prayertimes-go/pkg/diyanetcalc/provider.go

package diyanetcalc

import (
	"context"
	"errors"
	"fmt"
	"math"
	"time"

	"prayertimes/pkg/prayer"
)

const (
	daysToGenerate = 30
	degPerHour     = 15.0
)

var errNotSupported = errors.New("not supported in calculation provider")

// Diyanet prayer times calculator.
//
// Implements the Turkish Presidency of Religious Affairs (Diyanet) methodology,
// standardized in the 1983 reform. Prayer times are indexed to the Sun's apparent
// altitude angle at the observer's location, solved via spherical trigonometry.

// ---------------------------------------------------------------------------
// Diyanet angular criteria (post-1983 reform)
// ---------------------------------------------------------------------------

// Imsak (Fajr): Sun is 18deg below the horizon = start of astronomical twilight.
// Pre-1983 used -19deg plus a temkin buffer; now -18deg with zero temkin.
const imsakAngle = -18.0

// Isha (Yatsi): Sun is 17deg below the horizon = shafaq al-ahmar (red twilight)
// has fully disappeared from the western sky.
const ishaAngle = -17.0

// Sunrise / Sunset (Tulu / Gurup): geometric horizon alone is insufficient.
// Two physical corrections are combined into a single -0.833deg value (50 arcmin):
//   - Atmospheric refraction: ~0.567deg — air bends sunlight over the horizon
//   - Solar semi-diameter:    ~0.267deg — Sun is "up" when its upper limb clears
const sunAngle = -0.833

// ---------------------------------------------------------------------------
// Temkin — precautionary time buffers (minutes, post-1983 standardized values)
//
// Temkin ensures a single published time remains valid across the full
// geographical extent of a city (highest peak to lowest valley, east to west).
// Pre-1983 values were often 10–20 min; the reform moderated them.
//
//	Imsak:   0 min — no buffer; avoids starting Fajr too early / fast too late
//	Sunrise: -7 min — subtracted, ensuring the Sun has fully cleared the horizon
//	Dhuhr:  +5 min — added, ensuring the Sun has clearly begun its descent
//	Asr:    +4 min — accounts for local elevation and horizon obstacles
//	Maghrib:+7 min — ensures the Sun has completely set before breaking fast
//	Isha:    0 min — no buffer needed at this twilight stage
//
// ---------------------------------------------------------------------------
type temkinMinutes struct {
	Imsak   float64
	Sunrise float64
	Dhuhr   float64
	Asr     float64
	Maghrib float64
	Isha    float64
}

var temkin = temkinMinutes{
	Imsak:   0,
	Sunrise: -7,
	Dhuhr:   5,
	Asr:     4,
	Maghrib: 7,
	Isha:    0,
}

type Provider struct{}

func New() Provider {
	return Provider{}
}

func (Provider) SearchLocations(_ context.Context, _ string) ([]prayer.Location, error) {
	return nil, fmt.Errorf("failed to search locations: %w", errNotSupported)
}

func (Provider) Get(_ context.Context, _ string) ([]prayer.Times, error) {
	return nil, fmt.Errorf("failed to get prayer times by location id: %w", errNotSupported)
}

func (Provider) GetByCoords(_ context.Context, coords prayer.Coordinates) ([]prayer.Times, error) {
	offset := estimateUTCOffsetHours(coords.Longitude)
	todayUTC := time.Now().UTC().Truncate(24 * time.Hour)

	results := make([]prayer.Times, 0, daysToGenerate)
	for i := 0; i < daysToGenerate; i++ {
		day := todayUTC.AddDate(0, 0, i)
		calculated := prayerTimes(coords.Latitude, coords.Longitude, day)

		results = append(results, prayer.Times{
			Date:    day.Format(time.DateOnly),
			Fajr:    formatHHMM(calculated.Imsak, offset),
			Sunrise: formatHHMM(calculated.Sunrise, offset),
			Dhuhr:   formatHHMM(calculated.Dhuhr, offset),
			Asr:     formatHHMM(calculated.Asr, offset),
			Sunset:  formatHHMM(calculated.Sunset, offset),
			Maghrib: formatHHMM(calculated.Maghrib, offset),
			Isha:    formatHHMM(calculated.Isha, offset),
		})
	}

