aos/flatbuffers/static_flatbuffers.cc

#include "aos/flatbuffers/static_flatbuffers.h"

#include <stddef.h>
#include <stdint.h>

#include <algorithm>
#include <map>
#include <optional>
#include <ostream>
#include <utility>
#include <vector>

#include "absl/log/check.h"
#include "absl/log/log.h"
#include "absl/strings/numbers.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/str_format.h"
#include "absl/strings/str_join.h"
#include "absl/strings/str_replace.h"
#include "absl/strings/substitute.h"
#include "flatbuffers/base.h"
#include "flatbuffers/string.h"
#include "flatbuffers/vector.h"

#include "aos/flatbuffers/base.h"
#include "aos/json_to_flatbuffer.h"

namespace aos::fbs {
namespace {
// Represents a given field within a type with all of the data that we actually
// care about.
struct FieldData {
  // Field name.
  std::string name;
  // Whether it is an inline data type (scalar/struct vs vector/table/string).
  bool is_inline = true;
  // Whether the elements are inline (vector of ints vs vector of strings).
  bool elements_are_inline = true;
  // Whether this is a struct or not.
  bool is_struct = false;
  // Whether this is a repeated type (vector or string).
  bool is_repeated = false;
  // Full C++ type of this field.
  std::string full_type = "";
  // Full flatbuffer type for this field.
  // Only specified for Tables.
  std::optional<std::string> fbs_type = std::nullopt;
  // Size of this field in the inline field data (i.e., size of the field for
  // is_inline fields; 4 bytes for the offset for vectors/tables/strings).
  size_t inline_size = 0u;
  // Alignment of the inline data.
  size_t inline_alignment = 0u;
  // vtable offset of the field.
  size_t vtable_offset = 0u;
  // Size of the elements in the vector, if this is a vector.
  size_t element_size = 0u;
};

const reflection::Object *GetObject(const reflection::Schema *schema,
                                    const int index) {
  return (index == -1) ? schema->root_table() : schema->objects()->Get(index);
}

// Returns the flatbuffer field attribute with the specified name, if available.
std::optional<std::string_view> GetAttribute(const reflection::Field *field,
                                             std::string_view attribute) {
  if (!field->has_attributes()) {
    return std::nullopt;
  }
  const reflection::KeyValue *kv =
      field->attributes()->LookupByKey(attribute.data());
  if (kv == nullptr) {
    return std::nullopt;
  }
  return kv->value()->string_view();
}

// Returns the implied value of an attribute that specifies a length (i.e., 0 if
// the attribute is not specified; the integer value otherwise).
int64_t GetLengthAttributeOrZero(const reflection::Field *field,
                                 std::string_view attribute) {
  std::optional<std::string_view> str = GetAttribute(field, attribute);
  if (!str.has_value()) {
    return 0;
  }
  int64_t value;
  CHECK(absl::SimpleAtoi(str.value(), &value))
      << ": Field " << field->name()->string_view()
      << " must specify a positive integer for the " << attribute
      << " attribute. Got \"" << str.value() << "\".";
  CHECK_LE(0, value) << ": Field " << field->name()->string_view()
                     << " must have a non-negative " << attribute << ".";
  return value;
}

const std::string ScalarCppType(const reflection::BaseType type) {
  switch (type) {
    case reflection::BaseType::Bool:
      return "bool";
    case reflection::BaseType::Byte:
      return "int8_t";
    case reflection::BaseType::UByte:
      return "uint8_t";
    case reflection::BaseType::Short:
      return "int16_t";
    case reflection::BaseType::UShort:
      return "uint16_t";
    case reflection::BaseType::Int:
      return "int32_t";
    case reflection::BaseType::UInt:
      return "uint32_t";
    case reflection::BaseType::Long:
      return "int64_t";
    case reflection::BaseType::ULong:
      return "uint64_t";
    case reflection::BaseType::Float:
      return "float";
    case reflection::BaseType::Double:
      return "double";
    case reflection::BaseType::UType:
    case reflection::BaseType::String:
    case reflection::BaseType::Vector:
    case reflection::BaseType::Obj:
    case reflection::BaseType::None:
    case reflection::BaseType::Union:
    case reflection::BaseType::Array:
    case reflection::BaseType::MaxBaseType:
      LOG(FATAL) << ": Type " << reflection::EnumNameBaseType(type)
                 << " not a scalar.";
  }
  LOG(FATAL) << "Unreachable";
}

const std::string FlatbufferNameToCppName(const std::string_view input) {
  return absl::StrReplaceAll(input, {{".", "::"}});
}

const std::string AosNameForRawFlatbuffer(const std::string_view base_name) {
  return absl::StrCat(base_name, "Static");
}

