Blog multi-tag query: whitelist and blacklist

* Can query blog posts by multiple tags now.
* e.g. /tagged/blog,updates,-photos would query all posts that have tags
  "blog" OR "updates" but NOT show any post with tag "photos"

pkg/models/posts.go

@@ -141,14 +141,16 @@ func (m postMan) GetPostsByTag(tag, privacy string, page, perPage int) (PagedPos
 		pp.PerPage = 20
 	}
 
-	// Get the distinct post IDs for this tag.
-	var tags []TaggedPost
-	var postIDs []uint
-	r := DB.Where("tag = ?", tag).Find(&tags)
-	for _, taggedPost := range tags {
-		postIDs = append(postIDs, taggedPost.PostID)
+	// Multi-tag query.
+	whitelist, blacklist, err := ParseMultitag(tag)
+	if err != nil {
+		return pp, err
 	}
 
+	// Query the whitelist of post IDs which match the whitelist tags.
+	postIDs := getPostIDsByTag("tag IN (?)", whitelist)
+	notPostIDs := getPostIDsByTag("tag IN (?)", blacklist)
+	postIDs = narrowWhitelistByBlacklist(postIDs, notPostIDs)
 	if len(postIDs) == 0 {
 		return pp, errors.New("no posts found")
 	}
@@ -162,7 +164,7 @@ func (m postMan) GetPostsByTag(tag, privacy string, page, perPage int) (PagedPos
 	query.Model(&Post{}).Count(&pp.Total)
 
 	// Query the paginated slice of results.
-	r = query.
+	r := query.
 		Offset((page - 1) * perPage).
 		Limit(perPage).
 		Find(&pp.Posts)
@@ -182,6 +184,43 @@ func (m postMan) GetPostsByTag(tag, privacy string, page, perPage int) (PagedPos
 	return pp, r.Error
 }
 
+// getPostIDsByTag helper function returns the post IDs that either whitelist,
+// or blacklist, a set of tags.
+func getPostIDsByTag(query string, value []string) []int {
+	var tags []TaggedPost
+	var result []int
+
+	if len(value) == 0 {
+		return result
+	}
+
+	DB.Where(query, value).Find(&tags)
+	for _, tag := range tags {
+		result = append(result, tag.PostID)
+	}
+
+	return result
+}
+
+// narrowWhitelistByBlacklist removes IDs in whitelist that appear in blacklist.
+func narrowWhitelistByBlacklist(wl []int, bl []int) []int {
+	// Map the blacklist into a hash map.
+	var blacklist = map[int]interface{}{}
+	for _, id := range bl {
+		blacklist[id] = nil
+	}
+
+	// Limit the whitelist by the blacklist.
+	var result []int
+	for _, id := range wl {
+		if _, ok := blacklist[id]; !ok {
+			result = append(result, id)
+		}
+	}
+
+	return result
+}
+
 // GetArchive queries the archive view of the blog.
 // Set private=true to return private posts, false returns public only.
 func (m postMan) GetArchive(private bool) ([]*PostArchive, error) {
@@ -271,6 +310,32 @@ func (m postMan) CountComments(posts ...Post) (map[int]int, error) {
 	return result, nil
 }
 
+// ParseMultitag parses a tag string to return arrays for IN and NOT IN queries from DB.
+//
+// Example input: "blog,updates,-photo,-ask"
+// Returns: ["blog", "updates"], ["photo", "ask"]
+func ParseMultitag(tagline string) (whitelist, blacklist []string, err error) {
+	words := strings.Split(tagline, ",")
+	for _, word := range words {
+		word = strings.TrimSpace(word)
+		if len(word) == 0 {
+			continue
+		}
+
+		// Negation
+		if strings.HasPrefix(word, "-") {
+			blacklist = append(blacklist, strings.TrimPrefix(word, "-"))
+		} else {
+			whitelist = append(whitelist, word)
+		}
+	}
+
+	if len(whitelist) == 0 && len(blacklist) == 0 {
+		err = errors.New("parsing error")
+	}
+	return
+}
+
 // CountComments on the posts in a PagedPosts list.
 func (pp *PagedPosts) CountComments() error {
 	counts, err := Posts.CountComments(pp.Posts...)

pkg/models/tags.go

@@ -8,7 +8,7 @@ import (
 type TaggedPost struct {
 	ID     int `gorm:"primary_key"`
 	Tag    string
-	PostID uint // foreign key to Post
+	PostID int // foreign key to Post
 }
 
 // SummarizeTags returns the list of all tags ordered by frequency used.