feat(decart): interp

internal/engine/models.go

@@ -15,10 +15,10 @@ func Sum(models ...Model) Model {
 	if len(models) == 1 {
 		return models[0]
 	}
-	return func(t float64, s State) State {
-		var sum State
+	return func(t float64, s State) numerics.Rate {
+		var sum numerics.Rate
 		for _, m := range models {
-			sum = numerics.AddGeo(sum, m(t, s))
+			sum = numerics.AddRate(sum, m(t, s))
 		}
 		return sum
 	}
@@ -27,7 +27,7 @@ func Sum(models ...Model) Model {
 // 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} }
+	return func(_ float64, _ State) numerics.Rate { return numerics.Rate{Vertical: rate} }
 }
 
 // ParachuteDescent returns a model where vertical velocity grows with
@@ -40,8 +40,8 @@ func ConstantRate(rate float64) Model {
 //
 // using the NASA atmosphere model for rho. Equivalent to Tawhiri's drag_descent.
 func ParachuteDescent(seaLevelRate float64) Model {
-	return func(_ float64, s State) State {
-		return State{Altitude: numerics.DragTerminalVelocity(seaLevelRate, s.Altitude)}
+	return func(_ float64, s State) numerics.Rate {
+		return numerics.Rate{Vertical: numerics.DragTerminalVelocity(seaLevelRate, s.Altitude)}
 	}
 }
 
@@ -64,33 +64,34 @@ func Piecewise(segments []RateSegment) Model {
 	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 {
+	return func(t float64, _ State) numerics.Rate {
 		idx := sort.Search(len(sorted), func(i int) bool { return sorted[i].Until > t })
 		if idx == len(sorted) {
-			return State{Altitude: finalRate}
+			return numerics.Rate{Vertical: finalRate}
 		}
-		return State{Altitude: sorted[idx].Rate}
+		return numerics.Rate{Vertical: sorted[idx].Rate}
 	}
 }
 
 // WindTransport returns a model that moves laterally at the wind velocity
 // sampled from field. The vertical component is zero. Sampling and the
 // non-fatal "above_model" event live here (orchestration); the m/s → deg/s
-// conversion is numerics.WindToGeoRate.
+// wind is handed straight to the integrator as a horizontal velocity.
 //
 // 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 {
-	return func(t float64, s State) State {
+	return func(t float64, s State) numerics.Rate {
 		sample, err := field.Wind(t, s.Lat, s.Lng, s.Altitude)
 		if err != nil {
-			return State{}
+			return numerics.Rate{}
 		}
 		if sample.AboveModel && events != nil {
 			events.Emit("above_model", t, s,
 				"altitude exceeded the highest pressure level of the wind dataset; samples extrapolated")
 		}
-		dLat, dLng := numerics.WindToGeoRate(sample.U, sample.V, s.Lat, s.Altitude)
-		return State{Lat: dLat, Lng: dLng}
+		// The wind is already a horizontal velocity; it is handed over as-is.
+		// Converting it to deg/s here is what used to blow up near the poles.
+		return numerics.Rate{East: sample.U, North: sample.V}
 	}
 }