Read int64 from node.js buffer with precision loss

Question

Read int64 from node.js buffer with precision loss

What would be the best way to read an Int64BE from a node.js buffer into a Number primitive, like readInt32BE reads an Int32?

I know that I'll lose precision with numbers +/- 9'007'199'254'740'992, but i won't get such high numbers in the protocol I want to implement.

javascript
node.js
int64

Answer 1

Javascript uses 32 bit integers and 32 bit floats. So you have to read two 32 bit integers and shift one to the left while converting it to a float. In this code this is achieved by a multiplication with a float.

Since the low part can be negative but must be treated as unsigned, a correction is added.

function readInt64BEasFloat(buffer, offset) {
  var low = readInt32BE(buffer, offset + 4);
  var n = readInt32BE(buffer, offset) * 4294967296.0 + low;
  if (low < 0) n += 4294967296;
  return n;
}

Answer 2

Don't try and code the conversion yourself, use a tested version like node-int64.

var Int64 = require('node-int64');
function readInt64BEasFloat(buffer, offset) {
  var int64 = new Int64(buffer, offset);
  return int64.toNumber(true);
}

Answer 3

As of Node v0.12.7 you can use buf.readIntBE(offset, 8).

Here's the docs => https://nodejs.org/docs/v0.12.7/api/buffer.html#buffer_buf_readuintle_offset_bytelength_noassert

Here's the commit where the feature was added => https://github.com/joyent/node/commit/83d7d9e6d81fc35764a9000b6d7b17a0e7d8cc92

Hope that helps