Start on ATP

atalk/atp/atp.go

@@ -0,0 +1,146 @@
+/*
+   Copyright 2024 Josh Deprez
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package atp
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+)
+
+const (
+	ciFuncMask   = 0b11000000
+	ciFuncTReq   = 0b01000000
+	ciFuncTResp  = 0b10000000
+	ciFuncTRel   = 0b11000000
+	ciXOBit      = 0b00100000
+	ciEOMBit     = 0b00010000
+	ciSTSBit     = 0b00001000
+	ciTRelTOMask = 0b00000111
+)
+
+type TReq struct {
+	// Control information: 0 1 (XO) 0 0 (Timeout indicator)
+	ExactlyOnce          bool
+	TRelTimeoutIndicator uint8 // only if XO is set
+	Bitmap               uint8
+	TransactionID        uint16
+	UserBytes            [4]byte
+	Data                 []byte
+}
+
+func (p *TReq) Marshal() ([]byte, error) {
+	if p.TRelTimeoutIndicator != 0 && !p.ExactlyOnce {
+		return nil, fmt.Errorf("TRel timeout indicator %d only valid for XO TReq", p.TRelTimeoutIndicator)
+	}
+	if p.TRelTimeoutIndicator > 4 {
+		return nil, fmt.Errorf("invalid TRel timeout indicator [%d > 4]", p.TRelTimeoutIndicator)
+	}
+
+	b := bytes.NewBuffer(nil)
+	ci := byte(ciFuncTReq)
+	if p.ExactlyOnce {
+		ci |= ciXOBit
+	}
+	ci |= p.TRelTimeoutIndicator
+	b.WriteByte(ci)
+	b.WriteByte(p.Bitmap)
+	write16(b, p.TransactionID)
+	b.Write(p.UserBytes[:])
+	b.Write(p.Data)
+	return b.Bytes(), nil
+}
+
+type TResp struct {
+	// Control information: 1 0 0 (EOM) (STS) 0 0 0
+	EndOfMessage          bool
+	SendTransactionStatus bool
+	Sequence              uint8
+	TransactionID         uint16
+	UserBytes             [4]byte
+	Data                  []byte
+}
+
+func (p *TResp) Marshal() ([]byte, error) {
+	b := bytes.NewBuffer(nil)
+	ci := byte(ciFuncTResp)
+	if p.EndOfMessage {
+		ci |= ciEOMBit
+	}
+	if p.SendTransactionStatus {
+		ci |= ciSTSBit
+	}
+	b.WriteByte(ci)
+	b.WriteByte(p.Sequence)
+	write16(b, p.TransactionID)
+	b.Write(p.UserBytes[:])
+	b.Write(p.Data)
+	return b.Bytes(), nil
+}
+
+type TRel struct {
+	// Control information: 1 1 0 0 0 0 0 0
+	TransactionID uint16
+}
+
+func (p *TRel) Marshal() ([]byte, error) {
+	return []byte{
+		ciFuncTRel, 0,
+		byte(p.TransactionID >> 8),
+		byte(p.TransactionID & 0xFF),
+		0, 0, 0, 0,
+	}, nil
+}
+
+func UnmarshalPacket(data []byte) (any, error) {
+	if len(data) < 8 {
+		return nil, fmt.Errorf("insufficient input length %d for ATP packet", len(data))
+	}
+	switch data[0] & ciFuncMask {
+	case ciFuncTReq:
+		return &TReq{
+			ExactlyOnce:          data[0]&ciXOBit != 0,
+			TRelTimeoutIndicator: data[0] & ciTRelTOMask,
+			Bitmap:               data[1],
+			TransactionID:        binary.BigEndian.Uint16(data[2:4]),
+			UserBytes:            [4]byte(data[4:8]),
+			Data:                 data[8:],
+		}, nil
+
+	case ciFuncTResp:
+		return &TResp{
+			EndOfMessage:          data[0]&ciEOMBit != 0,
+			SendTransactionStatus: data[0]&ciSTSBit != 0,
+			Sequence:              data[1],
+			TransactionID:         binary.BigEndian.Uint16(data[2:4]),
+			UserBytes:             [4]byte(data[4:8]),
+			Data:                  data[8:],
+		}, nil
+
+	case ciFuncTRel:
+		return &TRel{
+			TransactionID: binary.BigEndian.Uint16(data[2:4]),
+		}, nil
+
+	default:
+		return nil, fmt.Errorf("unknown ATP function in control information byte %b", data[0])
+	}
+}
+
+func write16[I ~uint16](b *bytes.Buffer, n I) {
+	b.Write([]byte{byte(n >> 8), byte(n & 0xff)})
+}