// Protocol Buffers - Google's data interchange format
// Copyright 2023 Google LLC.  All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd

#include "upb/mini_descriptor/decode.h"

#include <inttypes.h>
#include <stdalign.h>
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "upb/base/descriptor_constants.h"
#include "upb/base/status.h"
#include "upb/base/string_view.h"
#include "upb/mem/alloc.h"
#include "upb/mem/arena.h"
#include "upb/message/internal/map_entry.h"
#include "upb/message/internal/types.h"
#include "upb/mini_descriptor/internal/base92.h"
#include "upb/mini_descriptor/internal/decoder.h"
#include "upb/mini_descriptor/internal/modifiers.h"
#include "upb/mini_descriptor/internal/wire_constants.h"
#include "upb/mini_table/extension.h"
#include "upb/mini_table/field.h"
#include "upb/mini_table/internal/field.h"
#include "upb/mini_table/internal/message.h"
#include "upb/mini_table/internal/sub.h"
#include "upb/mini_table/message.h"
#include "upb/mini_table/sub.h"

// Our awkward dance for including fasttable only when it is enabled.
#include "upb/port/def.inc"
#if UPB_FASTTABLE
#define UPB_INCLUDE_FAST_DECODE
#endif
#include "upb/port/undef.inc"

#ifdef UPB_INCLUDE_FAST_DECODE
#include "upb/wire/decode_fast/function_array.h"
#include "upb/wire/decode_fast/select.h"
#endif

#undef UPB_INCLUDE_FAST_DECODE

// Must be last.
#include "upb/port/def.inc"

// We reserve unused hasbits to make room for upb_Message fields.
#define kUpb_Reserved_Hasbytes sizeof(struct upb_Message)

// 64 is the first hasbit that we currently use.
#define kUpb_Reserved_Hasbits (kUpb_Reserved_Hasbytes * 8)

#define kUpb_OneOfLayoutItem_IndexSentinel ((uint16_t)-1)

// Stores the field number of the present value of the oneof
#define kUpb_OneOf_CaseFieldRep (kUpb_FieldRep_4Byte)

// The maximum field number that can be encoded on the wire.
// Note that this limit does not apply to MessageSet, which can have field
// numbers up to INT32_MAX.
#define kUpb_MaxFieldNumber ((1 << 29) - 1)

typedef struct {
  // Index of the corresponding field. The field's offset will be the index of
  // the next field in a linked list.
  uint16_t field_index;
  // This enum is stored in bytes to avoid trailing padding while preserving
  // two-byte alignment.
  uint8_t /* upb_FieldRep*/ rep;
} upb_OneOfLayoutItem;

typedef struct {
  upb_OneOfLayoutItem* data;
  size_t size;
  size_t buf_capacity_bytes;
} upb_OneOfLayoutItemVector;

typedef struct {
  upb_MdDecoder base;
  upb_MiniTableField* fields;
  upb_MiniTablePlatform platform;
  upb_OneOfLayoutItemVector oneofs;
  upb_Arena* arena;
  // Initially tracks the count of each field rep type; then, during assignment,
  // tracks the base offset for the next processed field of the given rep.
  uint16_t rep_counts_offsets[kUpb_FieldRep_Max + 1];
  uint32_t sub_count;
  bool is_extension;

  // When building fasttables, we don't know the total size of the table until
  // we're done decoding, so we allocate a temporary table on the stack and
  // allocate the real thing from the arena later.
  upb_MiniTable table;
} upb_MtDecoder;

// In each field's offset, we temporarily store a presence classifier:
enum PresenceClass {
  kNoPresence = 0,
  kHasbitPresence = 1,
  kRequiredPresence = 2,
  kOneofBase = 3,
  // Negative values refer to a specific oneof with that number.  Positive
  // values >= kOneofBase indicate that this field is in a oneof, and specify
  // the next field in this oneof's linked list.
};

static bool upb_MtDecoder_FieldIsPackable(upb_MiniTableField* field) {
  return (field->UPB_PRIVATE(mode) & kUpb_FieldMode_Array) &&
         upb_FieldType_IsPackable(field->UPB_PRIVATE(descriptortype));
}

typedef struct {
  uint16_t submsg_count;
  uint16_t subenum_count;
} upb_SubCounts;

static void upb_MiniTable_SetTypeAndSub(upb_MtDecoder* d,
                                        upb_MiniTableField* field,
                                        upb_FieldType type,
                                        uint64_t msg_modifiers,
                                        bool is_proto3_enum) {
  if (is_proto3_enum) {
    UPB_ASSERT(type == kUpb_FieldType_Enum);
    type = kUpb_FieldType_Int32;
    field->UPB_PRIVATE(mode) |= kUpb_LabelFlags_IsAlternate;
  } else if (type == kUpb_FieldType_String &&
             !(msg_modifiers & kUpb_MessageModifier_ValidateUtf8)) {
    type = kUpb_FieldType_Bytes;
    field->UPB_PRIVATE(mode) |= kUpb_LabelFlags_IsAlternate;
  }

  field->UPB_PRIVATE(descriptortype) = type;

  if (upb_MtDecoder_FieldIsPackable(field) &&
      (msg_modifiers & kUpb_MessageModifier_DefaultIsPacked)) {
    field->UPB_PRIVATE(mode) |= kUpb_LabelFlags_IsPacked;
  }

