fix: create data directory before writing CSVs

main.go

@@ -10,6 +10,7 @@ import (
 	"math/big"
 	"math/rand"
 	"os"
+	"path/filepath"
 	"strconv"
 	"time"
 
@@ -203,9 +204,13 @@ func main() {
 }
 
 func writeToFile(filename string, data []stats) error {
+	if err := os.MkdirAll(filepath.Dir(filename), 0755); err != nil {
+		return fmt.Errorf("failed to create output directory: %w", err)
+	}
+
 	file, err := os.Create(filename)
 	if err != nil {
-		log.Fatalf("Failed to create file: %v", err)
+		return fmt.Errorf("failed to create file: %w", err)
 	}
 	defer file.Close()
 
@@ -214,7 +219,7 @@ func writeToFile(filename string, data []stats) error {
 
 	header := []string{"sent_at", "txn_hash", "included_in_block", "inclusion_delay_ms"}
 	if err := writer.Write(header); err != nil {
-		log.Fatalf("Failed to write to file: %v", err)
+		return fmt.Errorf("failed to write header: %w", err)
 	}
 
 	for _, d := range data {
@@ -225,10 +230,14 @@ func writeToFile(filename string, data []stats) error {
 			strconv.FormatInt(d.InclusionDelay.Milliseconds(), 10),
 		}
 		if err := writer.Write(row); err != nil {
-			log.Fatalf("Failed to write to file: %v", err)
+			return fmt.Errorf("failed to write row: %w", err)
 		}
 	}
 
+	if err := writer.Error(); err != nil {
+		return fmt.Errorf("failed to flush csv: %w", err)
+	}
+
 	return nil
 }
 

main_test.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestWriteToFileCreatesParentDirectory(t *testing.T) {
+	filename := filepath.Join(t.TempDir(), "nested", "flashblocks-us-east.csv")
+
+	err := writeToFile(filename, []stats{
+		{
+			SentAt:          time.Unix(1, 0),
+			TxnHash:         "0xabc",
+			IncludedInBlock: 123,
+			InclusionDelay:  250 * time.Millisecond,
+		},
+	})
+	if err != nil {
+		t.Fatalf("writeToFile returned error: %v", err)
+	}
+
+	if _, err := os.Stat(filename); err != nil {
+		t.Fatalf("expected output file to be created: %v", err)
+	}
+}