From 80cce4671ffd38352ea0d3fb66992d298fe12928 Mon Sep 17 00:00:00 2001
From: BlackMark <me@blackmark.me>
Date: Sat, 16 May 2020 17:01:19 +0200
Subject: [PATCH] Implement optional automatic setting of day of week

---
 ds3231.hpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/ds3231.hpp b/ds3231.hpp
index 62bf497..2f48644 100644
--- a/ds3231.hpp
+++ b/ds3231.hpp
@@ -24,7 +24,7 @@ struct Time {
 struct DateTime : Date, Time {
 };
 
-template <typename I2cDriver>
+template <typename I2cDriver, bool SetDayOfWeek = true>
 class DS3231 {
   public:
 	static constexpr auto I2C_ADDRESS = 0x68;
@@ -125,6 +125,13 @@ class DS3231 {
 			static_assert(type::always_false_v<decltype(Address)>, "Invalid register address");
 		}
 	}
+
+	static uint8_t calcDayOfWeek(uint16_t year, uint8_t month, uint16_t day)
+	{
+		day += month < 3 ? year-- : year - 2;
+		const auto dayOfWeek = (23 * month / 9 + day + 4 + year / 4 - year / 100 + year / 400);
+		return dayOfWeek % 7;
+	}
 };
 
 } // namespace rtc