  // We initially set `submsg_ofs` to the index of the sub in the list of subs.
  // Later, we'll update it to be a relative byte offset.
  if (type == kUpb_FieldType_Message || type == kUpb_FieldType_Group ||
      type == kUpb_FieldType_Enum) {
    field->UPB_PRIVATE(submsg_ofs) = d->sub_count++;
  } else {
    field->UPB_PRIVATE(submsg_ofs) = kUpb_NoSub;
  }
}

static const char kUpb_EncodedToType[] = {
    [kUpb_EncodedType_Double] = kUpb_FieldType_Double,
    [kUpb_EncodedType_Float] = kUpb_FieldType_Float,
    [kUpb_EncodedType_Int64] = kUpb_FieldType_Int64,
    [kUpb_EncodedType_UInt64] = kUpb_FieldType_UInt64,
    [kUpb_EncodedType_Int32] = kUpb_FieldType_Int32,
    [kUpb_EncodedType_Fixed64] = kUpb_FieldType_Fixed64,
    [kUpb_EncodedType_Fixed32] = kUpb_FieldType_Fixed32,
    [kUpb_EncodedType_Bool] = kUpb_FieldType_Bool,
    [kUpb_EncodedType_String] = kUpb_FieldType_String,
    [kUpb_EncodedType_Group] = kUpb_FieldType_Group,
    [kUpb_EncodedType_Message] = kUpb_FieldType_Message,
    [kUpb_EncodedType_Bytes] = kUpb_FieldType_Bytes,
    [kUpb_EncodedType_UInt32] = kUpb_FieldType_UInt32,
    [kUpb_EncodedType_OpenEnum] = kUpb_FieldType_Enum,
    [kUpb_EncodedType_SFixed32] = kUpb_FieldType_SFixed32,
    [kUpb_EncodedType_SFixed64] = kUpb_FieldType_SFixed64,
    [kUpb_EncodedType_SInt32] = kUpb_FieldType_SInt32,
    [kUpb_EncodedType_SInt64] = kUpb_FieldType_SInt64,
    [kUpb_EncodedType_ClosedEnum] = kUpb_FieldType_Enum,
};

static void upb_MiniTable_SetField(upb_MtDecoder* d, uint8_t ch,
                                   upb_MiniTableField* field,
                                   uint64_t msg_modifiers) {
  static const char kUpb_EncodedToFieldRep[] = {
      [kUpb_EncodedType_Double] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_Float] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_Int64] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_UInt64] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_Int32] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_Fixed64] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_Fixed32] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_Bool] = kUpb_FieldRep_1Byte,
      [kUpb_EncodedType_String] = kUpb_FieldRep_StringView,
      [kUpb_EncodedType_Bytes] = kUpb_FieldRep_StringView,
      [kUpb_EncodedType_UInt32] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_OpenEnum] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_SFixed32] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_SFixed64] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_SInt32] = kUpb_FieldRep_4Byte,
      [kUpb_EncodedType_SInt64] = kUpb_FieldRep_8Byte,
      [kUpb_EncodedType_ClosedEnum] = kUpb_FieldRep_4Byte,
  };

  char pointer_rep = d->platform == kUpb_MiniTablePlatform_32Bit
                         ? kUpb_FieldRep_4Byte
                         : kUpb_FieldRep_8Byte;

  int8_t type = _upb_FromBase92(ch);
  if (ch >= _upb_ToBase92(kUpb_EncodedType_RepeatedBase)) {
    type -= kUpb_EncodedType_RepeatedBase;
    field->UPB_PRIVATE(mode) = kUpb_FieldMode_Array;
    field->UPB_PRIVATE(mode) |= pointer_rep << kUpb_FieldRep_Shift;
    field->UPB_PRIVATE(offset) = kNoPresence;
  } else {
    field->UPB_PRIVATE(mode) = kUpb_FieldMode_Scalar;
    field->UPB_PRIVATE(offset) = kHasbitPresence;
    if (type == kUpb_EncodedType_Group || type == kUpb_EncodedType_Message) {
      field->UPB_PRIVATE(mode) |= pointer_rep << kUpb_FieldRep_Shift;
    } else if ((unsigned long)type >= sizeof(kUpb_EncodedToFieldRep)) {
      upb_MdDecoder_ErrorJmp(&d->base, "Invalid field type: %d", (int)type);
    } else {
      field->UPB_PRIVATE(mode) |= kUpb_EncodedToFieldRep[type]
                                  << kUpb_FieldRep_Shift;
    }
  }
  if ((unsigned long)type >= sizeof(kUpb_EncodedToType)) {
    upb_MdDecoder_ErrorJmp(&d->base, "Invalid field type: %d", (int)type);
  }
  upb_MiniTable_SetTypeAndSub(d, field, kUpb_EncodedToType[type], msg_modifiers,
                              type == kUpb_EncodedType_OpenEnum);
}

static void upb_MtDecoder_ModifyField(upb_MtDecoder* d,
                                      uint32_t message_modifiers,
                                      uint32_t field_modifiers,
                                      upb_MiniTableField* field) {
  if (field_modifiers & kUpb_EncodedFieldModifier_FlipPacked) {
    if (!upb_MtDecoder_FieldIsPackable(field)) {
      upb_MdDecoder_ErrorJmp(&d->base,
                             "Cannot flip packed on unpackable field %" PRIu32,
                             upb_MiniTableField_Number(field));
    }
    field->UPB_PRIVATE(mode) ^= kUpb_LabelFlags_IsPacked;
  }

  if (field_modifiers & kUpb_EncodedFieldModifier_FlipValidateUtf8) {
    if (field->UPB_PRIVATE(descriptortype) != kUpb_FieldType_Bytes ||
        !(field->UPB_PRIVATE(mode) & kUpb_LabelFlags_IsAlternate)) {
      upb_MdDecoder_ErrorJmp(&d->base,
                             "Cannot flip ValidateUtf8 on field %" PRIu32
                             ", type=%d, mode=%d",
                             upb_MiniTableField_Number(field),
                             (int)field->UPB_PRIVATE(descriptortype),
                             (int)field->UPB_PRIVATE(mode));
    }
    field->UPB_PRIVATE(descriptortype) = kUpb_FieldType_String;
    field->UPB_PRIVATE(mode) &= ~kUpb_LabelFlags_IsAlternate;
  }

  bool singular = field_modifiers & kUpb_EncodedFieldModifier_IsProto3Singular;
  bool required = field_modifiers & kUpb_EncodedFieldModifier_IsRequired;

