Add good bit of functionality, some tests

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

listings.go

@@ -0,0 +1,104 @@
+package geddit
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+)
+
+// ListingsService handles communication with the link (post)
+// related methods of the Reddit API
+type ListingsService interface {
+	Get(ctx context.Context, ids ...string) (*Listing, *Response, error)
+}
+
+// ListingsServiceOp implements the Vote interface
+type ListingsServiceOp struct {
+	client *Client
+}
+
+var _ ListingsService = &ListingsServiceOp{}
+
+type listingRoot struct {
+	Kind string `json:"kind,omitempty"`
+	Data *struct {
+		Dist     int                      `json:"dist"`
+		Children []map[string]interface{} `json:"children,omitempty"`
+		After    string                   `json:"after,omitempty"`
+		Before   string                   `json:"before,omitempty"`
+	} `json:"data,omitempty"`
+}
+
+// Listing holds various types of things that all come from the Reddit API
+type Listing struct {
+	Links      []*Submission `json:"links,omitempty"`
+	Comments   []*Comment    `json:"comments,omitempty"`
+	Subreddits []*Subreddit  `json:"subreddits,omitempty"`
+}
+
+// Get gets a list of things based on their IDs
+// Only links, comments, and subreddits are allowed
+func (s *ListingsServiceOp) Get(ctx context.Context, ids ...string) (*Listing, *Response, error) {
+	type query struct {
+		IDs []string `url:"id,comma"`
+	}
+
+	path := "api/info"
+	path, err := addOptions(path, query{ids})
+	if err != nil {
+		return nil, nil, err
+	}
+
+	req, err := s.client.NewRequest(http.MethodGet, path, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	root := new(listingRoot)
+	resp, err := s.client.Do(ctx, req, root)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	if root.Data == nil {
+		return nil, resp, nil
+	}
+
+	l := new(Listing)
+
+	for _, result := range root.Data.Children {
+		kind, ok1 := result["kind"].(string)
+		data, ok2 := result["data"]
+
+		if ok1 && ok2 {
+			byteValue, err := json.Marshal(data)
+			if err != nil {
+				return nil, resp, err
+			}
+
+			var v interface{}
+			switch kind {
+			case kindComment:
+				v = new(Comment)
+				l.Comments = append(l.Comments, v.(*Comment))
+			case kindLink:
+				v = new(Submission)
+				l.Links = append(l.Links, v.(*Submission))
+			case kindSubreddit:
+				v = new(Subreddit)
+				l.Subreddits = append(l.Subreddits, v.(*Subreddit))
+			default:
+				continue
+			}
+
+			err = json.Unmarshal(byteValue, v)
+			if err != nil {
+				return nil, resp, err
+			}
+		}
+	}
+
+	return l, resp, nil
+}
+
+// todo: do by_id next