diff --git a/src/binary_annotator.h b/src/binary_annotator.h
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+/*
+ * Copyright 2021 Google Inc. All rights reserved.
+ *
+ * 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 FLATBUFFERS_BINARY_ANNOTATOR_H_
+#define FLATBUFFERS_BINARY_ANNOTATOR_H_
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include "flatbuffers/base.h"
+#include "flatbuffers/reflection.h"
+#include "flatbuffers/stl_emulation.h"
+#include "flatbuffers/util.h"
+
+namespace flatbuffers {
+
+enum class BinaryRegionType {
+  Unknown = 0,
+  UOffset = 1,
+  SOffset = 2,
+  VOffset = 3,
+  Bool = 4,
+  Byte = 5,
+  Char = 6,
+  Uint8 = 7,
+  Int8 = 8,
+  Uint16 = 9,
+  Int16 = 10,
+  Uint32 = 11,
+  Int32 = 12,
+  Uint64 = 13,
+  Int64 = 14,
+  Float = 15,
+  Double = 16,
+  UType = 17,
+};
+
+template<typename T>
+static inline std::string ToHex(T i, size_t width = sizeof(T)) {
+  std::stringstream stream;
+  stream << std::hex << std::uppercase << std::setfill('0') << std::setw(width)
+         << i;
+  return stream.str();
+}
+
+// Specialized version for uint8_t that don't work well with std::hex.
+static inline std::string ToHex(uint8_t i) {
+  return ToHex(static_cast<int>(i), 2);
+}
+
+enum class BinaryRegionStatus {
+  OK = 0,
+  WARN = 100,
+  WARN_NO_REFERENCES,
+  WARN_CORRUPTED_PADDING,
+  WARN_PADDING_LENGTH,
+  ERROR = 200,
+  // An offset is pointing outside the binary bounds.
+  ERROR_OFFSET_OUT_OF_BINARY,
+  // Expecting to read N bytes but not enough remain in the binary.
+  ERROR_INCOMPLETE_BINARY,
+  // When a length of a vtable/vector is longer than possible.
+  ERROR_LENGTH_TOO_LONG,
+  // When a length of a vtable/vector is shorter than possible.
+  ERROR_LENGTH_TOO_SHORT,
+  // A field mark required is not present in the vtable.
+  ERROR_REQUIRED_FIELD_NOT_PRESENT,
+  // A realized union type is not within the enum bounds.
+  ERROR_INVALID_UNION_TYPE,
+  // Occurs when there is a cycle in offsets.
+  ERROR_CYCLE_DETECTED,
+};
+
+enum class BinaryRegionCommentType {
+  Unknown = 0,
+  SizePrefix,
+  // The offset to the root table.
+  RootTableOffset,
+  // The optional 4-char file identifier.
+  FileIdentifier,
+  // Generic 0-filled padding
+  Padding,
+  // The size of the vtable.
+  VTableSize,
+  // The size of the referring table.
+  VTableRefferingTableLength,
+  // Offsets to vtable fields.
+  VTableFieldOffset,
+  // Offsets to unknown vtable fields.
+  VTableUnknownFieldOffset,
+  // The vtable offset of a table.
+  TableVTableOffset,
+  // A "inline" table field value.
+  TableField,
+  // A table field that is unknown.
+  TableUnknownField,
+  // A table field value that points to another section.
+  TableOffsetField,
+  // A struct field value.
+  StructField,
+  // A array field value.
+  ArrayField,
+  // The length of the string.
+  StringLength,
+  // The string contents.
+  StringValue,
+  // The explicit string terminator.
+  StringTerminator,
+  // The length of the vector (# of items).
+  VectorLength,
+  // A "inline" value of a vector.
+  VectorValue,
+  // A vector value that points to another section.
+  VectorTableValue,
+  VectorStringValue,
+  VectorUnionValue,
+};
+
+struct BinaryRegionComment {
+  BinaryRegionStatus status = BinaryRegionStatus::OK;
+
+  // If status is non OK, this may be filled in with additional details.
+  std::string status_message;
+
+  BinaryRegionCommentType type = BinaryRegionCommentType::Unknown;
+
+  std::string name;
+
+  std::string default_value;
+
+  size_t index = 0;
+};
+
+struct BinaryRegion {
+  // Offset into the binary where this region begins.
+  uint64_t offset = 0;
+
+  // The length of this region in bytes.