  // Validate.
  if ((singular || required) && field->UPB_PRIVATE(offset) != kHasbitPresence) {
    upb_MdDecoder_ErrorJmp(&d->base,
                           "Invalid modifier(s) for repeated field %" PRIu32,
                           upb_MiniTableField_Number(field));
  }
  if (singular && required) {
    upb_MdDecoder_ErrorJmp(
        &d->base, "Field %" PRIu32 " cannot be both singular and required",
        upb_MiniTableField_Number(field));
  }

  if (singular && upb_MiniTableField_IsSubMessage(field)) {
    upb_MdDecoder_ErrorJmp(&d->base,
                           "Field %" PRIu32 " cannot be a singular submessage",
                           upb_MiniTableField_Number(field));
  }

  if (singular) field->UPB_PRIVATE(offset) = kNoPresence;
  if (required) {
    field->UPB_PRIVATE(offset) = kRequiredPresence;
  }
}

static void upb_MtDecoder_PushOneof(upb_MtDecoder* d,
                                    upb_OneOfLayoutItem item) {
  if (item.field_index == kUpb_OneOfLayoutItem_IndexSentinel) {
    upb_MdDecoder_ErrorJmp(&d->base, "Empty oneof");
  }
  if ((d->oneofs.size + 1) * sizeof(*d->oneofs.data) >
      d->oneofs.buf_capacity_bytes) {
    size_t new_cap = UPB_MAX(8, d->oneofs.size * 2) * sizeof(*d->oneofs.data);
    d->oneofs.data =
        upb_grealloc(d->oneofs.data, d->oneofs.buf_capacity_bytes, new_cap);
    upb_MdDecoder_CheckOutOfMemory(&d->base, d->oneofs.data);
    d->oneofs.buf_capacity_bytes = new_cap;
  }
  item.field_index -= kOneofBase;

  d->rep_counts_offsets[kUpb_OneOf_CaseFieldRep]++;
  d->rep_counts_offsets[item.rep]++;
  d->oneofs.data[d->oneofs.size++] = item;
}

static size_t upb_MtDecoder_SizeOfRep(upb_FieldRep rep,
                                      upb_MiniTablePlatform platform) {
  enum { string_view_size_32 = 8, string_view_size_64 = 16 };
  UPB_STATIC_ASSERT(sizeof(upb_StringView) ==
                        UPB_SIZE(string_view_size_32, string_view_size_64),
                    "StringView size mismatch");
  static const uint8_t kRepToSize32[] = {
      [kUpb_FieldRep_1Byte] = 1,
      [kUpb_FieldRep_4Byte] = 4,
      [kUpb_FieldRep_StringView] = string_view_size_32,
      [kUpb_FieldRep_8Byte] = 8,
  };
  static const uint8_t kRepToSize64[] = {
      [kUpb_FieldRep_1Byte] = 1,
      [kUpb_FieldRep_4Byte] = 4,
      [kUpb_FieldRep_StringView] = string_view_size_64,
      [kUpb_FieldRep_8Byte] = 8,
  };
  return platform == kUpb_MiniTablePlatform_32Bit ? kRepToSize32[rep]
                                                  : kRepToSize64[rep];
}

static size_t upb_MtDecoder_AlignOfRep(upb_FieldRep rep,
                                       upb_MiniTablePlatform platform) {
  enum { string_view_align_32 = 4, string_view_align_64 = 8 };
  UPB_STATIC_ASSERT(UPB_ALIGN_OF(upb_StringView) ==
                        UPB_SIZE(string_view_align_32, string_view_align_64),
                    "StringView size mismatch");

  static const uint8_t kRepToAlign32[] = {
      [kUpb_FieldRep_1Byte] = 1,
      [kUpb_FieldRep_4Byte] = 4,
      [kUpb_FieldRep_StringView] = string_view_align_32,
      [kUpb_FieldRep_8Byte] = 8,
  };
  static const uint8_t kRepToAlign64[] = {
      [kUpb_FieldRep_1Byte] = 1,
      [kUpb_FieldRep_4Byte] = 4,
      [kUpb_FieldRep_StringView] = string_view_align_64,
      [kUpb_FieldRep_8Byte] = 8,
  };
  return platform == kUpb_MiniTablePlatform_32Bit ? kRepToAlign32[rep]
                                                  : kRepToAlign64[rep];
}

static const char* upb_MtDecoder_DecodeOneofField(upb_MtDecoder* d,
                                                  const char* ptr,
                                                  char first_ch,
                                                  upb_OneOfLayoutItem* item) {
  uint32_t field_num;
  ptr = upb_MdDecoder_DecodeBase92Varint(
      &d->base, ptr, first_ch, kUpb_EncodedValue_MinOneofField,
      kUpb_EncodedValue_MaxOneofField, &field_num);
  upb_MiniTableField* f =
      (void*)upb_MiniTable_FindFieldByNumber(&d->table, field_num);

  if (!f) {
    upb_MdDecoder_ErrorJmp(&d->base,
                           "Couldn't add field number %" PRIu32
                           " to oneof, no such field number.",
                           field_num);
  }
  if (f->UPB_PRIVATE(offset) != kHasbitPresence) {
    upb_MdDecoder_ErrorJmp(
        &d->base,
        "Cannot add repeated, required, or singular field %" PRIu32
        " to oneof.",
        field_num);
  }