const std::string IncludePathForFbs(
    std::string_view fbs_file, std::string_view include_suffix = "static") {
  // Special case for the reflection_generated.h, which is checked into the
  // repo.
  // Note that we *do* autogenerated the reflection_static.h but that because
  // it uses a special import path, we end up overriding the include anyways
  // (note that we could muck around with the paths on the bazel side to instead
  // get a cc_library with the correct include paths specified, although it is
  // not clear that that would be any simpler than the extra else-if).
  if (fbs_file == "reflection/reflection.fbs") {
    if (include_suffix == "generated") {
      return "flatbuffers/reflection_generated.h";
    } else if (include_suffix == "static") {
      return "aos/flatbuffers/reflection/reflection_static.h";
    } else {
      LOG(FATAL) << "This should be unreachable.";
    }
  }
  fbs_file.remove_suffix(4);
  return absl::StrCat(fbs_file, "_", include_suffix, ".h");
}

std::string ScalarOrEnumType(const reflection::Schema *schema,
                             const reflection::BaseType type, int index) {
  return (index < 0) ? ScalarCppType(type)
                     : FlatbufferNameToCppName(
                           schema->enums()->Get(index)->name()->string_view());
}

void PopulateTypeData(const reflection::Schema *schema,
                      const reflection::Field *field_fbs, FieldData *field) {
  VLOG(1) << aos::FlatbufferToJson(field_fbs);
  const reflection::Type *type = field_fbs->type();
  field->inline_size = type->base_size();
  field->inline_alignment = type->base_size();
  field->element_size = type->element_size();
  switch (type->base_type()) {
    case reflection::BaseType::Bool:
    case reflection::BaseType::Byte:
    case reflection::BaseType::UByte:
    case reflection::BaseType::Short:
    case reflection::BaseType::UShort:
    case reflection::BaseType::Int:
    case reflection::BaseType::UInt:
    case reflection::BaseType::Long:
    case reflection::BaseType::ULong:
    case reflection::BaseType::Float:
    case reflection::BaseType::Double:
      // We have a scalar field, so things are relatively
      // straightforwards.
      field->is_inline = true;
      field->elements_are_inline = true;
      field->is_struct = false;
      field->is_repeated = false;
      field->full_type =
          ScalarOrEnumType(schema, type->base_type(), type->index());
      return;
    case reflection::BaseType::String: {
      field->is_inline = false;
      field->elements_are_inline = true;
      field->is_struct = false;
      field->is_repeated = true;
      field->full_type =
          absl::StrFormat("::aos::fbs::String<%d>",
                          GetLengthAttributeOrZero(field_fbs, "static_length"));
      return;
    }
    case reflection::BaseType::Vector: {
      // We need to extract the name of the elements of the vector.
      std::string element_type;
      bool elements_are_inline = true;
      field->is_repeated = true;
      if (type->base_type() == reflection::BaseType::Vector) {
        switch (type->element()) {
          case reflection::BaseType::Obj: {
            const reflection::Object *element_object =
                GetObject(schema, type->index());
            element_type =
                FlatbufferNameToCppName(element_object->name()->string_view());
            elements_are_inline = element_object->is_struct();
            if (!element_object->is_struct()) {
              element_type = AosNameForRawFlatbuffer(element_type);
              field->fbs_type = element_object->name()->string_view();
            }
            break;
          }
          case reflection::BaseType::String:
            element_type =
                absl::StrFormat("::aos::fbs::String<%d>",
                                GetLengthAttributeOrZero(
                                    field_fbs, "static_vector_string_length"));
            elements_are_inline = false;
            break;
          case reflection::BaseType::Vector:
            LOG(FATAL) << "Vectors of vectors do not exist in flatbuffers.";
          default:
            element_type =
                ScalarOrEnumType(schema, type->element(), type->index());
        };
      }
      field->is_inline = false;
      field->elements_are_inline = elements_are_inline;
      field->is_struct = false;
      field->full_type =
          absl::StrFormat("::aos::fbs::Vector<%s, %d, %s, %s>", element_type,
                          GetLengthAttributeOrZero(field_fbs, "static_length"),
                          elements_are_inline ? "true" : "false",
                          GetAttribute(field_fbs, "force_align").value_or("0"));
      return;
    }
    case reflection::BaseType::Obj: {
      const reflection::Object *object = GetObject(schema, type->index());
      field->is_inline = object->is_struct();
      field->elements_are_inline = field->is_inline;
      field->is_struct = object->is_struct();
      field->is_repeated = false;
      const std::string flatbuffer_name =
          FlatbufferNameToCppName(object->name()->string_view());
      if (field->is_inline) {
        field->full_type = flatbuffer_name;
        field->inline_size = object->bytesize();
        field->inline_alignment = object->minalign();
      } else {
        field->fbs_type = object->name()->string_view();
        field->full_type = AosNameForRawFlatbuffer(flatbuffer_name);
      }
      return;
    }
    case reflection::BaseType::None:
    case reflection::BaseType::UType:
    case reflection::BaseType::Union:
    case reflection::BaseType::Array:
    case reflection::BaseType::MaxBaseType:
      LOG(FATAL) << ": Type " << reflection::EnumNameBaseType(type->base_type())
                 << " not supported currently.";
  };
}

std::string MakeMoveConstructor(std::string_view type_name) {
  return absl::StrFormat(R"code(
  // We need to provide a MoveConstructor to allow this table to be
  // used inside of vectors, but we do not want it readily available to
  // users. See TableMover for more details.
  %s(%s &&) = default;
  friend struct ::aos::fbs::internal::TableMover<%s>;
  )code",
                         type_name, type_name, type_name);
}

std::string MakeConstructor(std::string_view type_name) {
  const std::string constructor_body =
      R"code(
    CHECK_EQ(buffer.size(), kSize);
    CHECK_EQ(0u, reinterpret_cast<size_t>(buffer.data() + kAlignOffset) % kAlign);
    PopulateVtable();
)code";
  return absl::StrFormat(R"code(
  // Constructors for creating a flatbuffer object.
  // Users should typically use the Builder class to create these objects,
  // in order to allow it to populate the root table offset.

