16-bit binary arithmetic in Javascript - javascript

Javascript has only one number type: a 64-bit floating point.
Using Javascript, I need to implement a hashing algorithm which is designed to be written in C with 16 bit unsigned integers.
The main operation is something like this (pseudocode):
uint16 n = 0;
string s = "abcd1234";
for (int i = 0; i < s.length; i += 1) {
n ^= (n << 2) + (n >> 3) + s[i];
}
return n;
Of course, this produces one result when using uint16 values and a different result if n is a 64-bit floating point.
My best solution to the problem so far is to convert the results of each bitwise operation to <= 16 bits, using a function like this (javascript):
function uint16 (n) {
return parseInt(n.toString(2).slice(-16), 2);
}
and performing the operation something like this (javascript):
for (var i = 0; i < s.length; i +=1 ) {
n ^= uint16(uint16(n << 2) + uint16(n >>> 3) + s.charCodeAt(i));
}
but I'm not 100% confident this will always produce the correct results.
Is there any standard way to emulate 16-bit unsigned bitwise operations on a number value in Javascript?

You can use bitwise AND.
It operates on 32 bit integers, but you can just "and" with 0xffff.
function uint16 (n) {
return n & 0xFFFF;
}
Additionally, bit shift operations (<<, >>>) also operate on 32 bit integers, so you only really need to call the uint16 function before assignment:
for (var i = 0; i < s.length; i +=1 ) {
n ^= uint16((n << 2) + (n >>> 3) + s.charCodeAt(i));
}

I suspect #Amit's version will execute faster but this is a workaround I have had success with:
//create a typed container, we'll work only in the first index.
const idx = 0;
const n = new Uint16Array(1);
for (int i = 0; i < s.length; i += 1) {
n[idx] ^= (n[idx] << 2) + (n[idx] >> 3) + s[i];
}

Related

Hash algorithm issue with converting code from javascript to python

To get same hash function in javascript and python I tried to convert my javaScript function to python and completely stack with gooooogle int as result in python variant.
javascript
function str_hash(var s) {
var hash = 0, i, chr;
if (s.length === 0) return hash;
for (i = 0; i < s.length; i++)
{
chr = s.charCodeAt(i);
hash = ((hash << 5) - hash) + chr;
hash |= 0;
}
return hash;
};
output is like: -34998534 whatever
my python try:
def get_hash(s):
h = 0
if not s:
return h
for i in range(0, len(s)):
h = ((h << 5) - h) + i
h |= 0
return h
print(get_hash('PUT LONG STRING HERE'))
output is like: 349832894283058945028049523548405975892375489743847490218348927483984793048218934148973940218340298489273942374902938490238482942930729487210948239407329403082738940214 whatever
Any ideas how to fix it?
here is the python equivalent code
def str_hash(s):
hash = 0
for i in range(len(s)):
chr = ord(s[i])
hash = ((hash << 5) - hash) + chr
hash = hash & 0xFFFFFFFF
return hash
print(str_hash("PUT LONG STRING HERE"))
the << operator in javascript is called the bitwise left shift operator. and the native code for this is explained in the below example
let num = 4
let shiftBy = 2
print(num * 2 ** shiftBy) // num * Math.pow(2,shiftBy)
print(num << shiftBy)

Javascript bitwise operation

I was doing a Leetcode problem for 231. Power of Two and one of the solution is to use Bitwise Operators to get the Rightmost 1-bit, here is the answer.
/**
* #param {number} n
* #return {boolean}
*/
var isPowerOfTwo = function(n) {
if (n === 0) return false;
const x = n;
return (x & (-x)) === x;
};
But when i tested it with input -2147483648, the function return wrong answer but it is good for Python for exemple.
It is because of some problems with Javascript to deal with signed integer?
Thanks for your response.
Edit:
Oups, i ignore the fact that power of two couldn't be negative.
We can just use n > 0 and simultaneously couple our two statements here.
This'll get accepted in JavaScript:
const isPowerOfTwo = function(n) {
return n > 0 && (n & (n - 1)) === 0;
};
console.log(isPowerOfTwo(-2147483648))
Python doesn't have that integer overflow.
class Solution:
def isPowerOfTwo(self, n):
if n < 1:
return False
while not n & 1:
n >>= 1
return n == 1
We can also simply return false if n < 1 first. Here are implementations of the same idea in Java/C++:
Java
public final class Solution {
public static final boolean isPowerOfTwo(int n) {
if (n < 1) {
return false;
}
while ((n & 1) == 0) {
n >>= 1;
}
return n == 1;
}
}
C++
static const struct Solution {
static const bool isPowerOfTwo(int n) {
if (n < 1) {
return false;
}
while (!(n & 1)) {
n >>= 1;
}
return n == 1;
}
};
References
For additional details, please see the Discussion Board where you can find plenty of well-explained accepted solutions with a variety of languages including low-complexity algorithms and asymptotic runtime/memory analysis1, 2.
Yes for all negative integer the result is wrong because of the sign.
This format stores numbers in 64 bits, where the number (the fraction) is stored in bits 0 to 51, the exponent in bits 52 to 62, and the sign in bit 63.
For more information look https://www.w3schools.com/js/js_numbers.asp

