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3.js
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3.js
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JG.solution(3, () => {
/* START
Largest prime factor
The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?
END */
const factors = findFactors(600851475143)
// TODO: think there's an infinite loop with some numbers still
// ... or just really inefficent
function findFactors (num, primes) {
primes = primes || [2] // first prime number is 2
const factors = []
let prime
let leftovers
let primeIndex = 0
while (true) {
if (primes.length < primeIndex + 1) primes.push(nextPrime(primes))
prime = primes[primeIndex]
if (num % prime === 0) {
if (factors.indexOf(prime) === -1) factors.push(prime)
leftovers = num / prime // leftovers === 1 means num is prime
if (leftovers === 1 || primes.indexOf(leftovers) !== -1) {
if (leftovers !== 1 && factors.indexOf(leftovers) === -1) {
factors.push(leftovers)
}
break // found the last factor, we're done! :D
} else {
// factor was found, but not the last one
num = leftovers
primeIndex = 0
}
} else {
primeIndex++
}
}
return factors
}
function nextPrime (primes) {
let test = primes[primes.length - 1]
// keep testing new numbers to see if they're prime
let factorFound = true
while (factorFound === true) {
factorFound = false
test++
const sqrt = Math.sqrt(test) // no point testing numbers bigger than the sqrt
for (let prime of primes) {
if (prime > sqrt) break
if (test % prime === 0) {
// factor found, so 'test' isn't a new prime
factorFound = true
break
}
}
}
return test // we got out the loop so we're the next prime!
}
return factors[factors.length - 1] // largest one only
})