[RFC PATCH 10/14] rtc: improve and move week day computing function to rtc header

["Lee, Chun-Yi" <joeyli.kernel@gmail.com>]

Due to rtc-acpid and efi time used the same logic for computing
week day, so this patch moves code to rtc.h header file.

Additionally using a leap year algorithm to replace the for-loop
block in compute_wday for improve the performance. The first
version of algorithm is from Oliver Neukum.
---
 drivers/rtc/rtc-acpitad.c |   13 +------------
 include/linux/efi.h       |   13 +------------
 include/linux/rtc.h       |   38 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+), 24 deletions(-)

diff --git a/drivers/rtc/rtc-acpitad.c b/drivers/rtc/rtc-acpitad.c
index 065a033..bdf7ae1 100644
--- a/drivers/rtc/rtc-acpitad.c
+++ b/drivers/rtc/rtc-acpitad.c
@@ -32,23 +32,12 @@ compute_yday(struct acpi_time *acpit)
 static int
 compute_wday(struct acpi_time *acpit)
 {
-	int y;
-	int ndays = 0;
-
 	if (acpit->year < 1900) {
 		pr_err("ACPI year < 1900, invalid date\n");
 		return -1;
 	}
 
-	for (y = 1900; y < acpit->year; y++)
-		ndays += 365 + (is_leap_year(y) ? 1 : 0);
-
-	ndays += compute_yday(acpit);
-
-	/*
-	 * 1=1/1/1900 was a Monday
-	 */
-	return (ndays + 1) % 7;
+	return rtc_wday(acpit->day, acpit->month - 1, acpit->year);
 }
 
 static void
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 3859f3e..1c78ae7 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -177,23 +177,12 @@ compute_yday(efi_time_t *eft)
 static inline int
 compute_wday(efi_time_t *eft)
 {
-	int y;
-	int ndays = 0;
-
 	if (eft->year < 1998) {
 		pr_err("EFI year < 1998, invalid date\n");
 		return -1;
 	}
 
-	for (y = EFI_RTC_EPOCH; y < eft->year; y++)
-		ndays += 365 + (is_leap_year(y) ? 1 : 0);
-
-	ndays += compute_yday(eft);
-
-	/*
-	 * 4=1/1/1998 was a Thursday
-	 */
-	return (ndays + 4) % 7;
+	return rtc_wday(eft->day, eft->month - 1, eft->year);
 }
 
 static inline void
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index e6380ec..511884f 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -196,6 +196,44 @@ static inline bool is_leap_year(unsigned int year)
 	return (!(year % 4) && (year % 100)) || !(year % 400);
 }
 
+#define SINCE1900 25    /* valid from 2000 */
+
+static inline int rtc_wday(unsigned int day, unsigned int month, unsigned int year)
+{
+	int ndays, correction;
+	int base;
+
+	if (year < 1900) {
+		pr_err("rtc: year < 1900, invalid date\n");
+		return -1;
+	}
+
+	if (year >= 2000)
+		base = year - 2000;
+	else
+		base = year - 1900;
+
+	correction = 0;
+	if (base >= 0) {
+		correction += base / 4;
+		correction -= base / 100;
+		correction += base / 400;
+		if (year >= 2000)
+			correction += SINCE1900;
+
+		/* later rtc_year_days will add the leap day of current year */
+		correction -= ((is_leap_year(year)) ? 1 : 0);
+	}
+
+	ndays = (year - 1900) * 365 + correction;
+	ndays += rtc_year_days(day, month, year);
+
+	/*
+	 * 1=1/1/1900 was a Monday
+	 */
+	return (ndays + 1) % 7;
+}
+
 #ifdef CONFIG_RTC_HCTOSYS_DEVICE
 extern int rtc_hctosys_ret;
 #else
-- 
1.6.4.2