// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

package nfs4

import (
	"errors"
	"fmt"
	"strconv"

	"sourcedock.dev/petrbalvin/nfs/internal/xdr"
)

// ErrUnknownAttr marks an attribute number in a response that this build
// cannot decode, which means the two sides do not agree on the attribute
// table: a bug, not a protocol condition.
var ErrUnknownAttr = errors.New("nfs4: unknown attribute in response")

// Argument builders. Each appends one complete argop: the operation number
// followed by its arguments. The client joins them into a COMPOUND.

// AppendPutRootfh appends the PUTROOTFH argop.
func AppendPutRootfh(b []byte) []byte {
	return xdr.AppendUint32(b, OpPutRootfh)
}

// AppendSavefh appends the SAVEFH argop.
func AppendSavefh(b []byte) []byte {
	return xdr.AppendUint32(b, OpSavefh)
}

// AppendRestorefh appends the RESTOREFH argop.
func AppendRestorefh(b []byte) []byte {
	return xdr.AppendUint32(b, OpRestorefh)
}

// AppendPutfh appends the PUTFH argop with the handle.
func AppendPutfh(b []byte, fh []byte) []byte {
	b = xdr.AppendUint32(b, OpPutfh)
	return xdr.AppendVarOpaque(b, fh)
}

// AppendGetfh appends the GETFH argop.
func AppendGetfh(b []byte) []byte {
	return xdr.AppendUint32(b, OpGetfh)
}

// AppendLookup appends the LOOKUP argop with the component name.
func AppendLookup(b []byte, name string) []byte {
	b = xdr.AppendUint32(b, OpLookup)
	return xdr.AppendString(b, name)
}

// AppendGetattr appends the GETATTR argop asking for the attributes.
func AppendGetattr(b []byte, request Bitmap) []byte {
	b = xdr.AppendUint32(b, OpGetattr)
	return request.AppendTo(b)
}

// AppendAccess appends the ACCESS argop asking for the mask bits.
func AppendAccess(b []byte, mask uint32) []byte {
	b = xdr.AppendUint32(b, OpAccess)
	return xdr.AppendUint32(b, mask)
}

// AppendRead appends the READ argop: the anonymous stateid, the offset and
// the count.
func AppendRead(b []byte, stateid Stateid, off uint64, count uint32) []byte {
	b = xdr.AppendUint32(b, OpRead)
	b = stateid.Append(b)
	b = xdr.AppendUint64(b, off)
	return xdr.AppendUint32(b, count)
}

// AppendReaddir appends the READDIR argop: the cookie, the verifier of the
// listing the cookie belongs to, the response size budgets and the
// attributes wanted per entry.
func AppendReaddir(b []byte, cookie uint64, verifer [8]byte, dircount, maxcount uint32, request Bitmap) []byte {
	b = xdr.AppendUint32(b, OpReaddir)
	b = xdr.AppendUint64(b, cookie)
	b = append(b, verifer[:]...)
	b = xdr.AppendUint32(b, dircount)
	b = xdr.AppendUint32(b, maxcount)
	return request.AppendTo(b)
}

// AppendCreateArgs appends the CREATE argop. Regular files are created by
// OPEN and never by CREATE: kind is one of NF4Dir, NF4Lnk, NF4Fifo,
// NF4Sock, NF4Blk or NF4Chr, and linkdata and the device numbers apply
// only to the kinds that carry them. The arguments end in the object
// attributes, a full fattr4 of bitmap and list, RFC 8881 section 18.4;
// a zero perm encodes an empty attribute set.
func AppendCreateArgs(b []byte, kind uint32, name, linkdata string, major, minor uint32, perm uint32) []byte {
	b = xdr.AppendUint32(b, OpCreate)
	b = xdr.AppendUint32(b, kind)
	switch kind {
	case NF4Lnk:
		b = xdr.AppendString(b, linkdata)
	case NF4Blk, NF4Chr:
		b = xdr.AppendUint32(b, major)
		b = xdr.AppendUint32(b, minor)
	}
	b = xdr.AppendString(b, name)
	if perm != 0 {
		b = AppendFattr(b, OfBits(AttrMode), Attrs{Mode: perm})
	} else {
		b = AppendFattr(b, Bitmap{}, Attrs{})
	}
	return b
}

// DecodeCreateArgs decodes a CREATE argop body: everything after the
// operation number. The arguments end in the object attributes, a full
// fattr4 of bitmap and list, RFC 8881 section 18.4.
func DecodeCreateArgs(body []byte) (kind uint32, name, linkdata string, major, minor uint32, perm uint32, err error) {
	d := xdr.NewDecoder(body)
	if kind, err = d.Uint32(); err != nil {
		return
	}
	switch kind {
	case NF4Lnk:
		if linkdata, err = d.String(); err != nil {
			return
		}
	case NF4Blk, NF4Chr:
		if major, err = d.Uint32(); err != nil {
			return
		}
		if minor, err = d.Uint32(); err != nil {
			return
		}
	}
	if name, err = d.String(); err != nil {
		return
	}
	request, err := ReadBitmap(d)
	if err != nil {
		return
	}
	blob, err := d.VarOpaque()
	if err != nil {
		return
	}
	attrs, err := DecodeFattrAttrs(blob, request)
	if err != nil {
		return
	}
	perm = attrs.Mode
	return kind, name, linkdata, major, minor, perm, nil
}

// DecodeFattrAttrs decodes an fattr4 blob against its bitmap into the
// attribute values it carries. It is the dispatcher's view of what a
// client sent.
func DecodeFattrAttrs(blob []byte, response Bitmap) (Attrs, error) {
	return decodeFattrBlob(blob, response)
}

