add 04

src/euler04/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "euler04"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[profile.release]
+debug = true
+strip = false

src/euler04/src/main.rs

@@ -0,0 +1,83 @@
+const MIN_NUMBER: i64 = 100;
+const MAX_NUMBER: i64 = 999;
+
+fn main() {
+    let mut buffer = Vec::new();
+
+    let mut largest_palindrome = 0;
+    for i in MIN_NUMBER..=MAX_NUMBER {
+        for j in MIN_NUMBER..=MAX_NUMBER {
+            let product = i * j;
+            if is_palindrome(product, &mut buffer) && product > largest_palindrome {
+                largest_palindrome = product;
+            }
+        }
+    }
+    println!("{largest_palindrome}");
+}
+
+fn is_palindrome(number: i64, buffer: &mut Vec<char>) -> bool {
+    to_digits(number, buffer);
+    let number_digits = buffer.len();
+
+    #[cfg(test)]
+    dbg!(number, &buffer);
+
+    are_items_palindromic(number_digits, buffer)
+}
+
+fn are_items_palindromic<T>(number_digits: usize, buffer: &[T]) -> bool
+where
+    T: PartialEq,
+{
+    let mut digits_used = 0;
+
+    let mut iter = buffer.iter();
+    while number_digits - digits_used > 1 {
+        let first = iter.next().unwrap();
+        let last = iter.next_back().unwrap();
+        if first != last {
+            return false;
+        }
+
+        digits_used += 2;
+    }
+    true
+}
+
+fn to_digits(number: i64, buffer: &mut Vec<char>) {
+    buffer.clear();
+
+    #[cfg(test)]
+    dbg!(number);
+
+    number
+        .to_string()
+        .chars()
+        .for_each(|digit| buffer.push(digit))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::are_items_palindromic;
+
+    #[test]
+    fn items_palindromes() {
+        assert!(are_items_palindromic(2, &[0, 0]));
+        assert!(!are_items_palindromic(2, &[9, 9, 8, 0, 0, 1]));
+        assert!(are_items_palindromic(2, &[9, 0, 6, 6, 0, 9]));
+    }
+
+    #[test]
+    fn is_palindrome() {
+        let mut buffer = Vec::new();
+        assert!(!crate::is_palindrome(998001, &mut buffer));
+    }
+
+    #[test]
+    fn to_digits() {
+        let mut buffer: Vec<char> = Vec::new();
+        crate::to_digits(1, &mut buffer);
+        assert_eq!(buffer, vec!['1']);
+    }
+}