  // Oneof storage must be large enough to accommodate the largest member.
  int rep = f->UPB_PRIVATE(mode) >> kUpb_FieldRep_Shift;
  size_t new_size = upb_MtDecoder_SizeOfRep(rep, d->platform);
  size_t new_align = upb_MtDecoder_AlignOfRep(rep, d->platform);
  size_t current_size = upb_MtDecoder_SizeOfRep(item->rep, d->platform);
  size_t current_align = upb_MtDecoder_AlignOfRep(item->rep, d->platform);

  if (new_size > current_size ||
      (new_size == current_size && new_align > current_align)) {
    UPB_ASSERT(new_align >= current_align);
    item->rep = rep;
  } else {
    UPB_ASSERT(current_align >= new_align);
  }
  // Prepend this field to the linked list.
  f->UPB_PRIVATE(offset) = item->field_index;
  item->field_index = (f - d->fields) + kOneofBase;
  return ptr;
}

static const char* upb_MtDecoder_DecodeOneofs(upb_MtDecoder* d,
                                              const char* ptr) {
  upb_OneOfLayoutItem item = {
      .rep = 0, .field_index = kUpb_OneOfLayoutItem_IndexSentinel};
  while (ptr < d->base.end) {
    char ch = *ptr++;
    if (ch == kUpb_EncodedValue_FieldSeparator) {
      // Field separator, no action needed.
    } else if (ch == kUpb_EncodedValue_OneofSeparator) {
      // End of oneof.
      upb_MtDecoder_PushOneof(d, item);
      item.field_index =
          kUpb_OneOfLayoutItem_IndexSentinel;  // Move to next oneof.
    } else {
      ptr = upb_MtDecoder_DecodeOneofField(d, ptr, ch, &item);
    }
  }

  // Push final oneof.
  upb_MtDecoder_PushOneof(d, item);
  return ptr;
}

static const char* upb_MtDecoder_ParseModifier(upb_MtDecoder* d,
                                               const char* ptr, char first_ch,
                                               upb_MiniTableField* last_field,
                                               uint64_t* msg_modifiers) {
  uint32_t mod;
  ptr = upb_MdDecoder_DecodeBase92Varint(&d->base, ptr, first_ch,
                                         kUpb_EncodedValue_MinModifier,
                                         kUpb_EncodedValue_MaxModifier, &mod);
  if (last_field) {
    upb_MtDecoder_ModifyField(d, *msg_modifiers, mod, last_field);
  } else {
    if (d->is_extension) {
      upb_MdDecoder_ErrorJmp(&d->base,
                             "Extensions cannot have message modifiers");
    }
    *msg_modifiers = mod;
  }

  return ptr;
}

size_t upb_MtDecoder_PtrSize(upb_MtDecoder* d) {
  return d->platform == kUpb_MiniTablePlatform_32Bit ? 4 : 8;
}

static void upb_MtDecoder_AllocateSubs(upb_MtDecoder* d,
                                       upb_MiniTableSubInternal* subs) {
  // The `ofs` variable tracks byte offset between the current field and the
  // current entry in the `subs` array. Whenever we move to the next entry in
  // the `fields` array, the offset decreases by the size of the field, but
  // whenever we move to the next entry in the `subs` array, the offset
  // *increases* by the size of the entry in the `subs` array.
  UPB_ASSERT((char*)subs >= (char*)d->fields);
  size_t ofs = (char*)subs - (char*)d->fields;
  uintptr_t ptr_size = upb_MtDecoder_PtrSize(d);
  for (int i = 0; i < d->table.UPB_PRIVATE(field_count);
       i++, ofs -= sizeof(upb_MiniTableField)) {
    upb_MiniTableField* f = &d->fields[i];
    if (f->UPB_PRIVATE(submsg_ofs) == kUpb_NoSub) continue;
    size_t u32_ofs = ofs / kUpb_SubmsgOffsetBytes;
    UPB_ASSERT((ofs % 4) == 0);
    UPB_ASSERT((i * sizeof(upb_MiniTableField) + ofs) % ptr_size == 0);
    if (u32_ofs > UINT16_MAX) {
      upb_MdDecoder_ErrorJmp(&d->base, "Submessage offset overflow");
    }
    f->UPB_PRIVATE(submsg_ofs) = u32_ofs;
    ofs += ptr_size;
  }
}

static const char* upb_MtDecoder_Parse(upb_MtDecoder* d, const char* ptr,
                                       size_t len, void* fields,
                                       size_t field_size,
                                       uint16_t* field_count) {
  uint64_t msg_modifiers = 0;
  uint32_t last_field_number = 0;
  upb_MiniTableField* last_field = NULL;
  bool need_dense_below = !d->is_extension;

  d->base.end = UPB_PTRADD(ptr, len);

