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* feat: Project Euler Problem 21 * test: Project Euler 21 * fix: test description in Project Euler 21
34 lines
1.0 KiB
JavaScript
34 lines
1.0 KiB
JavaScript
import { aliquotSum } from '../Maths/AliquotSum.js'
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/**
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* Problem 21 - Amicable numbers
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*
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* @see {@link https://projecteuler.net/problem=21}
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*
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* Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
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* If d(a) = b and d(b) = a, where a != b, then a and b are an amicable pair and each of a and b are called amicable numbers.
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* For example, the proper divisors of 220 are 1,2,4,5,10,11,20,22,44,55 and 110; therefore d(220) = 284.
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* The proper divisors of 284 are 1,2,4,71 and 142; so d(284) = 220.
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* Evaluate the sum of all amicable numbers under 10000
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*
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* @author PraneethJain
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*/
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function problem21 (n) {
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if (n < 2) {
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throw new Error('Invalid Input')
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}
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let result = 0
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for (let a = 2; a < n; ++a) {
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const b = aliquotSum(a) // Sum of all proper divisors of a
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// Check if b > a to ensure each pair isn't counted twice, and check if sum of proper divisors of b is equal to a
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if (b > a && aliquotSum(b) === a) {
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result += a + b
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}
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}
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return result
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}
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export { problem21 }
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