feat: add cache layer over api

internal/geodata/fetcher_test.go

@@ -0,0 +1,43 @@
+package geodata
+
+import (
+	"os"
+	"testing"
+)
+
+func TestFetchLive(t *testing.T) {
+	tzMap, err := fetchLive()
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testTzMap(tzMap, t)
+}
+
+func TestCache(t *testing.T) {
+	// delete cache
+	cachePath := getCachePath()
+	if err := os.Remove(cachePath); err != nil && !os.IsNotExist(err) {
+		t.Fatal(err)
+	}
+
+	_, err := Fetch() // this populates cache
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tzMap, err := loadFromCache()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testTzMap(tzMap, t)
+}
+
+func testTzMap(tzMap TimezoneMap, t *testing.T) {
+	entry0100 := tzMap["+0100"]
+	if len(entry0100) == 0 {
+		t.Fatalf("no countries found to timezone")
+	}
+}