  // The buffer provided to these constructors should be aligned to kAlign
  // and kSize in length.
  // The parent/allocator may not be nullptr.
  %s(std::span<uint8_t> buffer, ::aos::fbs::ResizeableObject *parent) : Table(buffer, parent) {
    %s
  }
  %s(std::span<uint8_t> buffer, ::aos::fbs::Allocator *allocator) : Table(buffer, allocator) {
    %s
  }
  %s(std::span<uint8_t> buffer, ::std::unique_ptr<::aos::fbs::Allocator> allocator) : Table(buffer, ::std::move(allocator)) {
    %s
  }
)code",
                         type_name, constructor_body, type_name,
                         constructor_body, type_name, constructor_body);
}

std::string MemberName(const FieldData &data) {
  return absl::StrCat(data.name, "_");
}

std::string ObjectAbsoluteOffsetName(const FieldData &data) {
  return absl::StrCat("object_absolute_offset_", data.name);
}

std::string InlineAbsoluteOffsetName(const FieldData &data) {
  return absl::StrCat("kInlineAbsoluteOffset_", data.name);
}

// Generate the clear_* method for the requested field.
std::string MakeClearer(const FieldData &field) {
  std::string logical_clearer;
  if (!field.is_inline) {
    logical_clearer = MemberName(field) + ".reset();";
  }
  return absl::StrFormat(R"code(
  // Clears the %s field. This will cause has_%s() to return false.
  void clear_%s() {
    %s
    ClearField(%s, %d, %d);
  }
  )code",
                         field.name, field.name, field.name, logical_clearer,
                         InlineAbsoluteOffsetName(field), field.inline_size,
                         field.vtable_offset);
}

// Generate the has_* method for the requested field.
std::string MakeHaser(const FieldData &field) {
  return absl::StrFormat(R"code(
  // Returns true if the %s field is set and can be accessed.
  bool has_%s() const {
    return AsFlatbuffer().has_%s();
  }
  )code",
                         field.name, field.name, field.name);
}

// Generates the accessors for fields which are stored inline in the flatbuffer
// table (scalars, structs, and enums) .
std::string MakeInlineAccessors(const FieldData &field,
                                const size_t inline_absolute_offset) {
  constexpr size_t kVtablePointerSize = sizeof(soffset_t);
  CHECK_EQ(
      (inline_absolute_offset - kVtablePointerSize) % field.inline_alignment,
      0u)
      << ": Unaligned field " << field.name << " on " << field.full_type
      << " with inline offset of " << inline_absolute_offset
      << " and alignment of " << field.inline_alignment
      << " and an alignment offset of " << kVtablePointerSize;
  const std::string setter =
      absl::StrFormat(R"code(
  // Sets the %s field, causing it to be populated if it is not already.
  // This will populate the field even if the specified value is the default.
  void set_%s(const %s &value) {
    SetField<%s>(%s, %d, value);
  }
  )code",
                      field.name, field.name, field.full_type, field.full_type,
                      InlineAbsoluteOffsetName(field), field.vtable_offset);
  const std::string getters = absl::StrFormat(
      R"code(
  // Returns the value of %s if set; nullopt otherwise.
  std::optional<%s> %s() const {
    return has_%s() ? std::make_optional(Get<%s>(%s)) : std::nullopt;
  }
  // Returns a pointer to modify the %s field.
  // The pointer may be invalidated by mutations/movements of the underlying buffer.
  // Returns nullptr if the field is not set.
  %s* mutable_%s() {
    return has_%s() ? MutableGet<%s>(%s) : nullptr;
  }
  )code",
      field.name, field.full_type, field.name, field.name, field.full_type,
      InlineAbsoluteOffsetName(field), field.name, field.full_type, field.name,
      field.name, field.full_type, InlineAbsoluteOffsetName(field));
  const std::string clearer = MakeClearer(field);
  return setter + getters + clearer + MakeHaser(field);
}

