feat: implement file system persistence with save and load functionality

internal/persistence/fileSaver.go

@@ -0,0 +1,62 @@
+package persistence
+
+import (
+	"encoding/json"
+	"os"
+
+	"github.com/StevanFreeborn/term-tier/internal/core"
+)
+
+type FileSystem interface {
+	WriteFile(name string, data []byte, perm os.FileMode) error
+	ReadFile(name string) ([]byte, error)
+}
+
+type osFileSystem struct{}
+
+func (osFileSystem) WriteFile(name string, data []byte, perm os.FileMode) error {
+	return os.WriteFile(name, data, perm)
+}
+
+func (osFileSystem) ReadFile(name string) ([]byte, error) {
+	return os.ReadFile(name)
+}
+
+type fileSaver struct {
+	fs FileSystem
+}
+
+func NewFileSaver() core.Saver {
+	return &fileSaver{
+		fs: osFileSystem{},
+	}
+}
+
+func NewFileSaverWithFS(fs FileSystem) core.Saver {
+	return &fileSaver{
+		fs: fs,
+	}
+}
+
+func (s *fileSaver) Save(filename string, state core.SaveState) error {
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return s.fs.WriteFile(filename, data, 0644)
+}
+
+func (s *fileSaver) Load(filename string) (core.SaveState, error) {
+	data, err := s.fs.ReadFile(filename)
+	if err != nil {
+		return core.SaveState{}, err
+	}
+
+	var state core.SaveState
+	if err := json.Unmarshal(data, &state); err != nil {
+		return core.SaveState{}, err
+	}
+
+	return state, nil
+}