From fa7df64f523968c9e9d679950943befe45460269 Mon Sep 17 00:00:00 2001
From: Rhys Weatherley <rhys.weatherley@gmail.com>
Date: Mon, 11 Jun 2012 10:05:24 +1000
Subject: [PATCH] Calculating the day of week from a date

---
 libraries/RTC/RTC.cpp                      | 41 ++++++++++++++++++++++
 libraries/RTC/RTC.h                        | 13 +++++++
 libraries/RTC/examples/DumpRTC/DumpRTC.pde |  5 +++
 libraries/RTC/examples/TestRTC/TestRTC.pde |  5 +++
 4 files changed, 64 insertions(+)

diff --git a/libraries/RTC/RTC.cpp b/libraries/RTC/RTC.cpp
index 83c6e906..4edfa6bf 100644
--- a/libraries/RTC/RTC.cpp
+++ b/libraries/RTC/RTC.cpp
@@ -66,6 +66,21 @@ static uint8_t monthLengths[] = {
     31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
 };
 
+static unsigned int monthOffsets[] = {
+    0,
+    31,
+    31 + 28,
+    31 + 28 + 31,
+    31 + 28 + 31 + 30,
+    31 + 28 + 31 + 30 + 31,
+    31 + 28 + 31 + 30 + 31 + 30,
+    31 + 28 + 31 + 30 + 31 + 30 + 31,
+    31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
+    31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
+    31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
+    31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30
+};
+
 inline bool isLeapYear(unsigned int year)
 {
     if ((year % 100) == 0)
@@ -368,6 +383,32 @@ void RTC::adjustYears(RTCDate *date, uint8_t flags)
         date->day = len;
 }
 
+/**
+ * \enum RTC::DayOfWeek
+ * \brief Day of the week corresponding to a date.
+ *
+ * \sa dayOfWeek()
+ */
+
+/**
+ * \brief Returns the day of the week corresponding to \a date.
+ *
+ * This function is only guaranteed to produce meaningful values
+ * for years between 2000 and 2099.
+ */
+RTC::DayOfWeek RTC::dayOfWeek(const RTCDate *date)
+{
+    // The +4 here adjusts for Jan 1, 2000 being a Saturday.
+    unsigned long daynum = date->day + 4;
+    daynum += monthOffsets[date->month - 1];
+    if (date->month > 2 && isLeapYear(date->year))
+        ++daynum;
+    daynum += 365UL * (date->year - 2000);
+    if (date->year > 2000)
+        daynum += ((date->year - 2001) / 4) + 1;
+    return (DayOfWeek)((daynum % 7) + 1);
+}
+
 /**
  * \class RTCTime RTC.h <RTC.h>
  * \brief Stores time information from a realtime clock chip.
diff --git a/libraries/RTC/RTC.h b/libraries/RTC/RTC.h
index 923287d7..9338ab3a 100644
--- a/libraries/RTC/RTC.h
+++ b/libraries/RTC/RTC.h
@@ -52,6 +52,17 @@ public:
     RTC();
     ~RTC();
 
+    enum DayOfWeek
+    {
+        Monday = 1,
+        Tuesday,
+        Wednesday,
+        Thursday,
+        Friday,
+        Saturday,
+        Sunday,
+    };
+
     virtual bool hasUpdates();
 
     virtual void readTime(RTCTime *value);
@@ -82,6 +93,8 @@ public:
     static void adjustMonths(RTCDate *date, uint8_t flags);
     static void adjustYears(RTCDate *date, uint8_t flags);
 
+    static DayOfWeek dayOfWeek(const RTCDate *date);
+
 private:
     unsigned long midnight;
     RTCDate date;
diff --git a/libraries/RTC/examples/DumpRTC/DumpRTC.pde b/libraries/RTC/examples/DumpRTC/DumpRTC.pde
index d5d2d821..b2aa3645 100644
--- a/libraries/RTC/examples/DumpRTC/DumpRTC.pde
+++ b/libraries/RTC/examples/DumpRTC/DumpRTC.pde
@@ -11,6 +11,10 @@ This example is placed into the public domain.
 SoftI2C i2c(A4, A5);
 DS3232RTC rtc(i2c);
 
+const char *days[] = {
+    "Mon, ", "Tue, ", "Wed, ", "Thu, ", "Fri, ", "Sat, ", "Sun, "
+};
+
 const char *months[] = {
     " Jan ", " Feb ", " Mar ", " Apr ", " May ", " Jun ",
     " Jul ", " Aug ", " Sep ", " Oct ", " Nov ", " Dec "
@@ -52,6 +56,7 @@ void loop() {
     Serial.println();
 
     Serial.print("Date: ");
+    Serial.print(days[RTC::dayOfWeek(&date) - 1]);
     Serial.print(date.day, DEC);
     Serial.print(months[date.month - 1]);
     Serial.print(date.year);
diff --git a/libraries/RTC/examples/TestRTC/TestRTC.pde b/libraries/RTC/examples/TestRTC/TestRTC.pde
index 3c440086..bf458eb2 100644
--- a/libraries/RTC/examples/TestRTC/TestRTC.pde
+++ b/libraries/RTC/examples/TestRTC/TestRTC.pde
@@ -38,6 +38,10 @@ DS3232RTC rtc(i2c);
 char buffer[64];
 size_t buflen;
 
+const char *days[] = {
+    "Mon, ", "Tue, ", "Wed, ", "Thu, ", "Fri, ", "Sat, ", "Sun, "
+};
+
 const char *months[] = {
     " Jan ", " Feb ", " Mar ", " Apr ", " May ", " Jun ",
     " Jul ", " Aug ", " Sep ", " Oct ", " Nov ", " Dec "
@@ -192,6 +196,7 @@ void cmdDate(const char *args)
 
     // Read the current date.
     rtc.readDate(&date);
+    Serial.print(days[RTC::dayOfWeek(&date) - 1]);
     Serial.print(date.day, DEC);
     Serial.print(months[date.month - 1]);
     Serial.println(date.year, DEC);