Was thinking a bit about this in the past days.

I'm using BN.js to handle numbers between 8 and 256bits for the Ethereum ABI in http://github.com/axic/ethereumjs-abi. It supports signed (two's complement) and unsigned representations.

Currently I have two helper functions (toTwos() & fromTwos()) to convert the internal representation and then call toString() on them:

function toTwos(num, width) {
  if (num.isNeg())
    num = new BN(1).iushln(width).isub(num.iabs());
    // Or bitwise negate + plus 1
  return num;
}

function fromTwos(num, width) {
  if (num.testn(width - 1)) // top bit set => negative number
    num = new BN(1).iushln(width).isub(num).ineg();
    // Or bitwise negate + plus 1
  return num;
}

Perhaps I would suggest to have the following API:

.toTwos(width) - mandatory width parameter in bits

e.g. .toTwos(256) will convert the lower 256 bits into 256 bits of two's complement and leaves all the upper bits intact. It is up to the user to take care of this.

.fromTwos(width) - mandatory width parameter in bits

Works as the previous one, only touches the bottom width bits.

Feature request: two's complement #73

Description

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions