Provide a ListOpts helper

With `query.ListOpts`, it should be easier to list resources based on
repeated properties. For example: get information about multiple ports
by ID with a single call.

ListOpts is currently implemented for three Network resources:
* ports
* networks
* subnets

query/list.go

@@ -0,0 +1,85 @@
+package query
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func New() *ListOpts {
+	return &ListOpts{}
+}
+
+// ListOpts can be used to list multiple resources.
+type ListOpts struct {
+	query url.Values
+}
+
+// And adds an arbitrary number of permutations of a single property to filter
+// in. When a single ListOpts is called multiple times with the same property
+// name, the resulting query contains the resulting intersection (AND). Note
+// that how these properties are combined in OpenStack depend on the property.
+// For example: passing multiple "id" behaves like an OR. Instead, passing
+// multiple "tags" will only return resources that have ALL those tags. This
+// helper function only combines the parameters in the most straightforward
+// way; please refer to the OpenStack documented behaviour to know how these
+// parameters are treated.
+//
+// ListOpts is currently implemented for three Network resources:
+//
+// * ports
+// * networks
+// * subnets
+func (o *ListOpts) And(property string, values ...interface{}) *ListOpts {
+	if o.query == nil {
+		o.query = url.Values{}
+	}
+	if existingValues, ok := o.query[property]; ok {
+		// There already are values of the same property: we AND them
+		// with the new ones. We only keep the values that exist in
+		// both `o.query` AND in `values`.
+
+		// First, to avoid nested loops, we build a hashmap with the
+		// new values.
+		newValuesSet := make(map[string]struct{})
+		for _, newValue := range values {
+			newValuesSet[fmt.Sprint(newValue)] = struct{}{}
+		}
+
+		// intersectedValues is a slice which will contain the values
+		// that we want to keep. They will be at most as many as what
+		// we already have; that's what we set the slice capacity to.
+		intersectedValues := make([]string, 0, len(existingValues))
+
+		// We add each existing value to intersectedValues if and only
+		// if it's also present in the new set.
+		for _, existingValue := range existingValues {
+			if _, ok := newValuesSet[existingValue]; ok {
+				intersectedValues = append(intersectedValues, existingValue)
+			}
+		}
+		o.query[property] = intersectedValues
+		return o
+	}
+
+	for _, v := range values {
+		o.query.Add(property, fmt.Sprint(v))
+	}
+
+	return o
+}
+
+func (o ListOpts) String() string {
+	return "?" + o.query.Encode()
+}
+
+func (o ListOpts) ToPortListQuery() (string, error) {
+	return o.String(), nil
+}
+
+func (o ListOpts) ToNetworkListQuery() (string, error) {
+	return o.String(), nil
+}
+
+func (o ListOpts) ToSubnetListQuery() (string, error) {
+	return o.String(), nil
+}

query/list_test.go

@@ -0,0 +1,44 @@
+package query_test
+
+import (
+	"fmt"
+
+	"github.com/gophercloud/gophercloud/openstack/networking/v2/networks"
+	"github.com/gophercloud/gophercloud/openstack/networking/v2/ports"
+	"github.com/gophercloud/gophercloud/openstack/networking/v2/subnets"
+	"github.com/gophercloud/utils/query"
+)
+
+var _ networks.ListOptsBuilder = (*query.ListOpts)(nil)
+var _ ports.ListOptsBuilder = (*query.ListOpts)(nil)
+var _ subnets.ListOptsBuilder = (*query.ListOpts)(nil)
+
+func ExampleListOpts_And_by_id() {
+	q := query.New().
+		And("id", 123, 321, 12345)
+	fmt.Println(q)
+	//Output: ?id=123&id=321&id=12345
+}
+
+func ExampleListOpts_And_by_name() {
+	q := query.New().
+		And("name", "port-1", "port-&321", "the-other-port")
+	fmt.Println(q)
+	//Output: ?name=port-1&name=port-%26321&name=the-other-port
+}
+
+func ExampleListOpts_And_by_Name_and_tag() {
+	q := query.New().
+		And("name", "port-1", "port-3").
+		And("tags", "my-tag")
+	fmt.Println(q)
+	//Output: ?name=port-1&name=port-3&tags=my-tag
+}
+
+func ExampleListOpts_And_by_id_twice() {
+	q := query.New().
+		And("id", 1, 2, 3).
+		And("id", 2, 3, 4)
+	fmt.Println(q)
+	//Output: ?id=2&id=3
+}