From 584424241c61079940d94a5aed1bed583c555098 Mon Sep 17 00:00:00 2001
From: Daniel <67126972+ddaniel27@users.noreply.github.com>
Date: Mon, 17 Jun 2024 23:08:16 -0500
Subject: [PATCH] [Solution] Project euler challenge 19 with tests (#1659)
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* [Solution] Project euler challenge 19 with tests

* update leap year function

* Remove unnecessary, confusingly placed comments

---------

Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>
---
 Project-Euler/Problem019.js           | 48 +++++++++++++++++++++++++++
 Project-Euler/test/Problem019.test.js |  8 +++++
 2 files changed, 56 insertions(+)
 create mode 100644 Project-Euler/Problem019.js
 create mode 100644 Project-Euler/test/Problem019.test.js

diff --git a/Project-Euler/Problem019.js b/Project-Euler/Problem019.js
new file mode 100644
index 000000000..5b488c7f9
--- /dev/null
+++ b/Project-Euler/Problem019.js
@@ -0,0 +1,48 @@
+/**
+ * Problem 19 - Counting Sundays
+ *
+ * @see {@link https://projecteuler.net/problem=19}
+ *
+ * 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)?
+ *
+ * @author ddaniel27
+ */
+import { isLeapYear } from '../Maths/LeapYear'
+
+function problem19() {
+  let sundaysCount = 0 // Count of Sundays
+  let dayOfWeek = 2 // 1st Jan 1900 was a Monday, so 1st Jan 1901 was a Tuesday
+
+  for (let year = 1901; year <= 2000; year++) {
+    for (let month = 1; month <= 12; month++) {
+      if (dayOfWeek === 0) {
+        // If it's a Sunday (0 is Sunday, 1 is Monday, ..., 6 is Saturday)
+        sundaysCount++
+      }
+
+      let daysInMonth = 31
+      if (month === 4 || month === 6 || month === 9 || month === 11) {
+        // April, June, September, November
+        daysInMonth = 30
+      } else if (month === 2) {
+        // February
+        daysInMonth = isLeapYear(year) ? 29 : 28
+      }
+
+      dayOfWeek = (dayOfWeek + daysInMonth) % 7 // Calculate the day of the week
+    }
+  }
+
+  return sundaysCount
+}
+
+export { problem19 }
diff --git a/Project-Euler/test/Problem019.test.js b/Project-Euler/test/Problem019.test.js
new file mode 100644
index 000000000..8845e5a0a
--- /dev/null
+++ b/Project-Euler/test/Problem019.test.js
@@ -0,0 +1,8 @@
+import { problem19 } from '../Problem019.js'
+
+describe('checking sundays during the twentieth century', () => {
+  // Project Euler Challenge Check
+  test('result should be 171', () => {
+    expect(problem19()).toBe(171)
+  })
+})