  while (ptr < d->base.end) {
    char ch = *ptr++;
    if (ch <= kUpb_EncodedValue_MaxField) {
      if (d->is_extension && last_field) {
        // For extensions, consume only a single field and then return.
        return --ptr;
      }
      upb_MiniTableField* field = fields;
      *field_count += 1;
      fields = (char*)fields + field_size;
      uint32_t number = ++last_field_number;
      if (number == 0 || (number > kUpb_MaxFieldNumber && !d->is_extension)) {
        upb_MdDecoder_ErrorJmp(&d->base, "Invalid field number: %" PRIu32,
                               number);
      }
      field->UPB_PRIVATE(number) = number;
      last_field = field;
      upb_MiniTable_SetField(d, ch, field, msg_modifiers);
    } else if (kUpb_EncodedValue_MinModifier <= ch &&
               ch <= kUpb_EncodedValue_MaxModifier) {
      ptr = upb_MtDecoder_ParseModifier(d, ptr, ch, last_field, &msg_modifiers);
      if (msg_modifiers & kUpb_MessageModifier_IsExtendable) {
        d->table.UPB_PRIVATE(ext) |= kUpb_ExtMode_Extendable;
      }
    } else if (ch == kUpb_EncodedValue_End) {
      if (d->is_extension) {
        upb_MdDecoder_ErrorJmp(&d->base, "Extensions cannot have oneofs.");
      }
      ptr = upb_MtDecoder_DecodeOneofs(d, ptr);
    } else if (kUpb_EncodedValue_MinSkip <= ch &&
               ch <= kUpb_EncodedValue_MaxSkip) {
      if (need_dense_below) {
        d->table.UPB_PRIVATE(dense_below) = d->table.UPB_PRIVATE(field_count);
        need_dense_below = false;
      }
      uint32_t skip;
      ptr = upb_MdDecoder_DecodeBase92Varint(&d->base, ptr, ch,
                                             kUpb_EncodedValue_MinSkip,
                                             kUpb_EncodedValue_MaxSkip, &skip);
      last_field_number += skip;
      last_field_number--;  // Next field seen will increment.
    } else {
      upb_MdDecoder_ErrorJmp(&d->base, "Invalid char: %c", ch);
    }
  }

  if (need_dense_below) {
    d->table.UPB_PRIVATE(dense_below) = d->table.UPB_PRIVATE(field_count);
  }

  return ptr;
}

static void upb_MtDecoder_ParseMessage(upb_MtDecoder* d, const char* data,
                                       size_t len) {
  const size_t bytes_per_field =
      sizeof(upb_MiniTableField) + sizeof(upb_MiniTableSubInternal);
  // Buffer length is an upper bound on the number of fields. We will return
  // what we don't use.
  if ((SIZE_MAX - 4) / bytes_per_field < len) {
    upb_MdDecoder_ErrorJmp(&d->base, "MiniDescriptor is too large");
  }
  // Max size used per field is a upb_MiniTableField and a
  // upb_MiniTableSubInternal.  There could also be up to 4 bytes of padding,
  // since sizeof(upb_MiniTableField) == 12 and
  // alignof(upb_MiniTableSubInternal) == 8.
  UPB_STATIC_ASSERT(UPB_ALIGN_OF(upb_MiniTableSubInternal) -
                            UPB_ALIGN_OF(upb_MiniTableField) <=
                        4,
                    "alignment difference is too large");
  const size_t initial_bytes = bytes_per_field * len + 4;
  d->fields = upb_Arena_Malloc(d->arena, initial_bytes);
  upb_MdDecoder_CheckOutOfMemory(&d->base, d->fields);

  d->table.UPB_PRIVATE(field_count) = 0;
  d->table.UPB_PRIVATE(fields) = d->fields;
  upb_MtDecoder_Parse(d, data, len, d->fields, sizeof(*d->fields),
                      &d->table.UPB_PRIVATE(field_count));
  size_t field_bytes =
      UPB_ALIGN_UP(d->table.UPB_PRIVATE(field_count) * sizeof(*d->fields),
                   upb_MtDecoder_PtrSize(d));
  upb_MiniTableSubInternal* subs =
      UPB_PTR_AT(d->fields, field_bytes, upb_MiniTableSubInternal);
  memset(subs, 0, sizeof(upb_MiniTableSubInternal) * d->sub_count);

  // We now know how much space we actually used, so shrink the allocation to
  // that size.
  size_t final_bytes =
      field_bytes + sizeof(upb_MiniTableSubInternal) * d->sub_count;
  upb_Arena_ShrinkLast(d->arena, d->fields, initial_bytes, final_bytes);
  upb_MtDecoder_AllocateSubs(d, subs);
}

static void upb_MtDecoder_CalculateAlignments(upb_MtDecoder* d) {
  // Add alignment counts for non-oneof fields (oneofs were added already)
  int n = d->table.UPB_PRIVATE(field_count);
  for (int i = 0; i < n; i++) {
    upb_MiniTableField* f = &d->fields[i];
    if (f->UPB_PRIVATE(offset) >= kOneofBase) continue;
    d->rep_counts_offsets[f->UPB_PRIVATE(mode) >> kUpb_FieldRep_Shift]++;
  }

