cmd/golink: add stat loading and saving to DB

add new DB.LoadStats and DB.SaveStats methods.  DB.SaveSaveStats takes
an incremental count of clicks since the last call.  This is a noop for
FileDB (the full count is still saved in the Link struct on disk), but
will allow for the SqliteDB to store stats in a separate table with
timestamps to display only recent clicks.

Change-Id: I18c7db871854923b873ae100c844940242a32bb3

db.go

@@ -20,6 +20,10 @@ type Link struct {
 	Clicks   int       `json:",omitempty"` // number of times this link has been served
 }
 
+// ClickStats is the number of clicks a set of links have received in a given
+// time period. It is keyed by link short name, with values of total clicks.
+type ClickStats map[string]int
+
 // DB provides storage for Links.
 type DB interface {
 	// LoadAll returns all stored Links.
@@ -36,6 +40,14 @@ type DB interface {
 
 	// Save saves a Link.
 	Save(*Link) error
+
+	// LoadStats returns click stats for links.
+	LoadStats() (ClickStats, error)
+
+	// SaveStats records click stats for links.  The provided map includes
+	// incremental clicks that have occurred since the last time SaveStats
+	// was called.
+	SaveStats(ClickStats) error
 }
 
 // FileDB stores Links in JSON files on disk.
@@ -120,3 +132,36 @@ func (f *FileDB) Save(link *Link) error {
 	}
 	return nil
 }
+
+func (f *FileDB) LoadStats() (ClickStats, error) {
+	links, err := db.LoadAll()
+	if err != nil {
+		return nil, err
+	}
+
+	stats := make(ClickStats)
+	for _, link := range links {
+		if link.Clicks > 0 {
+			stats[link.Short] = link.Clicks
+		}
+	}
+
+	return stats, nil
+}
+
+func (f *FileDB) SaveStats(stats ClickStats) error {
+	for short, clicks := range stats {
+		if clicks <= 0 {
+			continue
+		}
+		link, err := f.Load(short)
+		if err != nil {
+			return err
+		}
+		link.Clicks += clicks
+		if err := f.Save(link); err != nil {
+			return err
+		}
+	}
+	return nil
+}

golink.go

@@ -38,10 +38,10 @@ var (
 
 var stats struct {
 	mu     sync.Mutex
-	clicks map[string]int // short link -> number of times visited
+	clicks ClickStats // short link -> number of times visited
 
-	// dirty identifies short links with clicks that have not yet been stored.
-	dirty map[string]bool
+	// dirty identifies short link clicks that have not yet been stored.
+	dirty ClickStats
 }
 
 //go:embed link-snapshot.json
@@ -140,20 +140,16 @@ func init() {
 
 // initStats initializes the in-memory stats counter with counts from db.
 func initStats() error {
-	links, err := db.LoadAll()
+	stats.mu.Lock()
+	defer stats.mu.Unlock()
+
+	clicks, err := db.LoadStats()
 	if err != nil {
 		return err
 	}
 
-	stats.mu.Lock()
-	defer stats.mu.Unlock()
-
-	stats.clicks = make(map[string]int)
-	for _, link := range links {
-		if link.Clicks > 0 {
-			stats.clicks[link.Short] = link.Clicks
-		}
-	}
+	stats.clicks = clicks
+	stats.dirty = make(ClickStats)
 
 	return nil
 }
@@ -163,19 +159,10 @@ func flushStats() error {
 	stats.mu.Lock()
 	defer stats.mu.Unlock()
 
-	for short := range stats.dirty {
-		link, err := db.Load(short)
-		if err != nil {
-			return err
-		}
-		if link.Clicks != stats.clicks[short] {
-			link.Clicks = stats.clicks[short]
-			if err := db.Save(link); err != nil {
-				return err
-			}
-		}
-		delete(stats.dirty, short)
+	if err := db.SaveStats(stats.dirty); err != nil {
+		return err
 	}
+	stats.dirty = make(ClickStats)
 	return nil
 }
 
@@ -260,13 +247,13 @@ func serveGo(w http.ResponseWriter, r *http.Request) {
 
 	stats.mu.Lock()
 	if stats.clicks == nil {
-		stats.clicks = make(map[string]int)
+		stats.clicks = make(ClickStats)
 	}
 	stats.clicks[link.Short]++
 	if stats.dirty == nil {
-		stats.dirty = make(map[string]bool)
+		stats.dirty = make(ClickStats)
 	}
-	stats.dirty[link.Short] = true
+	stats.dirty[link.Short]++
 	stats.mu.Unlock()
 
 	target, err := expandLink(link.Long, expandEnv{Now: time.Now().UTC(), Path: remainder})