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

// The session establishment layer of RFC 8881 sections 18.35 to 18.37 and
// 18.46: EXCHANGE_ID names the client, CREATE_SESSION makes the session
// with its slot table, SEQUENCE drives the slots, DESTROY_SESSION tears
// one down.

package nfs4

import (
	"encoding/binary"
	"errors"

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

// EXCHGID4_FLAG values, RFC 8881 section 18.35.
const (
	ExchgIDSuppMovedRefer   = 0x00000001
	ExchgIDSuppMovedMigr    = 0x00000002
	ExchgIDSuppFenceOps     = 0x00000004
	ExchgIDBindPrincStateid = 0x00000100
	ExchgIDUseNonPnfs       = 0x00010000
	ExchgIDUsePnfsMds       = 0x00020000
	ExchgIDUsePnfsDs        = 0x00040000
	ExchgIDConfirmedR       = 0x80000000
)

// CREATE_SESSION4_FLAG values.
const (
	CreateSessionFlagPersist      = 0x00000001
	CreateSessionFlagConnBackChan = 0x00000002
	CreateSessionFlagConnRdma     = 0x00000004
)

// state protection choices of the EXCHANGE_ID unions, the
// state_protect_how4 enumeration of RFC 8881 section 18.35. Only the
// plain SP4_NONE is spoken by this build.
const (
	StateProtectNone     = 0 // SP4_NONE
	StateProtectMachCred = 1 // SP4_MACH_CRED
	StateProtectSSV      = 2 // SP4_SSV
)

// A SessionID is the 16 byte session identifier RFC 7863 fixes
// (NFS4_SESSIONID_SIZE): an 8 byte server prefix followed by the 8 byte
// client id, both chosen by the server.
type SessionID [16]byte

// ClientIDOf splits the session id into its client id half.
func (s SessionID) ClientIDOf() uint64 {
	return binary.BigEndian.Uint64(s[8:16])
}

// MakeSessionID builds a session id from the server prefix and the client
// id.
func MakeSessionID(prefix [8]byte, clientid uint64) SessionID {
	var s SessionID
	copy(s[:8], prefix[:])
	binary.BigEndian.PutUint64(s[8:], clientid)
	return s
}

// MakeNumberedSessionID builds a session id whose server half carries a
// per-session number, so every CREATE_SESSION mints a distinct id as
// RFC 8881 section 18.36 requires: four prefix bytes, the number and
// the client id.
func MakeNumberedSessionID(prefix [4]byte, number uint32, clientid uint64) SessionID {
	var s SessionID
	copy(s[:4], prefix[:])
	binary.BigEndian.PutUint32(s[4:], number)
	binary.BigEndian.PutUint64(s[8:], clientid)
	return s
}

// A ChannelAttrs is the channel_attrs4 of a session: the sizes and limits
// of one connection direction.
type ChannelAttrs struct {
	HeaderPad        uint32
	MaxRequest       uint32
	MaxResponse      uint32
	MaxRespResourced uint32
	MaxOperations    uint32
	MaxRequests      uint32
	RdmaIRDEnabled   bool
	RdmaIRSizes      []uint32
}

// DefaultForeChannel is the fore channel this server grants: enough slots
// for a real client, bounded to what one connection streams.
var DefaultForeChannel = ChannelAttrs{
	MaxRequest:       1 << 22,
	MaxResponse:      1 << 22,
	MaxRespResourced: 1 << 22,
	MaxOperations:    16,
	MaxRequests:      8,
}

// DefaultBackChannel declares the minimal back channel: one operation and
// one request, with no channel attributes, which the client reads as
// nothing offered.
var DefaultBackChannel = ChannelAttrs{MaxRequest: 1 << 20, MaxResponse: 1 << 20, MaxRespResourced: 1 << 20, MaxOperations: 1, MaxRequests: 1}

// ErrOpMisordered marks a SEQUENCE that arrived on a slot with a sequence
// that is neither a retry nor the next one.
var ErrOpMisordered = errors.New("nfs4: sequence misordered")

