Do some string library work

libc/str/wcsstr.c

@@ -0,0 +1,42 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/str/str.h"
+
+/**
+ * Searches for substring.
+ *
+ * @param haystack is the search area, as a NUL-terminated string
+ * @param needle is the desired substring, also NUL-terminated
+ * @return pointer to first substring within haystack, or NULL
+ * @asyncsignalsafe
+ * @see memmem()
+ */
+wchar_t *wcsstr(const wchar_t *haystack, const wchar_t *needle) {
+  size_t i;
+  for (;;) {
+    for (i = 0;;) {
+      if (!needle[i]) return (/*unconst*/ wchar_t *)haystack;
+      if (!haystack[i]) break;
+      if (needle[i] != haystack[i]) break;
+      ++i;
+    }
+    if (!*haystack++) break;
+  }
+  return NULL;
+}