// Generates the accessors for fields which are not inline fields and have an
// offset to the actual field content stored inline in the flatbuffer table.
std::string MakeOffsetDataAccessors(const FieldData &field) {
  const std::string setter = absl::Substitute(
      R"code(
  // Creates an empty object for the $0 field, which you can
  // then populate/modify as desired.
  // The field must not be populated yet.
  $1* add_$0() {
    CHECK(!$2.has_value());
    constexpr size_t kVtableIndex = $3;
    // If this object does not normally have its initial memory statically allocated,
    // allocate it now (this is used for zero-length vectors).
    if constexpr ($1::kPreallocatedSize == 0) {
      const size_t object_absolute_offset = $4;
      std::optional<std::span<uint8_t>> inserted_bytes =
          InsertBytes(buffer().data() + object_absolute_offset, $1::kSize, ::aos::fbs::SetZero::kYes);
      if (!inserted_bytes.has_value()) {
        return nullptr;
      }
      // Undo changes to the object absolute offset that will have been made by
      // the InsertBytes call.
      // The InsertBytes() call normally goes through and "fixes" any offsets
      // that will have been affected by the memory insertion. Unfortunately,
      // if this object currently takes up zero bytes then the InsertBytes()
      // cannot distinguish between this offset and the (identical) offsets for any other objects
      // that may have been "sharing" this location. The effect of this logic
      // is that the first object that gets populated at any given location will
      // bump all other objects to later. This is fine, although it does mean
      // that the order in which objects appear in memory may vary depending
      // on the order in which they are constructed (if they start out sharing a start pointer).
      $4 = object_absolute_offset;

      // Construct the *Static object that we will use for managing this subtable.
      $5.emplace(BufferForObject($4, $1::$7), this);
    } else {
      // Construct the *Static object that we will use for managing this subtable.
      $5.emplace(BufferForObject($4, $1::kSize), this);
    }
    // Actually set the appropriate fields in the flatbuffer memory itself.
    SetField<::flatbuffers::uoffset_t>($6, kVtableIndex, $4 - $6);
    return &$5.value().t;
  }
  )code",
      field.name,                       // $0
      field.full_type,                  // $1
      MemberName(field),                // $2
      field.vtable_offset,              // $3
      ObjectAbsoluteOffsetName(field),  // $4
      MemberName(field),                // $5
      InlineAbsoluteOffsetName(field),  // $6
      (field.elements_are_inline
           // When building vectors of inline elements, we want this object to
           // consume as much of the memory that was allocated as possible. This
           // lets the vector use the padding for storage, saving space.  Round
           // this down to the size of memory that the max number of elements
           // fit in perfectly so the padding after that isn't owned.
           ? "RoundedLength(inserted_bytes.value().size())"
           // For vectors of elements, we need to pad out the inline portion of
           // the vector storing offsets to the alignment of the actual elements
           // so we can insert elements at the end without having to allocate
           // padding.  This saves space in the long run, and lets us consume
           // the padding for offsets if needed.
           : "kSize")  // $7
  );
  const std::string getters = absl::Substitute(
      R"code(
  // Returns a pointer to the $0 field, if set. nullptr otherwise.
  const $1* $0() const {
    return $2.has_value() ? &$2.value().t : nullptr;
  }
  $1* mutable_$0() {
    return $2.has_value() ? &$2.value().t : nullptr;
  }
  )code",
      field.name,        // $0
      field.full_type,   // $1
      MemberName(field)  // $2
  );
  return setter + getters + MakeClearer(field) + MakeHaser(field);
}

std::string MakeAccessors(const FieldData &field,
                          size_t inline_absolute_offset) {
  return field.is_inline ? MakeInlineAccessors(field, inline_absolute_offset)
                         : MakeOffsetDataAccessors(field);
}

std::string MakeMembers(const FieldData &field,
                        std::string_view offset_data_absolute_offset,
                        size_t inline_absolute_offset) {
  if (field.is_inline) {
    return absl::StrFormat(
        R"code(
    // Offset from the start of the buffer to the inline data for the %s field.
    static constexpr size_t %s = %d;
    )code",
        field.name, InlineAbsoluteOffsetName(field), inline_absolute_offset);
  } else {
    return absl::StrFormat(
        R"code(
    // Members relating to the %s field.
    //
    // *Static object used for managing this subtable. Will be nullopt
    // when the field is not populated.
    // We use the TableMover to be able to make this object moveable.
    std::optional<::aos::fbs::internal::TableMover<%s>> %s;
    // Offset from the start of the buffer to the start of the actual
    // data for this field. Will be updated even when the table is not
    // populated, so that we know where to construct it when requested.
    static constexpr size_t kDefaultObjectAbsoluteOffset%s = %s;
    size_t %s = kDefaultObjectAbsoluteOffset%s;
    // Offset from the start of the buffer to the offset in the inline data for
    // this field.
    static constexpr size_t %s = %d;
    )code",
        field.name, field.full_type, MemberName(field), field.name,
        offset_data_absolute_offset, ObjectAbsoluteOffsetName(field),
        field.name, InlineAbsoluteOffsetName(field), inline_absolute_offset);
  }
}

std::string MakeFullClearer(const std::vector<FieldData> &fields) {
  std::vector<std::string> clearers;
  for (const FieldData &field : fields) {
    clearers.emplace_back(absl::StrFormat("clear_%s();", field.name));
  }
  return absl::StrFormat(R"code(
  // Clears every field of the table, removing any existing state.
  void Clear() { %s })code",
                         absl::StrJoin(clearers, "\n"));
}

