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use chrono::{DateTime, Datelike, Duration, NaiveDate, Utc};
use crate::astronomy::solar::SolarTime;
use crate::astronomy::unit::{Angle, Coordinates};
use crate::models::parameters::Parameters;
/// Rule for approximating Fajr and Isha at high latitudes
#[derive(PartialEq, Debug, Copy, Clone)]
#[non_exhaustive]
pub enum HighLatitudeRule {
/// Fajr won't be earlier than the midpoint of the night and Isha
/// won't be later than the midpoint of the night. This is the default
/// value to prevent Fajr and Isha crossing boundaries.
MiddleOfTheNight,
/// Fajr will never be earlier than the beginning of the last seventh of
/// the night and Isha will never be later than the end of the first seventh of the night.
///
/// This is recommended for locations above 48° latitude to prevent prayer
/// times that would be difficult to perform.
SeventhOfTheNight,
/// The fajr/isha angle α determines a fraction t = α ÷ 60 of the night.
/// Isha begins after the first t part; Fajr before the last t part.
/// Example: 15° → t = 0.25 → Isha after the first quarter of the night.
///
/// This can be used to prevent difficult fajr and isha times at certain locations.
TwilightAngle,
/// MWL 2009 Local Relative Estimation.
///
/// Designed for the Muslim World League's 2009 high-latitude
/// methodology (between 48.6° and 66.6° latitude) using their
/// standard angles (18° Fajr / 17° Isha). The percentage is
/// resolved automatically inside [`PrayerTimes::try_new`] by
/// scanning a full year at the working latitude.
LocalRelativeEstimation,
/// Deferred variant resolved inside [`PrayerTimes::try_new`].
///
/// Evaluated via [`recommended()`](HighLatitudeRule::recommended) against
/// the working latitude (original or fallback-resolved).
Recommended,
}
impl HighLatitudeRule {
/// Return the recommended [`HighLatitudeRule`] for the given coordinates.
///
/// Based on MWL 2009 latitude zones:
/// - |latitude| ≤ 48.6° → [`MiddleOfTheNight`](HighLatitudeRule::MiddleOfTheNight) (Zone 1 — no effect, angles always reachable)
/// - 48.6° < |latitude| ≤ 66.6° → [`LocalRelativeEstimation`](HighLatitudeRule::LocalRelativeEstimation) (Zone 2)
/// - |latitude| > 66.6° → [`MiddleOfTheNight`](HighLatitudeRule::MiddleOfTheNight) (Zone 3 — LRE not designed for polar)
#[must_use]
pub fn recommended(coordinates: Coordinates) -> Self {
let abs_lat = coordinates.latitude.abs();
if abs_lat > 66.6 {
Self::MiddleOfTheNight
} else if abs_lat > 48.6 {
Self::LocalRelativeEstimation
} else {
Self::MiddleOfTheNight
}
}
/// Scan a full calendar year and return the average Isha proportion
/// of the night (`ratio = isha_length / night_length`).
///
/// Per the MWL 2009 Arabic spec, only days where the sign is present
/// AND not disturbed (day-to-day jump ≤ 10 min) are included in the
/// average — days of disappearance or disturbance are excluded.
