isolate record reading and writing from processing

internal/stdout.go

@@ -0,0 +1,31 @@
+package internal
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"time"
+)
+
+type ReportLogger struct {
+	counter int
+	writer  io.Writer
+}
+
+func NewStdOutLogger() *ReportLogger {
+	return &ReportLogger{
+		writer: os.Stdout,
+	}
+}
+
+func NewReportLogger(w io.Writer) *ReportLogger {
+	return &ReportLogger{
+		writer: w,
+	}
+}
+
+func (rl *ReportLogger) Write(ri ReportItem) error {
+	rl.counter++
+	_, err := fmt.Fprintf(rl.writer, "%6d - realised %+f on %s\n", rl.counter, ri.RealisedPnL(), ri.SellTimestamp.Format(time.RFC3339))
+	return err
+}