// AppendWriteArgs appends the WRITE argop: the stateid, the offset, the
// stability the client asks for and the data.
func AppendWriteArgs(b []byte, stateid Stateid, off uint64, stable uint32, data []byte) []byte {
	b = xdr.AppendUint32(b, OpWrite)
	b = stateid.Append(b)
	b = xdr.AppendUint64(b, off)
	b = xdr.AppendUint32(b, stable)
	return xdr.AppendVarOpaque(b, data)
}

// DecodeWriteArgs decodes a WRITE argop body.
func DecodeWriteArgs(body []byte) (stateid Stateid, off uint64, stable uint32, data []byte, err error) {
	d := xdr.NewDecoder(body)
	raw, err := d.Raw(16)
	if err != nil {
		return
	}
	copy(stateid[:], raw)
	if off, err = d.Uint64(); err != nil {
		return
	}
	if stable, err = d.Uint32(); err != nil {
		return
	}
	data, err = d.VarOpaque()
	return
}

// AppendWriteRes appends the result of a successful WRITE: how many bytes
// landed, at what stability, and the server verifier that pairs with it.
func AppendWriteRes(b []byte, count, committed uint32, verf [8]byte) []byte {
	b = xdr.AppendUint32(b, count)
	b = xdr.AppendUint32(b, committed)
	return append(b, verf[:]...)
}

// DecodeWriteRes decodes a successful WRITE result.
func DecodeWriteRes(body []byte) (count, committed uint32, verf [8]byte, err error) {
	d := xdr.NewDecoder(body)
	if count, err = d.Uint32(); err != nil {
		return
	}
	if committed, err = d.Uint32(); err != nil {
		return
	}
	raw, err := d.Raw(8)
	if err != nil {
		return
	}
	copy(verf[:], raw)
	return
}

// AppendCreateRes appends the result of a successful CREATE: the change
// information and the bitmap of the attributes set. This server reports
// the change information as not atomic and names no attributes, both of
// which tell the client to re-read them, which it prefers over pretending.
func AppendCreateRes(b []byte) []byte {
	b = xdr.AppendBool(b, false) // atomic
	b = xdr.AppendUint64(b, 0)   // before
	b = xdr.AppendUint64(b, 0)   // after
	return Bitmap{}.AppendTo(b)
}

// AppendRemoveArgs appends the REMOVE argop: the name relative to the
// current file handle.
func AppendRemoveArgs(b []byte, name string) []byte {
	b = xdr.AppendUint32(b, OpRemove)
	return xdr.AppendString(b, name)
}

// AppendChangeInfo appends a change_info4 that reports the change as not
// atomic, telling the client to re-read rather than cache. It is the whole
// REMOVE result and the first half of the RENAME result.
func AppendChangeInfo(b []byte) []byte {
	b = xdr.AppendBool(b, false)  // atomic
	b = xdr.AppendUint64(b, 0)    // before
	return xdr.AppendUint64(b, 0) // after
}

// AppendRemoveRes appends the result of a successful REMOVE.
func AppendRemoveRes(b []byte) []byte {
	return AppendChangeInfo(b)
}

// AppendRenameArgs appends the RENAME argop. The old name is resolved
// against the saved file handle and the new one against the current file
// handle, which is what SAVEFH sets up.
func AppendRenameArgs(b []byte, oldName, newName string) []byte {
	b = xdr.AppendUint32(b, OpRename)
	b = xdr.AppendString(b, oldName)
	return xdr.AppendString(b, newName)
}

// AppendRenameRes appends the result of a successful RENAME: the change
// information of the source directory and of the target directory.
func AppendRenameRes(b []byte) []byte {
	b = AppendChangeInfo(b)
	return AppendChangeInfo(b)
}

// Result payload builders. Each appends the bytes that follow the OK
// status of one nfs_resop4.

// AppendGetfhRes appends the handle a successful GETFH returns.
func AppendGetfhRes(b []byte, fh []byte) []byte {
	return xdr.AppendVarOpaque(b, fh)
}

// AppendGetattrRes appends the fattr4 a successful GETATTR returns.
func AppendGetattrRes(b []byte, request Bitmap, a Attrs) []byte {
	return AppendFattr(b, request, a)
}

// AppendAccessRes appends the two masks a successful ACCESS returns: what
// the server supports and what the caller was granted.
func AppendAccessRes(b []byte, supported, access uint32) []byte {
	b = xdr.AppendUint32(b, supported)
	return xdr.AppendUint32(b, access)
}

// AppendReadRes appends the result of a successful READ: whether the file
// ended and the bytes read.
func AppendReadRes(b []byte, eof bool, data []byte) []byte {
	b = xdr.AppendBool(b, eof)
	return xdr.AppendVarOpaque(b, data)
}

// A DirEntryRes is one entry of a READDIR result.
type DirEntryRes struct {
	Cookie uint64
	Name   string
	Attrs  Attrs
}

// AppendReadDirRes appends the result of a successful READDIR: the
// verifier, the entries and the end of listing flag.
func AppendReadDirRes(b []byte, verifer [8]byte, entries []DirEntryRes, request Bitmap, eof bool) []byte {
	b = append(b, verifer[:]...)
	for _, e := range entries {
		b = xdr.AppendBool(b, true) // value_follows
		b = xdr.AppendUint64(b, e.Cookie)
		b = xdr.AppendString(b, e.Name)
		b = AppendFattr(b, request, e.Attrs)
	}
	b = xdr.AppendBool(b, false) // value_follows: end of entries
	return xdr.AppendBool(b, eof)
}

// Result decoders for the client.

