add the visitor API endpoint

api/database/database.go

@@ -17,6 +17,18 @@ func (db *Type) Load() (err error) {
 		return fmt.Errorf("cannot access the database: %s", err.Error())
 	}
 
+	// see database/visitor.go
+	_, err = db.sql.Exec(`
+    CREATE TABLE IF NOT EXISTS visitor_count(
+      id    TEXT NOT NULL UNIQUE,
+      count INTEGER NOT NULL
+    );
+  `)
+
+	if err != nil {
+		return fmt.Errorf("failed to create the visitor_count table: %s", err.Error())
+	}
+
 	// see database/service.go
 	_, err = db.sql.Exec(`
     CREATE TABLE IF NOT EXISTS services(

api/database/visitor.go

@@ -0,0 +1,48 @@
+package database
+
+import (
+	"database/sql"
+)
+
+func (db *Type) VisitorGet() (uint64, error) {
+	var (
+		row   *sql.Row
+		count uint64
+		err   error
+	)
+
+	if row = db.sql.QueryRow("SELECT count FROM visitor_count WHERE id = 0"); row == nil {
+		return 0, nil
+	}
+
+	if err = row.Scan(&count); err != nil && err != sql.ErrNoRows {
+		return 0, err
+	}
+
+	if err == sql.ErrNoRows {
+		return 0, nil
+	}
+
+	return count, nil
+}
+
+func (db *Type) VisitorIncrement() (err error) {
+	if _, err = db.sql.Exec("UPDATE visitor_count SET count = count + 1 WHERE id = 0"); err != nil && err != sql.ErrNoRows {
+		return err
+	}
+
+	// TODO: err is always nil even if there is no rows for some reason, check sql.Result instead
+
+	if err == sql.ErrNoRows {
+		_, err = db.sql.Exec(
+			`INSERT INTO visitor_count(
+			  id, count
+		  ) values(?, ?)`,
+			0, 0,
+		)
+
+		return err
+	}
+
+	return nil
+}