converting array of binary doubles in protocol buffer to javascript numbers

问题

I'm using protocol buffers over zeroMQ to send graph data from C++ to a Javascript frontend:

message RawData
{
   Type type = 1;
   bytes data = 2;
}

when i call RawData.getData() i get something like this (usually much longer):

Data: 0, 0, 0, 0, 0, 0, 0, 0, 64, 1, 118, 62, 112, 8, 12, 63

This is two 8 bytes numbers, where each index in the array is 1 byte.

How do I convert this into double representation in Javascript?

EDIT: i ended up changing the protocol buffer message to repeated double data = 2;

this removed the need for conversion

回答1:

Just divide the result in 8 elements parts and use ArrayBuffer and Dataview to mount the number. Example:

function splitData(data) {
    return data.reduce(function(a, b, c){
        if(c % 8 == 0  && c !== 0){
            a.push([]); };
        a[a.length - 1].push(b);
        return a;
    }, [[]]);
}

function computeValues(data, littleEndian = false) {
    var res = [];
	splitData(data).forEach(numberElement=>{
        var buffer = new ArrayBuffer(8);
        var dv = new DataView(buffer);
        numberElement.forEach((val, ix)=>dv.setUint8(littleEndian ? 7 - ix : ix, val));
        res.push(dv.getFloat64(0));
    });
    return res;
}


var data = [0, 0, 0, 0, 0, 0, 0, 0, 64, 1, 118, 62, 112, 8, 12, 63];
console.log(computeValues(data));
console.log(computeValues(data, true));

The data.reduce function is taken from this SO answer: Splitting a JS array into N arrays

JSFiddle: https://jsfiddle.net/z3fL39sr/12/

littleEndian parameter refers to the order of the bytes. If the most significant came first, set littleEndian to true. This page explains about endianess if you need it: https://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Data/endian.html

来源：https://stackoverflow.com/questions/42985471/converting-array-of-binary-doubles-in-protocol-buffer-to-javascript-numbers

标签

javascript

c++

protocol-buffers

zeromq