feat: Add counter-based iterator

cmd/stfs-seek-tape/main.go

@@ -39,17 +39,7 @@ func main() {
 	}
 	defer f.Close()
 
-	if _, _, err := syscall.Syscall(
-		syscall.SYS_IOCTL,
-		f.Fd(),
-		MTIOCTOP,
-		uintptr(unsafe.Pointer(
-			&Operation{
-				Op:    MTSEEK,
-				Count: int32(*record),
-			},
-		)),
-	); err != 0 {
+	if err := seekToRecordOnTape(f, int32(*record)); err != nil {
 		panic(err)
 	}
 
@@ -67,3 +57,21 @@ func main() {
 
 	log.Println(hdr)
 }
+
+func seekToRecordOnTape(f *os.File, record int32) error {
+	if _, _, err := syscall.Syscall(
+		syscall.SYS_IOCTL,
+		f.Fd(),
+		MTIOCTOP,
+		uintptr(unsafe.Pointer(
+			&Operation{
+				Op:    MTSEEK,
+				Count: record,
+			},
+		)),
+	); err != 0 {
+		return err
+	}
+
+	return nil
+}

cmd/stfs-tvf-simple/main.go

@@ -7,8 +7,6 @@ import (
 	"io"
 	"log"
 	"os"
-	"syscall"
-	"unsafe"
 )
 
 // See https://github.com/benmcclelland/mtio
@@ -32,6 +30,24 @@ type Operation struct {
 	Count int32 // Operation count
 }
 
+type Counter struct {
+	reader io.Reader
+
+	bytesRead int
+}
+
+func (r *Counter) Read(p []byte) (n int, err error) {
+	n, err = r.reader.Read(p)
+
+	r.bytesRead += n
+
+	return n, err
+}
+
+func (r *Counter) GetBytesRead() int {
+	return r.bytesRead
+}
+
 func main() {
 	file := flag.String("file", "/dev/nst0", "File (tape drive or tar file) to open")
 	recordSize := flag.Int("recordSize", 20, "Amount of 512-bit blocks per record")
@@ -119,54 +135,28 @@ func main() {
 		}
 	} else {
 		br := bufio.NewReaderSize(f, blockSize**recordSize)
+		counter := &Counter{reader: br}
+
+		record := int64(0)
+		block := int64(0)
 
-		currentRecord := 0
-	o:
 		for {
-			for currentBlock := 0; currentBlock < *recordSize; currentBlock++ {
-				if currentRecord%4096 == 0 {
-					tell, err := getCurrentRecordFromTape(f)
-					if err != nil {
-						panic(err)
-					}
-
-					if tell < int64(currentRecord) {
-						// EOD
-
-						break o
-					}
-				}
-
-				tr := tar.NewReader(br)
-				hdr, err := tr.Next()
-				if err != nil {
-					continue
-				}
-
-				if hdr.Format == tar.FormatUnknown {
-					continue
-				}
-
-				log.Println("Record:", currentRecord, "Block:", currentBlock, "Header:", hdr)
-
-				// TODO: Seek to (curr + hdr.Size)/blockSize if recordToSeekTo != currentRecord, re-write `br`
+			tr := tar.NewReader(counter)
+			hdr, err := tr.Next()
+			if err != nil {
+				continue
 			}
 
-			currentRecord++
+			if hdr.Format == tar.FormatUnknown {
+				continue
+			}
+
+			log.Println("Record:", record, "Block:", block, "Header:", hdr)
+
+			nextBytes := int64(counter.GetBytesRead()) + hdr.Size + blockSize - 1
+
+			record = nextBytes / (blockSize * int64(*recordSize))
+			block = (nextBytes - (record * int64(*recordSize) * blockSize)) / blockSize
 		}
 	}
 }
-
-func getCurrentRecordFromTape(f *os.File) (int64, error) {
-	pos := &Position{}
-	if _, _, err := syscall.Syscall(
-		syscall.SYS_IOCTL,
-		f.Fd(),
-		MTIOCPOS,
-		uintptr(unsafe.Pointer(pos)),
-	); err != 0 {
-		return 0, err
-	}
-
-	return pos.BlkNo, nil
-}