// ExchangeIDArgs is the decoded EXCHANGE_ID4args.
type ExchangeIDArgs struct {
	Verifier [8]byte
	OwnerID  []byte
	Flags    uint32
	Protect  uint32
}

// AppendExchangeIDArgs encodes the EXCHANGE_ID4args. The implementation id
// array is sent empty.
func AppendExchangeIDArgs(b []byte, verifier [8]byte, ownerID []byte, flags uint32) []byte {
	b = xdr.AppendUint32(b, OpExchangeID)
	b = append(b, verifier[:]...)
	b = xdr.AppendVarOpaque(b, ownerID)
	b = xdr.AppendUint32(b, flags)
	b = xdr.AppendUint32(b, StateProtectNone)
	return xdr.AppendUint32(b, 0) // empty eia_client_impl_id
}

// DecodeExchangeIDArgs decodes the EXCHANGE_ID4args from the decoder,
// which is positioned after the operation number.
func DecodeExchangeIDArgs(d *xdr.Decoder) (ExchangeIDArgs, error) {
	var a ExchangeIDArgs
	verf, err := d.Raw(8)
	if err != nil {
		return a, err
	}
	copy(a.Verifier[:], verf)
	if a.OwnerID, err = d.VarOpaque(); err != nil {
		return a, err
	}
	if a.Flags, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Protect, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Protect != StateProtectNone {
		return a, ErrStateProtectNotSupp
	}
	// The client implementation id array is walked and ignored.
	n, err := d.Uint32()
	if err != nil {
		return a, err
	}
	for range n {
		if _, err = d.String(); err != nil {
			return a, err
		}
		if _, err = d.String(); err != nil {
			return a, err
		}
		if _, err = d.Uint64(); err != nil {
			return a, err
		}
		if _, err = d.Uint32(); err != nil {
			return a, err
		}
	}
	return a, nil
}

// AppendExchangeIDRes encodes a successful EXCHANGE_ID4res: the client id,
// the create session sequence, the flags, the plain state protection, the
// server owner, the server scope and one implementation id entry, RFC 8881
// section 18.35. The field order and the entry shape follow what real
// clients decode: the minor id is a uint64 and the major id a bounded
// opaque, the scope follows the owner, and the entries carry no dummy.
func AppendExchangeIDRes(b []byte, clientid uint64, sequence, flags uint32, majorID []byte) []byte {
	b = xdr.AppendUint64(b, clientid)
	b = xdr.AppendUint32(b, sequence)
	b = xdr.AppendUint32(b, flags)
	b = xdr.AppendUint32(b, StateProtectNone)
	// eir_server_owner: the major id stays constant across restarts so
	// the client recognises the server.
	b = xdr.AppendUint64(b, 0)
	b = xdr.AppendVarOpaque(b, majorID)
	// eir_server_scope: the servers that share this one's state. The
	// Linux client fails the exchange when the field is missing.
	b = xdr.AppendVarOpaque(b, majorID)
	// eir_server_impl_id: one entry of domain, name and the zeroed date.
	b = xdr.AppendUint32(b, 1)
	b = xdr.AppendString(b, "sourcedock.dev")
	b = xdr.AppendString(b, "nfsd")
	b = xdr.AppendInt64(b, 0)
	b = xdr.AppendUint32(b, 0)
	return b
}

// AppendSessionChannel encodes a channel_attrs4.
func AppendSessionChannel(b []byte, c ChannelAttrs) []byte {
	b = xdr.AppendUint32(b, c.HeaderPad)
	b = xdr.AppendUint32(b, c.MaxRequest)
	b = xdr.AppendUint32(b, c.MaxResponse)
	b = xdr.AppendUint32(b, c.MaxRespResourced)
	b = xdr.AppendUint32(b, c.MaxOperations)
	b = xdr.AppendUint32(b, c.MaxRequests)
	if c.RdmaIRDEnabled {
		b = xdr.AppendUint32(b, uint32(len(c.RdmaIRSizes)))
		for _, v := range c.RdmaIRSizes {
			b = xdr.AppendUint32(b, v)
		}
		return b
	}
	// ca_rdma_ird is a counted array, RFC 8881 section 18.36: an empty
	// one counts zero. Every real client decoder rejects anything else;
	// the Linux client answers EINVAL for a count above one.
	return xdr.AppendUint32(b, 0)
}

