# <img src="./logo.png" alt="bn.js" width="160" height="160" />

> BigNum in pure javascript

[![Build Status](https://secure.travis-ci.org/indutny/bn.js.png)](http://travis-ci.org/indutny/bn.js)

## Install
`npm install --save bn.js`

## Usage

```js
const BN = require('bn.js');

var a = new BN('dead', 16);
var b = new BN('101010', 2);

var res = a.add(b);
console.log(res.toString(10));  // 57047
```

**Note**: decimals are not supported in this library.

## Sponsors

[![Scout APM](./sponsors/scout-apm.png)](https://scoutapm.com/)
My Open Source work is supported by [Scout APM](https://scoutapm.com/) and
[other sponsors](https://github.com/sponsors/indutny).

## Notation

### Prefixes

There are several prefixes to instructions that affect the way they work. Here
is the list of them in the order of appearance in the function name:

* `i` - perform operation in-place, storing the result in the host object (on
  which the method was invoked). Might be used to avoid number allocation costs
* `u` - unsigned, ignore the sign of operands when performing operation, or
  always return positive value. Second case applies to reduction operations
  like `mod()`. In such cases if the result will be negative - modulo will be
  added to the result to make it positive

### Postfixes

* `n` - the argument of the function must be a plain JavaScript
  Number. Decimals are not supported. The number passed must be smaller than 0x4000000 (67_108_864). Otherwise, an error is thrown.
* `rn` - both argument and return value of the function are plain JavaScript
  Numbers. Decimals are not supported.

### Examples

* `a.iadd(b)` - perform addition on `a` and `b`, storing the result in `a`
* `a.umod(b)` - reduce `a` modulo `b`, returning positive value
* `a.iushln(13)` - shift bits of `a` left by 13

## Instructions

Prefixes/postfixes are put in parens at the end of the line. `endian` - could be
either `le` (little-endian) or `be` (big-endian).

### Utilities

* `a.clone()` - clone number
* `a.toString(base, length)` - convert to base-string and pad with zeroes
* `a.toNumber()` - convert to Javascript Number (limited to 53 bits)
* `a.toJSON()` - convert to JSON compatible hex string (alias of `toString(16)`)
* `a.toArray(endian, length)` - convert to byte `Array`, and optionally zero
  pad to length, throwing if already exceeding
* `a.toArrayLike(type, endian, length)` - convert to an instance of `type`,
  which must behave like an `Array`
* `a.toBuffer(endian, length)` - convert to Node.js Buffer (if available). `length` in bytes. For
  compatibility with browserify and similar tools, use this instead:
  `a.toArrayLike(Buffer, endian, length)`
* `a.bitLength()` - get number of bits occupied
* `a.zeroBits()` - return number of less-significant consequent zero bits
  (example: `1010000` has 4 zero bits)
* `a.byteLength()` - return number of bytes occupied
* `a.isNeg()` - true if the number is negative
* `a.isEven()` - no comments
* `a.isOdd()` - no comments
* `a.isZero()` - no comments
* `a.cmp(b)` - compare numbers and return `-1` (a `<` b), `0` (a `==` b), or `1` (a `>` b)
  depending on the comparison result (`ucmp`, `cmpn`)
* `a.lt(b)` - `a` less than `b` (`n`)
* `a.lte(b)` - `a` less than or equals `b` (`n`)
* `a.gt(b)` - `a` greater than `b` (`n`)
* `a.gte(b)` - `a` greater than or equals `b` (`n`)
* `a.eq(b)` - `a` equals `b` (`n`)
* `a.toTwos(width)` - convert to two's complement representation, where `width` is bit width
* `a.fromTwos(width)` - convert from two's complement representation, where `width` is the bit width
* `BN.isBN(object)` - returns true if the supplied `object` is a BN.js instance
* `BN.max(a, b)` - return `a` if `a` bigger than `b`
* `BN.min(a, b)` - return `a` if `a` less than `b`

### Arithmetics

* `a.neg()` - negate sign (`i`)
* `a.abs()` - absolute value (`i`)
* `a.add(b)` - addition (`i`, `n`, `in`)
* `a.sub(b)` - subtraction (`i`, `n`, `in`)
* `a.mul(b)` - multiply (`i`, `n`, `in`)
* `a.sqr()` - square (`i`)
* `a.pow(b)` - raise `a` to the power of `b`
* `a.div(b)` - divide (`divn`, `idivn`)
* `a.mod(b)` - reduct (`u`, `n`) (but no `umodn`)
* `a.divmod(b)` - quotient and modulus obtained by dividing
* `a.divRound(b)` - rounded division

### Bit operations

* `a.or(b)` - or (`i`, `u`, `iu`)
* `a.and(b)` - and (`i`, `u`, `iu`, `andln`) (NOTE: `andln` is going to be replaced
  with `andn` in future)
* `a.xor(b)` - xor (`i`, `u`, `iu`)
* `a.setn(b, value)` - set specified bit to `value`
* `a.shln(b)` - shift left (`i`, `u`, `iu`)
* `a.shrn(b)` - shift right (`i`, `u`, `iu`)
* `a.testn(b)` - test if specified bit is set
* `a.maskn(b)` - clear bits with indexes higher or equal to `b` (`i`)
* `a.bincn(b)` - add `1 << b` to the number
* `a.notn(w)` - not (for the width specified by `w`) (`i`)

### Reduction

* `a.gcd(b)` - GCD
* `a.egcd(b)` - Extended GCD results (`{ a: ..., b: ..., gcd: ... }`)
* `a.invm(b)` - inverse `a` modulo `b`

## Fast reduction

When doing lots of reductions using the same modulo, it might be beneficial to
use some tricks: like [Montgomery multiplication][0], or using special algorithm
for [Mersenne Prime][1].

### Reduction context

To enable this trick one should create a reduction context:

```js
var red = BN.red(num);
```
where `num` is just a BN instance.

Or:

```js
var red = BN.red(primeName);
```

Where `primeName` is either of these [Mersenne Primes][1]:

* `'k256'`
* `'p224'`
* `'p192'`
* `'p25519'`

Or:

```js
var red = BN.mont(num);
```

To reduce numbers with [Montgomery trick][0]. `.mont()` is generally faster than
`.red(num)`, but slower than `BN.red(primeName)`.

### Converting numbers

Before performing anything in reduction context - numbers should be converted
to it. Usually, this means that one should:

* Convert inputs to reducted ones
* Operate on them in reduction context
* Convert outputs back from the reduction context

Here is how one may convert numbers to `red`:

```js
var redA = a.toRed(red);
```
Where `red` is a reduction context created using instructions above

Here is how to convert them back:

```js
var a = redA.fromRed();
```

### Red instructions

Most of the instructions from the very start of this readme have their
counterparts in red context:

* `a.redAdd(b)`, `a.redIAdd(b)`
* `a.redSub(b)`, `a.redISub(b)`
* `a.redShl(num)`
* `a.redMul(b)`, `a.redIMul(b)`
* `a.redSqr()`, `a.redISqr()`
* `a.redSqrt()` - square root modulo reduction context's prime
* `a.redInvm()` - modular inverse of the number
* `a.redNeg()`
* `a.redPow(b)` - modular exponentiation

### Number Size

Optimized for elliptic curves that work with 256-bit numbers.
There is no limitation on the size of the numbers.

## LICENSE

This software is licensed under the MIT License.

[0]: https://en.wikipedia.org/wiki/Montgomery_modular_multiplication
[1]: https://en.wikipedia.org/wiki/Mersenne_prime
