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+// Copyright 2016 The Draco Authors.
+//
+// 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 DRACO_COMPRESSION_MESH_MESH_EDGEBREAKER_SHARED_H_
+#define DRACO_COMPRESSION_MESH_MESH_EDGEBREAKER_SHARED_H_
+
+#include <stdint.h>
+
+namespace draco {
+
+// Shared declarations used by both edgebreaker encoder and decoder.
+
+// A variable length encoding for storing all possible topology configurations
+// during traversal of mesh's surface. The configurations are based on visited
+// state of neighboring triangles around a currently processed face corner.
+// Note that about half of the encountered configurations is expected to be of
+// type TOPOLOGY_C. It's guaranteed that the encoding will use at most 2 bits
+// per triangle for meshes with no holes and up to 6 bits per triangle for
+// general meshes. In addition, the encoding will take up to 4 bits per triangle
+// for each non-position attribute attached to the mesh.
+//
+//     *-------*          *-------*          *-------*
+//    / \     / \        / \     / \        / \     / \
+//   /   \   /   \      /   \   /   \      /   \   /   \
+//  /     \ /     \    /     \ /     \    /     \ /     \
+// *-------v-------*  *-------v-------*  *-------v-------*
+//  \     /x\     /          /x\     /    \     /x\
+//   \   /   \   /          /   \   /      \   /   \
+//    \ /  C  \ /          /  L  \ /        \ /  R  \
+//     *-------*          *-------*          *-------*
+//
+//     *       *
+//    / \     / \
+//   /   \   /   \
+//  /     \ /     \
+// *-------v-------*          v
+//  \     /x\     /          /x\
+//   \   /   \   /          /   \
+//    \ /  S  \ /          /  E  \
+//     *-------*          *-------*
+//
+// TODO(ostava): Get rid of the topology bit pattern. It's important only for
+// encoding but the algorithms should use EdgebreakerSymbol instead.
+enum EdgebreakerTopologyBitPattern {
+  TOPOLOGY_C = 0x0,  // 0
+  TOPOLOGY_S = 0x1,  // 1 0 0
+  TOPOLOGY_L = 0x3,  // 1 1 0
+  TOPOLOGY_R = 0x5,  // 1 0 1
+  TOPOLOGY_E = 0x7,  // 1 1 1
+  // A special symbol that's not actually encoded, but it can be used to mark
+  // the initial face that triggers the mesh encoding of a single connected
+  // component.
+  TOPOLOGY_INIT_FACE,
+  // A special value used to indicate an invalid symbol.
+  TOPOLOGY_INVALID
+};
+
+enum EdgebreakerSymbol {
+  EDGEBREAKER_SYMBOL_C = 0,
+  EDGEBREAKER_SYMBOL_S,
+  EDGEBREAKER_SYMBOL_L,
+  EDGEBREAKER_SYMBOL_R,
+  EDGEBREAKER_SYMBOL_E,
+  EDGEBREAKER_SYMBOL_INVALID
+};
+
+// Bit-length of symbols in the EdgebreakerTopologyBitPattern stored as a
+// lookup table for faster indexing.
+constexpr int32_t edge_breaker_topology_bit_pattern_length[] = {1, 3, 0, 3,
+                                                                0, 3, 0, 3};
+
+// Zero-indexed symbol id for each of topology pattern.
+constexpr EdgebreakerSymbol edge_breaker_topology_to_symbol_id[] = {
+    EDGEBREAKER_SYMBOL_C,       EDGEBREAKER_SYMBOL_S,
+    EDGEBREAKER_SYMBOL_INVALID, EDGEBREAKER_SYMBOL_L,
+    EDGEBREAKER_SYMBOL_INVALID, EDGEBREAKER_SYMBOL_R,
+    EDGEBREAKER_SYMBOL_INVALID, EDGEBREAKER_SYMBOL_E};
+
+// Reverse mapping between symbol id and topology pattern symbol.
+constexpr EdgebreakerTopologyBitPattern edge_breaker_symbol_to_topology_id[] = {
+    TOPOLOGY_C, TOPOLOGY_S, TOPOLOGY_L, TOPOLOGY_R, TOPOLOGY_E};
+
+// Types of edges used during mesh traversal relative to the tip vertex of a
+// visited triangle.
+enum EdgeFaceName : uint8_t { LEFT_FACE_EDGE = 0, RIGHT_FACE_EDGE = 1 };
+
+// Struct used for storing data about a source face that connects to an
+// already traversed face that was either the initial face or a face encoded
+// with either topology S (split) symbol. Such connection can be only caused by
+// topology changes on the traversed surface (if its genus != 0, i.e. when the
+// surface has topological handles or holes).
+// For each occurrence of such event we always encode the split symbol id,
+// source symbol id and source edge id (left, or right). There will be always
+// exactly two occurrences of this event for every topological handle on the
+// traversed mesh and one occurrence for a hole.
+struct TopologySplitEventData {
+  uint32_t split_symbol_id;
+  uint32_t source_symbol_id;
+  // We need to use uint32_t instead of EdgeFaceName because the most recent
+  // version of gcc does not allow that when optimizations are turned on.
+  uint32_t source_edge : 1;
+};
+
+// Hole event is used to store info about the first symbol that reached a
+// vertex of so far unvisited hole. This can happen only on either the initial
+// face or during a regular traversal when TOPOLOGY_S is encountered.
+struct HoleEventData {
+  int32_t symbol_id;
+  HoleEventData() : symbol_id(0) {}
+  explicit HoleEventData(int32_t sym_id) : symbol_id(sym_id) {}
+};
+
+// List of supported modes for valence based edgebreaker coding.
+enum EdgebreakerValenceCodingMode {
+  EDGEBREAKER_VALENCE_MODE_2_7 = 0,  // Use contexts for valences in range 2-7.
+};
+
+}  // namespace draco
+
+#endif  // DRACO_COMPRESSION_MESH_MESH_EDGEBREAKER_SHARED_H_