Reverse Bits JavaScript

The problem is standard but the solution in JavaScript takes a lot more effort to code.
I got the solution but my answer is coming half of what is desired.
Problem Description
Reverse the bits of a 32-bit unsigned integer A.
Problem Constraints
0 <= A <= 2^32
Input Format
The first and only argument of input contains an integer A.
Output Format
Return a single unsigned integer denoting minimum xor value.
Example Input
Input 1:
0
Input 2:
3
Example Output
Output 1:
0
Output 2:
3221225472
My solution
function modulo(a, b) {
return a - Math.floor(a/b)*b;
}
function ToUint32(x) {
return modulo(parseInt(x), Math.pow(2, 32));
}
function revereBits(A){
A = A.toString(2);
while (A.length < 31){
A = "0"+A;
}
var reverse = 0;
var NO_OF_BITS = A.length;
for(var i = NO_OF_BITS; i >= 1; i--){
var temp = (parseInt(A, 2) & (1 << i - 1));
if(temp){
reverse |= 1 << (NO_OF_BITS - i);
}
}
if( reverse << 1 < 0 ) reverse = ToUint32(reverse << 1);
return reverse;
}
Now, in the line
if( reverse << 1 < 0 ) reverse = ToUint32(reverse << 1);
You see that I have to double the answer. I cannot, however, get the part of why is this required.
I took the approach from https://www.geeksforgeeks.org/write-an-efficient-c-program-to-reverse-bits-of-a-number/
Had to make few adjustments to it. For example, run the loop from 31 to 1 rather than 0 to 31. The latter gives negative values in first left shift operation for i = 0 itself.
Can someone please help in fixing this solution and point to the problem in this?
UPDATE - Problem is related to Bit manipulation. So guys, please don't answer or comment for anything consisting of in-built string functions of Javascript. Cheers!
You should be able to do it using just bitwise operators, and a typed array to solve the sign issue:
Update
Changing slightly approach of the rev function after #bryc comment. Since having multiple function for "history" purpose makes the answer difficult to read, I'm putting first the latest code.
However, I'm keeping the comments about the different steps – the rest can be found in the edit history.
function rev(x) {
x = ((x >> 1) & 0x55555555) | ((x & 0x55555555) << 1);
x = ((x >> 2) & 0x33333333) | ((x & 0x33333333) << 2);
x = ((x >> 4) & 0x0F0F0F0F) | ((x & 0x0F0F0F0F) << 4);
x = ((x >> 8) & 0x00FF00FF) | ((x & 0x00FF00FF) << 8);
x = (x >>> 16) | (x << 16);
return x >>> 0;
}
This is the same code you would write in other languages as well to reverse bits, the only difference here is the addition of the typed array.
As #harold pointed out in the comment, the zero-fill right shift returns an unsigned (it's the only bitwise operator to do so) therefore we can omit the typed array and just add >>> 0 before the return.
In fact, doing >>> 0 is commonly used to simulate the ToUint32 in JS polyfilll; e.g.:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every
// 2. Let lenValue be the result of calling the Get internal method
// of O with the argument "length".
// 3. Let len be ToUint32(lenValue).
var len = O.length >>> 0;
function reverseBits(integer, bitLength) {
if (bitLength > 32) {
throw Error(
"Bit manipulation is limited to <= 32 bit numbers in JavaScript."
);
}
let result = 0;
for (let i = 0; i < bitLength; i++) {
result |= ((integer >> i) & 1) << (bitLength - 1 - i);
}
return result >>> 0; // >>> 0 makes it unsigned even if bit 32 (the sign bit) was set
}
Try this:
function reverseBits(num) {
let reversed = num.toString(2);
const padding = "0";
reversed = padding.repeat(32 - reversed.length) + reversed;
return parseInt(reversed.split('').reverse().join(''), 2);
}
console.log(reverseBits(0)); // 0
console.log(reverseBits(3)); // 3221225472

Using bitwise operators with large numbers in javascript [duplicate]

