Project Euler - Problem 20 (fix + test) #747

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		@@ -1,21 +1,36 @@
	/*
	Factorial digit sum
	/**
	* Problem 20 - Factorial digit sum
	*
	* @see {@link https://projecteuler.net/problem=20}
	*
	* n! means n ×ばつ (n − 1) ×ばつ ... ×ばつ 3 ×ばつ 2 ×ばつ 1
	*
	* For example, 10! = 10 ×ばつ 9 ×ばつ ... ×ばつ 3 ×ばつかける 2 ×ばつかける 1 =わ 3628800,
	* and the sum of the digits in the number 10! is 3 +たす 6 +たす 2 +たす 8 +たす 8 +たす 0 +たす 0 =わ 27
	*
	* Find the sum of the digits in the number 100!
	*/

	n! means n ×ばつ (n − 1) ×ばつ ... ×ばつ 3 ×ばつ 2 ×ばつ 1
	const factorialDigitSum = (n = 100) => {
	// Consider each digit*10^exp separately, right-to-left ([units, tens, ...]).
	const digits = [1]

	For example, 10! = 10 ×ばつ 9 ×ばつ ... ×ばつ 3 ×ばつかける 2 ×ばつかける 1 =わ 3628800,
	and the sum of the digits in the number 10! is 3 +たす 6 +たす 2 +たす 8 +たす 8 +たす 0 +たす 0 =わ 27.
	for (let x = 2; x <= n; x++) {
	let carry = 0
	for (let exp = 0; exp < digits.length; exp++) {
	const prod = digits[exp] * x + carry
	carry = Math.floor(prod / 10)
	digits[exp] = prod % 10
	}
	while (carry > 0) {
	digits.push(carry % 10)
	carry = Math.floor(carry / 10)
	}
	}

	Find the sum of the digits in the number 100!
	*/
	// (digits are reversed but we only want the sum so it doesn't matter)

	const findFactorialDigitSum = (num) => {
	let result = 0
	const stringifiedNumber = factorize(num).toLocaleString('fullwide', { useGrouping: false })
	stringifiedNumber.split('').map(num => { result += Number(num) })
	return result
	return digits.reduce((prev, current) => prev + current, 0)
		}

	const factorize = (num) => num === 0 ? 1 : num * factorize(num - 1)

	console.log(findFactorialDigitSum(100))
	export { factorialDigitSum }

16 changes: 16 additions & 0 deletions Project-Euler/test/Problem020.test.js

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	import { factorialDigitSum } from '../Problem020'

	describe('Check Problem 20 - Factorial digit sum', () => {
	it('Factorial digit sum of 10!', () => {
	expect(factorialDigitSum(10)).toBe(27)
	})

	it('Factorial digit sum of 100!', () => {
	expect(factorialDigitSum()).toBe(648)
	expect(factorialDigitSum(100)).toBe(648)
	})

	it('Factorial digit sum of 1000!', () => {
	expect(factorialDigitSum(1000)).toBe(10539)
	})
	})

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