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//+build ignore
// Copyright 2015, Klaus Post, see LICENSE for details.
//
// Simple decoder example.
//
// The decoder reverses the process of "simple-encoder.go"
//
// To build an executable use:
//
// go build simple-decoder.go
//
// Simple Encoder/Decoder Shortcomings:
// * If the file size of the input isn't diviable by the number of data shards
// the output will contain extra zeroes
//
// * If the shard numbers isn't the same for the decoder as in the
// encoder, invalid output will be generated.
//
// * If values have changed in a shard, it cannot be reconstructed.
//
// * If two shards have been swapped, reconstruction will always fail.
// You need to supply the shards in the same order as they were given to you.
//
// The solution for this is to save a metadata file containing:
//
// * File size.
// * The number of data/parity shards.
// * HASH of each shard.
// * Order of the shards.
//
// If you save these properties, you should abe able to detect file corruption
// in a shard and be able to reconstruct your data if you have the needed number of shards left.
package main
import (
"flag"
"fmt"
"io/ioutil"
"os"
"github.com/klauspost/reedsolomon"
)
var dataShards = flag.Int("data", 4, "Number of shards to split the data into")
var parShards = flag.Int("par", 2, "Number of parity shards")
var outFile = flag.String("out", "", "Alternative output path/file")
func init() {
flag.Usage = func() {
fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0])
fmt.Fprintf(os.Stderr, " simple-decoder [-flags] basefile.ext\nDo not add the number to the filename.\n")
fmt.Fprintf(os.Stderr, "Valid flags:\n")
flag.PrintDefaults()
}
}
func main() {
// Parse flags
flag.Parse()
args := flag.Args()
if len(args) != 1 {
fmt.Fprintf(os.Stderr, "Error: No filenames given\n")
flag.Usage()
os.Exit(1)
}
fname := args[0]
// Create matrix
enc, err := reedsolomon.New(*dataShards, *parShards)
checkErr(err)
// Create shards and load the data.
shards := make([][]byte, *dataShards+*parShards)
for i := range shards {
infn := fmt.Sprintf("%s.%d", fname, i)
fmt.Println("Opening", infn)
shards[i], err = ioutil.ReadFile(infn)
if err != nil {
fmt.Println("Error reading file", err)
shards[i] = nil
}
}
// Verify the shards
ok, err := enc.Verify(shards)
if ok {
fmt.Println("No reconstruction needed")
} else {
fmt.Println("Verification failed. Reconstructing data")
err = enc.Reconstruct(shards)
if err != nil {
fmt.Println("Reconstruct failed -", err)
os.Exit(1)
}
ok, err = enc.Verify(shards)
if !ok {
fmt.Println("Verification failed after reconstruction, data likely corrupted.")
os.Exit(1)
}
checkErr(err)
}
// Join the shards and write them
outfn := *outFile
if outfn == "" {
outfn = fname
}
fmt.Println("Writing data to", outfn)
f, err := os.Create(outfn)
checkErr(err)
// We don't know the exact filesize.
err = enc.Join(f, shards, len(shards[0])**dataShards)
checkErr(err)
}
func checkErr(err error) {
if err != nil {
fmt.Fprintf(os.Stderr, "Error: %s", err.Error())
os.Exit(2)
}
}
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