	return results, nil
}

// Convert a calendar date to a Julian Day Number.
//
// JDN is a continuous day count from Jan 1, 4713 BC used in astronomy to
// avoid calendar-system ambiguities. The -1524.5 offset shifts the epoch to
// noon UT, Jan 1, 4713 BC (the standard astronomical Julian Date epoch).
// The Gregorian calendar correction term b = 2 - a + a//4 accounts for
// the century-year leap-day rules introduced in 1582.
func julianDay(d time.Time) float64 {
	y, m, day := d.Date()
	if m <= 2 {
		// January/February treated as months 13/14 of the prior year
		y--
		m += 12
	}
	a := y / 100
	b := 2 - a + a/4 // Gregorian correction

	return math.Floor(365.25*float64(y+4716)) + math.Floor(30.6001*float64(m+1)) + float64(day+b) - 1524.5
}

// Compute solar declination and equation of time for a given Julian Day.
//
// Returns:
//
//	delta — solar declination in degrees: the Sun's angular distance
//	        north (+) or south (-) of the celestial equator. Controls
//	        seasonal day length and the Sun's maximum altitude.
//	eot   — equation of time in minutes: difference between apparent solar
//	        time (sundial) and mean solar time (clock). Caused by Earth's
//	        elliptical orbit and axial tilt; ranges roughly ±16 min.
//
// Algorithm uses low-precision USNO solar coordinates (~0.01deg accuracy):
//
//	d  — days since J2000.0 epoch (Jan 1.5, 2000 = JD 2451545.0)
//	g  — mean anomaly: Sun's angular position in its elliptical orbit
//	q  — mean longitude of the Sun
//	L  — ecliptic longitude: corrected for orbital eccentricity via the
//	     equation of centre (1.915deg*sin g + 0.020deg*sin 2g)
//	e  — obliquity of the ecliptic: Earth's axial tilt (~23.44deg, slowly
//	     decreasing at 0.00000036deg/day)
func sunParams(jd float64) (delta float64, eot float64) {
	d := jd - 2451545.0 // days since J2000.0

	g := toRadians(357.529 + 0.98560028*d) // mean anomaly
	q := toRadians(280.459 + 0.98564736*d) // mean longitude

	// Ecliptic longitude: equation of centre adds up to ~1.9deg correction for
	// the difference between uniform circular and actual elliptical motion.
	L := q + toRadians(1.915*math.Sin(g)+0.020*math.Sin(2*g))
	e := toRadians(23.439 - 0.00000036*d) // obliquity of ecliptic

	// Declination: project ecliptic longitude onto the celestial equator.
	delta = toDegrees(math.Asin(math.Sin(e) * math.Sin(L)))

	// Right ascension in hours (atan2 handles all four quadrants correctly).
	ra := toDegrees(math.Atan2(math.Cos(e)*math.Sin(L), math.Cos(L))) / degPerHour

	// EoT = mean sun hour angle minus apparent sun RA, normalized to ±30 min.
	// round() removes the large integer offset (q accumulates many full rotations)
	// before converting to minutes; without it the raw difference is ~600 hours.
	diff := toDegrees(q)/degPerHour - ra
	eot = (diff - math.Round(diff)) * 60

	return delta, eot
}

// Solve for the hour angle H (hours) at which the Sun reaches a given altitude.
//
// Derived from the spherical law of cosines for the astronomical triangle:
//
//	sin(a) = sin(phi)*sin(delta) + cos(phi)*cos(delta)*cos(H)
//
// Rearranged:
//
//	cos(H) = (sin(a) − sin(phi)*sin(delta)) / (cos(phi)*cos(delta))
//
// H is converted from degrees to hours by dividing by 15 (360deg/24h = 15deg/h).
// Returns false when |cos H| > 1, i.e. the Sun never reaches that altitude
// (midnight sun or polar night) — Diyanet handles these with the Takdir method.
func hourAngle(altitudeDeg, lat, delta float64) (hours float64, ok bool) {
	cosH := (math.Sin(toRadians(altitudeDeg)) - math.Sin(toRadians(lat))*math.Sin(toRadians(delta))) /
		(math.Cos(toRadians(lat)) * math.Cos(toRadians(delta)))

	if math.Abs(cosH) > 1 {
		return 0, false
	}

	return toDegrees(math.Acos(cosH)) / degPerHour, true
}

// Compute the solar altitude at which Asr begins (Asr-i Avval / First Asr).
//
// Diyanet follows the majority-school definition: Asr starts when an object's
// shadow length equals the object's height plus its shortest noon shadow (fey-i zeval).
// The shadow factor is 1 (Asr-i Avval; Hanafi uses 2 for Asr-i Sani).
//
// The required altitude satisfies:  cot(a) = 1 + tan(|phi − delta|)
// which is:  a = atan(1 / (1 + tan(|phi − delta|)))
// where |phi − delta| is the Sun's angular distance from the zenith at solar noon.
func asrAltitude(lat, delta float64) float64 {
	return toDegrees(math.Atan(1.0 / (1.0 + math.Tan(toRadians(math.Abs(lat-delta))))))
}