pub fn compute_pct(coordinates: Coordinates, params: &Parameters) -> f64 {
let year = Utc::now().year();
let days_in_year = if year % 4 == 0 && (year % 100 != 0 || year % 400 == 0) {
366
} else {
365
};
let jan1 = NaiveDate::from_ymd_opt(year, 1, 1).expect("valid date");
let mut total = 0.0;
let mut count = 0usize;
let mut prev_isha: Option<DateTime<Utc>> = None;
let mut prev_was_reachable = false;
for day_offset in 0..days_in_year {
let date = jan1 + Duration::days(day_offset);
let tomorrow = date + Duration::days(1);
let Ok(solar_today) = SolarTime::new(
date.and_hms_opt(0, 0, 0).expect("valid time").and_utc(),
coordinates,
) else {
prev_isha = None;
prev_was_reachable = false;
continue;
};
let Ok(solar_tomorrow) = SolarTime::new(
tomorrow.and_hms_opt(0, 0, 0).expect("valid time").and_utc(),
coordinates,
) else {
prev_isha = None;
prev_was_reachable = false;
continue;
};
let isha_angle = Angle::new(-params.isha_angle);
let isha_time = solar_today.time_for_solar_angle(isha_angle, true);
let include = if let Some(current) = isha_time
&& let Some(prev) = prev_isha
{
if prev_was_reachable {
// Both days reachable — exclude if jump > 10 min
let prev_today = date.and_time(prev.time()).and_utc();
let raw_diff = (current - prev_today).num_seconds() as f64 / 60.0;
let diff = if raw_diff.abs() > 720.0 {
let wrapped = if raw_diff > 0.0 {
raw_diff - 1440.0
} else {
raw_diff + 1440.0
};
wrapped.abs()
} else {
raw_diff.abs()
};
diff <= 10.0
} else {
// Previous day was unreachable → reappearance day → exclude
false
}
} else if isha_time.is_some() {
// First reachable day of the year — no baseline to judge
true
} else {
false
};
if include {
let night = solar_tomorrow
.sunrise
.expect("compute_pct only runs where sunrise exists")
.signed_duration_since(
solar_today
.sunset
.expect("compute_pct only runs where sunset exists"),
);
let night_secs = night.num_seconds() as f64;
if night_secs > 0.0 {
let isha_len = isha_time
.expect("include is only true when isha_time is Some")
.signed_duration_since(
solar_today
.sunset
.expect("compute_pct only runs where sunset exists"),
);
let ratio = isha_len.num_seconds() as f64 / night_secs;
total += ratio;
count += 1;
}
}
prev_isha = isha_time;
prev_was_reachable = isha_time.is_some();
}
if count == 0 {
0.5
} else {
total / count as f64
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn recommended_rule_lre_above_48_degrees() {
let location = Coordinates::new(48.983226, -3.216649);
assert_eq!(
HighLatitudeRule::recommended(location),
HighLatitudeRule::LocalRelativeEstimation
);
}
#[test]
fn recommended_rule_middle_of_night_below_48_degrees() {
let location = Coordinates::new(45.983226, -3.216649);
assert_eq!(
HighLatitudeRule::recommended(location),
HighLatitudeRule::MiddleOfTheNight
);
}
#[test]
fn recommended_rule_middle_of_night_above_66_degrees() {
let location = Coordinates::new(70.0, 20.0);
assert_eq!(
HighLatitudeRule::recommended(location),
HighLatitudeRule::MiddleOfTheNight
);
}
#[test]
fn compute_pct_brussels_is_reasonable() {
let location = Coordinates::new(50.85, 4.35);
let params = Parameters::new(18.0, 17.0);
let pct = HighLatitudeRule::compute_pct(location, ¶ms);
assert!(
pct > 0.1 && pct < 0.9,
"Brussels pct should be between 0.1 and 0.9, got {pct}"
);
}
#[test]
fn compute_pct_oslo_is_reasonable() {
let location = Coordinates::new(59.9094, 10.7349);
let params = Parameters::new(18.0, 17.0);
let pct = HighLatitudeRule::compute_pct(location, ¶ms);
assert!(
pct > 0.1 && pct < 0.9,
"Oslo pct should be between 0.1 and 0.9, got {pct}"
);
}
#[test]
fn compute_pct_equator_is_reasonable() {
let location = Coordinates::new(0.0, 0.0);
let params = Parameters::new(18.0, 17.0);
let pct = HighLatitudeRule::compute_pct(location, ¶ms);
// At the equator night and day are ~12 h year round.
// 17° Isha is ~(17/60) ≈ 0.28 of the night after sunset.
// Pct should be reasonable (0 < pct < 1).
assert!(
pct > 0.0 && pct < 1.0,
"Equator pct must be between 0 and 1, got {pct}"
);
// At the equator with 17° angle, expect roughly 0.25–0.35
assert!(pct < 0.5, "Equator pct should be < 0.5, got {pct}");
}
}