# `Calendrical.Islamic.Rgsa`

Implementation of the *Saudi Arabian sighting-based* Islamic
calendar (CLDR `:islamic_rgsa`).

Like `Calendrical.Islamic.Observational`, this calendar determines
the start of each lunar month from actual crescent visibility, but
the observation point is **Mecca, Saudi Arabia** (21.4225° N,
39.8262° E, 277 m) — the *al-Masjid al-Ḥarām* — rather than Cairo.
This matches the location used for the canonical Saudi religious
determination of dates such as the start of Ramadan and Eid.

This calendar differs from `Calendrical.Islamic.UmmAlQura` in two
important ways:

* **`UmmAlQura`** uses the official tabular calendar published by
  KACST (the Umm al-Qura Calendar). It is a precomputed lookup
  table — fast, deterministic, and the legal Saudi civil calendar.

* **`Rgsa`** computes month starts from astronomical visibility at
  Mecca on demand. It is the algorithmic equivalent of the Saudi
  *religious sighting* practice and may diverge from `UmmAlQura`
  in months where the astronomical prediction does not match the
  KACST table.

Days are assumed to begin at midnight.

## Visibility model

As with `Calendrical.Islamic.Observational`, crescent visibility
is computed by `Astro.new_visible_crescent/3` using the Odeh (2006)
criterion by default.

## Reference

- CLDR `:islamic_rgsa` calendar type. The `rgsa` suffix denotes
  "*Religious Saudi Arabia*".
- Dershowitz & Reingold, *Calendrical Calculations* (4th ed.),
  Chapter 14.

# `day`

```elixir
@type day() :: 1..30
```

# `month`

```elixir
@type month() :: 1..12
```

# `year`

```elixir
@type year() :: integer()
```

# `calendar_base`

Identifies whether this calendar is month
or week based.

# `calendar_year`

```elixir
@spec calendar_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendar.year()
```

Returns the calendar year as displayed
on rendered calendars.

# `cldr_calendar_type`

Defines the CLDR calendar type for this calendar.

This type is used in support of `Calendrical.
localize/3`.

# `cyclic_year`

```elixir
@spec cyclic_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendar.year()
```

Returns the cyclic year as displayed
on rendered calendars.

# `date_from_iso_days`

```elixir
@spec date_from_iso_days(integer()) :: {year(), month(), day()}
```

Returns a Saudi sighting-based Islamic `{year, month, day}` for
the given ISO day number.

# `date_to_iso_days`

```elixir
@spec date_to_iso_days(year(), month(), day()) :: integer()
```

Returns the number of ISO days for the given Saudi sighting-based
Islamic `year`, `month`, and `day`.

# `day_of_era`

```elixir
@spec day_of_era(Calendar.year(), Calendar.month(), Calendar.day()) ::
  {day :: Calendar.day(), era :: Calendar.era()}
```

Calculates the day and era from the given
`year`, `month`, and `day`.

By default we consider on two eras: before the epoch
and on-or-after the epoch.

# `day_of_year`

```elixir
@spec day_of_year(Calendar.year(), Calendar.month(), Calendar.day()) :: Calendar.day()
```

Calculates the day of the year from the given
`year`, `month`, and `day`.

# `days_in_month`

```elixir
@spec days_in_month(Calendar.month()) ::
  Calendar.month()
  | {:ambiguous, Range.t() | [pos_integer()]}
  | {:error, :undefined}
```

Returns how many days there are in the given month.

Must be implemented in derived calendars because
we cannot know what the calendar format is.

# `days_in_month`

```elixir
@spec days_in_month(Calendar.year(), Calendar.month()) :: Calendar.month()
@spec days_in_month(year(), month()) :: 29..30
```

Returns how many days there are in the given year
and month.

# `days_in_week`

Returns the number days in a a week.

# `days_in_year`

Returns the number days in a given year.

The year is the number of years since the
epoch.

