feat(euler): 19 - counting sundays

-

challenges/euler/readme.md

@@ -30,7 +30,7 @@ The source code can be found in the [src](src) directory. My thoughts about some
 -   [x] [16 - Power digit sum](src/16%20-%20Power%20digit%20sum.ts)
 -   [x] [17 - Number letter counts](src/17%20-%20Number%20letter%20counts.ts)
 -   [x] [18 - Maximum path sum I](src/18%20-%20Maximum%20path%20sum%20I.ts)
--   [ ] 19 - Counting Sundays
+-   [x] [19 - Counting Sundays](src/19%20-%20Counting%20Sundays.ts)
 -   [ ] 20 - Factorial digit sum
 -   [ ] 21 - Amicable numbers
 -   [ ] 22 - Names scores

challenges/euler/src/19 - Counting Sundays.ts

@@ -0,0 +1,62 @@
+// You are given the following information, but you may prefer to do some research for yourself.
+// 1 Jan 1900 was a Monday.
+// Thirty days has September,
+// April, June and November.
+// All the rest have thirty-one,
+// Saving February alone,
+// Which has twenty-eight, rain or shine.
+// And on leap years, twenty-nine.
+// A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
+// How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
+export = {};
+
+interface Date {
+    year: number;
+    month: number;
+    date: number;
+}
+
+/**
+ * Find all of the Sundays in a given
+ */
+const sundaysInAYear = (year: number): Date[] => {
+    const time = new Date(year, 0, 1);
+
+    while (time.getDay() != 0) {
+        time.setDate(time.getDate() + 1);
+    }
+
+    const dates = [];
+
+    while (time.getFullYear() === year) {
+        const month = time.getMonth() + 1;
+        const date = time.getDate();
+
+        dates.push({
+            year,
+            month,
+            date
+        });
+
+        time.setDate(time.getDate() + 7);
+    }
+
+    return dates;
+};
+
+// Output
+let dates: Date[] = [];
+
+for (let year = 1901; year <= 2000; year++) {
+    dates = [...dates, ...sundaysInAYear(year)];
+}
+
+let count = 0;
+
+dates.forEach(({ date }) => {
+    if (date == 1) {
+        count += 1;
+    }
+});
+
+console.log(count);