Merge cars command (#125)

* add merge car method

* add merge car method

* header

* duplicate const

cmd-merge-cars.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"bufio"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/urfave/cli/v2"
+)
+
+const varintSize = 10
+
+func newCmd_MergeCars() *cli.Command {
+	var outputFile string
+	return &cli.Command{
+		Name:        "merge-cars",
+		Description: "Merges split car files into a single file",
+		Usage:       "Merges split car files into a single file",
+		ArgsUsage:   "<list of car files>",
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:        "output-file",
+				Aliases:     []string{"o"},
+				Usage:       "Output file name",
+				Required:    true,
+				Destination: &outputFile,
+			},
+		},
+		Action: func(c *cli.Context) error {
+			paths := c.Args().Slice()
+
+			out, err := os.Create(outputFile)
+			if err != nil {
+				return fmt.Errorf("failed to create output file: %w", err)
+			}
+			defer out.Close()
+
+			w := bufio.NewWriter(out)
+
+			if _, err := io.WriteString(w, nulRootCarHeader); err != nil {
+				return fmt.Errorf("failed to write empty header: %w", err)
+			}
+
+			for _, path := range paths {
+				f, err := os.Open(path)
+				if err != nil {
+					return fmt.Errorf("failed to open input file %s: %w", path, err)
+				}
+				defer f.Close()
+
+				r := bufio.NewReader(f)
+				err = discardHeader(r)
+				if err != nil {
+					return fmt.Errorf("failed to discard header: %w", err)
+				}
+
+				io.Copy(w, r)
+
+			}
+
+			return nil
+		},
+	}
+}
+
+func discardHeader(streamBuf *bufio.Reader) error {
+	maybeHeaderLen, err := streamBuf.Peek(varintSize)
+	if err != nil {
+		return fmt.Errorf("failed to read header: %s\n", err)
+	}
+
+	hdrLen, viLen := binary.Uvarint(maybeHeaderLen)
+	if hdrLen <= 0 || viLen < 0 {
+		return fmt.Errorf("unexpected header len = %d, varint len = %d\n", hdrLen, viLen)
+	}
+
+	_, err = io.CopyN(io.Discard, streamBuf, int64(viLen))
+	if err != nil {
+		return fmt.Errorf("failed to discard header varint: %s\n", err)
+	}
+
+	// ignoring header decoding for now
+	_, err = io.CopyN(io.Discard, streamBuf, int64(hdrLen))
+	if err != nil {
+		return fmt.Errorf("failed to discard header header: %s\n", err)
+	}
+
+	return nil
+}

main.go

@@ -57,6 +57,7 @@ func main() {
 			newCmd_Version(),
 			newCmd_rpc(),
 			newCmd_check_deals(),
+			newCmd_MergeCars(),
 			newCmd_SplitCar(),
 		},
 	}