4.rs

/*
Problem 4 - Largest palindrome product

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.
*/

use rayon::prelude::*;

fn largest_pallindromic_number(lower_bound: usize, upper_bound: usize) -> usize {
    let mut products: Vec<usize> = vec![];

    for i in lower_bound..(upper_bound + 1) {
        for j in lower_bound..(upper_bound + 1) {
            products.push(i * j);
        }
    }

    // Filter for pallindromic numbers
    let mut pallindromic = products
        .par_iter()
        .filter(|x| x.to_string() == x.to_string().chars().rev().collect::<String>())
        .collect::<Vec<&usize>>();

    // Return the largest value
    pallindromic.sort();
    pallindromic.reverse();

    return *pallindromic[0];
}

pub fn main() {
    let number = largest_pallindromic_number(100, 999);

    println!("The largest pallindromic number made from the product of two three-digit numbers is {number}");
}