Create is_leap.py

mccright · mccright · commit 4d55c36a6eb9 · 2025-09-25T17:09:38.000-05:00
This is just a reminder about checking for leap years.  It has two model functions.
diff --git a/is_leap.py b/is_leap.py
@@ -0,0 +1,42 @@
+#!/usr/bin/python3
+
+import sys
+
+# Leap years occur every four years, with 
+# exceptions for years divisible by 100 but not by 400. 
+# The most recent leap years were 2020 and 2024, and 
+# the next one will be in 2028
+#
+# Yes, maybe I could better emulate Falk Hüffner's work using
+# the Python ctypes module...but it seemed like there was a material
+# performance and simplicity hit.  I'll leave that to someone else.
+# https://runebook.dev/en/articles/python/library/ctypes/ctypes.c_uint32
+
+
+def is_leap_year(year: int) -> bool:
+    # This is the standard way...
+    return (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0)
+
+
+def is_leap_year_fast(y: int) -> bool: 
+    # This interesting function is based on a C function by Falk Hüffner. See:
+    # https://hueffner.de/falk/blog/a-leap-year-check-in-three-instructions.html
+    return ((y * 1073750999) & 3221352463) <= 126976
+
+
+def main(num_args: int, usage: str):
+    if len(sys.argv) != num_args:
+        this_script = sys.argv[0]
+        usage_message = usage.replace("script_name", str(this_script))
+        print(usage_message)
+        sys.exit(1)
+    else:
+        # use abs() to deal convert negative numbers to positive
+        if int(sys.argv[1]) < 0:
+            print(f"You entered a negative number.\nWe will convert it and check for leap.")
+        print(f"\t{is_leap_year(abs(int(sys.argv[1])))}")
+        print(f"\t{is_leap_year_fast(abs(int(sys.argv[1])))}")
+
+
+if __name__ == '__main__':
+    main(2, 'USAGE: python3 script_name <year_to_test, only use with positive numbers>')
