Streaming new posts from subreddits

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

reddit/stream.go

@@ -0,0 +1,93 @@
+package reddit
+
+import (
+	"context"
+	"sync"
+	"time"
+)
+
+// StreamService allows streaming new content from Reddit as it appears.
+type StreamService struct {
+	client *Client
+}
+
+// Posts streams posts from the specified subreddit.
+// It returns 2 channels and a function:
+//   - a channel into which new posts will be sent
+//   - a channel into which any errors will be sent
+//   - a function that the client can call once to stop the streaming and close the channels
+// Because of the 100 post limit imposed by Reddit when fetching posts, some high-traffic
+// streams might drop submissions between API requests, such as when streaming r/all.
+func (s *StreamService) Posts(subreddit string) (<-chan *Post, <-chan error, func()) {
+	posts := make(chan *Post)
+	errs := make(chan error)
+	ticker := time.NewTicker(time.Second * 5)
+
+	// originally used the "before" parameter, but if that post gets deleted, subsequent requests
+	// would just return empty listings; easier to just keep track of all post ids encountered
+	ids := set{}
+
+	go func() {
+		for ; ; <-ticker.C {
+			result, err := s.getPosts(subreddit)
+			if err != nil {
+				errs <- err
+				continue
+			}
+
+			for _, post := range result.Posts {
+				id := post.FullID
+
+				// if this post id is already part of the set, it means that it and the ones
+				// after it in the list have already been streamed, so break out of the loop
+				if ids.Exists(id) {
+					break
+				}
+
+				ids.Add(id)
+				posts <- post
+			}
+		}
+	}()
+
+	var once sync.Once
+	return posts, errs, func() {
+		once.Do(func() {
+			ticker.Stop()
+			close(posts)
+			close(errs)
+		})
+	}
+}
+
+func (s *StreamService) getPosts(subreddit string) (*Posts, error) {
+	opts := &ListOptions{
+		Limit: 100,
+	}
+
+	result, _, err := s.client.Subreddit.NewPosts(context.Background(), subreddit, opts)
+	if err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}
+
+type set map[string]struct{}
+
+func (s set) Add(v string) {
+	s[v] = struct{}{}
+}
+
+func (s set) Delete(v string) {
+	delete(s, v)
+}
+
+func (s set) Len() int {
+	return len(s)
+}
+
+func (s set) Exists(v string) bool {
+	_, ok := s[v]
+	return ok
+}