// DecodeGetattrBody decodes an fattr4 into the attribute values it
// carries.
func DecodeGetattrBody(body []byte) (Bitmap, Attrs, error) {
	d := xdr.NewDecoder(body)
	response, err := ReadBitmap(d)
	if err != nil {
		return nil, Attrs{}, err
	}
	blob, err := d.VarOpaque()
	if err != nil {
		return nil, Attrs{}, err
	}
	attrs, err := decodeFattrBlob(blob, response)
	return response, attrs, err
}

func decodeFattrBlob(blob []byte, response Bitmap) (Attrs, error) {
	d := xdr.NewDecoder(blob)
	var a Attrs
	for _, n := range bitsOf(response) {
		var err error
		switch n {
		case AttrSupportedAttrs:
			if _, err = ReadBitmap(d); err != nil {
				return a, err
			}
		case AttrType:
			if a.Type, err = d.Uint32(); err != nil {
				return a, err
			}
		case AttrFHExpireType:
			if a.FHExpireType, err = d.Uint32(); err != nil {
				return a, err
			}
		case AttrChange:
			if a.Change, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrSize:
			if a.Size, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrLinkSupport:
			if a.LinkSupport, err = d.Bool(); err != nil {
				return a, err
			}
		case AttrSymlinkSupport:
			if a.SymlinkSupport, err = d.Bool(); err != nil {
				return a, err
			}
		case AttrNamedAttr:
			if a.NamedAttr, err = d.Bool(); err != nil {
				return a, err
			}
		case AttrFSID:
			if a.FSID[0], err = d.Uint64(); err != nil {
				return a, err
			}
			if a.FSID[1], err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrUniqueHandles:
			if a.UniqueHandles, err = d.Bool(); err != nil {
				return a, err
			}
		case AttrFileHandle:
			if a.FileHandle, err = d.VarOpaque(); err != nil {
				return a, err
			}
		case AttrFileID:
			if a.FileID, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrMaxName:
			if a.Limits.MaxName, err = d.Uint32(); err != nil {
				return a, err
			}
		case AttrMaxRead:
			if a.Limits.MaxRead, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrMaxWrite:
			if a.Limits.MaxWrite, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrMode:
			if a.Mode, err = d.Uint32(); err != nil {
				return a, err
			}
		case AttrNumlinks:
			if a.Numlinks, err = d.Uint32(); err != nil {
				return a, err
			}
		case AttrOwner:
			if a.Owner, err = d.String(); err != nil {
				return a, err
			}
		case AttrOwnerGroup:
			if a.OwnerGroup, err = d.String(); err != nil {
				return a, err
			}
		case AttrSpaceUsed:
			if a.SpaceUsed, err = d.Uint64(); err != nil {
				return a, err
			}
		case AttrTimeAccess:
			if a.TimeAccess, err = readNfsTime(d); err != nil {
				return a, err
			}
		case AttrTimeMetadata:
			if a.TimeMetadata, err = readNfsTime(d); err != nil {
				return a, err
			}
		case AttrTimeModify:
			if a.TimeModify, err = readNfsTime(d); err != nil {
				return a, err
			}
		case AttrMountedOnFileID:
			if a.MountedOnFileID, err = d.Uint64(); err != nil {
				return a, err
			}
		default:
			return a, ErrUnknownAttr
		}
	}
	return a, nil
}

func readNfsTime(d *xdr.Decoder) (NfsTime, error) {
	sec, err := d.Int64()
	if err != nil {
		return NfsTime{}, err
	}
	nsec, err := d.Uint32()
	if err != nil {
		return NfsTime{}, err
	}
	return NfsTime{sec, nsec}, nil
}

// ErrNotSuppName is kept out of the API; unknown attributes carry
// ErrUnknownAttr.

func bitsOf(m Bitmap) []uint32 {
	var out []uint32
	for w, word := range m.Words() {
		for bit := range 32 {
			if word&(1<<bit) != 0 {
				out = append(out, uint32(w*32+bit))
			}
		}
	}
	return out
}

// DecodeAccessBody decodes a successful ACCESS body: the supported and the
// granted masks.
func DecodeAccessBody(body []byte) (supported, access uint32, err error) {
	d := xdr.NewDecoder(body)
	if supported, err = d.Uint32(); err != nil {
		return 0, 0, err
	}
	access, err = d.Uint32()
	return supported, access, err
}

// DecodeReadBody decodes a successful READ body: the end of file flag and
// the data.
func DecodeReadBody(body []byte) (eof bool, data []byte, err error) {
	d := xdr.NewDecoder(body)
	if eof, err = d.Bool(); err != nil {
		return false, nil, err
	}
	data, err = d.VarOpaque()
	return eof, data, err
}

// A ReadDirEntry is one entry of a decoded READDIR body.
type ReadDirEntry struct {
	Cookie uint64
	Name   string
	Attrs  Attrs
}

// DecodeReadDirBody decodes a successful READDIR body: the entries in
// order, then the end of listing flag. The verifier precedes the entries
// and is returned with them.
func DecodeReadDirBody(body []byte) (verifier [8]byte, entries []ReadDirEntry, eof bool, err error) {
	d := xdr.NewDecoder(body)
	raw, err := d.Raw(8)
	if err != nil {
		return verifier, nil, false, err
	}
	copy(verifier[:], raw)
	for {
		more, err := d.Bool()
		if err != nil {
			return verifier, nil, false, err
		}
		if !more {
			break
		}
		var e ReadDirEntry
		if e.Cookie, err = d.Uint64(); err != nil {
			return verifier, nil, false, err
		}
		if e.Name, err = d.String(); err != nil {
			return verifier, nil, false, err
		}
		resp, err := ReadBitmap(d)
		if err != nil {
			return verifier, nil, false, err
		}
		blob, err := d.VarOpaque()
		if err != nil {
			return verifier, nil, false, err
		}
		if e.Attrs, err = decodeFattrBlob(blob, resp); err != nil {
			return verifier, nil, false, err
		}
		entries = append(entries, e)
	}
	if eof, err = d.Bool(); err != nil {
		return verifier, nil, false, err
	}
	return verifier, entries, eof, nil
}