// Creates the FromFlatbuffer() method that copies from a flatbuffer object API
// object (i.e., the FlatbufferT types).
std::string MakeObjectCopier(const std::vector<FieldData> &fields) {
  std::vector<std::string> copiers;
  for (const FieldData &field : fields) {
    if (field.is_struct) {
      // Structs are stored as unique_ptr<FooStruct>
      copiers.emplace_back(absl::StrFormat(R"code(
      if (other.%s) {
        set_%s(*other.%s);
      }
      )code",
                                           field.name, field.name, field.name));
    } else if (field.is_inline) {
      // Inline non-struct elements are stored as FooType.
      copiers.emplace_back(absl::StrFormat(R"code(
      set_%s(other.%s);
      )code",
                                           field.name, field.name));
    } else if (field.is_repeated) {
      // strings are stored as std::string's.
      // vectors are stored as std::vector's.
      copiers.emplace_back(absl::StrFormat(R"code(
      // Unconditionally copy strings/vectors, even if it will just end up
      // being 0-length (this maintains consistency with the flatbuffer Pack()
      // behavior).
      {
        %s* added_%s = add_%s();
        CHECK(added_%s != nullptr);
        if (!added_%s->FromFlatbuffer(other.%s)) {
          // Fail if we were unable to copy (e.g., if we tried to copy in a long
          // vector and do not have the space for it).
          return false;
        }
      }
      )code",
                                           field.full_type, field.name,
                                           field.name, field.name, field.name,
                                           field.name));
    } else {
      // Tables are stored as unique_ptr<FooTable>
      copiers.emplace_back(absl::StrFormat(R"code(
      if (other.%s) {
        %s* added_%s = add_%s();
        CHECK(added_%s != nullptr);
        if (!added_%s->FromFlatbuffer(*other.%s)) {
          // Fail if we were unable to copy (e.g., if we tried to copy in a long
          // vector and do not have the space for it).
          return false;
        }
      }
      )code",
                                           field.name, field.full_type,
                                           field.name, field.name, field.name,
                                           field.name, field.name));
    }
  }
  return absl::StrFormat(
      R"code(
  // Copies the contents of the provided flatbuffer into this flatbuffer,
  // returning true on success.
  // Because the Flatbuffer Object API does not provide any concept of an
  // optionally populated scalar field, all scalar fields will be populated
  // after a call to FromFlatbufferObject().
  // This is a deep copy, and will call FromFlatbufferObject on
  // any constituent objects.
  [[nodiscard]] bool FromFlatbuffer(const Flatbuffer::NativeTableType &other) {
    Clear();
    %s
    return true;
  }
  [[nodiscard]] bool FromFlatbuffer(const flatbuffers::unique_ptr<Flatbuffer::NativeTableType>& other) {
    return FromFlatbuffer(*other);
  }
)code",
      absl::StrJoin(copiers, "\n"));
}

// Creates the FromFlatbuffer() method that copies from an actual flatbuffer
// object.
std::string MakeCopier(const std::vector<FieldData> &fields) {
  std::vector<std::string> copiers;
  for (const FieldData &field : fields) {
    if (field.is_struct) {
      copiers.emplace_back(absl::StrFormat(R"code(
      if (other.has_%s()) {
        set_%s(*other.%s());
      }
      )code",
                                           field.name, field.name, field.name));
    } else if (field.is_inline) {
      copiers.emplace_back(absl::StrFormat(R"code(
      if (other.has_%s()) {
        set_%s(other.%s());
      }
      )code",
                                           field.name, field.name, field.name));
    } else {
      copiers.emplace_back(absl::StrFormat(R"code(
      if (other.has_%s()) {
        %s* added_%s = add_%s();
        CHECK(added_%s != nullptr);
        if (!added_%s->FromFlatbuffer(other.%s())) {
          // Fail if we were unable to copy (e.g., if we tried to copy in a long
          // vector and do not have the space for it).
          return false;
        }
      }
      )code",
                                           field.name, field.full_type,
                                           field.name, field.name, field.name,
                                           field.name, field.name));
    }
  }
  return absl::StrFormat(
      R"code(
  // Copies the contents of the provided flatbuffer into this flatbuffer,
  // returning true on success.
  // This is a deep copy, and will call FromFlatbuffer on any constituent
  // objects.
  [[nodiscard]] bool FromFlatbuffer(const Flatbuffer &other) {
    Clear();
    %s
    return true;
  }
  // Equivalent to FromFlatbuffer(const Flatbuffer&); this overload is provided
  // to ease implementation of the aos::fbs::Vector internals.
  [[nodiscard]] bool FromFlatbuffer(const Flatbuffer *other) {
    CHECK(other != nullptr);
    return FromFlatbuffer(*other);
  }
)code",
      absl::StrJoin(copiers, "\n"));
}

