByteAddress](found) -% cast[ByteAddress](s)
+  return -1
+
+func find*(s, sub: cstring, start: Natural = 0, last = 0): int =
+  ## Searches for `sub` in `s` inside the range `start..last` (both ends included).
+  ## If `last` is unspecified, it defaults to `s.high` (the last element).
+  ##
+  ## Searching is case-sensitive. If `sub` is not in `s`, -1 is returned.
+  ## Otherwise the index returned is relative to `s[0]`, not `start`.
+  ## Use `s[start..last].find` for a `start`-origin index.
+  if sub.len > s.len - start: return -1
+  if sub.len == 1: return find(s, sub[0], start, last)
+  if last == 0 and s.len > start:
+    let found = c_strstr(s[start].unsafeAddr, sub)
+    if not found.isNil:
+      result = cast[ByteAddress](found) -% cast[ByteAddress](s)
+    else:
+      result = -1

tests/stdlib/tstrimpl.nim

@@ -0,0 +1,6 @@
+import std/private/strimpl
+
+doAssert find(cstring"Hello Nim", cstring"Nim") == 6
+doAssert find(cstring"Hello Nim", cstring"N") == 6
+doAssert find(cstring"Hello Nim", cstring"I") == -1
+doAssert find(cstring"Hello Nim", cstring"O") == -1