# `epoch`

# `epoch_day_of_week`

# `extended_year`

```elixir
@spec extended_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendar.year()
```

Returns the extended year as displayed
on rendered calendars.

# `first_day_of_week`

# `iso_week_of_year`

```elixir
@spec iso_week_of_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  {:error, :not_defined}
```

Calculates the ISO week of the year from the
given `year`, `month`, and `day`.

By default this function always returns
`{:error, :not_defined}`.

# `last_day_of_week`

# `location`

```elixir
@spec location() :: Geo.PointZ.t()
```

Returns the geographic location used to determine crescent
visibility for this calendar.

# `month`

Returns a `t:Date.Range.t/0` representing
a given month of a year.

# `month_of_year`

```elixir
@spec month_of_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendar.month() | {Calendar.month(), Calendrical.leap_month?()}
```

Returns the month of the year from the given
`year`, `month`, and `day`.

# `months_in_leap_year`

Returns the number of months in a leap year.

# `months_in_ordinary_year`

Returns the number of months in a normal year.

# `months_in_year`

Returns the number of months in a
given `year`.

# `naive_datetime_from_iso_days`

```elixir
@spec naive_datetime_from_iso_days(Calendar.iso_days()) ::
  {Calendar.year(), Calendar.month(), Calendar.day(), Calendar.hour(),
   Calendar.minute(), Calendar.second(), Calendar.microsecond()}
```

Converts the `t:Calendar.iso_days` format to the
datetime format specified by this calendar.

# `naive_datetime_to_iso_days`

```elixir
@spec naive_datetime_to_iso_days(
  Calendar.year(),
  Calendar.month(),
  Calendar.day(),
  Calendar.hour(),
  Calendar.minute(),
  Calendar.second(),
  Calendar.microsecond()
) :: Calendar.iso_days()
```

Returns the `t:Calendar.iso_days` format of
the specified date.

# `periods_in_year`

Returns the number of periods in a given
`year`. A period corresponds to a month
in month-based calendars and a week in
week-based calendars.

# `plus`

Adds an `increment` number of `date_part`s
to a `year-month-day`.

`date_part` can be `:months` only.

# `quarter`

Returns a `t:Date.Range.t/0` representing
a given quarter of a year.

# `quarter_of_year`

```elixir
@spec quarter_of_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendrical.quarter()
```

Returns the quarter of the year from the given
`year`, `month`, and `day`.

# `related_gregorian_year`

```elixir
@spec related_gregorian_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  Calendar.year()
```

Returns the related gregorain year as displayed
on rendered calendars.

# `valid_date?`

Determines if the `date` given is valid according to
this calendar.

# `week`

Returns a `t:Date.Range.t/0` representing
a given week of a year.

# `week_of_month`

```elixir
@spec week_of_month(Calendar.year(), Calendar.month(), Calendar.day()) ::
  {pos_integer(), pos_integer()} | {:error, :not_defined}
```

Calculates the week of the year from the given
`year`, `month`, and `day`.

By default this function always returns
`{:error, :not_defined}`.

# `week_of_year`

```elixir
@spec week_of_year(Calendar.year(), Calendar.month(), Calendar.day()) ::
  {:error, :not_defined}
```

Calculates the week of the year from the given
`year`, `month`, and `day`.

By default this function always returns
`{:error, :not_defined}`.

# `weeks_in_year`

Returns the number of weeks in a
given `year`.

# `year`

Returns a `t:Date.Range.t/0` representing
a given year.

# `year_of_era`

```elixir
@spec year_of_era(Calendar.year()) :: {year :: Calendar.year(), era :: Calendar.era()}
```

Calculates the year and era from the given `year`.

# `year_of_era`

```elixir
@spec year_of_era(Calendar.year(), Calendar.month(), Calendar.day()) ::
  {year :: Calendar.year(), era :: Calendar.era()}
```

Calculates the year and era from the given `date`.

---

*Consult [api-reference.md](api-reference.md) for complete listing*