util: add printf specifier shared helper code.

clover needs to walk valid specifiers in C++, vtn needs
to find string specifiers in C, let's do both.

This writes the format walker in C++, and wraps it with C.

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8254>

src/util/u_printf.cpp

@@ -0,0 +1,53 @@
+//
+// Copyright 2020 Serge Martin
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+// OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+// OTHER DEALINGS IN THE SOFTWARE.
+//
+// Extract from Serge's printf clover code by airlied.
+
+#include "u_printf.h"
+
+size_t util_printf_next_spec_pos(const std::string &s, size_t pos)
+{
+   size_t next_tok, spec_pos;
+   do {
+      pos = s.find_first_of('%', pos);
+
+      if (pos == std::string::npos)
+         return -1;
+
+      if (s[pos + 1] == '%') {
+         pos += 2;
+         continue;
+      }
+
+      next_tok = s.find_first_of('%', pos + 1);
+      spec_pos = s.find_first_of("cdieEfFgGaAosuxXp", pos + 1);
+      if (spec_pos != std::string::npos)
+         if (spec_pos < next_tok)
+            return spec_pos;
+
+      pos++;
+   } while (1);
+}
+
+size_t util_printf_next_spec_pos(const char *str, size_t pos)
+{
+   return util_printf_next_spec_pos(std::string(str), pos);
+}