// Copyright 2019 The Go Authors. All rights reserved.

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

// Package messageset encodes and decodes the obsolete MessageSet wire format.

package messageset

import (

	"math"

	"google.golang.org/protobuf/encoding/protowire"

	"google.golang.org/protobuf/internal/errors"

	pref "google.golang.org/protobuf/reflect/protoreflect"

)

// The MessageSet wire format is equivalent to a message defined as follows,

// where each Item defines an extension field with a field number of 'type_id'

// and content of 'message'. MessageSet extensions must be non-repeated message

// fields.

//

//	message MessageSet {

//		repeated group Item = 1 {

//			required int32 type_id = 2;

//			required string message = 3;

//		}

//	}

const (

	FieldItem    = protowire.Number(1)

	FieldTypeID  = protowire.Number(2)

	FieldMessage = protowire.Number(3)

)

// ExtensionName is the field name for extensions of MessageSet.

//

// A valid MessageSet extension must be of the form:

//	message MyMessage {

//		extend proto2.bridge.MessageSet {

//			optional MyMessage message_set_extension = 1234;

//		}

//		...

//	}

const ExtensionName = "message_set_extension"

// IsMessageSet returns whether the message uses the MessageSet wire format.

func IsMessageSet(md pref.MessageDescriptor) bool {

	xmd, ok := md.(interface{ IsMessageSet() bool })

	return ok && xmd.IsMessageSet()

}

// IsMessageSetExtension reports this field properly extends a MessageSet.

func IsMessageSetExtension(fd pref.FieldDescriptor) bool {

	switch {

	case fd.Name() != ExtensionName:

		return false

	case !IsMessageSet(fd.ContainingMessage()):

		return false

	case fd.FullName().Parent() != fd.Message().FullName():

		return false

	}

	return true

}

// SizeField returns the size of a MessageSet item field containing an extension

// with the given field number, not counting the contents of the message subfield.

func SizeField(num protowire.Number) int {

	return 2*protowire.SizeTag(FieldItem) + protowire.SizeTag(FieldTypeID) + protowire.SizeVarint(uint64(num))

}

// Unmarshal parses a MessageSet.

//

// It calls fn with the type ID and value of each item in the MessageSet.

// Unknown fields are discarded.

//

// If wantLen is true, the item values include the varint length prefix.

// This is ugly, but simplifies the fast-path decoder in internal/impl.

func Unmarshal(b []byte, wantLen bool, fn func(typeID protowire.Number, value []byte) error) error {

	for len(b) > 0 {

		num, wtyp, n := protowire.ConsumeTag(b)

		if n < 0 {

			return protowire.ParseError(n)

		}

		b = b[n:]

		if num != FieldItem || wtyp != protowire.StartGroupType {

			n := protowire.ConsumeFieldValue(num, wtyp, b)

			if n < 0 {

				return protowire.ParseError(n)

			}

			b = b[n:]

			continue

		}

		typeID, value, n, err := ConsumeFieldValue(b, wantLen)

		if err != nil {

			return err

		}

		b = b[n:]

		if typeID == 0 {

			continue

		}

		if err := fn(typeID, value); err != nil {

			return err

		}

	}

	return nil

}

// ConsumeFieldValue parses b as a MessageSet item field value until and including

// the trailing end group marker. It assumes the start group tag has already been parsed.

// It returns the contents of the type_id and message subfields and the total

// item length.

//

// If wantLen is true, the returned message value includes the length prefix.

func ConsumeFieldValue(b []byte, wantLen bool) (typeid protowire.Number, message []byte, n int, err error) {

	ilen := len(b)

	for {

		num, wtyp, n := protowire.ConsumeTag(b)

		if n < 0 {

			return 0, nil, 0, protowire.ParseError(n)

		}

		b = b[n:]

		switch {

		case num == FieldItem && wtyp == protowire.EndGroupType:

			if wantLen && len(message) == 0 {

				// The message field was missing, which should never happen.