  // Reserve properly aligned space for each type of field representation
  // present in this message. When we iterate over the fields, they will obtain
  // their offset from within the region matching their alignment requirements.
  size_t base = d->table.UPB_PRIVATE(size);
  // Start with the lowest alignment requirement, going up, because:
  // 1. If there are presence bits, we won't be aligned to start, but adding
  //    some lower-alignment fields may get us closer without wasting space to
  //    padding.
  // 2. The allocator enforces 8 byte alignment, so moving intermediate padding
  //    to trailing padding doesn't save us anything.
  for (upb_FieldRep rep = kUpb_FieldRep_1Byte; rep <= kUpb_FieldRep_Max;
       rep++) {
    uint16_t count = d->rep_counts_offsets[rep];
    if (count) {
      base = UPB_ALIGN_UP(base, upb_MtDecoder_AlignOfRep(rep, d->platform));
      // This entry now tracks the base offset for this field representation
      // type, instead of the count
      d->rep_counts_offsets[rep] = base;
      base += upb_MtDecoder_SizeOfRep(rep, d->platform) * count;
    }
  }
  static const size_t max = UINT16_MAX;
  if (base > max) {
    upb_MdDecoder_ErrorJmp(
        &d->base, "Message size exceeded maximum size of %zu bytes", max);
  }
  d->table.UPB_PRIVATE(size) = (uint16_t)base;
}

static size_t upb_MiniTable_DivideRoundUp(size_t n, size_t d) {
  return (n + d - 1) / d;
}

static void upb_MtDecoder_AssignHasbits(upb_MtDecoder* d) {
  int n = d->table.UPB_PRIVATE(field_count);
  size_t last_hasbit = kUpb_Reserved_Hasbits - 1;

  // First assign required fields, which must have the lowest hasbits.
  for (int i = 0; i < n; i++) {
    upb_MiniTableField* field =
        (upb_MiniTableField*)&d->table.UPB_PRIVATE(fields)[i];
    if (field->UPB_PRIVATE(offset) == kRequiredPresence) {
      field->presence = ++last_hasbit;
    } else if (field->UPB_PRIVATE(offset) == kNoPresence) {
      field->presence = 0;
    }
  }
  if (last_hasbit >= kUpb_Reserved_Hasbits + 63) {
    upb_MdDecoder_ErrorJmp(&d->base, "Too many required fields");
  }

  d->table.UPB_PRIVATE(required_count) =
      last_hasbit - (kUpb_Reserved_Hasbits - 1);
  UPB_ASSERT(d->table.UPB_PRIVATE(required_count) < 64);

  // Next assign non-required hasbit fields.
  for (int i = 0; i < n; i++) {
    upb_MiniTableField* field =
        (upb_MiniTableField*)&d->table.UPB_PRIVATE(fields)[i];
    if (field->UPB_PRIVATE(offset) == kHasbitPresence) {
      field->presence = ++last_hasbit;
    }
  }

  d->table.UPB_PRIVATE(size) =
      last_hasbit ? upb_MiniTable_DivideRoundUp(last_hasbit + 1, 8) : 0;
}

static size_t upb_MtDecoder_Place(upb_MtDecoder* d, upb_FieldRep rep) {
  size_t size = upb_MtDecoder_SizeOfRep(rep, d->platform);
  size_t offset = d->rep_counts_offsets[rep];
  d->rep_counts_offsets[rep] += size;
  return offset;
}

static void upb_MtDecoder_AssignOffsets(upb_MtDecoder* d) {
  upb_MiniTableField* field_end =
      UPB_PTRADD(d->fields, d->table.UPB_PRIVATE(field_count));
  for (upb_MiniTableField* field = d->fields; field < field_end; field++) {
    if (field->UPB_PRIVATE(offset) >= kOneofBase) continue;
    field->UPB_PRIVATE(offset) =
        upb_MtDecoder_Place(d, field->UPB_PRIVATE(mode) >> kUpb_FieldRep_Shift);
  }

  upb_OneOfLayoutItem* oneof_end = UPB_PTRADD(d->oneofs.data, d->oneofs.size);

  for (upb_OneOfLayoutItem* item = d->oneofs.data; item < oneof_end; item++) {
    upb_MiniTableField* f = &d->fields[item->field_index];
    uint16_t case_offset = upb_MtDecoder_Place(d, kUpb_OneOf_CaseFieldRep);
    uint16_t data_offset = upb_MtDecoder_Place(d, item->rep);
    while (true) {
      f->presence = ~case_offset;
      uint16_t next_offset = f->UPB_PRIVATE(offset);
      f->UPB_PRIVATE(offset) = data_offset;
      if (next_offset == kUpb_OneOfLayoutItem_IndexSentinel) break;
      UPB_ASSERT(next_offset - kOneofBase < d->table.UPB_PRIVATE(field_count));
      f = &d->fields[next_offset - kOneofBase];
    }
  }

  // Since messages are always allocated on arenas, we can save repeatedly
  // realigning by doing alignment at minitable construction time. We don't want
  // to align to UPB_MALLOC_ALIGN because it can change with sanitizers, and if
  // we're generating code we don't want to calculate size differently depending
  // on the proto compiler's host or build configuration.
  d->table.UPB_PRIVATE(size) =
      UPB_ALIGN_UP(d->table.UPB_PRIVATE(size), kUpb_Message_Align);
}

static void upb_MtDecoder_ValidateEntryField(upb_MtDecoder* d,
                                             const upb_MiniTableField* f,
                                             uint32_t expected_num) {
  const char* name = expected_num == 1 ? "key" : "val";
  const uint32_t f_number = upb_MiniTableField_Number(f);
  if (f_number != expected_num) {
    upb_MdDecoder_ErrorJmp(&d->base,
                           "map %s did not have expected number (%d vs %d)",
                           name, expected_num, f_number);
  }

  if (!upb_MiniTableField_IsScalar(f)) {
    upb_MdDecoder_ErrorJmp(
        &d->base, "map %s cannot be repeated or map, or be in oneof", name);
  }