// DecodeSessionChannel decodes a channel_attrs4 body.
func DecodeSessionChannel(d *xdr.Decoder) (ChannelAttrs, error) {
	var c ChannelAttrs
	var err error
	if c.HeaderPad, err = d.Uint32(); err != nil {
		return c, err
	}
	if c.MaxRequest, err = d.Uint32(); err != nil {
		return c, err
	}
	if c.MaxResponse, err = d.Uint32(); err != nil {
		return c, err
	}
	if c.MaxRespResourced, err = d.Uint32(); err != nil {
		return c, err
	}
	if c.MaxOperations, err = d.Uint32(); err != nil {
		return c, err
	}
	if c.MaxRequests, err = d.Uint32(); err != nil {
		return c, err
	}
	count, err := d.Uint32()
	if err != nil {
		return c, err
	}
	// ca_rdma_ird is a counted array: zero means no RDMA IRD support.
	if count > 0 {
		c.RdmaIRDEnabled = true
		for range count {
			v, verr := d.Uint32()
			if verr != nil {
				return c, verr
			}
			c.RdmaIRSizes = append(c.RdmaIRSizes, v)
		}
	}
	return c, nil
}

// AppendCreateSessionArgs encodes the CREATE_SESSION4args.
func AppendCreateSessionArgs(b []byte, clientid uint64, sequence, flags uint32, fore, back ChannelAttrs, cbProgram uint32) []byte {
	b = xdr.AppendUint32(b, OpCreateSession)
	b = xdr.AppendUint64(b, clientid)
	b = xdr.AppendUint32(b, sequence)
	b = xdr.AppendUint32(b, flags)
	b = AppendSessionChannel(b, fore)
	b = AppendSessionChannel(b, back)
	return xdr.AppendUint32(b, cbProgram)
}

// A CreateSessionArgs is the decoded CREATE_SESSION4args.
type CreateSessionArgs struct {
	ClientID  uint64
	Sequence  uint32
	Flags     uint32
	Fore      ChannelAttrs
	Back      ChannelAttrs
	CBProgram uint32
}

// DecodeCreateSessionArgs decodes the CREATE_SESSION4args from the
// decoder, which is positioned after the operation number.
func DecodeCreateSessionArgs(d *xdr.Decoder) (CreateSessionArgs, error) {
	var a CreateSessionArgs
	var err error
	if a.ClientID, err = d.Uint64(); err != nil {
		return a, err
	}
	if a.Sequence, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Flags, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Fore, err = DecodeSessionChannel(d); err != nil {
		return a, err
	}
	if a.Back, err = DecodeSessionChannel(d); err != nil {
		return a, err
	}
	if a.CBProgram, err = d.Uint32(); err != nil {
		return a, err
	}
	return a, nil
}

// AppendCreateSessionRes encodes a successful CREATE_SESSION4res.
func AppendCreateSessionRes(b []byte, id SessionID, sequence, flags uint32, fore, back ChannelAttrs) []byte {
	b = append(b, id[:]...)
	b = xdr.AppendUint32(b, sequence)
	b = xdr.AppendUint32(b, flags)
	b = AppendSessionChannel(b, fore)
	return AppendSessionChannel(b, back)
}

// AppendDestroySessionArgs appends the DESTROY_SESSION argop.
func AppendDestroySessionArgs(b []byte, id SessionID) []byte {
	b = xdr.AppendUint32(b, OpDestroySession)
	return append(b, id[:]...)
}

