beam: JDN-based day-of-week with Julian/Gregorian transition

wizard-work/beam/zeller_dow.py

@@ -0,0 +1,38 @@
+"""Day-of-week via Julian Day Number, handling Julian/Gregorian transition.
+
+Dates before 1582-10-15 use the Julian calendar; on/after use Gregorian.
+The gap 1582-10-05..14 did not exist.
+"""
+def _jdn_julian(y, m, d):
+    a = (14 - m) // 12
+    yy = y + 4800 - a
+    mm = m + 12 * a - 3
+    return d + (153 * mm + 2) // 5 + 365 * yy + yy // 4 - 32083
+
+def _jdn_gregorian(y, m, d):
+    a = (14 - m) // 12
+    yy = y + 4800 - a
+    mm = m + 12 * a - 3
+    return d + (153 * mm + 2) // 5 + 365 * yy + yy // 4 - yy // 100 + yy // 400 - 32045
+
+NAMES = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
+
+def dow(y, m, d):
+    if not (1 <= m <= 12 and 1 <= d <= 31):
+        raise ValueError("invalid month/day")
+    if (y, m, d) < (1582, 10, 15):
+        jdn = _jdn_julian(y, m, d)
+    else:
+        jdn = _jdn_gregorian(y, m, d)
+    return NAMES[jdn % 7]
+
+def main():
+    import sys
+    if len(sys.argv) != 4:
+        print("usage: zeller_dow.py YYYY MM DD"); return 2
+    y, m, d = int(sys.argv[1]), int(sys.argv[2]), int(sys.argv[3])
+    print(dow(y, m, d))
+    return 0
+
+if __name__ == "__main__":
+    raise SystemExit(main())