// A NfsTimeSet is the settime4 of a SETATTR: either the server's current
// time or the time the client names.
type NfsTimeSet struct {
	Server bool
	Time   NfsTime
}

// SetAttrUpdates carries the attribute changes a SETATTR names, decoded
// from the fattr4 against its bitmap.
type SetAttrUpdates struct {
	HasMode bool
	Mode    uint32
	HasSize bool
	Size    uint64
	UID     *uint32
	GID     *uint32
	Atime   *NfsTimeSet
	Mtime   *NfsTimeSet
}

// AppendSetattrArgs appends the SETATTR argop: the stateid and the fattr4
// of the changes. A caller that sets no state carries the anonymous stateid.
func AppendSetattrArgs(b []byte, stateid Stateid, request Bitmap, a Attrs) []byte {
	b = xdr.AppendUint32(b, OpSetattr)
	b = stateid.Append(b)
	return AppendFattr(b, request, a)
}

// AppendSetattrRes appends the SETATTR result: the bitmap of the
// attributes that were set. A failed operation reports what is known to
// have been applied; this server applies atomically per call or reports
// nothing.
func AppendSetattrRes(b []byte, attrsset Bitmap) []byte {
	return attrsset.AppendTo(b)
}

// AppendLinkArgs appends the LINK argop: the name for the new link, which
// lands in the current directory and points at the saved file handle.
func AppendLinkArgs(b []byte, name string) []byte {
	b = xdr.AppendUint32(b, OpLink)
	return xdr.AppendString(b, name)
}

// AppendLinkRes appends the result of a successful LINK.
func AppendLinkRes(b []byte) []byte {
	return AppendChangeInfo(b)
}

// DecodeSetattrBlob decodes the fattr4 of a SETATTR into the changes it
// names. Time attributes carry the settime4 shape, and the owner
// attributes are parsed from their numeric form, uid@domain. An attribute
// this build does not set is refused rather than silently skipped, because
// the encoding walks the blob in order and skipping would desynchronise
// the decode.
func DecodeSetattrBlob(blob []byte, request Bitmap) (SetAttrUpdates, error) {
	d := xdr.NewDecoder(blob)
	var u SetAttrUpdates
	for _, n := range bitsOf(request) {
		var err error
		switch n {
		case AttrMode:
			u.HasMode = true
			if u.Mode, err = d.Uint32(); err != nil {
				return u, err
			}
		case AttrSize:
			u.HasSize = true
			if u.Size, err = d.Uint64(); err != nil {
				return u, err
			}
		case AttrOwner:
			s, err := d.String()
			if err != nil {
				return u, err
			}
			id, err := parseOwner(s)
			if err != nil {
				return u, err
			}
			u.UID = &id
		case AttrOwnerGroup:
			s, err := d.String()
			if err != nil {
				return u, err
			}
			id, err := parseOwner(s)
			if err != nil {
				return u, err
			}
			u.GID = &id
		case AttrTimeAccessSet:
			u.Atime, err = decodeNfsTimeSet(d)
			if err != nil {
				return u, err
			}
		case AttrTimeModifySet:
			u.Mtime, err = decodeNfsTimeSet(d)
			if err != nil {
				return u, err
			}
		case AttrTimeAccess:
			sec, serr := d.Int64()
			if serr != nil {
				return u, serr
			}
			nsec, nerr := d.Uint32()
			if nerr != nil {
				return u, nerr
			}
			u.Atime = &NfsTimeSet{Time: NfsTime{sec, nsec}}
		case AttrTimeModify:
			sec, serr := d.Int64()
			if serr != nil {
				return u, serr
			}
			nsec, nerr := d.Uint32()
			if nerr != nil {
				return u, nerr
			}
			u.Mtime = &NfsTimeSet{Time: NfsTime{sec, nsec}}
		default:
			return u, ErrAttrNotSettable
		}
	}
	return u, nil
}

// decodeNfsTimeSet reads a settime4: a flag for server time, then, when the
// flag is false, the client's time.
func decodeNfsTimeSet(d *xdr.Decoder) (*NfsTimeSet, error) {
	server, err := d.Bool()
	if err != nil {
		return nil, err
	}
	if server {
		return &NfsTimeSet{Server: true}, nil
	}
	sec, err := d.Int64()
	if err != nil {
		return nil, err
	}
	nsec, err := d.Uint32()
	if err != nil {
		return nil, err
	}
	return &NfsTimeSet{Time: NfsTime{sec, nsec}}, nil
}

// parseOwner reads the numeric identifier of an owner string, which the
// protocol writes as uid@domain, with the domain optional.
func parseOwner(s string) (uint32, error) {
	name := s
	if i := indexByte(s, '@'); i >= 0 {
		name = s[:i]
	}
	v, err := strconv.ParseUint(name, 10, 32)
	if err != nil {
		return 0, fmt.Errorf("%w: owner %q is not numeric", ErrAttrNotSettable, s)
	}
	return uint32(v), nil
}

func indexByte(s string, b byte) int {
	for i := range len(s) {
		if s[i] == b {
			return i
		}
	}
	return -1
}

