Problem004.js

// https://projecteuler.net/problem=4
/* A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
 Find the largest palindrome made from the product of two 3-digit numbers.
*/
export const largestPalindromic = (digits) => {
 let i
 let n
 let m
 let d
 let limit
 let number = 0

 for (i = 1; i < digits; i++) {
 number = 10 * number + 9
 }
 const inf = number // highest (digits - 1) number, in this example highest 2 digit number
 const sup = 10 * number + 9 // highest (digits) number, in this example highest 3 digit number

 const isPalindromic = (n) => {
 let p = 0
 const q = n
 let r
 while (n > 0) {
 r = n % 10
 p = 10 * p + r
 n = Math.floor(n / 10)
 }
 return p === q // returning whether the number is palindromic or not
 }

 for (n = sup * sup, m = inf * inf; n > m; n--) {
 if (isPalindromic(n)) {
 limit = Math.ceil(Math.sqrt(n))
 d = sup
 while (d >= limit) {
 if (n % d === 0 && n / d > inf) {
 return n
 }
 d -= 1
 }
 }
 }
 return NaN // returning not a number, if any such case arise
}