package propagator

import (
	"fmt"
	"math"
	"sort"
	"sync"
	"time"

	"SatMaster/backend/tle"

	"github.com/akhenakh/sgp4"
)

const (
	EarthRadius = 6371.0 // km
	RadToDeg    = 180.0 / math.Pi
)

// SatPosition holds the computed position of a satellite at a given moment.
type SatPosition struct {
	Name      string  `json:"name"`
	Latitude  float64 `json:"lat"`
	Longitude float64 `json:"lon"`
	Altitude  float64 `json:"alt"`       // km
	Azimuth   float64 `json:"az"`        // degrees 0=N
	Elevation float64 `json:"el"`        // degrees above horizon
	Range     float64 `json:"range"`     // km from observer
	RangeRate float64 `json:"rangeRate"` // km/s positive=receding
	Footprint float64 `json:"footprint"` // km radius on ground
}

// Pass represents one overhead pass of a satellite.
type Pass struct {
	SatName   string      `json:"satName"`
	AOS       time.Time   `json:"aos"`
	LOS       time.Time   `json:"los"`
	MaxEl     float64     `json:"maxEl"`
	MaxElTime time.Time   `json:"maxElTime"`
	AosAz     float64     `json:"aosAz"`
	LosAz     float64     `json:"losAz"`
	MaxElAz   float64     `json:"maxElAz"`
	Duration  float64     `json:"duration"` // seconds
	Points    []PassPoint `json:"points"`
}

// PassPoint is one az/el sample in a pass track.
type PassPoint struct {
	Time time.Time `json:"t"`
	Az   float64   `json:"az"`
	El   float64   `json:"el"`
}

// Observer holds the QTH location.
type Observer struct {
	Lat float64 // degrees
	Lon float64 // degrees
	Alt float64 // meters above sea level
}

// Engine propagates satellite positions using SGP4.
type Engine struct {
	mu       sync.RWMutex
	observer Observer
}

func NewEngine() *Engine {
	return &Engine{
		observer: Observer{Lat: 45.75, Lon: 4.85, Alt: 200}, // Default: Lyon area
	}
}

func (e *Engine) SetObserver(lat, lon, altM float64) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.observer = Observer{Lat: lat, Lon: lon, Alt: altM}
}

func (e *Engine) ObserverPosition() Observer {
	e.mu.RLock()
	defer e.mu.RUnlock()
	return e.observer
}

// Position computes the current position of one satellite.
func (e *Engine) Position(sat *tle.Satellite, t time.Time) *SatPosition {
	e.mu.RLock()
	obs := e.observer
	e.mu.RUnlock()
	return computePosition(sat, obs, t)
}

// AllPositions computes positions for all loaded satellites.
func (e *Engine) AllPositions(sats []*tle.Satellite, t time.Time) []SatPosition {
	e.mu.RLock()
	obs := e.observer
	e.mu.RUnlock()

	result := make([]SatPosition, 0, len(sats))
	for _, sat := range sats {
		if pos := computePosition(sat, obs, t); pos != nil {
			result = append(result, *pos)
		}
	}
	return result
}

// ComputePasses predicts upcoming passes over `hours` hours.
func (e *Engine) ComputePasses(sat *tle.Satellite, start time.Time, hours float64) []Pass {
	e.mu.RLock()
	obs := e.observer
	e.mu.RUnlock()

	var passes []Pass
	end := start.Add(time.Duration(float64(time.Hour) * hours))
	step := 10 * time.Second
	t := start

	var inPass bool
	var cur *Pass

	for t.Before(end) {
		pos := computePosition(sat, obs, t)
		if pos == nil {
			t = t.Add(step)
			continue
		}

		if pos.Elevation > 0 && !inPass {
			inPass = true
			aosT := bisectAOS(sat, obs, t.Add(-step), t)
			aosPos := computePosition(sat, obs, aosT)
			if aosPos == nil {
				aosPos = pos
			}
			cur = &Pass{
				SatName: sat.Name,
				AOS:     aosT,
				AosAz:   aosPos.Azimuth,
				Points:  []PassPoint{},
			}
		}

		if inPass && cur != nil {
			cur.Points = append(cur.Points, PassPoint{Time: t, Az: pos.Azimuth, El: pos.Elevation})
			if pos.Elevation > cur.MaxEl {
				cur.MaxEl = pos.Elevation
				cur.MaxElTime = t
				cur.MaxElAz = pos.Azimuth
			}
		}

		if pos.Elevation <= 0 && inPass {
			inPass = false
			if cur != nil {
				losT := bisectLOS(sat, obs, t.Add(-step), t)
				losPos := computePosition(sat, obs, losT)
				if losPos == nil {
					losPos = pos
				}
				cur.LOS = losT
				cur.LosAz = losPos.Azimuth
				cur.Duration = losT.Sub(cur.AOS).Seconds()
				if cur.MaxEl >= 1.0 {
					passes = append(passes, *cur)
				}
				cur = nil
			}
		}

		if pos.Elevation < -15 {
			step = 30 * time.Second
		} else {
			step = 10 * time.Second
		}
		t = t.Add(step)
	}

	sort.Slice(passes, func(i, j int) bool {
		return passes[i].AOS.Before(passes[j].AOS)
	})
	return passes
}

// ─── Core SGP4 computation ────────────────────────────────────────────────────

