Util: Add a function to calculate the gateway

2 files changed, 115 insertions, 0 deletions

diff --git a/util/util.go b/util/util.go
new file mode 100644
index 0000000..c7816df
--- /dev/null
+++ b/util/util.go
@@ -0,0 +1,33 @@
+// package util defines public utility functions to be used by applications
+// these are outside of the client package as they can be used even if a client hasn't been created yet
+package util
+
+import (
+	"net"
+
+	"github.com/eduvpn/eduvpn-common/i18nerr"
+)
+
+// CalculateGateway takes a CIDR encoded subnet `cidr` and returns the gateway and an error
+func CalculateGateway(cidr string) (string, error) {
+	_, ipn, err := net.ParseCIDR(cidr)
+	if err != nil {
+		return "", i18nerr.WrapInternalf(err, "failed to parse CIDR for calculating gateway: %v", cidr)
+	}
+
+	ret := make(net.IP, len(ipn.IP))
+	copy(ret, ipn.IP)
+
+	for i := len(ret) - 1; i >= 0; i-- {
+		ret[i]++
+		if ret[i] > 0 {
+			break
+		}
+	}
+
+	if !ipn.Contains(ret) {
+		return "", i18nerr.Newf("IP network does not contain incremented IP: %v", ret)
+	}
+
+	return ret.String(), nil
+}
diff --git a/util/util_test.go b/util/util_test.go
new file mode 100644
index 0000000..0f4888d
--- /dev/null
+++ b/util/util_test.go
@@ -0,0 +1,82 @@
+package util
+
+import (
+	"testing"
+
+	"github.com/eduvpn/eduvpn-common/internal/test"
+)
+
+func TestCalculateGateway(t *testing.T) {
+	cases := []struct {
+		in   string
+		want string
+		err  string
+	}{
+		// normal cases
+		{
+			in:   "10.10.10.5/24",
+			want: "10.10.10.1",
+			err:  "",
+		},
+		{
+			in:   "10.10.10.130/25",
+			want: "10.10.10.129",
+			err:  "",
+		},
+		{
+			in:   "fd42::5/112",
+			want: "fd42::1",
+			err:  "",
+		},
+		{
+			in:   "5502:df9::/64",
+			want: "5502:df9::1",
+			err:  "",
+		},
+		// unrealistic scenario but we have to handle these!
+		{
+			in:   "5502:df9::0/128",
+			want: "",
+			err:  "IP network does not contain incremented IP: 5502:df9::1",
+		},
+		{
+			in:   "5502:df9::ffff/128",
+			want: "",
+			err:  "IP network does not contain incremented IP: 5502:df9::1:0",
+		},
+		{
+			in:   "10.0.0.0/32",
+			want: "",
+			err:  "IP network does not contain incremented IP: 10.0.0.1",
+		},
+		{
+			in:   "10.0.0.255/32",
+			want: "",
+			err:  "IP network does not contain incremented IP: 10.0.1.0",
+		},
+		// parsing errors
+		{
+			in:   "10.0.0.1",
+			want: "",
+			err:  "An internal error occurred. The cause of the error is: invalid CIDR address: 10.0.0.1.",
+		},
+		{
+			in:   "bla",
+			want: "",
+			err:  "An internal error occurred. The cause of the error is: invalid CIDR address: bla.",
+		},
+		{
+			in:   "5502:df9::ffff",
+			want: "",
+			err:  "An internal error occurred. The cause of the error is: invalid CIDR address: 5502:df9::ffff.",
+		},
+	}
+
+	for _, c := range cases {
+		got, err := CalculateGateway(c.in)
+		test.AssertError(t, err, c.err)
+		if got != c.want {
+			t.Fatalf("got: %v not equal to want: %v", got, c.want)
+		}
+	}
+}