				// Be prepared for this case anyway.

				message = protowire.AppendVarint(message, 0)

			}

			return typeid, message, ilen - len(b), nil

		case num == FieldTypeID && wtyp == protowire.VarintType:

			v, n := protowire.ConsumeVarint(b)

			if n < 0 {

				return 0, nil, 0, protowire.ParseError(n)

			}

			b = b[n:]

			if v < 1 || v > math.MaxInt32 {

				return 0, nil, 0, errors.New("invalid type_id in message set")

			}

			typeid = protowire.Number(v)

		case num == FieldMessage && wtyp == protowire.BytesType:

			m, n := protowire.ConsumeBytes(b)

			if n < 0 {

				return 0, nil, 0, protowire.ParseError(n)

			}

			if message == nil {

				if wantLen {

					message = b[:n:n]

				} else {

					message = m[:len(m):len(m)]

				}

			} else {

				// This case should never happen in practice, but handle it for

				// correctness: The MessageSet item contains multiple message

				// fields, which need to be merged.

				//

				// In the case where we're returning the length, this becomes

				// quite inefficient since we need to strip the length off

				// the existing data and reconstruct it with the combined length.

				if wantLen {

					_, nn := protowire.ConsumeVarint(message)

					m0 := message[nn:]

					message = nil

					message = protowire.AppendVarint(message, uint64(len(m0)+len(m)))

					message = append(message, m0...)

					message = append(message, m...)

				} else {

					message = append(message, m...)

				}

			}

			b = b[n:]

		default:

			// We have no place to put it, so we just ignore unknown fields.

			n := protowire.ConsumeFieldValue(num, wtyp, b)

			if n < 0 {

				return 0, nil, 0, protowire.ParseError(n)

			}

			b = b[n:]

		}

	}

}

// AppendFieldStart appends the start of a MessageSet item field containing

// an extension with the given number. The caller must add the message

// subfield (including the tag).

func AppendFieldStart(b []byte, num protowire.Number) []byte {

	b = protowire.AppendTag(b, FieldItem, protowire.StartGroupType)

	b = protowire.AppendTag(b, FieldTypeID, protowire.VarintType)

	b = protowire.AppendVarint(b, uint64(num))

	return b

}

// AppendFieldEnd appends the trailing end group marker for a MessageSet item field.

func AppendFieldEnd(b []byte) []byte {

	return protowire.AppendTag(b, FieldItem, protowire.EndGroupType)

}

// SizeUnknown returns the size of an unknown fields section in MessageSet format.

//

// See AppendUnknown.

func SizeUnknown(unknown []byte) (size int) {

	for len(unknown) > 0 {

		num, typ, n := protowire.ConsumeTag(unknown)

		if n < 0 || typ != protowire.BytesType {

			return 0

		}

		unknown = unknown[n:]

		_, n = protowire.ConsumeBytes(unknown)

		if n < 0 {

			return 0

		}

		unknown = unknown[n:]

		size += SizeField(num) + protowire.SizeTag(FieldMessage) + n

	}

	return size

}

// AppendUnknown appends unknown fields to b in MessageSet format.

//

// For historic reasons, unresolved items in a MessageSet are stored in a

// message's unknown fields section in non-MessageSet format. That is, an

// unknown item with typeID T and value V appears in the unknown fields as

// a field with number T and value V.

//

// This function converts the unknown fields back into MessageSet form.

func AppendUnknown(b, unknown []byte) ([]byte, error) {

	for len(unknown) > 0 {

		num, typ, n := protowire.ConsumeTag(unknown)

		if n < 0 || typ != protowire.BytesType {

			return nil, errors.New("invalid data in message set unknown fields")

		}

		unknown = unknown[n:]

		_, n = protowire.ConsumeBytes(unknown)

		if n < 0 {

			return nil, errors.New("invalid data in message set unknown fields")

		}

		b = AppendFieldStart(b, num)

		b = protowire.AppendTag(b, FieldMessage, protowire.BytesType)

		b = append(b, unknown[:n]...)

		b = AppendFieldEnd(b)

		unknown = unknown[n:]

	}

	return b, nil

}