// parsedTLE caches the orbital elements for a satellite TLE.
// akhenakh/sgp4 ParseTLE accepts the 3-line TLE as a single string.
func makeTLEString(sat *tle.Satellite) string {
	return fmt.Sprintf("%s\n%s\n%s", sat.Name, sat.TLE1, sat.TLE2)
}

func computePosition(sat *tle.Satellite, obs Observer, t time.Time) *SatPosition {
	// Parse TLE — akhenakh/sgp4 accepts the 3-line block as one string
	tleObj, err := sgp4.ParseTLE(makeTLEString(sat))
	if err != nil {
		return nil
	}

	// Propagate to time t → ECI state with geodetic fields
	eciState, err := tleObj.FindPositionAtTime(t)
	if err != nil {
		return nil
	}

	// Convert ECI to geodetic (lat deg, lon deg, alt km)
	lat, lon, altKm := eciState.ToGeodetic()

	// Build observer location
	location := &sgp4.Location{
		Latitude:  obs.Lat, // degrees
		Longitude: obs.Lon, // degrees
		Altitude:  obs.Alt, // meters
	}

	// Build state vector for look angle calculation
	sv := &sgp4.StateVector{
		X:  eciState.Position.X,
		Y:  eciState.Position.Y,
		Z:  eciState.Position.Z,
		VX: eciState.Velocity.X,
		VY: eciState.Velocity.Y,
		VZ: eciState.Velocity.Z,
	}

	// GetLookAngle uses eciState.DateTime (the propagated time) not t
	observation, err := sv.GetLookAngle(location, eciState.DateTime)
	if err != nil {
		return nil
	}

	la := observation.LookAngles
	footprint := EarthRadius * math.Acos(EarthRadius/(EarthRadius+altKm))

	// Compute range rate by finite difference (2s) - library value is unreliable
	rangeRate := finiteRangeRate(tleObj, location, t)

	return &SatPosition{
		Name:      sat.Name,
		Latitude:  lat,
		Longitude: lon,
		Altitude:  altKm,
		Azimuth:   la.Azimuth,
		Elevation: la.Elevation,
		Range:     la.Range,
		RangeRate: rangeRate,
		Footprint: footprint,
	}
}

// finiteRangeRate computes range rate (km/s) by finite difference over 2 seconds.
// Positive = satellite receding, negative = approaching.
func finiteRangeRate(tleObj *sgp4.TLE, loc *sgp4.Location, t time.Time) float64 {
	dt := 2.0 // seconds
	t2 := t.Add(time.Duration(dt * float64(time.Second)))

	e1, err1 := tleObj.FindPositionAtTime(t)
	e2, err2 := tleObj.FindPositionAtTime(t2)
	if err1 != nil || err2 != nil {
		return 0
	}

	sv1 := &sgp4.StateVector{X: e1.Position.X, Y: e1.Position.Y, Z: e1.Position.Z,
		VX: e1.Velocity.X, VY: e1.Velocity.Y, VZ: e1.Velocity.Z}
	sv2 := &sgp4.StateVector{X: e2.Position.X, Y: e2.Position.Y, Z: e2.Position.Z,
		VX: e2.Velocity.X, VY: e2.Velocity.Y, VZ: e2.Velocity.Z}

	obs1, err1 := sv1.GetLookAngle(loc, e1.DateTime)
	obs2, err2 := sv2.GetLookAngle(loc, e2.DateTime)
	if err1 != nil || err2 != nil {
		return 0
	}

	return (obs2.LookAngles.Range - obs1.LookAngles.Range) / dt
}

// bisectAOS finds exact AOS time (precision: 1 second).
func bisectAOS(sat *tle.Satellite, obs Observer, lo, hi time.Time) time.Time {
	for hi.Sub(lo) > time.Second {
		mid := lo.Add(hi.Sub(lo) / 2)
		pos := computePosition(sat, obs, mid)
		if pos != nil && pos.Elevation > 0 {
			hi = mid
		} else {
			lo = mid
		}
	}
	return hi
}

// bisectLOS finds exact LOS time (precision: 1 second).
func bisectLOS(sat *tle.Satellite, obs Observer, lo, hi time.Time) time.Time {
	for hi.Sub(lo) > time.Second {
		mid := lo.Add(hi.Sub(lo) / 2)
		pos := computePosition(sat, obs, mid)
		if pos != nil && pos.Elevation > 0 {
			lo = mid
		} else {
			hi = mid
		}
	}
	return lo
}

// GroundtrackPoint is a lat/lon position at a given time.
type GroundtrackPoint struct {
	Lat  float64   `json:"lat"`
	Lon  float64   `json:"lon"`
	Time time.Time `json:"t"`
}

// ComputeGroundtrack returns the future orbit track for the next `minutes` minutes.
// Points every 30 seconds. Handles the antimeridian by splitting segments.
func (e *Engine) ComputeGroundtrack(sat *tle.Satellite, start time.Time, minutes float64) []GroundtrackPoint {
	e.mu.RLock()
	defer e.mu.RUnlock()

	var points []GroundtrackPoint
	end := start.Add(time.Duration(float64(time.Minute) * minutes))
	step := 30 * time.Second

	// Parse TLE once outside the loop for efficiency
	tleObj, err := sgp4.ParseTLE(fmt.Sprintf("%s\n%s\n%s", sat.Name, sat.TLE1, sat.TLE2))
	if err != nil {
		return points
	}

	for t := start; t.Before(end); t = t.Add(step) {
		eciState, err := tleObj.FindPositionAtTime(t)
		if err != nil {
			continue
		}
		lat, lon, _ := eciState.ToGeodetic()
		points = append(points, GroundtrackPoint{Lat: lat, Lon: lon, Time: t})
	}
	return points
}