Approx 1.5x speedup in weekday in Datetime using pre computed odd days

Lib/_pydatetime.py

@@ -38,6 +38,10 @@ def _get_class_module(self):
 _DAYS_IN_MONTH = [-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
 
 _DAYS_BEFORE_MONTH = [-1]  # -1 is a placeholder for indexing purposes.
+
+# This additional list stores the number of odd days before each month in the year.
+_ODD_DAYS_BEFORE_MONTH = [-1, 0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5]  #-1 is a placeholder for indexing purposes.
+
 dbm = 0
 for dim in _DAYS_IN_MONTH[1:]:
     _DAYS_BEFORE_MONTH.append(dbm)
@@ -53,6 +57,13 @@ def _days_before_year(year):
     y = year - 1
     return y*365 + y//4 - y//100 + y//400
 
+def _odd_days_before_year(year):
+    "year -> number of odd days before January 1st of year."
+    y = year - 1
+    r = y%400
+    m100 = y%100
+    return r//300 + (r%300)//200*3 + ((r%300)%200)//100*5 + (m100//4)*2 + (m100 - m100//4)
+
 def _days_in_month(year, month):
     "year, month -> number of days in that month in that year."
     assert 1 <= month <= 12, f"month must be in 1..12, not {month}"
@@ -74,6 +85,17 @@ def _ymd2ord(year, month, day):
             _days_before_month(year, month) +
             day)
 
+# This additional function computes the number of odd days before the start of the given year way more reduced here than just multplying years with 365 & 366 and month values.
+def _odd_ymd2ord(year, month, day):
+    "year, month, day -> ordinal, considering 01-Jan-0001 as day 1."
+    assert 1 <= month <= 12, f"month must be in 1..12, not {month}"
+    dim = _days_in_month(year, month)
+    assert 1 <= day <= dim, f"day must be in 1..{dim}, not {day}"
+    # return (_days_before_year(year) +
+    #         _days_before_month(year, month) +
+    #         day)
+    return (_odd_days_before_year(year) + _ODD_DAYS_BEFORE_MONTH[month - 1] + day + 1) if(_is_leap(year) and month > 2) else (_odd_days_before_year(year) + _ODD_DAYS_BEFORE_MONTH[month - 1] + day)
+
 _DI400Y = _days_before_year(401)    # number of days in 400 years
 _DI100Y = _days_before_year(101)    #    "    "   "   " 100   "
 _DI4Y   = _days_before_year(5)      #    "    "   "   "   4   "
@@ -1181,6 +1203,14 @@ def toordinal(self):
         """
         return _ymd2ord(self._year, self._month, self._day)
 
+    def to_odd_ordinal(self):
+            """Return reduced number of proleptic Gregorian ordinal for the year, month and day.
+
+            January 1 of year 1 is day 1.  Only the year, month and day values
+            contribute to the result.
+            """
+            return _odd_ymd2ord(self._year, self._month, self._day)
+
     def replace(self, year=None, month=None, day=None):
         """Return a new date with new values for the specified fields."""
         if year is None:
@@ -1258,7 +1288,7 @@ def __sub__(self, other):
 
     def weekday(self):
         "Return day of the week, where Monday == 0 ... Sunday == 6."
-        return (self.toordinal() + 6) % 7
+        return (self.to_odd_ordinal()) % 7
 
     # Day-of-the-week and week-of-the-year, according to ISO
 

Modules/_datetimemodule.c

@@ -424,6 +424,12 @@ static const int _days_before_month[] = {
     0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
 };
 
+/* This additional array stores the number of odd days before each month in the year. */
+static const int _odd_days_before_month[] = {
+    0, /* unused; this vector uses 1-based indexing */
+    0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5
+};
+
 /* year -> 1 if leap year, else 0. */
 static int
 is_leap(int year)
@@ -478,6 +484,21 @@ days_before_year(int year)
     return y*365 + y/4 - y/100 + y/400;
 }
 
+/*This additional function here computes the number of odd days before the start of the given year.*/
+static int
+odd_days_before_year(int year)
+{
+    int y = year - 1;
+    /* This is incorrect if year <= 0; we really want the floor
+     * here.  But so long as MINYEAR is 1, the smallest year this
+     * can see is 1.
+     */
+    assert (year >= 1);
+    int temp = (y%400);
+    int mod100 = (y%100);
+    return temp/300+(temp%300)/200*3+((temp%300)%200)/100*5 + (mod100/4)*2 + (mod100-mod100/4);
+}
+
 /* Number of days in 4, 100, and 400 year cycles.  That these have
  * the correct values is asserted in the module init function.
  */
@@ -576,11 +597,19 @@ ymd_to_ord(int year, int month, int day)
     return days_before_year(year) + days_before_month(year, month) + day;
 }
 
+/* This additional function computes the number of odd days before the start of the given year way more reduced here than just multplying years with 365 & 366 and month values. */
+/* year, month, day -> ordinal, considering 01-Jan-0001 as day 1. */
+static int
+odd_ymd_to_ord(int year, int month, int day)
+{
+    return (is_leap(year) && month > 2) ? (odd_days_before_year(year) + _odd_days_before_month[month - 1] + day+1) : (odd_days_before_year(year) + _odd_days_before_month[month - 1] + day);
+}
+
 /* Day of week, where Monday==0, ..., Sunday==6.  1/1/1 was a Monday. */
 static int
 weekday(int year, int month, int day)
 {
-    return (ymd_to_ord(year, month, day) + 6) % 7;
+    return (odd_ymd_to_ord(year, month, day)) % 7;
 }
 
 /* Ordinal of the Monday starting week 1 of the ISO year.  Week 1 is the