std::string MakeSubObjectList(const std::vector<FieldData> &fields) {
  size_t num_object_fields = 0;
  std::vector<std::string> object_offsets;
  std::vector<std::string> objects;
  std::vector<std::string> inline_offsets;
  for (const FieldData &field : fields) {
    if (!field.is_inline) {
      ++num_object_fields;
      object_offsets.push_back(
          absl::StrFormat("&%s", ObjectAbsoluteOffsetName(field)));
      objects.push_back(absl::StrFormat("&%s->t", MemberName(field)));
      inline_offsets.push_back(InlineAbsoluteOffsetName(field));
    }
  }
  if (num_object_fields == 0) {
    return R"code(
  // This object has no non-inline subobjects, so we don't have to do anything special.
  size_t NumberOfSubObjects() const final { return 0; }
  using ::aos::fbs::ResizeableObject::SubObject;
  SubObject GetSubObject(size_t) final { LOG(FATAL) << "No subobjects."; }
  )code";
  }
  return absl::StrFormat(R"code(
  size_t NumberOfSubObjects() const final { return %d; }
  using ::aos::fbs::ResizeableObject::SubObject;
  SubObject GetSubObject(size_t index) final {
    SubObject object;
    // Note: The below arrays are local variables rather than class members to
    // avoid having to deal with what happens to them if the object is moved.

    // Array of the members that we use for tracking where the buffers for
    // each subobject belong.
    // Pointers because these may need to be modified when memory is
    // inserted into the buffer.
    const std::array<size_t*, %d> subobject_object_offsets{%s};
    // Actual subobjects; note that the pointers will be invalid when the
    // field is not populated.
    const std::array<::aos::fbs::ResizeableObject*, %d> subobject_objects{%s};
    // Absolute offsets from the start of the buffer to where the inline
    // entry is for each table. These offsets do not need to change at
    // runtime (because memory is never inserted into the start of
    // a given table), but the offsets pointed to by these offsets
    // may need to be updated.
    const std::array<size_t, %d> subobject_inline_offsets{%s};
    object.inline_entry = MutableGet<::flatbuffers::uoffset_t>(subobject_inline_offsets[index]);
    object.object = (*object.inline_entry == 0) ? nullptr : subobject_objects[index];
    object.absolute_offset = subobject_object_offsets[index];
    return object;
  }
  )code",
                         num_object_fields, num_object_fields,
                         absl::StrJoin(object_offsets, ", "), num_object_fields,
                         absl::StrJoin(objects, ", "), num_object_fields,
                         absl::StrJoin(inline_offsets, ", "));
}

std::string AlignCppString(const std::string_view expression,
                           const std::string_view alignment,
                           const std::string_view offset) {
  return absl::StrCat("::aos::fbs::AlignOffset(", expression, ", ", alignment,
                      ", ", offset, ")");
}

std::string MakeInclude(std::string_view path, bool system = false) {
  return absl::StrFormat("#include %s%s%s\n", system ? "<" : "\"", path,
                         system ? ">" : "\"");
}

}  // namespace
GeneratedObject GenerateCodeForObject(const reflection::Schema *schema,
                                      int object_index) {
  return GenerateCodeForObject(schema, GetObject(schema, object_index));
}
GeneratedObject GenerateCodeForObject(const reflection::Schema *schema,
                                      const reflection::Object *object) {
  std::vector<FieldData> fields;
  for (const reflection::Field *field_fbs : *object->fields()) {
    if (field_fbs->deprecated()) {
      // Don't codegen anything for deprecated fields.
      continue;
    }
    FieldData field{.name = field_fbs->name()->str(),
                    .vtable_offset = field_fbs->offset()};
    PopulateTypeData(schema, field_fbs, &field);
    fields.push_back(field);
  }
  std::sort(fields.begin(), fields.end(),
            [](const FieldData &f1, const FieldData &f2) {
              return std::make_tuple(f1.inline_alignment, f1.element_size,
                                     f1.vtable_offset) >
                     std::make_tuple(f2.inline_alignment, f2.element_size,
                                     f2.vtable_offset);
            });
  const size_t nominal_min_align = object->minalign();
  std::string out_of_line_member_size = "";
  // inline_absolute_offset tracks the current position of the inline table
  // contents so that we can assign static offsets to each field.
  constexpr size_t kVtablePointerSize = sizeof(soffset_t);
  size_t inline_absolute_offset = kVtablePointerSize;
  // offset_data_relative_offset tracks the current size of the various
  // sub-tables/vectors/strings that get stored at the end of the buffer.
  // For simplicity, the offset data will start at a fully aligned offset
  // (which may be larger than the soffset_t at the start of the table).
  // Note that this is a string because it's irritating to actually pipe the
  // numbers for size/alignment up here, so we just accumulate them here and
  // then write the expression directly into the C++.
  const std::string offset_data_start_expression =
      "(kVtableStart + kVtableSize)";
  std::string offset_data_relative_offset = offset_data_start_expression;
  std::vector<std::string> accessors;
  std::vector<std::string> members;
  std::set<std::string> includes = {
      MakeInclude("optional", true),
      MakeInclude("aos/flatbuffers/static_table.h"),
      MakeInclude("aos/flatbuffers/static_vector.h")};
  for (const reflection::SchemaFile *file : *schema->fbs_files()) {
    includes.insert(
        MakeInclude(IncludePathForFbs(file->filename()->string_view())));
    includes.insert(MakeInclude(
        IncludePathForFbs(file->filename()->string_view(), "generated")));
    for (const flatbuffers::String *included : *file->included_filenames()) {
      includes.insert(MakeInclude(IncludePathForFbs(included->string_view())));
    }
  }
  std::vector<std::string> alignments;
  std::set<std::string> subobject_names;
  for (const FieldData &field : fields) {
    inline_absolute_offset = AlignOffset(
        inline_absolute_offset, field.inline_alignment, kVtablePointerSize);
    if (!field.is_inline) {
      alignments.push_back(field.full_type + "::kAlign");
      // We care about aligning each field relative to the alignment point in
      // this flatbuffer (which is kAlignOffset into the block of memory).  We
      // then need to report out the offset relative to the start, not the
      // alignment point.
      offset_data_relative_offset =
          AlignCppString(offset_data_relative_offset + " - kAlignOffset",
                         alignments.back(),
                         field.full_type + "::kAlignOffset") +
          " + kAlignOffset";
    } else {
      alignments.push_back(std::to_string(field.inline_alignment));
    }
    const std::string offset_data_absolute_offset = offset_data_relative_offset;
    accessors.emplace_back(MakeAccessors(field, inline_absolute_offset));
    members.emplace_back(MakeMembers(field, offset_data_absolute_offset,
                                     inline_absolute_offset));

