leap.h File Reference

Leap year function prototypes. More...

#include <stdbool.h>

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Data Structures
struct	leap_off
	Leap offset by year and day. More...
struct	leap_date
	Leap year date structure. More...

Macros
#define	LEAP_MCM   693961
	Leap offset at 1900.

Functions
bool	is_leap (int year)
	Determine if a year is a leap year.
int	leap_add (int year)
	Adds one for a leap year otherwise zero.
int	leap_thru (int year)
	Leap years completed from year 0 up to but not including the first day of the specified year.
int	leap_day (int year)
	Counts leap-adjusted days up to some year.
static bool	equal_leap_off (struct leap_off lhs, struct leap_off rhs)
	Compares two leap_off structures for equality.
struct leap_off	leap_off (int year, int day_off)
	Offsets year and day of year.
int	leap_mday (int year, int month)
	Day of month from year and month.
int	leap_yday (int year, int month)
	Day of year from year and month.
static bool	equal_leap_date (struct leap_date lhs, struct leap_date rhs)
	Compares two leap_date structures for equality.
struct leap_date	leap_date (int year, int day_off)
	Date from year and day of year.
static struct leap_date	leap_date_from_off (struct leap_off off)
	Date from leap offset.
struct leap_off	leap_from (int year, int month, int day)
	Day from year, month and day of month.
static struct leap_off	leap_from_date (struct leap_date date)
	Day from leap date.
struct leap_date	leap_abs_date (int day_off)
	Absolute date from day of year.
int	leap_abs_from (int year, int month, int day)
	Absolute date from year, month, and day of month.
static int	leap_abs_from_date (struct leap_date date)
	Absolute date from leap_date structure.

Detailed Description

Leap year function prototypes.

Provides functions to determine if a year is a leap year, count leap years up to a given year, and calculate leap-adjusted days.

Copyright

2025, Roy Ratcliffe, Northumberland, United Kingdom

Definition in file leap.h.

Macro Definition Documentation

LEAP_MCM

#define LEAP_MCM   693961

Leap offset at 1900.

MCM is Roman numerals for 1900.

Definition at line 19 of file leap.h.

Function Documentation

equal_leap_date()

static bool equal_leap_date ( struct leap_date  lhs, struct leap_date  rhs  )

inlinestatic

Compares two leap_date structures for equality.

Checks if two leap_date structures represent the same date by comparing their year, month, and day fields.

Parameters

lhs	The first leap_date structure.
rhs	The second leap_date structure.

Return values

true	if both structures represent the same date.
false	otherwise.

Definition at line 193 of file leap.h.

equal_leap_off()

static bool equal_leap_off ( struct leap_off  lhs, struct leap_off  rhs  )

inlinestatic

Compares two leap_off structures for equality.

Checks if two leap_off structures represent the same offset by comparing their year and day offset fields.

Parameters

lhs	The first leap_off structure.
rhs	The second leap_off structure.

Return values

true	if both structures represent the same offset.
false	otherwise.

Note

C cannot directly compare structures; the compiler reports "invalid operands to binary expression" errors. For example, the following code

(struct leap_off){2020, 59} == leap_off(2020, 59)

produces such an error. So, provide a helper function to compare two leap_off structures. Compile-time inlining should eliminate any performance penalty.

Definition at line 105 of file leap.h.

is_leap()

bool is_leap

(

int

year

)

Determine if a year is a leap year.

Is a year a leap year? A year is a leap year if it is divisible by four, except for years that are divisible by 100, unless they are also divisible by 400.

Parameters

year	The year to check.

Return values

true	if the year is a leap year.
false	if the year is not a leap year.

Definition at line 13 of file leap.c.

leap_abs_date()

struct leap_date leap_abs_date

(

int

day_off

)

Absolute date from day of year.

Converts a day offset (from year 0) into a (year, month, day) date. The absolute day is anchored to year 0 such that day 0 corresponds to 0-01-01, making leap_day(0) == 0. Absolute days differ from Unix epoch days (which start at 1970-01-01).

Parameters

day_off

The absolute day offset, starting from 0 for the first day of year 0.

Returns

The leap_date structure (year, month, day) corresponding to the absolute day.

Definition at line 135 of file leap.c.

leap_abs_from()

int leap_abs_from	(	int	year,
		int	month,
		int	day
	)

Absolute date from year, month, and day of month.

Converts a (year, month, day) date into an absolute day offset from year 0 such that day 0 corresponds to 0-01-01, making leap_day(0) == 0. The absolute day is anchored at year 0, not Unix epoch (1970-01-01).

Parameters

year	The year.
month	The month, starting from 1 for January.
day	Day of the month, starting from 1 for the first day of the month.

Returns

The absolute day offset from year 0.

Definition at line 137 of file leap.c.

leap_abs_from_date()

static int leap_abs_from_date ( struct leap_date  date )

inlinestatic

Absolute date from leap_date structure.

Returns the absolute date from the given leap_date structure.

Parameters

date	The leap_date structure.

Returns

The absolute date as the number of days since the epoch.

