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

// The RPC-over-RDMA version one framing of RFC 8166: the fixed header,
// the chunk lists and the message assembly. The transfer of the chunks
// themselves belongs to RDMA verbs, which no pure Go stack can reach
// without cgo; this layer speaks the framing over a stream transport
// (TCP in the tests), where every chunk list stands empty and the whole
// message rides inline, wire compatible with an RDMA peer that
// registers nothing.

package rdma

import (
	"encoding/binary"
	"errors"
)

// The protocol constants of RFC 8166 section 4.
const (
	Version = 1

	ProcMsg   = 0
	ProcNomsg = 1
	ProcError = 4

	ErrVers      = 1
	ErrChunk     = 2
	MaxSupported = 1
)

// The transport magic of the RDMA echo of the record marking layer: a
// frame on the stream transport carries one RDMA message.
const frameMagic = 0x52444d31 // "RDM1"

// ErrFrame marks a malformed or unsupported frame.
var ErrFrame = errors.New("rdma: bad frame")

// A Segment is one xdr_rdma_segment: the registered memory handle, the
// chunk length and the remote offset.
type Segment struct {
	Handle uint32
	Length uint32
	Offset uint64
}

// A ReadChunk is one xdr_read_chunk: the position in the XDR stream
// and the segment that carries the bytes.
type ReadChunk struct {
	Position uint32
	Segment  Segment
}

// A WriteChunk is one xdr_write_chunk: a segment list for one reply
// piece.
type WriteChunk struct {
	Segments []Segment
}

// A Header is the decoded RPC-over-RDMA frame: the fixed four fields,
// the chunk lists and the inline payload after them.
type Header struct {
	XID     uint32
	Version uint32
	Credit  uint32
	Proc    uint32
	Reads   []ReadChunk
	Writes  []WriteChunk
	Reply   *WriteChunk
	// Payload carries the inline RPC call or reply bytes; empty for
	// RDMA_NOMSG frames whose payload rides the chunks.
	Payload []byte
}

// appendSegment encodes one xdr_rdma_segment.
func appendSegment(b []byte, s Segment) []byte {
	b = binary.BigEndian.AppendUint32(b, s.Handle)
	b = binary.BigEndian.AppendUint32(b, s.Length)
	return binary.BigEndian.AppendUint64(b, s.Offset)
}

// appendReadList encodes the optional read list: entries until the
// zero handle terminator.
func appendReadList(b []byte, reads []ReadChunk) []byte {
	for _, r := range reads {
		b = binary.BigEndian.AppendUint32(b, r.Position)
		b = appendSegment(b, r.Segment)
	}
	return binary.BigEndian.AppendUint32(b, 0) // terminator
}

// appendWriteList encodes the optional write list: each chunk leads
// with its segment count and the list closes with a zero count word,
// RFC 8166 section 4.3.2.
func appendWriteList(b []byte, writes []WriteChunk) []byte {
	for _, w := range writes {
		b = binary.BigEndian.AppendUint32(b, uint32(len(w.Segments)))
		for _, s := range w.Segments {
			b = appendSegment(b, s)
		}
	}
	return binary.BigEndian.AppendUint32(b, 0)
}

// AppendFrame encodes one RPC-over-RDMA message: the fixed header, the
// empty chunk lists of an inline transfer and the payload. A nil
// payload makes an RDMA_NOMSG frame.
func AppendFrame(b []byte, h Header) []byte {
	b = binary.BigEndian.AppendUint32(b, h.XID)
	b = binary.BigEndian.AppendUint32(b, Version)
	b = binary.BigEndian.AppendUint32(b, h.Credit)
	if h.Payload == nil {
		b = binary.BigEndian.AppendUint32(b, ProcNomsg)
	} else {
		b = binary.BigEndian.AppendUint32(b, ProcMsg)
	}
	b = appendReadList(b, h.Reads)
	b = appendWriteList(b, h.Writes)
	// The reply chunk count is always present: zero when no reply chunk
	// rides the frame.
	if h.Reply != nil {
		b = binary.BigEndian.AppendUint32(b, uint32(len(h.Reply.Segments)))
		for _, s := range h.Reply.Segments {
			b = appendSegment(b, s)
		}
	} else {
		b = binary.BigEndian.AppendUint32(b, 0)
	}
	if h.Payload != nil {
		b = binary.BigEndian.AppendUint32(b, uint32(len(h.Payload)))
		b = append(b, h.Payload...)
	}
	return b
}

