Convert C# bitwise operations to Javascript

Question

I am trying to convert the following C# function into Javascript, but I haven't had much luck (see my attempt below).

Original C# code:

using System;
                    
public class Program
{
    const UInt16 MASK = 0xA001;
    public static void Main()
    {
        byte[] test = System.Text.Encoding.ASCII.GetBytes("A TEST STRING");
        ushort result = Program.GetChecksum(test); 
    }
    
    public static ushort GetChecksum(byte[] pBytes)
    {
       ushort result = 0;
       int vCount = pBytes.Length;
       byte b;
       for (int i = 0; i < vCount; i++)
       {
           b = pBytes[i];
           for (byte cnt = 1; cnt <= 8; cnt++)
           {
               if (((b % 2) != 0) ^ ((result % 2) != 0))
               {
                   result = (ushort)((result >> 1) ^ MASK);
               }
               else
               {
                   result = (ushort)(result >> 1);
               }
               b = (byte)(b >> 1);
           }
       }
       return result;
    }
}

My conversion attempt:

Javascript:

const MASK = 0xA001;

const GetChecksum = (b: Buffer) => {
    const result = Buffer.alloc(10).fill(0); // Don't know how much to alloc here
    for (let i = 0; i < b.length; i++)
    {
        const byte = b[i];
        for (let j = 1; j <= 8; j++)
        {
          const t1 = (byte % 2) !== 0;
          const t2 = (result[i] % 2) !== 0;
          
          result[i] = (!(t1 && t2) && (t1 || t2))
            ? ((result[i] >> 1) ^ MASK) 
            : (result[i] >> 1);

          result[i] = byte >> 1;
        }
    }
    return result;
}

const x =  GetChecksum(Buffer.from('THIS IS A TEST','ascii'));
console.log(x.toString('ascii'));

In particular, I'm not able to replicate the condition: (((b % 2) != 0) ^ ((result % 2) != 0)), although I think there are multiple issues in my attempt.

Any help here would be appreciated!

ibrahim mahrir · Accepted Answer · 2020-11-27 02:20:32Z

2

The javascript equivalent of that code is:

const MASK = 0xA001;

function getChecksum(bytes) {
    let result = 0;                                            // this is a number not a buffer
    
    for (let b of bytes) {                                     // for each byte in bytes (clearer than 'for' with an index 'i')
        for (let ctn = 1; ctn <= 8; ctn++) {                   // same as C#
            if (((b % 2) !== 0) ^ ((result % 2) !== 0)) {      // same as C#
               result = (result >> 1) ^ MASK;                  // same as C#
            } else {
               result = result >> 1;                           // same as C#
            }
            
            b = b >> 1;                                        // same as C#
        }
    }
    
    return result;
};

let buffer = Buffer.from("A TEST STRING", "ascii");
let result = getChecksum(buffer);
let checksumString = result.toString(16);

console.log(checksumString);

edited Nov 27, 2020 at 2:20

answered Nov 27, 2020 at 2:17

ibrahim mahrir

31.8k5 gold badges50 silver badges78 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Dai Over a year ago

Wouldn't you want const instead of let - and >>> (unsigned right-shift) instead of >>?

ibrahim mahrir Over a year ago

@Dai result is changing for each byte in the array so it can't be const, as for the unsigned right-shift, I'll think about once I figure out what the original code is doing exactly.

Dai Over a year ago

I meant using const for buffer and the outer result and checksumString.

ibrahim mahrir Over a year ago

@Dai there is no problem in using let for those too - and the unsigned right-shift won't be needed here because result, since it is a number (way larger than the 16-bit short used in the C# code), is big enough that most of the leftmost bits are always 0 so >> and >>> have the same results in this case. Also, the current byte b is unsigned by default so >>> won't make a difference there too

Ryan de Rivera · Accepted Answer · 2020-11-27 02:21:50Z

-1

I hope this gives you a clue:

if ((((b % 2) | 0) !== 0) ^ (((result % 2) >>> 0) !== 0)) {

answered Nov 27, 2020 at 2:21

Ryan de Rivera

1027 bronze badges

Collectives™ on Stack Overflow

Convert C# bitwise operations to Javascript

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related