feat: move stuff to numerics

internal/engine/models.go

@@ -1,7 +1,6 @@
 package engine
 
 import (
-	"math"
 	"sort"
 
 	"predictor-refactored/internal/numerics"
@@ -19,10 +18,7 @@ func Sum(models ...Model) Model {
 	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
+			sum = numerics.AddGeo(sum, m(t, s))
 		}
 		return sum
 	}
@@ -44,9 +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 {
-	k := seaLevelRate * 1.1045
 	return func(_ float64, s State) State {
-		return State{Altitude: -k / math.Sqrt(numerics.NasaDensity(s.Altitude))}
+		return State{Altitude: numerics.DragTerminalVelocity(seaLevelRate, s.Altitude)}
 	}
 }
 
@@ -79,15 +74,13 @@ func Piecewise(segments []RateSegment) Model {
 }
 
 // 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.
+// 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.
 //
 // 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 {
@@ -97,10 +90,7 @@ func WindTransport(field weather.WindField, events *EventSink) Model {
 			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)),
-		}
+		dLat, dLng := numerics.WindToGeoRate(sample.U, sample.V, s.Lat, s.Altitude)
+		return State{Lat: dLat, Lng: dLng}
 	}
 }

internal/engine/registry.go

@@ -241,25 +241,31 @@ func buildPiecewise(spec ModelSpec, deps BuildDeps) (BuiltModel, error) {
 	}, nil
 }
 
-// resolveSegments converts spec segments to engine.RateSegment using the
-// stage context to resolve relative references.
+// resolveSegments converts spec segments to engine.RateSegment, turning each
+// segment's reference-relative Until into an absolute UNIX time. References
+// are validated by buildPiecewise, so an unrecognised one here is treated as
+// absolute rather than re-erroring.
 func resolveSegments(in []PiecewiseSegmentSpec, ctx StageContext) []RateSegment {
 	out := make([]RateSegment, 0, len(in))
 	for _, s := range in {
-		var until float64
-		switch s.Reference {
-		case "", "absolute":
-			until = s.Until
-		case "profile_start":
-			until = ctx.ProfileStart + s.Until
-		case "propagator_start":
-			until = ctx.PropagatorStart + s.Until
-		}
-		out = append(out, RateSegment{Until: until, Rate: s.Rate})
+		out = append(out, RateSegment{Until: segmentBase(s.Reference, ctx) + s.Until, Rate: s.Rate})
 	}
 	return out
 }
 
+// segmentBase returns the absolute time a piecewise segment's Until is
+// measured from, per its reference.
+func segmentBase(reference string, ctx StageContext) float64 {
+	switch reference {
+	case "profile_start":
+		return ctx.ProfileStart
+	case "propagator_start":
+		return ctx.PropagatorStart
+	default: // "", "absolute"
+		return 0
+	}
+}
+
 // maybeAddWind sums a WindTransport model into base when the spec asks for it.
 func maybeAddWind(base Model, includeWind bool, deps BuildDeps) Model {
 	if !includeWind {