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// 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
}