reverse-number

Reverse a number without having to do any conversions!

Installation

To install, run npm install reverse-number --save

Usage

The following snippet shows just how easily you can reverse a number:

const reverse = require('reverse-number');

const my_number = 1234; // The number to be reversed

const my_number_reversed = reverse(my_number); // The reverse of your number

console.log(`${my_number} reversed is ${my_number_reversed}`); // Prints 1234 reversed is 4321

How it works

The reversing is one function with one statement. Apart from ?: and calling itself, it uses four operators: ~, &, << and >>>. It has no +, -, *, / or %, no comparisons and no numeric literals.

• Every constant comes from the input. n & ~n is 0, ~(n & ~n) is -1, and -1 >>> -1 is 1, because a shift count of -1 means 31. Every 1 in the code is written out as (~(n & ~n) >>> ~(n & ~n)).
• OR and XOR are built from ~ and &.
• To add, the function calls itself with a third argument and runs a ripple-carry adder, one call per round of carries. Subtracting adds the two's complement.
• Dividing by 10 is the shift-and-add method from Hacker's Delight, inlined everywhere it's needed.

The input checks are ordinary JavaScript.

Notes

1. Numbers must be whole and not negative.
2. Numbers must be at most 4294967295, the largest 32-bit unsigned integer, and so must their reverse. reverse(1000000003) is 3000000001, but reverse(1000000005) throws, as 5000000001 is too big.
3. Anything else throws an error saying which rule it broke.