feat(euler): 28 - number spiral diagonals

challenges/euler/readme.md

@@ -39,7 +39,7 @@ The source code can be found in the [src](src) directory. My thoughts about some
 -   [x] [25 - 1000-digit Fibonacci number](src/25%20-%201000-digit%20Fibonacci%20number.ts)
 -   [ ] 26 - Reciprocal cycles
 -   [ ] 27 - Quadratic primes
--   [ ] 28 - Number spiral diagonals
+-   [x] [28 - Number spiral diagonals](src/28%20-%20Number%20spiral%20diagonals.ts)
 -   [ ] 29 - Distinct powers
 -   [ ] 30 - Digit fifth powers
 -   [ ] 31 - Coin sums

challenges/euler/src/28 - Number spiral diagonals.ts

@@ -0,0 +1,32 @@
+// Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:
+// 21 22 23 24 25
+// 20  7  8  9 10
+// 19  6  1  2 11
+// 18  5  4  3 12
+// 17 16 15 14 13
+// It can be verified that the sum of the numbers on the diagonals is 101.
+// What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
+export = {};
+
+const sumOfSpiralDiagonals = (size: number) => {
+    let sum = 1;
+
+    // Matrix is size n
+    // Lowest number is n(n - 3) + 4
+    // Numbers increase by n - 1
+
+    for (let n = 3; n <= size; n += 2) {
+        const corners = [];
+
+        for (let i = n * (n - 3) + 3; corners.length < 4; i += n - 1) {
+            corners.push(i);
+        }
+
+        sum += corners.reduce((a, b) => a + b);
+    }
+
+    return sum;
+};
+
+// Output
+console.log(sumOfSpiralDiagonals(1001));