// ErrAttrNotSettable marks an attribute a SETATTR names that this build
// neither sets nor skips: the caller has to answer ATTRNOTSUPP.
var ErrAttrNotSettable = errors.New("nfs4: attribute not settable")

// AppendSetattrBlob appends the attrlist4 of a SETATTR: the values of the
// named changes in ascending attribute order, which is the order the
// decoder walks.
func AppendSetattrBlob(b []byte, u SetAttrUpdates) []byte {
	if u.HasSize {
		b = xdr.AppendUint64(b, u.Size)
	}
	if u.HasMode {
		b = xdr.AppendUint32(b, u.Mode)
	}
	if u.UID != nil {
		b = xdr.AppendString(b, strconv.FormatUint(uint64(*u.UID), 10))
	}
	if u.GID != nil {
		b = xdr.AppendString(b, strconv.FormatUint(uint64(*u.GID), 10))
	}
	if u.Atime != nil {
		b = appendSettime(b, *u.Atime)
	}
	if u.Mtime != nil {
		b = appendSettime(b, *u.Mtime)
	}
	return b
}

// SetattrBitmap returns the attribute bitmap that names the changes.
func (u SetAttrUpdates) SetattrBitmap() Bitmap {
	set := Bitmap{}
	if u.HasSize {
		set = set.With(AttrSize)
	}
	if u.HasMode {
		set = set.With(AttrMode)
	}
	if u.UID != nil {
		set = set.With(AttrOwner)
	}
	if u.GID != nil {
		set = set.With(AttrOwnerGroup)
	}
	if u.Atime != nil {
		set = set.With(AttrTimeAccessSet)
	}
	if u.Mtime != nil {
		set = set.With(AttrTimeModifySet)
	}
	return set
}

func appendSettime(b []byte, t NfsTimeSet) []byte {
	b = xdr.AppendBool(b, t.Server)
	if t.Server {
		return b
	}
	b = xdr.AppendInt64(b, t.Time.Seconds)
	return xdr.AppendUint32(b, t.Time.Nseconds)
}

// AppendSetattrArgsUpdates appends the whole SETATTR argop from the
// structured updates: the stateid, the derived bitmap and the values.
func AppendSetattrArgsUpdates(b []byte, stateid Stateid, u SetAttrUpdates) []byte {
	b = xdr.AppendUint32(b, OpSetattr)
	b = stateid.Append(b)
	return xdr.AppendVarOpaque(u.SetattrBitmap().AppendTo(b), AppendSetattrBlob(nil, u))
}

// AppendReadlinkArgs appends the READLINK argop, which carries no
// arguments: the current file handle names the symlink.
func AppendReadlinkArgs(b []byte) []byte {
	return xdr.AppendUint32(b, OpReadlink)
}

// AppendReadlinkRes appends the result of a successful READLINK: the
// target the symlink points at.
func AppendReadlinkRes(b []byte, linkdata string) []byte {
	return xdr.AppendString(b, linkdata)
}

// AppendCommitArgs appends the COMMIT argop: the range the client asks to
// flush and the verifier of the writes it made.
func AppendCommitArgs(b []byte, off uint64, count uint32, verf [8]byte) []byte {
	b = xdr.AppendUint32(b, OpCommit)
	b = xdr.AppendUint64(b, off)
	b = xdr.AppendUint32(b, count)
	return append(b, verf[:]...)
}

// AppendCommitRes appends the result of a successful COMMIT: the server
// verifier the flushed data is now stable under.
func AppendCommitRes(b []byte, verf [8]byte) []byte {
	return append(b, verf[:]...)
}

// A GSSInfo is the rpcsec_gss_info of the secinfo4 union's RPCSEC_GSS
// branch, RFC 7863: the mechanism object identifier, the quality of
// protection and the service.
type GSSInfo struct {
	OID     []byte
	QOP     uint32
	Service uint32
}

// A SecinfoEntry is one answer row of a SECINFO: a flavour the server
// accepts. Only the RPCSEC_GSS branch carries the rpcsec_gss_info;
// every other flavour, and the default branch, carry nothing.
type SecinfoEntry struct {
	Flavor uint32
	GSS    *GSSInfo
}

// AppendSecinfoArgs appends the SECINFO argop: the name the client asks
// the security for.
func AppendSecinfoArgs(b []byte, name string) []byte {
	b = xdr.AppendUint32(b, OpSecinfo)
	return xdr.AppendString(b, name)
}

// AppendSecinfoNoNameArgs appends the SECINFO_NO_NAME argop: the
// secinfo_style4 enum alone, the whole argument RFC 7863 defines.
func AppendSecinfoNoNameArgs(b []byte, style uint32) []byte {
	b = xdr.AppendUint32(b, OpSecinfoNoName)
	return xdr.AppendUint32(b, style)
}

// DecodeSecinfoNoNameArgs decodes the secinfo_style4 enum from the
// decoder, which is positioned after the operation number.
func DecodeSecinfoNoNameArgs(d *xdr.Decoder) (uint32, error) {
	style, err := d.Uint32()
	if err != nil {
		return 0, err
	}
	switch style {
	case StyleCurrentFH, StyleParent:
		return style, nil
	default:
		return 0, ErrBadStyle
	}
}

// ErrBadStyle marks a secinfo_style4 value outside the enumeration.
var ErrBadStyle = fmt.Errorf("nfs4: unknown secinfo style")

