Create problem28.rs

problem28.rs

@@ -0,0 +1,53 @@
+struct Spiral {
+    pub start_num: usize,
+    pub end_num: usize,
+    pub row: usize,
+    pub side_length: usize,
+}
+
+impl Spiral {
+    pub fn new() -> Self {
+        Spiral { row: 0, start_num: 1, end_num: 1, side_length: 1 }
+    }
+
+    pub fn next(&self) -> Self {
+        let current_row = self.row + 1;
+        let start_num = self.end_num + 1;
+
+        let side_length = current_row * 2 + 1;
+
+        let delta = (side_length - 1) * 4;
+
+        Spiral { 
+            row: current_row,
+            start_num,
+            end_num: start_num + delta - 1,
+            side_length,
+        }
+    }
+
+    pub fn diagonals_sum(&self) -> usize {
+        if self.row == 0 {
+            1
+        }
+        else {
+            self.end_num + 
+            (self.end_num - (self.side_length - 1)) + 
+            (self.end_num - (self.side_length - 1) * 2) +
+            (self.end_num - (self.side_length - 1) * 3)
+        }
+    }
+}
+
+fn main() {
+    let mut spiral = Spiral::new();
+    let mut count = 0;
+
+    for _ in 0..501 {
+        count += spiral.diagonals_sum();
+
+        spiral = spiral.next();
+    }
+
+    println!("{}", count);
+}