// Convert a decimal hour value (e.g. 10.5 = 10:30) to a UTC-aware datetime.
func decimalHoursToUTC(hours float64, d time.Time) time.Time {
	dayUTC := time.Date(d.Year(), d.Month(), d.Day(), 0, 0, 0, 0, time.UTC)
	return dayUTC.Add(time.Duration(hours * float64(time.Hour)))
}

type computedTimes struct {
	Imsak   *time.Time
	Sunrise *time.Time
	Dhuhr   *time.Time
	Asr     *time.Time
	Sunset  *time.Time
	Maghrib *time.Time
	Isha    *time.Time
}

// Compute Diyanet prayer times, returning UTC-aware datetimes.
//
// Solar noon (Dhuhr) is the central reference. All other times are offsets:
//
//	Morning times (Imsak, Sunrise):               noon − H + temkin
//	Afternoon/evening times (Asr, Maghrib, Isha): noon + H + temkin
//
// Internally computes solar noon at UTC (tz=0), so results are in UTC.
//
// Solar noon formula:  T_noon = 12 + TZ − lambda/15 − EoT/60
//
//	lambda/15  converts longitude to hours (15deg/h)
//	EoT        corrects the gap between mean solar time and apparent solar time
func prayerTimes(lat, lon float64, d time.Time) computedTimes {
	jd := julianDay(d)
	delta, eot := sunParams(jd)

	// UTC solar noon: tz=0, so T_noon = 12 − lambda/15 − EoT/60
	noonUTC := 12 - lon/degPerHour - eot/60.0

	compute := func(base, angle float64, sign int, temkinMinutes float64) *time.Time {
		h, ok := hourAngle(angle, lat, delta)
		if !ok {
			return nil
		}
		t := decimalHoursToUTC(base+float64(sign)*h+temkinMinutes/60.0, d)
		return &t
	}

	hAsr, hasAsr := hourAngle(asrAltitude(lat, delta), lat, delta)
	var asr *time.Time
	if hasAsr {
		t := decimalHoursToUTC(noonUTC+hAsr+temkin.Asr/60.0, d)
		asr = &t
	}

	// Sunset for output is the geometric sunset without temkin offset.
	geometricSunset := func(base, angle float64) *time.Time {
		h, ok := hourAngle(angle, lat, delta)
		if !ok {
			return nil
		}
		t := decimalHoursToUTC(base+h, d)
		return &t
	}

	tDhuhr := decimalHoursToUTC(noonUTC+temkin.Dhuhr/60.0, d)

	return computedTimes{
		Imsak:   compute(noonUTC, imsakAngle, -1, temkin.Imsak),
		Sunrise: compute(noonUTC, sunAngle, -1, temkin.Sunrise),
		Dhuhr:   &tDhuhr,
		Asr:     asr,
		Sunset:  geometricSunset(noonUTC, sunAngle),
		Maghrib: compute(noonUTC, sunAngle, +1, temkin.Maghrib),
		Isha:    compute(noonUTC, ishaAngle, +1, temkin.Isha),
	}
}

// Convert UTC prayer time datetimes to HH:MM strings at a given UTC offset.
//
// tzOffset is UTC offset in hours, e.g. 3 for Turkey (UTC+3).
func formatHHMM(dt *time.Time, tzOffset float64) string {
	if dt == nil {
		return ""
	}

	tz := time.FixedZone("estimated", int(tzOffset*3600))
	return dt.In(tz).Add(30 * time.Second).Format("15:04")
}

func estimateUTCOffsetHours(longitude float64) float64 {
	offset := math.Round(longitude / degPerHour)
	if offset < -12 {
		return -12
	}
	if offset > 14 {
		return 14
	}
	return offset
}

func toRadians(deg float64) float64 {
	return deg * math.Pi / 180.0
}

func toDegrees(rad float64) float64 {
	return rad * 180.0 / math.Pi
}