+  uint64_t length = 0;
+
+  // The underlying datatype of this region
+  BinaryRegionType type = BinaryRegionType::Unknown;
+
+  // If `type` is an array/vector, this is the number of those types this region
+  // encompasses.
+  uint64_t array_length = 0;
+
+  // If the is an offset to some other region, this is what it points to. The
+  // offset is relative to overall binary, not to this region.
+  uint64_t points_to_offset = 0;
+
+  // The comment on the region.
+  BinaryRegionComment comment;
+};
+
+enum class BinarySectionType {
+  Unknown = 0,
+  Header = 1,
+  Table = 2,
+  RootTable = 3,
+  VTable = 4,
+  Struct = 5,
+  String = 6,
+  Vector = 7,
+  Union = 8,
+  Padding = 9,
+};
+
+// A section of the binary that is grouped together in some logical manner, and
+// often is pointed too by some other offset BinaryRegion. Sections include
+// `tables`, `vtables`, `strings`, `vectors`, etc..
+struct BinarySection {
+  // User-specified name of the section, if applicable.
+  std::string name;
+
+  // The type of this section.
+  BinarySectionType type = BinarySectionType::Unknown;
+
+  // The binary regions that make up this section, in order of their offsets.
+  std::vector<BinaryRegion> regions;
+};
+
+inline static BinaryRegionType GetRegionType(reflection::BaseType base_type) {
+  switch (base_type) {
+    case reflection::UType: return BinaryRegionType::UType;
+    case reflection::Bool: return BinaryRegionType::Uint8;
+    case reflection::Byte: return BinaryRegionType::Uint8;
+    case reflection::UByte: return BinaryRegionType::Uint8;
+    case reflection::Short: return BinaryRegionType::Int16;
+    case reflection::UShort: return BinaryRegionType::Uint16;
+    case reflection::Int: return BinaryRegionType::Uint32;
+    case reflection::UInt: return BinaryRegionType::Uint32;
+    case reflection::Long: return BinaryRegionType::Int64;
+    case reflection::ULong: return BinaryRegionType::Uint64;
+    case reflection::Float: return BinaryRegionType::Float;
+    case reflection::Double: return BinaryRegionType::Double;
+    default: return BinaryRegionType::Unknown;
+  }
+}
+
+inline static std::string ToString(const BinaryRegionType type) {
+  switch (type) {
+    case BinaryRegionType::UOffset: return "UOffset32";
+    case BinaryRegionType::SOffset: return "SOffset32";
+    case BinaryRegionType::VOffset: return "VOffset16";
+    case BinaryRegionType::Bool: return "bool";
+    case BinaryRegionType::Char: return "char";
+    case BinaryRegionType::Byte: return "int8_t";
+    case BinaryRegionType::Uint8: return "uint8_t";
+    case BinaryRegionType::Uint16: return "uint16_t";
+    case BinaryRegionType::Uint32: return "uint32_t";
+    case BinaryRegionType::Uint64: return "uint64_t"; ;
+    case BinaryRegionType::Int8: return "int8_t";
+    case BinaryRegionType::Int16: return "int16_t";
+    case BinaryRegionType::Int32: return "int32_t";
+    case BinaryRegionType::Int64: return "int64_t";
+    case BinaryRegionType::Double: return "double";
+    case BinaryRegionType::Float: return "float";
+    case BinaryRegionType::UType: return "UType8";
+    case BinaryRegionType::Unknown: return "?uint8_t";
+    default: return "todo";
+  }
+}
+
+class BinaryAnnotator {
+ public:
+  explicit BinaryAnnotator(const uint8_t *const bfbs,
+                           const uint64_t bfbs_length,
+                           const uint8_t *const binary,
+                           const uint64_t binary_length)
+      : bfbs_(bfbs),
+        bfbs_length_(bfbs_length),
+        schema_(reflection::GetSchema(bfbs)),
+        binary_(binary),
+        binary_length_(binary_length) {}
+
+  std::map<uint64_t, BinarySection> Annotate();
+
+ private:
+  struct VTable {
+    struct Entry {
+      const reflection::Field *field = nullptr;
+      uint16_t offset_from_table = 0;
+    };
+
+    // Field ID -> {field def, offset from table}
+    std::map<uint16_t, Entry> fields;
+
+    uint16_t vtable_size = 0;
+    uint16_t table_size = 0;
+  };
+
+  uint64_t BuildHeader(uint64_t offset);
+
+  void BuildVTable(uint64_t offset, const reflection::Object *table,
+                   uint64_t offset_of_referring_table);
+
+  void BuildTable(uint64_t offset, const BinarySectionType type,
+                  const reflection::Object *table);
+
+  uint64_t BuildStruct(uint64_t offset, std::vector<BinaryRegion> &regions,
+                       const reflection::Object *structure);
+
+  void BuildString(uint64_t offset, const reflection::Object *table,
+                   const reflection::Field *field);
+
+  void BuildVector(uint64_t offset, const reflection::Object *table,
+                   const reflection::Field *field, uint64_t parent_table_offset,
+                   const VTable &vtable);
+
+  std::string BuildUnion(uint64_t offset, uint8_t realized_type,
+                         const reflection::Field *field);
+
+  void FixMissingRegions();
+  void FixMissingSections();
+
+  inline bool IsValidOffset(const uint64_t offset) const {
+    return offset < binary_length_;
+  }
+
+  // Determines if performing a GetScalar request for `T` at `offset` would read
+  // passed the end of the binary.