  uint32_t not_ok_types;
  if (expected_num == 1) {
    not_ok_types = (1 << kUpb_FieldType_Float) | (1 << kUpb_FieldType_Double) |
                   (1 << kUpb_FieldType_Message) | (1 << kUpb_FieldType_Group) |
                   (1 << kUpb_FieldType_Bytes) | (1 << kUpb_FieldType_Enum);
  } else {
    not_ok_types = 1 << kUpb_FieldType_Group;
  }

  if ((1 << upb_MiniTableField_Type(f)) & not_ok_types) {
    upb_MdDecoder_ErrorJmp(&d->base, "map %s cannot have type %d", name,
                           (int)f->UPB_PRIVATE(descriptortype));
  }
}

static void upb_MtDecoder_ParseMap(upb_MtDecoder* d, const char* data,
                                   size_t len) {
  upb_MtDecoder_ParseMessage(d, data, len);
  upb_MtDecoder_AssignHasbits(d);

  if (UPB_UNLIKELY(d->table.UPB_PRIVATE(field_count) != 2)) {
    upb_MdDecoder_ErrorJmp(&d->base, "%hu fields in map",
                           d->table.UPB_PRIVATE(field_count));
    UPB_UNREACHABLE();
  }

  if (d->oneofs.size != 0) {
    upb_MdDecoder_ErrorJmp(&d->base, "Map entry cannot have oneof");
  }

  upb_MtDecoder_ValidateEntryField(d, &d->table.UPB_PRIVATE(fields)[0], 1);
  upb_MtDecoder_ValidateEntryField(d, &d->table.UPB_PRIVATE(fields)[1], 2);

  d->fields[0].UPB_PRIVATE(offset) = offsetof(upb_MapEntry, k);
  d->fields[1].UPB_PRIVATE(offset) = offsetof(upb_MapEntry, v);
  d->table.UPB_PRIVATE(size) = sizeof(upb_MapEntry);

  // Map entries have a special bit set to signal it's a map entry, used in
  // upb_MiniTable_SetSubMessage() below.
  d->table.UPB_PRIVATE(ext) |= kUpb_ExtMode_IsMapEntry;
}

static void upb_MtDecoder_ParseMessageSet(upb_MtDecoder* d, const char* data,
                                          size_t len) {
  if (len > 0) {
    upb_MdDecoder_ErrorJmp(&d->base, "Invalid message set encode length: %zu",
                           len);
  }

  upb_MiniTable* ret = &d->table;
  ret->UPB_PRIVATE(size) = kUpb_Reserved_Hasbytes;
  ret->UPB_PRIVATE(field_count) = 0;
  ret->UPB_PRIVATE(ext) = kUpb_ExtMode_IsMessageSet;
  ret->UPB_PRIVATE(dense_below) = 0;
  ret->UPB_PRIVATE(table_mask) = -1;
  ret->UPB_PRIVATE(required_count) = 0;
}

static upb_MiniTable* upb_MtDecoder_DoBuildMiniTableWithBuf(
    upb_MtDecoder* decoder, const char* data, size_t len, void** buf,
    size_t* buf_size) {
  decoder->table.UPB_PRIVATE(size) = kUpb_Reserved_Hasbytes;
  decoder->table.UPB_PRIVATE(field_count) = 0;
  decoder->table.UPB_PRIVATE(ext) = kUpb_ExtMode_NonExtendable;
  decoder->table.UPB_PRIVATE(dense_below) = 0;
  decoder->table.UPB_PRIVATE(table_mask) = -1;
  decoder->table.UPB_PRIVATE(required_count) = 0;
#ifdef UPB_TRACING_ENABLED
  // MiniTables built from MiniDescriptors will not be able to vend the message
  // name unless it is explicitly set with upb_MiniTable_SetFullName().
  decoder->table.UPB_PRIVATE(full_name) = 0;
#endif

  // Strip off and verify the version tag.
  if (!len--) goto done;
  const char vers = *data++;

  switch (vers) {
    case kUpb_EncodedVersion_MapV1:
      upb_MtDecoder_ParseMap(decoder, data, len);
      break;

    case kUpb_EncodedVersion_MessageV1:
      upb_MtDecoder_ParseMessage(decoder, data, len);
      upb_MtDecoder_AssignHasbits(decoder);
      upb_MtDecoder_CalculateAlignments(decoder);
      upb_MtDecoder_AssignOffsets(decoder);
      break;

    case kUpb_EncodedVersion_MessageSetV1:
      upb_MtDecoder_ParseMessageSet(decoder, data, len);
      break;

    default:
      upb_MdDecoder_ErrorJmp(&decoder->base, "Invalid message version: %c",
                             vers);
  }

done:
  *buf = decoder->oneofs.data;
  *buf_size = decoder->oneofs.buf_capacity_bytes;
  size_t mt_size = sizeof(upb_MiniTable);

#if UPB_FASTTABLE
  upb_DecodeFast_TableEntry fasttable[32];
  int fasttable_size = upb_DecodeFast_BuildTable(&decoder->table, fasttable);
  mt_size += fasttable_size * sizeof(fasttable[0]);
#endif

  upb_MiniTable* ret = upb_Arena_Malloc(decoder->arena, mt_size);
  memcpy(ret, &decoder->table, sizeof(*ret));

