feat: add cache layer over api

internal/geodata/cache.go

@@ -0,0 +1,62 @@
+package geodata
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+const cacheDuration = 30 * 24 * time.Hour
+
+func getCachePath() string {
+	home, _ := os.UserHomeDir()
+	return filepath.Join(home, ".cache", "git-chronos", "geodata.json")
+}
+
+func loadFromCache() (TimezoneMap, error) {
+	path := getCachePath()
+
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	var cacheContainer cacheCountryData
+	if err := json.NewDecoder(file).Decode(&cacheContainer); err != nil {
+		return nil, err
+	}
+
+	if time.Since(cacheContainer.FetchedAt) > cacheDuration {
+		return nil, fmt.Errorf("cache expired")
+	}
+
+	return cacheContainer.Data, nil
+}
+
+func saveToCache(tzMap TimezoneMap) error {
+	path := getCachePath()
+	dir := filepath.Dir(path)
+
+	if err := os.MkdirAll(dir, 755); err != nil { // creates if not exists
+		return fmt.Errorf("failed to create cache dir")
+	}
+
+	file, err := os.Create(path) // create if not exists
+	if err != nil {
+		return err
+	}
+
+	defer file.Close()
+
+	container := cacheCountryData{
+		FetchedAt: time.Now(),
+		Data:      tzMap,
+	}
+
+	encoder := json.NewEncoder(file)
+	encoder.SetIndent("", " ") // so it looks better
+	return encoder.Encode(container)
+}