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Add sqrt function. #277

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Add sqrt function. #277

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16 changes: 16 additions & 0 deletions lib/bn.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2016,6 +2016,22 @@
return this.clone().imuln(num);
};

// https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method
BN.prototype.sqrt = function sqrt () {
var z = new BN(0);
if (this.gt(new BN(3))) {
z = this;
var x = this.div(new BN(2)).add(new BN(1));
while (x.lt(z)) {
z = x;
x = this.div(x).add(x).div(new BN(2));
}
} else if (!this.eq(new BN(0))) {
z = new BN(1);
}
return z;
};

// `this` * `this`
BN.prototype.sqr = function sqr () {
return this.mul(this);
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9 changes: 9 additions & 0 deletions test/arithmetic-test.js
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Expand Up @@ -317,6 +317,15 @@ describe('BN.js/Arithmetic', function () {
});
});

describe('.sqrt()', function () {
it('should raise number to the sqrt', function () {
var a = new BN('123456', 10);
var b = a.mul(a).sqrt();

assert.equal(b.toString(10), a.toString(10));
});
});

describe('.div()', function () {
it('should divide small numbers (<=26 bits)', function () {
assert.equal(new BN('256').div(new BN(10)).toString(10),
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