Can I construct a BigInt with any byte array (Scala)?
I'm trying to represent the result of an MD5 hash in the shortest possible string. It seems a waste to just turn it into a hex string and let G through Z go to waste.
One idea I have had is getting the MD5 hash of my input as an array of bytes and constructing a BigInt
with it. I can then call toString(36)
, and get the number as a base-36 in 开发者_如何学Goa string (-?[0-9a-z]*
, the number can be positive or negative). It works for me.
Problem is, I'm not sure that a BigInt
can be constructed with any array of bytes, and I can't prove it with testing (at least not in a timely way!). I assume so, because I understand that a BigInt can be of arbitrary size. I can't use this method until I know for sure that it will work for all possible outputs. So, can anyone tell me whether it will work for all inputs (or how to easily convert a byte array so it can be represented in base 36).
Clarification: I have the implementation, I'm asking about the behaviour over the whole domain (i.e. 00000000000000000000000000000000 to FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
Building on your feedback above, the following implementation will reliably encode/decode an arbitrary byte array:
package blevins.example
object BigIntEncoder {
val radix = 36
implicit def byteArrayToString(ba: Array[Byte]): String = {
new java.math.BigInteger(addByte(ba)).toString(radix)
}
implicit def stringToByteArray(s: String): Array[Byte] = {
stripByte(new java.math.BigInteger(s, radix).toByteArray)
}
def addByte(ba: Array[Byte]): Array[Byte] = {
val h = new Array[Byte](1)
h(0) = 0x01
h ++ ba
}
def stripByte(ba: Array[Byte]): Array[Byte] = {
ba.slice(1,ba.size)
}
}
Note that we are adding an extra 0x01 byte to the head of the array to avoid any side effects from taking the two-complement of the byte array.
EDIT: The testing involved to prove this out is documented here: http://cleverlytitled.blogspot.com/2009/10/scalacheck.html
Wouldn't Base64 encoding be shorter than Base36? You can find plenty of implementations around.
But, to actually answer the question:
// Make a big randomly-filled byte array
val random = scala.util.Random
val arraySize = 8543
val bytes: Array[Byte] = new Array[Byte](arraySize) // make some big array
random.nextBytes(bytes) // fill it randomly
// Make a BigInt out of it and the corresponding base36 string representation
val bi: BigInt = new BigInt(new java.math.BigInteger(bytes))
val strRep: String = bi.toString(36)
// Make a new BigInt out of the string rep. Does it match?
val bi2: BigInt = new BigInt(new java.math.BigInteger(strRep, 36))
if (bi == bi2) {
println("yippee!!")
}
// Make a new byte array out of the BigInt. Does it match the original array?
val bytes2: Array[Byte] = bi2.toByteArray
if (bytes deepEquals bytes2) {
println("yippee again!!")
}
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