    inline_absolute_offset += field.inline_size;
    if (!field.is_inline) {
      offset_data_relative_offset = absl::StrFormat(
          "kDefaultObjectAbsoluteOffset%s + %s::kPreallocatedSize", field.name,
          field.full_type);
    }
    if (field.fbs_type.has_value()) {
      // Is this not getting populate for the root schema?
      subobject_names.insert(field.fbs_type.value());
    }
  }

  const std::string alignment = absl::StrCat(
      "static constexpr size_t kAlign = std::max<size_t>({kMinAlign, ",
      absl::StrJoin(alignments, ", "), "});\n");
  // Same here, we want to align the end relative to the alignment point, but
  // then we want to report out the size including the offset.
  const std::string size = absl::StrCat(
      "static constexpr size_t kSize = ",
      AlignCppString(offset_data_relative_offset + " - kAlignOffset", "kAlign",
                     "kAlignOffset"),
      " + kAlignOffset;");
  const size_t inline_data_size = inline_absolute_offset;
  const std::string constants = absl::StrFormat(
      R"code(
  // Space taken up by the inline portion of the flatbuffer table data, in
  // bytes.
  static constexpr size_t kInlineDataSize = %d;
  // Space taken up by the vtable for this object, in bytes.
  static constexpr size_t kVtableSize =
     sizeof(::flatbuffers::voffset_t) * (2 + %d);
  // Offset from the start of the internal memory buffer to the start of the
  // vtable.
  static constexpr size_t kVtableStart = ::aos::fbs::AlignOffset(
      kInlineDataSize, alignof(::flatbuffers::voffset_t));
  // Required alignment of this object. The buffer that this object gets
  // constructed into must be aligned to this value.
  %s
  // Offset into this object to measure the alignment at.
  static constexpr size_t kAlignOffset = sizeof(::flatbuffers::soffset_t);
  static_assert(
      %d <= kAlign,
      "Flatbuffer schema minalign should not exceed our required alignment.");
  // Offset from the start of the memory buffer to the start of any out-of-line
  // data (subtables, vectors, strings).
  static constexpr size_t kOffsetDataStart = %s;
  // Various overrides to support the Table parent class.
  size_t FixedVtableOffset() const final { return kVtableStart; }
  size_t VtableSize() const final { return kVtableSize; }
  size_t InlineTableSize() const final { return kInlineDataSize; }
  size_t OffsetDataStart() const final { return kOffsetDataStart; }
  size_t Alignment() const final { return kAlign; }
  // Exposes the name of the flatbuffer type to allow interchangeable use
  // of the Flatbuffer and FlatbufferStatic types in various AOS methods.
  static const char *GetFullyQualifiedName() {
    return Flatbuffer::GetFullyQualifiedName();
  }
)code",
      inline_data_size, object->fields()->size(), alignment, nominal_min_align,
      offset_data_start_expression);
  const std::string_view fbs_type_name = object->name()->string_view();
  const std::string type_namespace = FlatbufferNameToCppName(
      fbs_type_name.substr(0, fbs_type_name.find_last_of(".")));
  const std::string type_name = AosNameForRawFlatbuffer(
      fbs_type_name.substr(fbs_type_name.find_last_of(".") + 1));
  const std::string object_code = absl::StrFormat(
      R"code(
namespace %s {
class %s : public ::aos::fbs::Table {
  public:
  // The underlying "raw" flatbuffer type for this type.
  typedef %s Flatbuffer;
  typedef flatbuffers::unique_ptr<Flatbuffer::NativeTableType> FlatbufferObjectType;
  // Returns this object as a flatbuffer type. This reference may not be valid
  // following mutations to the underlying flatbuffer, due to how memory may get
  // may get moved around.
  const Flatbuffer &AsFlatbuffer() const { return *GetFlatbuffer<Flatbuffer>(); }
%s
%s
  virtual ~%s() {}
%s
%s
%s
%s
  private:
%s
%s
%s
 public:
  // Nominal size of this object, in bytes. The object may grow beyond this
  // size, but will always start at this size and so the initial buffer must
  // match this size.
  %s
  // Always statically allocate memory for tables (set for consistency with
  // static_vector.h).
  static constexpr size_t kPreallocatedSize = kSize;
  // Size required for a buffer that includes a root table offset at the start.
  static constexpr size_t kRootSize =
      ::aos::fbs::AlignOffset(kSize + sizeof(::flatbuffers::uoffset_t), kAlign);
};
}
  )code",
      type_namespace, type_name, FlatbufferNameToCppName(fbs_type_name),
      constants, MakeConstructor(type_name), type_name,
      absl::StrJoin(accessors, ""), MakeFullClearer(fields), MakeCopier(fields),
      MakeObjectCopier(fields), MakeMoveConstructor(type_name),
      absl::StrJoin(members, ""), MakeSubObjectList(fields), size);