// AppendSequenceArgs appends the SEQUENCE argop.
func AppendSequenceArgs(b []byte, id SessionID, sequence, slot, highestSlot uint32, cacheThis bool) []byte {
	b = xdr.AppendUint32(b, OpSequence)
	b = append(b, id[:]...)
	b = xdr.AppendUint32(b, sequence)
	b = xdr.AppendUint32(b, slot)
	b = xdr.AppendUint32(b, highestSlot)
	return xdr.AppendBool(b, cacheThis)
}

// A SequenceArgs is the decoded SEQUENCE4args.
type SequenceArgs struct {
	SessionID   SessionID
	Sequence    uint32
	Slot        uint32
	HighestSlot uint32
	CacheThis   bool
}

// DecodeSequenceArgs decodes the SEQUENCE4args from the decoder, which
// is positioned after the operation number.
func DecodeSequenceArgs(d *xdr.Decoder) (SequenceArgs, error) {
	var a SequenceArgs
	raw, err := d.Raw(16)
	if err != nil {
		return a, err
	}
	copy(a.SessionID[:], raw)
	if a.Sequence, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.Slot, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.HighestSlot, err = d.Uint32(); err != nil {
		return a, err
	}
	if a.CacheThis, err = d.Bool(); err != nil {
		return a, err
	}
	return a, nil
}

// NegotiateChannel clamps the requested channel attributes to the
// server's limits, RFC 8881 section 18.36: every negotiated value is the
// smaller of the request and the limit. A zero request carries meaning,
// ca_maxresponsesize_cached of zero above all, so it is answered with
// zero. Real clients reject a reply that exceeds their request.
func NegotiateChannel(requested ChannelAttrs, limits ChannelAttrs) ChannelAttrs {
	nv := func(requested, limit uint32) uint32 {
		if requested > limit {
			return limit
		}
		return requested
	}
	return ChannelAttrs{
		MaxRequest:       nv(requested.MaxRequest, limits.MaxRequest),
		MaxResponse:      nv(requested.MaxResponse, limits.MaxResponse),
		MaxRespResourced: nv(requested.MaxRespResourced, limits.MaxRespResourced),
		MaxOperations:    nv(requested.MaxOperations, limits.MaxOperations),
		MaxRequests:      nv(requested.MaxRequests, limits.MaxRequests),
	}
}

// AppendSequenceRes encodes the successful SEQUENCE4resok of RFC 7863:
// the session id, the echoed identifiers and the status flags. The
// target highest slot mirrors the highest slot this build grants; a
// caller with its own view passes it instead.
func AppendSequenceRes(b []byte, id SessionID, sequence, slot, highestSlot, statusFlags uint32) []byte {
	b = append(b, id[:]...)
	b = xdr.AppendUint32(b, sequence)
	b = xdr.AppendUint32(b, slot)
	b = xdr.AppendUint32(b, highestSlot)
	b = xdr.AppendUint32(b, highestSlot) // sr_target_highest_slotid
	return xdr.AppendUint32(b, statusFlags)
}

// DecodeSequenceRes decodes the successful SEQUENCE4resok.
func DecodeSequenceRes(d *xdr.Decoder) (id SessionID, sequence, slot, highestSlot, targetHighestSlot, statusFlags uint32, err error) {
	raw, err := d.Raw(16)
	if err != nil {
		return
	}
	copy(id[:], raw)
	if sequence, err = d.Uint32(); err != nil {
		return
	}
	if slot, err = d.Uint32(); err != nil {
		return
	}
	if highestSlot, err = d.Uint32(); err != nil {
		return
	}
	if targetHighestSlot, err = d.Uint32(); err != nil {
		return
	}
	statusFlags, err = d.Uint32()
	return
}

// ErrStateProtectNotSupp marks an EXCHANGE_ID that named a state
// protection this build does not speak.
var ErrStateProtectNotSupp = errors.New("nfs4: state protection not supported")