Solution to Problem 4

src/bin/4.rs

@@ -0,0 +1,66 @@
+// A palindromic number reads the same both ways. The largest palindrome made
+// from the product of two 2-digit numbers is 9009 = 91 × 99.
+
+/// Checks if `num` is a palindrome.
+/// Increasingly compares beginning and ending digits
+fn is_palindrome(num: u64) -> bool {
+    let mut number = num;
+    let mut leading_div = 1;
+    while number > 10 {
+        number /= 10;
+        leading_div *= 10;
+    }
+    number = num;
+    while number > 10 {
+        let ending = number % 10;
+        let leading = number / leading_div;
+        if ending != leading {
+            return false;
+        }
+        number = (number % leading_div) / 10;
+        leading_div /= 100;
+    }
+    true
+}
+
+/// Finds the largest palindrome made from the product of two n-`digit` numbers.
+/// This is an extremely naive approach bruteforce method.
+/// It finds all possible palindromes and then pick the largest one.
+///
+/// TODO: Find a more efficient method.
+fn problem_4_sol_1(digits: u32) -> Result<u64, &'static str> {
+    let a = 10u64.pow(digits - 1)..10u64.pow(digits);
+    let mut max_palindrome = 0;
+    for a_value in a {
+        for b_value in (10u64.pow(digits - 1)..10u64.pow(digits)).rev() {
+            let candidate = a_value * b_value;
+            if is_palindrome(candidate) && candidate > max_palindrome {
+                max_palindrome = candidate;
+            }
+        }
+    }
+    if max_palindrome == 0 {
+        Err("No palindrome product found")
+    } else {
+        Ok(max_palindrome)
+    }
+}
+
+fn main() {
+    let digits = 3;
+    println!("Largest palindromic product for 2 numbers with {} digits {}",
+             digits, problem_4_sol_1(digits).ok().unwrap());
+}
+
+#[test]
+fn test_is_palindrome() {
+    assert_eq!(true, is_palindrome(9009));
+    assert_eq!(true, is_palindrome(1234554321));
+    assert_eq!(true, is_palindrome(1));
+    assert_eq!(false, is_palindrome(12));
+}
+
+#[test]
+fn test_largest_palindrome() {
+    assert_eq!(9009, problem_4_sol_1(2).ok().unwrap());
+}