make use of the store optional

internal/ephemeral_store.go

@@ -0,0 +1,12 @@
+package internal
+
+// EphemeralStore loads an empty state and discards everything on save
+type EphemeralStore struct{}
+
+func (EphemeralStore) Load() (map[string]*FillerQueue, error) {
+	return make(map[string]*FillerQueue), nil
+}
+
+func (EphemeralStore) Save(map[string]*FillerQueue) error {
+	return nil
+}

internal/report.go

@@ -53,6 +53,7 @@ type ReportWriter interface {
 
 type optionals struct {
 	selector Selector
+	store    Store
 }
 
 func applyOptions(defaults optionals, opts []Option) optionals {
@@ -64,23 +65,38 @@ func applyOptions(defaults optionals, opts []Option) optionals {
 
 type Option func(*optionals)
 
+// Selector returns true if a record should be selected for processing, false otherwise.
+type Selector func(Record) bool
+
 func WithSelector(s Selector) Option {
 	return func(o *optionals) {
 		o.selector = s
 	}
 }
 
-// Selector returns true if a record should be selected for processing, false otherwise.
+type Store interface {
-type Selector func(Record) bool
+	Load() (map[string]*FillerQueue, error)
+	Save(map[string]*FillerQueue) error
+}
+
+func WithStore(s Store) Option {
+	return func(o *optionals) {
+		o.store = s
+	}
+}
 
 // BuildReport reads records from a RecordReader and, if the record passes the Selector, it is
 // processed into the ReportWriter.
 func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter, options ...Option) error {
 	optionals := applyOptions(optionals{
 		selector: Any(),
+		store:    EphemeralStore{},
 	}, options)
 
-	buys := make(map[string]*FillerQueue)
+	buys, err := optionals.store.Load()
+	if err != nil {
+		return fmt.Errorf("loading state: %w", err)
+	}
 
 	var buysCount, sellsCount int64
 	var lastTimestamp time.Time
@@ -102,6 +118,11 @@ func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter,
 			rec, err := reader.ReadRecord(ctx)
 			if err != nil {
 				if errors.Is(err, io.EOF) {
+					err = optionals.store.Save(buys)
+					if err != nil {
+						return fmt.Errorf("saving state: %w", err)
+					}
+
 					return nil
 				}
 				return err
@@ -125,9 +146,10 @@ func BuildReport(ctx context.Context, reader RecordReader, writer ReportWriter,
 			if err != nil {
 				return fmt.Errorf("processing record: %w", err)
 			}
-
 		}
 	}
+
+	return nil
 }
 
 // processRecord either adds buys to the queue or consumes buys from the queue when processing a