How to replicate Javascript's 32-bit signed integer arithmetic in C (or Perl) with a few differences

Question

How to replicate Javascript's 32-bit signed integer arithmetic in C (or Perl) with a few differences

I have encountered a discrepancy when translating simple JS code to C and/or Perl, specifically related to arithmetic operations (+ - * / << >>) on integers causing overflow. My goal is to replicate the exact behavior of JS, including handling overflows. The JS variables are not explicitly BigInt, but just standard JS var.

JS (via node.js or Firefox's developer tools' console):

function calc(a, b){
 return (a<<b) + (a<<b);
}
var x = calc(1, 30);
result:
2147483648

C:

#include <stdio.h>
#include <stdint.h>
int main(void){
  int32_t lop = 1<<30; //1073741824;
  int32_t rop = 1<<30; //1073741824;
  int32_t res = lop + rop;
  printf("1<<30 + 1<<30 : %d\n", res);
}
result:
1<<30 + 1<<30 : -2147483648

Perl:

sub to32bit { unpack("l", pack("L", $_[0])) } # Corion @ PerlMonks
print to32bit(to32bit(1<<30)+to32bit(1<<30));
result:
-2147483648

Is there truly a discrepancy in my approach, or am I missing something?

How can I ensure that the C/Perl code behaves exactly like JS?

My aim is to replicate JS's precise behavior in C/Perl without altering the original JS code.

For further discussion, refer to this link.

Edit: February 2024 - A new CPAN module Math::JS (available at ) has been developed by Sisyphus.

javascript c perl overflow integer-arithmetic

Answer 1

Answer №1

JavaScript is built on ECMAScript, which dictates that a left-shift operation should be carried out by converting the left operand into a 32-bit two’s complement integer before performing the operation and converting the right operand into a 32-bit unsigned integer. It also specifies that addition should be done using numbers in the IEEE-754 binary64 (“double precision”) form.

If your C implementation utilizes binary64 for double, as is common practice, and has access to int32_t and uint32_t (found in the <stdint.h> header), then JavaScript's (a<<b) + (a<<b) can be mostly represented as:

(double) ((int32_t) a << (uint32_t) b) + (double) ((int32_t) a << (uint32_t) b)

This statement of "mostly equivalent" stems from potential variations between ECMAScript and C in handling exceptional cases, infinities, NaNs, and type conversions during operations.

The ECMAScript specification clearly outlines the semantics of operations. For those translating JavaScript into another language, it is advisable to refer to the ECMAScript Language Specification.

Answer 2

JavaScript is built on ECMAScript, which dictates that a left-shift operation should be carried out by converting the left operand into a 32-bit two’s complement integer before performing the operation and converting the right operand into a 32-bit unsigned integer. It also specifies that addition should be done using numbers in the IEEE-754 binary64 (“double precision”) form.

If your C implementation utilizes binary64 for double, as is common practice, and has access to int32_t and uint32_t (found in the <stdint.h> header), then JavaScript's (a<<b) + (a<<b) can be mostly represented as:

(double) ((int32_t) a << (uint32_t) b) + (double) ((int32_t) a << (uint32_t) b)

This statement of "mostly equivalent" stems from potential variations between ECMAScript and C in handling exceptional cases, infinities, NaNs, and type conversions during operations.

The ECMAScript specification clearly outlines the semantics of operations. For those translating JavaScript into another language, it is advisable to refer to the ECMAScript Language Specification.

Answer 3

Answer №2

When working with Javascript numbers, it's important to note that they are essentially doubles. During bit arithmetics, they are first truncated to int32's and then promoted back to doubles. Therefore, your C code should look something like this:

double js_shift_left(double a, double b) {
    return (double) ((int32_t)a << (int32_t)b);
}

double calculate(double a, double b) {
    return js_shift_left(a, b) + js_shift_left(a, b);
}

int main(void){
  double result = calculate(1, 30);
  printf("result : %f\n", result);
}

Answer 4

When working with Javascript numbers, it's important to note that they are essentially doubles. During bit arithmetics, they are first truncated to int32's and then promoted back to doubles. Therefore, your C code should look something like this:

double js_shift_left(double a, double b) {
    return (double) ((int32_t)a << (int32_t)b);
}

double calculate(double a, double b) {
    return js_shift_left(a, b) + js_shift_left(a, b);
}

int main(void){
  double result = calculate(1, 30);
  printf("result : %f\n", result);
}

How to replicate Javascript's 32-bit signed integer arithmetic in C (or Perl) with a few differences

Answer №1

Answer №2

Similar questions

Manage the border around the image by incorporating a timer countdown - starting from a complete circle, transitioning to a partial arc, and finally disappearing completely

What is the best way to perform this task in Angular2?

The Xero Node OAuth Authorize Callback URL is malfunctioning after granting access

Utilizing a custom font to emphasize the extended phrase in the following sentence

Don't waste time creating multiple popups for changing content - streamline your process

Guide to Repairing Uncaught TypeError: players.setAttribute is not a recognized method?

Utilize a Python script to transmit data to JavaScript through JSON in order to dynamically alter the content of

Select a random class from an array of classes in JavaScript

Prop in a React component is undergoing mutation

Tips for injecting animation into a DIV

Is it possible to assign a deconstructed array to a variable and then further deconstruct it?

Reactjs Promise left hanging in limbo

Guide on how to toggle disabled input text box upon checking checkbox using JavaScript

How to retrieve a random element from an array within a for loop using Angular 2

The automated Login Pop Up button appears on its own and does not immediately redirect to the login form

Dealing with a checkbox click event in Vuejs when there is no parent element involvement

Moving an item to the top of an array in JavaScript/Vue.js: A simple guide

A sophisticated approach to implementing a search functionality within a complex JSON structure containing nested arrays using JavaScript

Try enabling automatic status bar filling in Onsen UI when working with AngularJS

Generate a JSON Object array by collecting data from various elements to make it dynamic