// AppendSecinfoRes appends a successful SECINFO or SECINFO_NO_NAME
// result: one entry per accepted flavour, the RPCSEC_GSS branch with
// its rpcsec_gss_info and every other branch empty.
func AppendSecinfoRes(b []byte, entries []SecinfoEntry) []byte {
	b = xdr.AppendUint32(b, uint32(len(entries)))
	for _, e := range entries {
		b = xdr.AppendUint32(b, e.Flavor)
		if e.Flavor == SecFlavorGSS {
			b = xdr.AppendVarOpaque(b, e.GSS.OID)
			b = xdr.AppendUint32(b, e.GSS.QOP)
			b = xdr.AppendUint32(b, e.GSS.Service)
		}
	}
	return b
}

// DecodeSecinfoRes decodes a successful SECINFO result.
func DecodeSecinfoRes(body []byte) ([]SecinfoEntry, error) {
	d := xdr.NewDecoder(body)
	n, err := d.Uint32()
	if err != nil {
		return nil, err
	}
	if n > maxOps {
		return nil, fmt.Errorf("nfs4: %d secinfo entries exceeds %d", n, maxOps)
	}
	entries := make([]SecinfoEntry, 0, n)
	for range n {
		var e SecinfoEntry
		if e.Flavor, err = d.Uint32(); err != nil {
			return nil, err
		}
		if e.Flavor == SecFlavorGSS {
			g := &GSSInfo{}
			if g.OID, err = d.VarOpaque(); err != nil {
				return nil, err
			}
			if g.QOP, err = d.Uint32(); err != nil {
				return nil, err
			}
			if g.Service, err = d.Uint32(); err != nil {
				return nil, err
			}
			e.GSS = g
		}
		entries = append(entries, e)
	}
	return entries, nil
}

// Share access and deny bits of the OPEN operation, RFC 8881 section
// 16.16. The deny bits sit four positions above their access twins.
const (
	ShareAccessRead  = 0x1
	ShareAccessWrite = 0x2
	ShareAccessBoth  = 0x3
	ShareDenyRead    = 0x10
	ShareDenyWrite   = 0x20
	ShareDenyBoth    = 0x30
)

// Open claim types of the open_claim4 union.
const (
	ClaimNull           = 0
	ClaimPrevious       = 1
	ClaimDelegateC      = 2
	ClaimDelegatePrev   = 3
	ClaimDelegateCFh    = 4
	ClaimDelegatePrevFh = 5
	ClaimFH             = 6
)

// OPEN result flags.
const (
	OpenResultConfirm       = 0x1
	OpenResultLockTypePosix = 0x2
)

// AppendOpenArgs encodes the OPEN4args for a CLAIM_NULL open of name,
// optionally creating the file. The legacy seqid field is sent as zero,
// as the standard requires from a 4.1 client.
func AppendOpenArgs(b []byte, ownerClientid uint64, owner []byte, access, deny uint32, create bool, perm uint32, name string) []byte {
	b = xdr.AppendUint32(b, OpOpen)
	b = xdr.AppendUint32(b, 0) // seqid, deprecated
	b = xdr.AppendUint32(b, access)
	b = xdr.AppendUint32(b, deny)
	b = xdr.AppendUint64(b, ownerClientid)
	b = xdr.AppendVarOpaque(b, owner)
	if create {
		b = xdr.AppendUint32(b, 1) // OPEN4_CREATE
		b = xdr.AppendUint32(b, CreateUnchecked)
		b = AppendFattr(b, OfBits(AttrMode), Attrs{Mode: perm})
	} else {
		b = xdr.AppendUint32(b, 0) // OPEN4_NOCREATE
	}
	b = xdr.AppendUint32(b, ClaimNull)
	return xdr.AppendString(b, name)
}

// AppendOpenArgsPrevious encodes the OPEN4args for a CLAIM_PREVIOUS open:
// the reclaim of a delegation-backed open after the client restarted. The
// openflag carries NOCREATE and the claim carries no extra fields.
func AppendOpenArgsPrevious(b []byte, ownerClientid uint64, owner []byte, access, deny uint32) []byte {
	b = xdr.AppendUint32(b, OpOpen)
	b = xdr.AppendUint32(b, 0) // seqid, deprecated
	b = xdr.AppendUint32(b, access)
	b = xdr.AppendUint32(b, deny)
	b = xdr.AppendUint64(b, ownerClientid)
	b = xdr.AppendVarOpaque(b, owner)
	b = xdr.AppendUint32(b, 0) // OPEN4_NOCREATE
	return xdr.AppendUint32(b, ClaimPrevious)
}

// AppendOpenArgsExclusive41 encodes the OPEN4args of an EXCLUSIVE4_1
// create, RFC 8881 section 18.16: the verifier followed by the create
// attribute set.
func AppendOpenArgsExclusive41(b []byte, ownerClientid uint64, owner []byte, verifier [8]byte, perm uint32, name string) []byte {
	b = xdr.AppendUint32(b, OpOpen)
	b = xdr.AppendUint32(b, 0) // seqid, deprecated
	b = xdr.AppendUint32(b, ShareAccessBoth)
	b = xdr.AppendUint32(b, 0)
	b = xdr.AppendUint64(b, ownerClientid)
	b = xdr.AppendVarOpaque(b, owner)
	b = xdr.AppendUint32(b, 1) // OPEN4_CREATE
	b = xdr.AppendUint32(b, CreateExclusive1)
	b = append(b, verifier[:]...)
	b = AppendFattr(b, OfBits(AttrMode), Attrs{Mode: perm})
	b = xdr.AppendUint32(b, ClaimNull)
	return xdr.AppendString(b, name)
}