Definition at line 275 of file leap.h.

leap_add()

int leap_add

(

int

year

)

Adds one for a leap year otherwise zero.

Answers 1 if the year is a leap year, otherwise answers 0. This is the same result as is_leap() except that the result is an explicit integer; 1 for leap years, 0 for non-leap years.

Compute the number of days in a given year as:

365 + leap_add(year);

Parameters

year	The year to check.

Return values

1	if the year is a leap year.
0	if the year is not a leap year.

Definition at line 28 of file leap.c.

leap_date()

struct leap_date leap_date	(	int	year,
		int	day_off
	)

Date from year and day of year.

Adjust the day so that it sits in-between 1 and 365 (or 366) inclusively. Returns the year, month, and day of the month for the given year and day of year. The day of the month is adjusted to be 1-based, meaning it starts from 1 for the first day of the month.

Parameters

year	The year.
day_off	Day of the year offset, starting from 0 for first of January.

Return values

leap_date.year	The year.
leap_date.month	The month, starting from 1 for January.
leap_date.day	The day of the month, starting from 1 for the first day of the month.

Definition at line 104 of file leap.c.

leap_date_from_off()

static struct leap_date leap_date_from_off ( struct leap_off  off )

inlinestatic

Date from leap offset.

Converts a leap_off structure to a leap_date structure.

Parameters

off	The leap_off structure containing year and day of year.

Returns

The corresponding leap_date structure.

Definition at line 218 of file leap.h.

leap_day()

int leap_day

(

int

year

)

Counts leap-adjusted days up to some year.

Counts the number of days completed up to but not including the first day of the year.

Parameters

year	The year to check.

Returns

The number of leap-adjust days completed up to but not including the first day of the given year.

Definition at line 48 of file leap.c.

leap_from()

struct leap_off leap_from	(	int	year,
		int	month,
		int	day
	)

Day from year, month and day of month.

Adjusts the month so that it sits in-between 1 and 12 inclusively. The month offsets the year. Returns the absolute date from the given year, month, and day of month.

Parameters

year	The year.
month	The month, starting from 1 for January.
day	The one-based day of the month, starting from 1 for the first day of the month.

Returns

The absolute date as the number of days since the epoch.

Definition at line 129 of file leap.c.

leap_from_date()

static struct leap_off leap_from_date ( struct leap_date  date )

inlinestatic

Day from leap date.

Converts a leap_date structure to a leap_off structure.

Parameters

date	The leap_date structure containing year, month, and day of month.

Returns

The corresponding leap_off structure. The absolute date as the number of days since the epoch.

Definition at line 240 of file leap.h.

leap_mday()

int leap_mday	(	int	year,
		int	month
	)

Day of month from year and month.

Returns the number of days in the month for the given year and month. Accounts for leap years in February.

Parameters

year	The year.
month	The month ordinal, starting from 1 for January.

Return values

The	number of days in the month, accounting for leap years in February.

Definition at line 80 of file leap.c.

leap_off()

struct leap_off leap_off	(	int	year,
		int	day_off
	)

Offsets year and day of year.

Year and day of year from year and day of year. Adjust the day so that it sits in-between 0 and 365 or 366, inclusively.

The day adjusts to be within the range of 0 to 365 or 366 inclusively, and the year is adjusted accordingly.

This function ensures that the day of the year does not exceed the number of days in the year, accounting for leap years. If the day is negative or exceeds the number of days in the year, it adjusts the year and day accordingly. The day is adjusted to be 0-based, meaning it starts from 0 for the first day of the year. The year is adjusted to account for the number of days in the year, which can be either 365 or 366 depending on whether it is a leap year.

Parameters

year	The year.
day_off	The day of the year offset, starting from 0. It can be negative or exceed the number of days in the year.

Return values

leap_off.year	The year adjusted to account for the number of days in the year. The resulting year becomes normalised according to the day adjustment whether positive or negative.
leap_off.day	The adjusted day of the year, starting from 0 for the first day. The day is guaranteed to be within the bounds of the year. 0 <= day < 365 or 366 depending on whether it is a leap year.

Definition at line 70 of file leap.c.

leap_thru()

int leap_thru

(

int

year

)

Leap years completed from year 0 up to but not including the first day of the specified year.

Counts the number of leap years that have occurred from year 0 up to but not including the first day of the given year. This is calculated as the number of years divisible by 4, minus those divisible by 100, plus those divisible by 400. This accounts for the rules of leap years in the Gregorian calendar.

Parameters

year	The target year up to which leap years should be counted.

Returns

The total number of leap years from year 0 through the specified year.

Definition at line 30 of file leap.c.

leap_yday()

int leap_yday	(	int	year,
		int	month
	)

Day of year from year and month.

Returns the day of the year for the given year and month. The day of the year is calculated by summing the days in the months up to the given month, and adding an extra day if the month is after February in a leap year.

Parameters

year	The year.
month	The month ordinal, starting from 1 for January.

Return values

The	day of the year, starting from 0 for first of January.

Definition at line 86 of file leap.c.