ceran - 6 months ago 30

Scala Question

I'm constructing

`BigInt`

`Long`

`val msb = -1L // some arbitrary long value, can be anything between Long.Min/MaxValue`

val lsb = 25L // a second arbitrary long value

val bb = ByteBuffer

.allocate(17)

.put(0.toByte) // 1 byte

.putLong(msb) // 8 bytes

.putLong(lsb) // 8 bytes

val number = BigInt(bb.array) // in this case: 340282366920938463444927863358058659865

The reason I'm adding another 0-

`Byte`

`BigInt`

So far, so good.

I'm having trouble with reversing this whole process - transforming the

`BigInt`

`Long`

`val arr = number.toByteArray`

val bb = ByteBuffer.wrap(arr)

val ignore = bb.getByte

val msb = bb.getLong

val lsb = bb.getLong

Imagine the

`BigInt`

`.toByteArray`

`getLong`

`BufferUnderflowException`

What's the easiest way to solve this problem? I tried several ways to manually fill up the buffer until there are 16 bytes available, but since this "padding" must correctly take the two's complement of the two numbers into account, I wasn't succesful.

Answer

Modulo operation can be helpful here:

```
....
val number = BigInt(bb.array) // in this case: 340282366920938463444927863358058659865
val modulo = BigInt(2).pow(64)
val lsb2 = (number / modulo).toLong //25
val msb2 = (number.mod(modulo)).toLong //-1
```