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diff --git a/indra/llwindow/llgamecontroltranslator.h b/indra/llwindow/llgamecontroltranslator.h
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+++ b/indra/llwindow/llgamecontroltranslator.h
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+/**
+ * @file llgamecontroltranslator.h
+ * @brief LLGameControlTranslator class definition
+ *
+ * $LicenseInfo:firstyear=2023&license=viewerlgpl$
+ * Second Life Viewer Source Code
+ * Copyright (C) 2023, Linden Research, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation;
+ * version 2.1 of the License only.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
+ * $/LicenseInfo$
+ */
+
+#pragma once
+
+#include <map>
+
+#include "stdtypes.h"
+#include "llgamecontrol.h"
+
+
+class LLGameControlTranslator
+{
+public:
+
+    using ActionToMaskMap = std::map< std::string, U32 >; // < action : mask >
+    using MaskToChannelMap = std::map< U32, LLGameControl::InputChannel >; // < mask : channel >
+    using NamedChannel = std::pair < std::string , LLGameControl::InputChannel >;
+    using NamedChannels = std::vector< NamedChannel >;
+
+
+    LLGameControlTranslator();
+    void setAvailableActions(ActionToMaskMap& action_to_mask);
+    LLGameControl::InputChannel getChannelByAction(const std::string& action) const;
+    void setMappings(NamedChannels& list);
+    bool updateMap(const std::string& name, const LLGameControl::InputChannel& channel);
+    // Note: to remove a mapping you can call updateMap() with a TYPE_NONE channel
+
+    // Given external action_flags (i.e. raw avatar input)
+    // compute the corresponding LLGameControl::State that would have produced those flags.
+    // Note: "action flags" are similar to, but not quite the same as, "control flags".
+    const LLGameControl::State& computeStateFromFlags(U32 action_flags);
+
+    // Given LLGameControl::State (i.e. from a real controller)
+    // compute corresponding action flags (e.g. for moving the avatar around)
+    U32 computeFlagsFromState(const std::vector<S32>& axes, U32 buttons);
+
+    U32 getMappedFlags() const { return mMappedFlags; }
+
+private:
+    bool updateMapInternal(const std::string& name, const LLGameControl::InputChannel& channel);
+    bool addOrRemoveMaskMapping(U32 mask, const LLGameControl::InputChannel& channel);
+
+private:
+    // mActionToMask is an invarient map between the possible actions
+    // and the action bit masks.  Only actions therein can have their
+    // bit masks mapped to channels.
+    ActionToMaskMap mActionToMask; // invariant map after init
+
+    // mMaskToChannel is a dynamic map between action bit masks
+    // and GameControl channels.
+    MaskToChannelMap mMaskToChannel; // dynamic map, per preference changes
+
+    // mCachedState is an optimization:
+    // it is only recomputed when external action_flags change
+    LLGameControl::State mCachedState;
+
+    U32 mMappedFlags { 0 };
+    U32 mPrevActiveFlags { 0 };
+};