// A OpenArgs is the decoded OPEN4args.
type OpenArgs struct {
	Access        uint32
	Deny          uint32
	ClientID      uint64
	Owner         []byte
	Create        bool
	Guarded       bool    // the create mode refuses an existing name
	Exclusive     bool    // the create mode carries a create verifier
	ExclusiveVerf [8]byte // the verifier of an exclusive create
	Truncate      bool    // the create attributes named size 0, the shell redirect
	Perm          uint32
	Name          string
	Previous      bool    // CLAIM_PREVIOUS: the reclaim of a pre restart open
	Claim         uint32  // the open_claim4 selector as it arrived
	DelegStateid  Stateid // the delegation stateid of the DELEGATE_CUR claims
}

// DecodeOpenArgs decodes the OPEN4args from the decoder, which is
// positioned after the operation number.
func DecodeOpenArgs(d *xdr.Decoder) (OpenArgs, error) {
	var a OpenArgs
	if _, err := d.Uint32(); err != nil { // legacy seqid
		return a, err
	}
	var err error
	if a.Access, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Deny, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.ClientID, err = d.Uint64(); err != nil {
		return a, err
	}
	if a.Owner, err = d.VarOpaque(); err != nil {
		return a, err
	}
	opentype, err := d.Uint32()
	if err != nil {
		return a, err
	}
	if opentype == 1 {
		a.Create = true
		how, err := d.Uint32()
		if err != nil {
			return a, err
		}
		switch how {
		case CreateGuarded:
			a.Guarded = true
			fallthrough
		case CreateUnchecked:
			request, err := ReadBitmap(d)
			if err != nil {
				return a, err
			}
			attrblob, err := d.VarOpaque()
			if err != nil {
				return a, err
			}
			attrs, err := DecodeFattrAttrs(attrblob, request)
			if err != nil {
				return a, err
			}
			a.Perm = attrs.Mode
			// A create that names size 0 truncates an existing file:
			// the shape of a shell redirect over the file.
			if request.Has(AttrSize) && attrs.Size == 0 {
				a.Truncate = true
			}
		case CreateExclusive, CreateExclusive1:
			// The exclusive forms carry an eight byte create verifier
			// and, for EXCLUSIVE4_1, an attribute set, RFC 8881 section
			// 18.16. The server answers both as guarded creates keyed
			// by the verifier: a retry after a lost reply replays the
			// same verifier and succeeds, a create over an existing
			// name with a different verifier answers EXIST.
			a.Guarded = true
			a.Exclusive = true
			verf, verr := d.Raw(8)
			if verr != nil {
				return a, verr
			}
			copy(a.ExclusiveVerf[:], verf)
			if how == CreateExclusive1 {
				request, err := ReadBitmap(d)
				if err != nil {
					return a, err
				}
				attrblob, err := d.VarOpaque()
				if err != nil {
					return a, err
				}
				attrs, err := DecodeFattrAttrs(attrblob, request)
				if err != nil {
					return a, err
				}
				a.Perm = attrs.Mode
			}
		}
	}
	claim, err := d.Uint32()
	if err != nil {
		return a, err
	}
	a.Claim = claim
	switch claim {
	case ClaimNull, ClaimDelegateC, ClaimDelegatePrev:
		if a.Name, err = d.String(); err != nil {
			return a, err
		}
		if claim == ClaimDelegateC {
			raw, derr := d.Raw(16)
			if derr != nil {
				return a, derr
			}
			copy(a.DelegStateid[:], raw)
		}
	case ClaimPrevious:
		a.Previous = true
	case ClaimDelegateCFh, ClaimDelegatePrevFh, ClaimFH:
		// The file handle claims name no component and carry no
		// stateid: the current file handle is the reference, and a
		// delegation claim of that form is identified by the client and
		// the handle alone, RFC 8881 section 18.16.1.
	default:
		return a, ErrNotSuppName
	}
	return a, nil
}

// AppendOpenRes encodes a successful OPEN4resok without a delegation: the
// stateid of the open, the change information of the directory, no result
// flags, an empty attribute set and the NONE delegation.
func AppendOpenRes(b []byte, stateid Stateid) []byte {
	return AppendOpenResDeleg(b, stateid, OpenDelegNone, Stateid{})
}

// AppendCloseArgs encodes the CLOSE4args: the deprecated v4.0 seqid rides
// ahead of the stateid and goes out as zero, RFC 8881 section 18.2.3.
func AppendCloseArgs(b []byte, stateid Stateid) []byte {
	b = xdr.AppendUint32(b, OpClose)
	b = xdr.AppendUint32(b, 0) // seqid, deprecated
	return stateid.Append(b)
}

// AppendCloseRes encodes a successful CLOSE4res: the stateid, now with the
// sequence bumped one past its last use.
func AppendCloseRes(b []byte, stateid Stateid) []byte {
	return stateid.Append(b)
}

// ErrNotSuppName marks an OPEN the server answers NOT_SUPP to: the
// exclusive creation mode and the delegation claims.
var ErrNotSuppName = errors.New("nfs4: open form not supported")

// AppendOpenDowngradeArgs encodes the OPEN_DOWNGRADE argop: the deprecated
// seqid, the stateid of the open and the reduced share access and deny
// bits.
func AppendOpenDowngradeArgs(b []byte, stateid Stateid, access, deny uint32) []byte {
	b = xdr.AppendUint32(b, OpOpenDowngrade)
	b = xdr.AppendUint32(b, 0) // seqid, deprecated
	b = stateid.Append(b)
	b = xdr.AppendUint32(b, access)
	return xdr.AppendUint32(b, deny)
}

// AppendOpenDowngradeRes encodes a successful OPEN_DOWNGRADE result: the
// change information of the file.
func AppendOpenDowngradeRes(b []byte) []byte {
	return AppendChangeInfo(b)
}