+  template<typename T> inline bool IsValidRead(const uint64_t offset) const {
+    return IsValidRead(offset, sizeof(T));
+  }
+
+  inline bool IsValidRead(const uint64_t offset, const uint64_t length) const {
+    return length < binary_length_ && IsValidOffset(offset + length - 1);
+  }
+
+  // Calculate the number of bytes remaining from the given offset. If offset is
+  // > binary_length, 0 is returned.
+  uint64_t RemainingBytes(const uint64_t offset) const {
+    return IsValidOffset(offset) ? binary_length_ - offset : 0;
+  }
+
+  template<typename T>
+  flatbuffers::Optional<T> ReadScalar(const uint64_t offset) const {
+    if (!IsValidRead<T>(offset)) { return flatbuffers::nullopt; }
+
+    return flatbuffers::ReadScalar<T>(binary_ + offset);
+  }
+
+  // Adds the provided `section` keyed by the `offset` it occurs at. If a
+  // section is already added at that offset, it doesn't replace the exisiting
+  // one.
+  void AddSection(const uint64_t offset, const BinarySection &section) {
+    sections_.insert(std::make_pair(offset, section));
+  }
+
+  bool IsInlineField(const reflection::Field *const field) {
+    if (field->type()->base_type() == reflection::BaseType::Obj) {
+      return schema_->objects()->Get(field->type()->index())->is_struct();
+    }
+    return IsScalar(field->type()->base_type());
+  }
+
+  bool IsUnionType(const reflection::BaseType type) {
+    return (type == reflection::BaseType::UType ||
+            type == reflection::BaseType::Union);
+  }
+
+  bool IsUnionType(const reflection::Field *const field) {
+    return IsUnionType(field->type()->base_type()) &&
+           field->type()->index() >= 0;
+  }
+
+  bool IsValidUnionValue(const reflection::Field *const field,
+                         const uint8_t value) {
+    return IsUnionType(field) &&
+           IsValidUnionValue(field->type()->index(), value);
+  }
+
+  bool IsValidUnionValue(const uint32_t enum_id, const uint8_t value) {
+    if (enum_id >= schema_->enums()->size()) { return false; }
+
+    const reflection::Enum *enum_def = schema_->enums()->Get(enum_id);
+
+    if (enum_def == nullptr) { return false; }
+
+    return value < enum_def->values()->size();
+  }
+
+  uint64_t GetElementSize(const reflection::Field *const field) {
+    if (IsScalar(field->type()->element())) {
+      return GetTypeSize(field->type()->element());
+    }
+
+    switch (field->type()->element()) {
+      case reflection::BaseType::Obj: {
+        auto obj = schema_->objects()->Get(field->type()->index());
+        return obj->is_struct() ? obj->bytesize() : sizeof(uint32_t);
+      }
+      default: return sizeof(uint32_t);
+    }
+  }
+
+  bool ContainsSection(const uint64_t offset);
+
+  // The schema for the binary file
+  const uint8_t *bfbs_;
+  const uint64_t bfbs_length_;
+  const reflection::Schema *schema_;
+
+  // The binary data itself.
+  const uint8_t *binary_;
+  const uint64_t binary_length_;
+
+  // Map of binary offset to vtables, to dedupe vtables.
+  std::map<uint64_t, VTable> vtables_;
+
+  // The annotated binary sections, index by their absolute offset.
+  std::map<uint64_t, BinarySection> sections_;
+};
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_BINARY_ANNOTATOR_H_
\ No newline at end of file