This question already has answers here:
Bitshift in javascript
(4 answers)
Closed 3 years ago.
I am writing a Javascript version of this Microsoft string decoding algorithm and its failing on large numbers. This seems to be because of sizing (int / long) issues. If I step through the code in C# I see that the JS implementation fails on this line
n |= (b & 31) << k;
This happens when the values are (and the C# result is 240518168576)
(39 & 31) << 35
If I play around with these values in C# I can replicate the JS issue if b is an int. And If I set b to be long it works correctly.
So then I checked the max size of a JS number, and compared it to the C# long result
240518168576 < Number.MAX_SAFE_INTEGER = true
So.. I can see that there is some kind of number size issue happening but do not know how to force JS to treat this number as a long.
Full JS code:
private getPointsFromEncodedString(encodedLine: string): number[][] {
const EncodingString = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";
var points: number[][] = [];
if (!encodedLine) {
return points;
}
var index = 0;
var xsum = 0;
var ysum = 0;
while (index < encodedLine.length) {
var n = 0;
var k = 0;
debugger;
while (true) {
if (index >= encodedLine.length) {
return points;
}
var b = EncodingString.indexOf(encodedLine[index++]);
if (b == -1) {
return points;
}
n |= (b & 31) << k;
k += 5;
if (b < 32) {
break;
}
}
var diagonal = ((Math.sqrt(8 * n + 5) - 1) / 2);
n -= diagonal * (diagonal + 1) / 2;
var ny = n;
var nx = diagonal - ny;
nx = (nx >> 1) ^ -(nx & 1);
ny = (ny >> 1) ^ -(ny & 1);
xsum += nx;
ysum += ny;
points.push([ysum * 0.000001, xsum * 0.000001]);
}
console.log(points);
return points;
}
Expected input output:
Encoded string
qkoo7v4q-lmB0471BiuuNmo30B
Decoded points:
35.89431, -110.72522
35.89393, -110.72578
35.89374, -110.72606
35.89337, -110.72662
Bitwise operators treat their operands as a sequence of 32 bits
(zeroes and ones), rather than as decimal, hexadecimal, or octal
numbers. For example, the decimal number nine has a binary
representation of 1001. Bitwise operators perform their operations on
such binary representations, but they return standard JavaScript
numerical values.
(39 & 31) << 35 tries to shift 35 bits when there only 32
Bitwise Operators
To solve this problem you could use BigInt to perform those operations and then downcast it back to Number
Number((39n & 31n) << 35n)
You can try this:
function getPointsFromEncodedString(encodedLine) {
const EncodingString = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";
var points = [];
if (!encodedLine) {
return points;
}
var index = 0;
var xsum = 0;
var ysum = 0;
while (index < encodedLine.length) {
var n = 0n;
var k = 0n;
while (true) {
if (index >= encodedLine.length) {
return points;
}
var b = EncodingString.indexOf(encodedLine[index++]);
if (b === -1) {
return points;
}
n |= (b & 31n) << k;
k += 5n;
if (b < 32n) {
break;
}
}
var diagonal = ((Math.sqrt(8 * Number(n) + 5) - 1) / 2);
n -= diagonal * (diagonal + 1) / 2;
var ny = n;
var nx = diagonal - ny;
nx = (nx >> 1) ^ -(nx & 1);
ny = (ny >> 1) ^ -(ny & 1);
xsum += Number(nx);
ysum += Number(ny);
points.push([ysum * 0.000001, xsum * 0.000001]);
}
console.log(points);
return points;
}

Javascript get single bit

I have an integer and i want to check if a single bit is 0 or 1.
What is the best practise for doing that?
An example of what i'm doing at this moment:
const myInt = 8; // Binary in 32 Bit integer = 00000000000000000000000000001000
const myBit = myInt << 28 >>> 31; // 00000000000000000000000000000001
if (myBit === 1) {
//do something
}
But i think that this isn't the best methode for doing this.
Have you any better idea?
EDIT:
It is always the same bit i want to check, but the integer is different
myInt = 8+4; // 1100
n = 3;
(myInt >> n) & 0x1; // 1
n = 2;
(myInt >> n) & 0x1; // 1
n = 1;
(myInt >> n) & 0x1; // 0
n = 0;
(myInt >> n) & 0x1; // 0
general solution shifts your number by N bits to right, and applies bitmask, that leaves only last bit, all other are set to 0
I think you can use the bitwise AND
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators
my32Bit = 123414123;
twoBy7 = 128;
//check the 7th bit
if (my32Bit & twoBy7) {
// should return 1 if the 7thbit is 1
}
You could take the bit and a left shift << with bitwise AND & operator.
var value = 10,
bit;
for (bit = 0; bit < 4; bit++) {
console.log(bit, !!(value & (1 << bit)));
}

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