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id	title	challengeType	forumTopicId	dashedName
5e4ce2f5ac708cc68c1df261	Linear congruential generator	5	385266	linear-congruential-generator

--description--

The [linear congruential generator](https://en.wikipedia.org/wiki/linear congruential generator) is a very simple example of a [random number generator](http://rosettacode.org/wiki/random number generator). All linear congruential generators use this formula:

r_{n + 1} = (a \times r_n + c) \bmod m

Where:

$ r_0 $ is a seed.
$r_1$, $r_2$, $r_3$, ..., are the random numbers.
$a$, $c$, $m$ are constants.

If one chooses the values of a, c and m with care, then the generator produces a uniform distribution of integers from 0 to m - 1.

LCG numbers have poor quality. r_n and r\_{n + 1} are not independent, as true random numbers would be. Anyone who knows r_n can predict r\_{n + 1}, therefore LCG is not cryptographically secure. The LCG is still good enough for simple tasks like [Miller-Rabin primality test](http://rosettacode.org/wiki/Miller-Rabin primality test), or [FreeCell deals](http://rosettacode.org/wiki/deal cards for FreeCell). Among the benefits of the LCG, one can easily reproduce a sequence of numbers, from the same r_0. One can also reproduce such sequence with a different programming language, because the formula is so simple.

--instructions--

Write a function that takes r_0,a,c,m,n as parameters and returns r_n.

--hints--

linearCongGenerator should be a function.

assert(typeof linearCongGenerator == 'function');

linearCongGenerator(324, 1145, 177, 2148, 3) should return a number.

assert(typeof linearCongGenerator(324, 1145, 177, 2148, 3) == 'number');

linearCongGenerator(324, 1145, 177, 2148, 3) should return 855.

assert.equal(linearCongGenerator(324, 1145, 177, 2148, 3), 855);

linearCongGenerator(234, 11245, 145, 83648, 4) should return 1110.

assert.equal(linearCongGenerator(234, 11245, 145, 83648, 4), 1110);

linearCongGenerator(85, 11, 1234, 214748, 5) should return 62217.

assert.equal(linearCongGenerator(85, 11, 1234, 214748, 5), 62217);

linearCongGenerator(0, 1103515245, 12345, 2147483648, 1) should return 12345.

assert.equal(linearCongGenerator(0, 1103515245, 12345, 2147483648, 1), 12345);

linearCongGenerator(0, 1103515245, 12345, 2147483648, 2) should return 1406932606.

assert.equal(
  linearCongGenerator(0, 1103515245, 12345, 2147483648, 2),
  1406932606
);

--seed--

--seed-contents--

function linearCongGenerator(r0, a, c, m, n) {

}

--solutions--

function linearCongGenerator(r0, a, c, m, n) {
    for (let i = 0; i < n; i++) {
        r0 = (a * r0 + c) % m;
    }
    return r0;
}

2.8 KiB Raw Blame History