feat: add persistent storage (#30)

* refactor: remove unused default timeout

* feat: add persistent storage

Allows you to save the state to a file and load it upon restarting the app to restore the previous state.

* chore: upgrade to go 1.18

* use tinykv with generics

* build: add "-buildvcs=false" flag

Git is not available in golang:1.18-alpine image

pkg/storage/periodic_sync.go

@@ -0,0 +1,62 @@
+package storage
+
+import (
+	"encoding/json"
+	"log"
+	"os"
+	"sync"
+	"time"
+)
+
+type PS interface {
+	Stop()
+}
+
+type PeriodicSync struct {
+	File     *os.File
+	Interval time.Duration
+	Content  interface{}
+
+	stop     chan struct{}
+	stopOnce sync.Once
+}
+
+func New(file *os.File, interval time.Duration, content interface{}) PS {
+	sync := &PeriodicSync{
+		File:     file,
+		Interval: interval,
+		Content:  content,
+		stop:     make(chan struct{}),
+	}
+
+	go sync.syncLoop()
+
+	return sync
+}
+
+func (sync *PeriodicSync) Stop() {
+	sync.stopOnce.Do(func() { close(sync.stop) })
+}
+
+func (sync *PeriodicSync) save() error {
+	sync.File.Truncate(0)
+	sync.File.Seek(0, 0)
+	return json.NewEncoder(sync.File).Encode(sync.Content)
+}
+
+func (sync *PeriodicSync) syncLoop() {
+	interval := sync.Interval
+	expireTime := time.NewTimer(interval)
+	for {
+		select {
+		case <-sync.stop:
+			return
+		case <-expireTime.C:
+			err := sync.save()
+			if err != nil {
+				log.Println("Error saving", err)
+			}
+			expireTime.Reset(interval)
+		}
+	}
+}