how to change integer to 2 bytes

Question

I have an algorithm problem I need to solve. But I do not really understand the question after spending a day to grasp it.

PROBLEM: For this task, you need to write a small program including a pair of functions that can

convert an integer into a special text encoding The Encoding Function

This function needs to accept a signed integer in the 14-bit range [-8192..+8191] and return a 4 character string.

The encoding process is as follows:

1.Add 8192 to the raw value, so its range is translated to [0..16383] 2.Pack that value into two bytes such that the most significant bit of each is cleared

Unencoded intermediate value (as a 16-bit integer):

00HHHHHH HLLLLLLL

Encoded value:

0HHHHHHH 0LLLLLLL

1 of 3

Format the two bytes as a single 4-character hexadecimal string and return it. Sample values:

Unencoded (decimal) | Intermediate (decimal) | Intermediate (hex) | Encoded (hex)

0 | 8192 | 2000 | 4000

-8192 | 0 | 0000 | 0000

8191 | 16383 | 3fff | 7F7F

2048 | 10240 | 2800 | 5000

-4096 | 4096 | 1000 | 2000

The part '2.Pack that value into two bytes such that the most significant bit of each is cleared' is my obstacle now.

what should I do with this instruction? Thank you so much in advance.

I tried to convert the number to 16bit by the code below. But I don't know what should do next.

let rawInput = parseInt(document.getElementById("inputValue").value);
let interDec= Number.parseInt(rawInput) + 8192
let interHex = interDec.toString(16)

Answer 1

When you added 8192 to integer, you have value of kind (binary representation) 00HHHHHH HLLLLLLL

1) You have to preserve right ( L ) bits - just separate them with bitwise AND using bitmask 0x7f ((binary 00000000 01111111 ))

2) Shift all value left, getting 0HHHHHHH LLLLLLL0 , now H bits stay at the final places

3) Separate H bits with bitwise AND using mask 0x7f00 (binary 01111111 00000000 )

4) Combine low and high parts together with binary OR

Python example

v = 0x3fff
ev = (v & 0x7f) | ((v<<1) & 0x7f00)
print(hex(ev))

>>>0x7f7f

Answer 2

Try - the +('0b'+binString) parse binary string to js number (the toString , slice and padStart are standard well documented js functions)

 function convert(num) { let b = (num+8192).toString(2).padStart(14,'0'); let L = ( +('0b'+b.slice(7)) ).toString(16).padStart(2,'0'); let H = ( +('0b'+b.slice(0,7)) ).toString(16).padStart(2,'0'); return H+L; } console.log(' 0 =>', convert( 0)); console.log('-8192 =>', convert(-8192)); console.log(' 8191 =>', convert( 8191)); console.log(' 2048 =>', convert( 2048)); console.log('-4096 =>', convert(-4096));