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diff --git a/ink_stroke_modeler/internal/stylus_state_modeler.h b/ink_stroke_modeler/internal/stylus_state_modeler.h
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+/*
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef INK_STROKE_MODELER_INTERNAL_STYLUS_STATE_MODELER_H_
+#define INK_STROKE_MODELER_INTERNAL_STYLUS_STATE_MODELER_H_
+
+#include <deque>
+#include <ostream>
+
+#include "ink_stroke_modeler/internal/internal_types.h"
+#include "ink_stroke_modeler/params.h"
+#include "ink_stroke_modeler/types.h"
+
+namespace ink {
+namespace stroke_model {
+
+// This class is used to model the state of the stylus for a given position,
+// based on the state of the stylus at the original input points.
+//
+// The stylus is modeled by storing the last max_input_samples positions and
+// states received via Update(); when queried, it treats the stored positions as
+// a polyline, and finds the closest segment. The returned stylus state is a
+// linear interpolation between the states associated with the endpoints of the
+// segment, correcting angles to account for the "wraparound" that occurs at 0
+// and 2π. The value used for interpolation is based on how far along the
+// segment the closest point lies.
+//
+// If Update() is called with a state in which a field (i.e. pressure, tilt, or
+// orientation) has a negative value (indicating no information), then the
+// results of Query() will be -1 for that field until Reset() is called. This is
+// tracked independently for each field; e.g., if you pass in tilt = -1, then
+// pressure and orientation will continue to be interpolated normally.
+class StylusStateModeler {
+ public:
+  // Adds a position and state pair to the model. During stroke modeling, these
+  // values will be taken from the raw input.
+  void Update(Vec2 position, const StylusState &state);
+
+  // Clear the model and reset.
+  void Reset(const StylusStateModelerParams &params);
+
+  // Query the model for the state at the given position. During stroke
+  // modeling, the position will be taken from the modeled input.
+  //
+  // If no Update() calls have been received since the last Reset(), this will
+  // return {.pressure = -1, .tilt = -1, .orientation = -1}.
+  StylusState Query(Vec2 position) const;
+
+ private:
+  struct PositionAndState {
+    Vec2 position{0};
+    StylusState state;
+
+    PositionAndState(Vec2 position_in, const StylusState &state_in)
+        : position(position_in), state(state_in) {}
+  };
+
+  bool received_unknown_pressure_ = false;
+  bool received_unknown_tilt_ = false;
+  bool received_unknown_orientation_ = false;
+
+  std::deque<PositionAndState> positions_and_states_;
+  StylusStateModelerParams params_;
+};
+
+}  // namespace stroke_model
+}  // namespace ink
+
+#endif  // INK_STROKE_MODELER_INTERNAL_STYLUS_STATE_MODELER_H_