package parser

import (
    "encoding/binary"
    "fmt"
    "net"
)

// ARP ref: https://zh.wikipedia.org/zh-cn/%E5%9C%B0%E5%9D%80%E8%A7%A3%E6%9E%90%E5%8D%8F%E8%AE%AE#%E6%8A%A5%E6%96%87%E6%A0%BC%E5%BC%8F
type ARP struct {
    BaseLayer
    HardwareType    uint16           // 硬件类型
    ProtoType       uint16           // 协议类型
    HwAddressSize   uint8            // 硬件地址长度
    ProtoAddrSize   uint8            // 协议地址长度
    Opcode          uint16           // 操作码
    SrcHwAddress    net.HardwareAddr // 源硬件地址
    SrcProtoAddress net.IP           // 源协议地址
    DstHwAddress    net.HardwareAddr // 目标硬件地址
    DstProtoAddress net.IP           // 目标协议地址
}

func (A *ARP) LayerType() int {
    return LayerTypeARP
}

// DecodeARP ref: https://zh.wikipedia.org/zh-cn/%E5%9C%B0%E5%9D%80%E8%A7%A3%E6%9E%90%E5%8D%8F%E8%AE%AE#%E6%8A%A5%E6%96%87%E6%A0%BC%E5%BC%8F
func DecodeARP(data []byte) (*ARP, error) {
    // 至少要有8bytes
    if len(data) < 8 {
        return nil, fmt.Errorf("[arp] 数据包太短: %d bytes", len(data))
    }

    arp := ARP{}
    arp.HardwareType = binary.BigEndian.Uint16(data[0:2])
    arp.ProtoType = binary.BigEndian.Uint16(data[2:4])
    arp.HwAddressSize = data[4]
    arp.ProtoAddrSize = data[5]
    arp.Opcode = binary.BigEndian.Uint16(data[6:8])
    arpLen := 8 + 2*arp.HwAddressSize + 2*arp.ProtoAddrSize
    if len(data) < int(arpLen) {
        return nil, fmt.Errorf("[arp] 实际长度小于声明长度：实际：%d，声明：%d", len(data), arpLen)
    }

    var offset uint8 = 8

    arp.SrcHwAddress = data[offset : offset+arp.HwAddressSize]
    offset += arp.HwAddressSize

    arp.SrcProtoAddress = data[offset : offset+arp.ProtoAddrSize]
    offset += arp.ProtoAddrSize

    arp.DstHwAddress = data[offset : offset+arp.HwAddressSize]
    offset += arp.HwAddressSize

    arp.DstProtoAddress = data[offset : offset+arp.ProtoAddrSize]

    arp.Header = data[:arpLen]
    arp.Payload = data[arpLen:]
    return &arp, nil
}