package engine

import (
	"math"
	"sort"

	"predictor-refactored/internal/weather"
)

// Sum composes models by summing their derivatives at each evaluation point.
//
// Useful for combining a vertical-rate model with a horizontal wind model
// into a single propagator. Equivalent to Tawhiri's LinearModel.
func Sum(models ...Model) Model {
	if len(models) == 1 {
		return models[0]
	}
	return func(t float64, s State) State {
		var sum State
		for _, m := range models {
			d := m(t, s)
			sum.Lat += d.Lat
			sum.Lng += d.Lng
			sum.Altitude += d.Altitude
		}
		return sum
	}
}

// ConstantRate returns a model with a constant vertical velocity (m/s).
// Positive rates are upward.
func ConstantRate(rate float64) Model {
	return func(_ float64, _ State) State { return State{Altitude: rate} }
}

// ParachuteDescent returns a model where vertical velocity grows with
// altitude because thinner air provides less drag. seaLevelRate is the
// descent speed at sea level (m/s, positive).
//
// Terminal velocity at altitude is computed as
//
//	v = -k / sqrt(rho(alt)),  k = seaLevelRate * 1.1045,
//
// using the NASA atmosphere model for rho. Equivalent to Tawhiri's drag_descent.
func ParachuteDescent(seaLevelRate float64) Model {
	k := seaLevelRate * 1.1045
	return func(_ float64, s State) State {
		return State{Altitude: -k / math.Sqrt(nasaDensity(s.Altitude))}
	}
}

// nasaDensity returns air density (kg/m^3) for an altitude in metres,
// using the NASA simple atmosphere model.
// See https://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html.
func nasaDensity(alt float64) float64 {
	var temp, pressure float64
	switch {
	case alt > 25000:
		temp = -131.21 + 0.00299*alt
		pressure = 2.488 * math.Pow((temp+273.1)/216.6, -11.388)
	case alt > 11000:
		temp = -56.46
		pressure = 22.65 * math.Exp(1.73-0.000157*alt)
	default:
		temp = 15.04 - 0.00649*alt
		pressure = 101.29 * math.Pow((temp+273.1)/288.08, 5.256)
	}
	return pressure / (0.2869 * (temp + 273.1))
}

// RateSegment is one entry in a Piecewise rate schedule. Until is the UNIX
// timestamp at which this segment ends — the model emits the segment's
// Rate for all t < Until. The final segment's Rate is held indefinitely.
type RateSegment struct {
	Until float64
	Rate  float64
}

// Piecewise returns a model that produces a piecewise-constant vertical
// rate over a sequence of intervals. The input is sorted ascending by
// Until on construction; later segments shadow earlier ones.
func Piecewise(segments []RateSegment) Model {
	if len(segments) == 0 {
		return ConstantRate(0)
	}
	sorted := append([]RateSegment(nil), segments...)
	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Until < sorted[j].Until })
	finalRate := sorted[len(sorted)-1].Rate

	return func(t float64, _ State) State {
		idx := sort.Search(len(sorted), func(i int) bool { return sorted[i].Until > t })
		if idx == len(sorted) {
			return State{Altitude: finalRate}
		}
		return State{Altitude: sorted[idx].Rate}
	}
}

// WindTransport returns a model that moves laterally at the wind velocity
// sampled from field. Vertical component is zero. Wind components in m/s
// are converted to deg/s on Earth's surface using R = 6371009 m.
//
// If events is non-nil, an "above_model" event is emitted whenever the
// wind field reports altitude above the highest pressure level.
func WindTransport(field weather.WindField, events *EventSink) Model {
	const earthR = 6371009.0
	const piOver180 = math.Pi / 180.0
	const degPerRad = 180.0 / math.Pi
	return func(t float64, s State) State {
		sample, err := field.Wind(t, s.Lat, s.Lng, s.Altitude)
		if err != nil {
			return State{}
		}
		if sample.AboveModel && events != nil {
			events.Emit("above_model", t, s,
				"altitude exceeded the highest pressure level of the wind dataset; samples extrapolated")
		}
		r := earthR + s.Altitude
		return State{
			Lat: degPerRad * sample.V / r,
			Lng: degPerRad * sample.U / (r * math.Cos(s.Lat*piOver180)),
		}
	}
}