  GeneratedObject result;
  result.name = fbs_type_name;
  result.include_declarations = includes;
  result.code = object_code;
  result.subobjects = subobject_names;
  return result;
}

namespace {

// Generated C++ code for an entire fbs file.
// This includes all of the actual C++ code that will be written to a file (call
// GenerateCode() to actually get the desired contents of the file).
struct GeneratedCode {
  // Prefix (for include guards).
  std::string contents_prefix;
  // Full set of required #include declarations.
  std::set<std::string> include_declarations;
  // Ordered list of objects (order is necessary to ensure that any dependencies
  // between objects are managed correctly).
  std::vector<GeneratedObject> objects;
  // Suffix (for include guards).
  std::string contents_suffix;

  // Combine the above things into the string that actually needs to be written
  // to a file.
  std::string GenerateCode() const;
  // Combines the code for multiple objects into one.
  static GeneratedCode MergeCode(const std::vector<GeneratedObject> &objects);
};

std::string GeneratedCode::GenerateCode() const {
  std::string result =
      contents_prefix + absl::StrJoin(include_declarations, "");
  for (const auto &object : objects) {
    result += object.code;
  }
  result += contents_suffix;
  return result;
}

GeneratedCode GeneratedCode::MergeCode(
    const std::vector<GeneratedObject> &objects) {
  GeneratedCode result;
  // TODO(james): Should we use #ifdef include guards instead?
  result.contents_prefix =
      "#pragma once\n// This is a generated file. Do not modify.\n";
  // We need to get the ordering of objects correct in order to ensure that
  // depended-on objects appear before their dependees.
  // In order to do this, we:
  // 1) Assume that any objects not in the provided vector must exist in
  //    #includes and so can be ignored.
  // 2) Create a list of all the objects we have been provided but which we have
  //    not yet added to the results vector.
  // 3) Until said list is empty, we iterate over it and find any object(s)
  //    which have no dependencies in the list itself, and add them to the
  //    result.
  // We aren't going to worry about efficient graph traversal here or anything.
  // We also don't currently attempt to support circular dependencies.
  std::map<std::string_view, const GeneratedObject *> remaining_objects;
  for (const auto &object : objects) {
    remaining_objects[object.name] = &object;
  }
  while (!remaining_objects.empty()) {
    std::string_view to_remove;
    for (const auto &pair : remaining_objects) {
      bool has_dependencies = false;
      for (const std::string_view subobject : pair.second->subobjects) {
        if (remaining_objects.contains(subobject)) {
          has_dependencies = true;
        }
      }
      if (has_dependencies) {
        continue;
      }
      to_remove = pair.first;
      result.objects.push_back(*pair.second);
      result.include_declarations.insert(
          pair.second->include_declarations.begin(),
          pair.second->include_declarations.end());
      break;
    }
    // In order to support circular dependencies, two main things have to
    // change:
    // 1. We have to dynamically allow depopulating table members (rather than
    //    just supporting dynamically lengthed vectors).
    // 2. Some of the codegen needs to be tweaked so that we can have the
    // generated
    //    C++ classes depend on one another.
    CHECK(!to_remove.empty())
        << ": Circular dependencies in flatbuffers schemas are not supported.";
    CHECK_EQ(1u, remaining_objects.erase(to_remove))
        << ": Failed to remove " << to_remove;
  }
  return result;
}
}  // namespace

std::string GenerateCodeForRootTableFile(const reflection::Schema *schema,
                                         std::string_view file_hint) {
  const reflection::Object *root_object = GetObject(schema, -1);
  const std::string_view root_file =
      (root_object == nullptr) ? file_hint
                               : root_object->declaration_file()->string_view();
  std::vector<GeneratedObject> objects;
  if (root_object != nullptr) {
    objects.push_back(GenerateCodeForObject(schema, root_object));
  }
  for (const reflection::Object *object : *schema->objects()) {
    if (object->is_struct()) {
      continue;
    }
    if (object->declaration_file()->string_view() == root_file) {
      objects.push_back(GenerateCodeForObject(schema, object));
    }
  }
  return GeneratedCode::MergeCode(objects).GenerateCode();
}
}  // namespace aos::fbs