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diff --git a/gcc/diagnostics/event-id.h b/gcc/diagnostics/event-id.h
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+/* A class for referring to events within a diagnostics::paths::path.
+   Copyright (C) 2019-2025 Free Software Foundation, Inc.
+   Contributed by David Malcolm <dmalcolm@redhat.com>
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_DIAGNOSTICS_EVENT_ID_H
+#define GCC_DIAGNOSTICS_EVENT_ID_H
+
+/* A class for referring to events within a diagnostics::paths::path.
+
+   They are stored as 0-based offsets into the events, but
+   printed (e.g. via %@) as 1-based numbers.
+
+   For example, a 3-event path has event offsets 0, 1, and 2,
+   which would be shown to the user as "(1)", "(2)" and "(3)".
+
+   This has its own header so that pretty-print.cc can use this
+   to implement "%@" without bringing in all of diagnostics::paths.
+
+   This has to be in the global namespace for compatibility with
+   c-format.cc in GCC 10 onwards.  */
+
+class diagnostic_event_id_t
+{
+ public:
+  diagnostic_event_id_t () : m_index (UNKNOWN_EVENT_IDX) {}
+  diagnostic_event_id_t (int zero_based_idx) : m_index (zero_based_idx) {}
+
+  bool known_p () const { return m_index != UNKNOWN_EVENT_IDX; }
+
+  int zero_based () const
+  {
+    gcc_assert (known_p ());
+    return m_index;
+  }
+
+  int one_based () const
+  {
+    gcc_assert (known_p ());
+    return m_index + 1;
+  }
+
+ private:
+  static const int UNKNOWN_EVENT_IDX = -1;
+  int m_index; // zero-based
+};
+
+namespace diagnostics {
+namespace paths {
+
+typedef diagnostic_event_id_t event_id_t;
+
+/* A type for compactly referring to a particular thread within a
+   diagnostics::paths::path.  Typically there is just one thread per path,
+   with id 0.  */
+typedef int thread_id_t;
+
+} // namespace paths
+} // namespace diagnostics
+
+
+/* A pointer to a diagnostic_event_id_t, for use with the "%@" format
+   code, which will print a 1-based representation for it, with suitable
+   colorization, e.g. "(1)".
+   The %@ format code requires that known_p be true for the event ID. */
+typedef diagnostic_event_id_t *diagnostic_event_id_ptr;
+
+#endif /* ! GCC_DIAGNOSTICS_EVENT_ID_H */