Computes proof of work for variable length md5 prefix match

md5_prefix_proof_of_work.go

/*
  MD5 Prefix Proof of Work
  Computes proof of work for variable length md5 prefix match
  Charset: a-zA-Z

  Usage:
    $. go run md5_prefix_proof_of_work.go <MD5 Prefix>

  Examples:
    $ go run md5_prefix_proof_of_work.go af232    // mUOtbyGE
    $ go run md5_prefix_proof_of_work.go 38ad22   // C0dutV6A

  Note: most runs take ~5 seconds to run; absolutely worst case: ~20 seconds

  Zillwc
 */

package main

import (
  "fmt"
  "os"
  "crypto/md5"
  "math/rand"
  "time"
  "encoding/hex"
  "strings"
)

var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")

func generateRandomCode(n int) string {
  b := make([]rune, n)
  for i := range b {
    b[i] = letters[rand.Intn(len(letters))]
  }

  return string(b)
}

func main() {
  rand.Seed(time.Now().UnixNano())
  prefix := os.Args[1]

  for {
    code := generateRandomCode(8)
    hash := md5.New()
    hash.Write([]byte(code))
    hashString := hex.EncodeToString(hash.Sum(nil))

    if strings.HasPrefix(hashString, prefix){
      fmt.Printf("%s\n", code)
      break
    }
  }
}

parallelized:

func main() {
  rand.Seed(time.Now().UnixNano())
  prefix := os.Args[1]

  found := false
  limit := 30
  semaphore := make(chan bool, limit)

  for !found {
    // wait if we've hit limit
    semaphore <- true

    go func() {
      // release lock at the end
      defer func(){
        <-semaphore
      }()

      attempt := generateRandomCode(8)
      hash := md5.New()
      hash.Write([]byte(attempt))
      hashString := hex.EncodeToString(hash.Sum(nil))

      if strings.HasPrefix(hashString, prefix){
        fmt.Printf("%s\n",attempt)
        found = true
      }
    }()
  }
}

maybe you can take the parallelization limit as a command line arg

As discussed on slack, the parallelized version is slower :/
Attaching full parallelized version for posterity:

package main

import (
  "fmt"
  "os"
  "crypto/md5"
  "math/rand"
  "time"
  "encoding/hex"
  "strings"
)

var prefix = os.Args[1]
var letters = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")

func generateRandomCode(n int) string {
  b := make([]rune, n)
  for i := range b {
    b[i] = letters[rand.Intn(len(letters))]
  }

  return string(b)
}

func main() {
  rand.Seed(time.Now().UnixNano())
  prefix := os.Args[1]

  found := false
  limit := 1000
  semaphore := make(chan bool, limit)

  for !found {
    semaphore <- true

    go func() {
      defer func(){
        <-semaphore
      }()

      attempt := generateRandomCode(8)
      hash := md5.New()
      hash.Write([]byte(attempt))
      hashString := hex.EncodeToString(hash.Sum(nil))

      if strings.HasPrefix(hashString, prefix){
        fmt.Printf("%s\n",attempt)
        found = true
      }
    }()
  }
}