// AppendDestroyClientIDArgs encodes the DESTROY_CLIENTID argop.
func AppendDestroyClientIDArgs(b []byte, clientid uint64) []byte {
	b = xdr.AppendUint32(b, OpDestroyClientID)
	return xdr.AppendUint64(b, clientid)
}

// AppendReclaimCompleteArgs encodes the RECLAIM_COMPLETE argop: the
// rca_one_client flag alone, the whole argument RFC 7863 defines. A
// client sends it false after it has finished reclaiming state.
func AppendReclaimCompleteArgs(b []byte, oneClient bool) []byte {
	b = xdr.AppendUint32(b, OpReclaimComplete)
	return xdr.AppendBool(b, oneClient)
}

// Open delegation types of the delegation4 union.
const (
	OpenDelegNone  = 0
	OpenDelegRead  = 1
	OpenDelegWrite = 2
)

// The nfs_space_limit4 limit_by discriminant, RFC 7863: this build
// grants a plain file size limit.
const LimitBySize = 1

// appendACE encodes the empty nfsace4 this build grants with every
// delegation: no access bits for anyone.
func appendACE(b []byte) []byte {
	b = xdr.AppendUint32(b, 0) // type
	b = xdr.AppendUint32(b, 0) // flag
	b = xdr.AppendUint32(b, 0) // access mask
	return xdr.AppendVarOpaque(b, nil)
}

// AppendDelegation encodes one delegation4 value, RFC 7863: the type
// followed by the type specific fields. The read form carries the
// delegation stateid, the recall flag and the permissions ace; the
// write form adds the space limit ahead of the ace.
func AppendDelegation(b []byte, delegType uint32, stateid Stateid, recall bool, maxSize uint64) []byte {
	b = xdr.AppendUint32(b, delegType)
	switch delegType {
	case OpenDelegRead:
		b = stateid.Append(b)
		b = xdr.AppendBool(b, recall)
		b = appendACE(b)
	case OpenDelegWrite:
		b = stateid.Append(b)
		b = xdr.AppendBool(b, recall)
		// nfs_space_limit4: the file size branch with the limit.
		b = xdr.AppendUint32(b, LimitBySize)
		b = xdr.AppendUint64(b, maxSize)
		b = appendACE(b)
	}
	return b
}

// readACE walks one nfsace4.
func readACE(d *xdr.Decoder) error {
	for range 3 {
		if _, err := d.Uint32(); err != nil {
			return err
		}
	}
	_, err := d.VarOpaque()
	return err
}

// DecodeOpenResDeleg decodes the delegation of a successful OPEN result:
// the stateid of the open, the delegation type and the delegation's own
// stateid.
func DecodeOpenResDeleg(body []byte) (openStateid Stateid, delegType uint32, delegStateid Stateid, err error) {
	d := xdr.NewDecoder(body)
	raw, rerr := d.Raw(16)
	if rerr != nil {
		err = rerr
		return
	}
	copy(openStateid[:], raw)
	// change_info4: atomic, before, after.
	if _, err = d.Bool(); err != nil {
		return
	}
	if _, err = d.Uint64(); err != nil {
		return
	}
	if _, err = d.Uint64(); err != nil {
		return
	}
	if _, err = d.Uint32(); err != nil { // rflags
		return
	}
	if _, err = ReadBitmap(d); err != nil { // attrset
		return
	}
	if delegType, err = d.Uint32(); err != nil {
		return
	}
	switch delegType {
	case OpenDelegRead:
		if delegStateid, err = readStateid(d); err != nil {
			return
		}
		if _, err = d.Bool(); err != nil { // recall
			return
		}
		if err = readACE(d); err != nil {
			return
		}
	case OpenDelegWrite:
		if delegStateid, err = readStateid(d); err != nil {
			return
		}
		if _, err = d.Bool(); err != nil { // recall
			return
		}
		// nfs_space_limit4: the limit_by union and its payload.
		limitBy, lerr := d.Uint32()
		if lerr != nil {
			err = lerr
			return
		}
		switch limitBy {
		case LimitBySize:
			if _, err = d.Uint64(); err != nil {
				return
			}
		case 2: // NFS_LIMIT_BLOCKS: num_blocks and bytes_per_block.
			if _, err = d.Uint32(); err != nil {
				return
			}
			if _, err = d.Uint32(); err != nil {
				return
			}
		default:
			err = ErrBadDeleg
			return
		}
		if err = readACE(d); err != nil {
			return
		}
	}
	return
}

// ErrBadDeleg marks a delegation4 body this decoder cannot walk.
var ErrBadDeleg = errors.New("nfs4: malformed delegation")

func readStateid(d *xdr.Decoder) (Stateid, error) {
	var s Stateid
	raw, err := d.Raw(16)
	if err != nil {
		return s, err
	}
	copy(s[:], raw)
	return s, nil
}

// AppendOpenResDeleg encodes a successful OPEN4res with a granted
// delegation: the stateid of the open, the change information, no result
// flags, an empty attribute set and the delegation union.
func AppendOpenResDeleg(b []byte, stateid Stateid, delegType uint32, delegSt Stateid) []byte {
	b = stateid.Append(b)
	b = xdr.AppendBool(b, false) // atomic
	b = xdr.AppendUint64(b, 0)   // before
	b = xdr.AppendUint64(b, 0)   // after
	b = xdr.AppendUint32(b, 0)   // rflags
	b = Bitmap{}.AppendTo(b)     // attrset
	return AppendDelegation(b, delegType, delegSt, false, 0)
}