gistfile1.txt

// Randomly corrupts a file by writing a minimum amount of zeros.
package main

import (
	"fmt"
	"math/rand"
	"os"
	"time"

	"gopkg.in/alecthomas/kingpin.v2"
)

var (
	seed = kingpin.Flag("seed", "Seed value for corruptor").Default(fmt.Sprintf("%v", time.Now().Unix())).Int64()

	file      = kingpin.Arg("file", "File to corrupt").Required().ExistingFile()
	num_bytes = kingpin.Arg("numbytes", "Number of bytes to corrupt").Bytes()
	max_span  = kingpin.Flag("max-span", "Maximum contiguous bytes to corrupt").Bytes()
)

func main() {
	kingpin.Parse()

	rnd := rand.New(rand.NewSource(*seed))

	filename := *file

	st, err := os.Stat(filename)
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	f, err := os.OpenFile(filename, os.O_EXCL|os.O_RDWR, os.FileMode(0777))
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	defer f.Close()

	bytesCorrupted := int64(0)
	bytesToCorrupt := int64(*num_bytes)
	fmt.Println(bytesToCorrupt)
	spans := 0

	for 0 < bytesToCorrupt {
		// Pick a start point in the file
		startPoint := rnd.Int63n(st.Size())

		// MaxSpan
		maxSpan := rnd.Int63n(st.Size() - startPoint)

		if bytesToCorrupt < maxSpan {
			maxSpan = bytesToCorrupt
		}

		if st.Size() < (startPoint + maxSpan) {
			maxSpan = st.Size() - startPoint
		}

		if maxSpan > int64(*max_span) {
			maxSpan = int64(*max_span)
		}

		// Pick a range in the span range to corrupt
		corruptSize := rnd.Int63n(maxSpan)

		// Write zeroes over the corrupt range
		if _, err := f.Seek(startPoint, os.SEEK_SET); err != nil {
			fmt.Println(err)
			os.Exit(1)
		}

		if _, err := f.Write(make([]byte, corruptSize, corruptSize)); err != nil {
			fmt.Println(err)
			os.Exit(1)
		}

		bytesToCorrupt -= corruptSize + 1
		bytesCorrupted += corruptSize + 1
		spans++
	}

	fmt.Printf("Zeroed out %d bytes in %d spans.\n", bytesCorrupted, spans)
}