// DecodeFrame decodes one RPC-over-RDMA frame from the payload of a
// stream frame. The chunk lists are parsed and skipped: an inline
// transport never carries registered memory.
func DecodeFrame(frame []byte) (Header, error) {
	h := Header{}
	if len(frame) < 16 {
		return h, ErrFrame
	}
	h.XID = binary.BigEndian.Uint32(frame[0:])
	if binary.BigEndian.Uint32(frame[4:]) != Version {
		return h, ErrFrame
	}
	h.Credit = binary.BigEndian.Uint32(frame[8:])
	h.Proc = binary.BigEndian.Uint32(frame[12:])
	off := 16

	// The read list: entries until a zero position.
	for {
		if off+4 > len(frame) {
			return h, ErrFrame
		}
		pos := binary.BigEndian.Uint32(frame[off:])
		off += 4
		if pos == 0 {
			break
		}
		if off+16 > len(frame) {
			return h, ErrFrame
		}
		var rc ReadChunk
		rc.Position = pos
		rc.Segment.Handle = binary.BigEndian.Uint32(frame[off:])
		rc.Segment.Length = binary.BigEndian.Uint32(frame[off+4:])
		rc.Segment.Offset = binary.BigEndian.Uint64(frame[off+8:])
		off += 16
		h.Reads = append(h.Reads, rc)
	}

	// The write list: chunks each led by their segment count, until a
	// zero count word closes the list. The reply chunk follows as one
	// more count-plus-segments group, RFC 8166 section 4.3.2.
	for {
		if off+4 > len(frame) {
			return h, ErrFrame
		}
		n := binary.BigEndian.Uint32(frame[off:])
		off += 4
		if n != 0 {
			var wc WriteChunk
			for i := uint32(0); i < n; i++ {
				if off+16 > len(frame) {
					return h, ErrFrame
				}
				var s Segment
				s.Handle = binary.BigEndian.Uint32(frame[off:])
				s.Length = binary.BigEndian.Uint32(frame[off+4:])
				s.Offset = binary.BigEndian.Uint64(frame[off+8:])
				off += 16
				wc.Segments = append(wc.Segments, s)
			}
			h.Writes = append(h.Writes, wc)
			continue
		}
		// The zero closes the write list; the word after it is the reply
		// chunk count.
		if off+4 > len(frame) {
			return h, ErrFrame
		}
		n = binary.BigEndian.Uint32(frame[off:])
		off += 4
		if n != 0 {
			h.Reply = &WriteChunk{}
			for i := uint32(0); i < n; i++ {
				if off+16 > len(frame) {
					return h, ErrFrame
				}
				var s Segment
				s.Handle = binary.BigEndian.Uint32(frame[off:])
				s.Length = binary.BigEndian.Uint32(frame[off+4:])
				s.Offset = binary.BigEndian.Uint64(frame[off+8:])
				off += 16
				h.Reply.Segments = append(h.Reply.Segments, s)
			}
		}
		break
	}
	if h.Proc == ProcMsg {
		if off+4 > len(frame) {
			return h, ErrFrame
		}
		n := binary.BigEndian.Uint32(frame[off:])
		off += 4
		if off+int(n) > len(frame) {
			return h, ErrFrame
		}
		h.Payload = frame[off : off+int(n)]
	}
	return h, nil
}

// AppendStreamFrame frames one RDMA message for a stream transport:
// the magic, the payload length and the message. A decode of a frame
// written this way answers DecodeFrame(frame).
func AppendStreamFrame(b []byte, h Header) []byte {
	msg := AppendFrame(nil, h)
	b = binary.BigEndian.AppendUint32(b, frameMagic)
	b = binary.BigEndian.AppendUint32(b, uint32(len(msg)))
	return append(b, msg...)
}

// ReadStreamFrame splits one framed message off the front of buf: it
// answers the frame payload, the bytes consumed, and whether a whole
// frame is present.
func ReadStreamFrame(buf []byte) (frame []byte, consumed int, ok bool, err error) {
	if len(buf) < 8 {
		return nil, 0, false, nil
	}
	if binary.BigEndian.Uint32(buf[0:]) != frameMagic {
		return nil, 0, false, ErrFrame
	}
	n := binary.BigEndian.Uint32(buf[4:])
	if int(n)+8 > len(buf) {
		return nil, 0, false, nil
	}
	return buf[8 : 8+n], 8 + int(n), true, nil
}