Reverse IP Converter in Go

README.md

Reverse IP

Converts an IP to the reverse DNS Style:

		{
			name:   "v4",
			IP:     net.ParseIP("1.2.3.4"),
			output: "4.3.2.1",
		},
		{
			name:   "v4",
			IP:     net.ParseIP("255.255.255.255"),
			output: "255.255.255.255",
		},
		{
			name:   "v6",
			IP:     net.ParseIP("2001::"),
			output: "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.2",
		},

=== RUN   Test/v4
    test_scratch_9_test.go:43: 22515540	        49.38 ns/op
    --- PASS: Test/v4 (2.15s)
=== RUN   Test/v4#01
    test_scratch_9_test.go:43: 17212569	        71.24 ns/op
    --- PASS: Test/v4#01 (1.30s)
=== RUN   Test/v6
    test_scratch_9_test.go:43: 23410729	        50.83 ns/op
    --- PASS: Test/v6 (1.24s)
PASS

reverse_ip.go

func reverseaddr(ip net.IP) string {
	const hexDigit = "0123456789abcdef"

	buf := make([]byte, 0, len(ip)*4)
	if v4 := ip.To4(); v4 != nil {
		for i := len(v4) - 1; i >= 0; i-- {
			buf = append(buf, '.')
			buf = strconv.AppendUint(buf, uint64(v4[i]), 10)
		}
	} else {
		for i := len(ip) - 1; i >= 0; i-- {
			buf = append(buf, '.', hexDigit[ip[i]&0xF], '.', hexDigit[ip[i]>>4])
		}
	}

	return b2s(buf[1:])
}

func b2s(b []byte) string {
	return *(*string)(unsafe.Pointer(&b))
}