#if UPB_FASTTABLE
  ret->UPB_PRIVATE(table_mask) = upb_DecodeFast_GetTableMask(fasttable_size);
  for (int i = 0; i < fasttable_size; i++) {
    ret->UPB_PRIVATE(fasttable)[i].field_data = fasttable[i].function_data;
    ret->UPB_PRIVATE(fasttable)[i].field_parser =
        upb_DecodeFast_GetFunctionPointer(fasttable[i].function_idx);
  }
#endif
  UPB_PRIVATE(upb_MiniTable_CheckInvariants)(ret);
  return ret;
}

static upb_MiniTable* upb_MtDecoder_BuildMiniTableWithBuf(
    upb_MtDecoder* const decoder, const char* const data, const size_t len,
    void** const buf, size_t* const buf_size) {
  if (UPB_SETJMP(decoder->base.err) != 0) {
    *buf = decoder->oneofs.data;
    *buf_size = decoder->oneofs.buf_capacity_bytes;
    return NULL;
  }

  return upb_MtDecoder_DoBuildMiniTableWithBuf(decoder, data, len, buf,
                                               buf_size);
}

upb_MiniTable* upb_MiniTable_BuildWithBuf(const char* data, size_t len,
                                          upb_MiniTablePlatform platform,
                                          upb_Arena* arena, void** buf,
                                          size_t* buf_size,
                                          upb_Status* status) {
  upb_MtDecoder decoder = {
      .base = {.status = status},
      .platform = platform,
      .sub_count = 0,
      .is_extension = false,
      .oneofs =
          {
              .data = *buf,
              .buf_capacity_bytes = *buf_size,
              .size = 0,
          },
      .arena = arena,
  };

  return upb_MtDecoder_BuildMiniTableWithBuf(&decoder, data, len, buf,
                                             buf_size);
}

static const char* upb_MtDecoder_DoBuildMiniTableExtension(
    upb_MtDecoder* decoder, const char* data, size_t len,
    upb_MiniTableExtension* ext, const upb_MiniTable* extendee,
    upb_MiniTableSub sub) {
  if (!(extendee->UPB_PRIVATE(ext) &
        (kUpb_ExtMode_Extendable | kUpb_ExtMode_IsMessageSet))) {
    upb_MdDecoder_ErrorJmp(&decoder->base, "Extendee is not extendable");
  }

  // If the string is non-empty then it must begin with a version tag.
  if (len) {
    if (*data != kUpb_EncodedVersion_ExtensionV1) {
      upb_MdDecoder_ErrorJmp(&decoder->base, "Invalid ext version: %c", *data);
    }
    data++;
    len--;
  }

  uint16_t count = 0;
  const char* ret =
      upb_MtDecoder_Parse(decoder, data, len, ext, sizeof(*ext), &count);
  if (!ret || count != 1) return NULL;

  upb_MiniTableField* f = &ext->UPB_PRIVATE(field);

  f->UPB_PRIVATE(mode) |= kUpb_LabelFlags_IsExtension;
  f->UPB_PRIVATE(offset) = 0;
  f->presence = 0;

  // In upb_MiniTableExtension, the `sub` member is a pointer-sized member that
  // directly follows the `field` member.
  f->UPB_PRIVATE(submsg_ofs) =
      UPB_ALIGN_UP(sizeof(upb_MiniTableField), upb_MtDecoder_PtrSize(decoder)) /
      kUpb_SubmsgOffsetBytes;

  if (extendee->UPB_PRIVATE(ext) & kUpb_ExtMode_IsMessageSet) {
    // Extensions of MessageSet must be messages.
    if (!upb_MiniTableField_IsSubMessage(f)) return NULL;

    // Extensions of MessageSet must be non-repeating.
    if (upb_MiniTableField_IsArray(f)) return NULL;
  }

  ext->UPB_PRIVATE(extendee) = extendee;
  ext->UPB_PRIVATE(sub) = sub;

  return ret;
}

static const char* upb_MtDecoder_BuildMiniTableExtension(
    upb_MtDecoder* const decoder, const char* const data, const size_t len,
    upb_MiniTableExtension* const ext, const upb_MiniTable* const extendee,
    const upb_MiniTableSub sub) {
  if (UPB_SETJMP(decoder->base.err) != 0) return NULL;
  return upb_MtDecoder_DoBuildMiniTableExtension(decoder, data, len, ext,
                                                 extendee, sub);
}

const char* _upb_MiniTableExtension_Init(const char* data, size_t len,
                                         upb_MiniTableExtension* ext,
                                         const upb_MiniTable* extendee,
                                         upb_MiniTableSub sub,
                                         upb_MiniTablePlatform platform,
                                         upb_Status* status) {
  upb_MtDecoder decoder = {
      .base = {.status = status},
      .arena = NULL,
      .platform = platform,
      .sub_count = 0,
      .is_extension = true,
  };

  return upb_MtDecoder_BuildMiniTableExtension(&decoder, data, len, ext,
                                               extendee, sub);
}

upb_MiniTableExtension* _upb_MiniTableExtension_Build(
    const char* data, size_t len, const upb_MiniTable* extendee,
    upb_MiniTableSub sub, upb_MiniTablePlatform platform, upb_Arena* arena,
    upb_Status* status) {
  upb_MiniTableExtension* ext =
      upb_Arena_Malloc(arena, sizeof(upb_MiniTableExtension));
  if (UPB_UNLIKELY(!ext)) return NULL;

  const char* ptr = _upb_MiniTableExtension_Init(data, len, ext, extendee, sub,
                                                 platform, status);
  if (UPB_UNLIKELY(!ptr)) return NULL;

  return ext;
}

upb_MiniTable* _upb_MiniTable_Build(const char* data, size_t len,
                                    upb_MiniTablePlatform platform,
                                    upb_Arena* arena, upb_Status* status) {
  void* buf = NULL;
  size_t size = 0;
  upb_MiniTable* ret = upb_MiniTable_BuildWithBuf(data, len, platform, arena,
                                                  &buf, &size, status);
  upb_gfree(buf);
  return ret;
}
