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feat(decart): interp

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gili8420 2026-08-03 22:10:10 +09:00
parent 1bd9143186
commit 19557f3a62
14 changed files with 681 additions and 208 deletions
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23
internal/numerics/ode.go
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@ -1,31 +1,8 @@
package numerics
// Field returns the time derivative of a geographic state at (t, y).
// The derivative is direction-independent; the integrator applies the sign
// of dt for reverse-time integration.
type Field func(t float64, y GeoVec) GeoVec
// Crossed reports whether a termination condition holds at (t, y).
type Crossed func(t float64, y GeoVec) bool
// RK4Step performs one classical Runge-Kutta-4 step from (t, y) with step dt.
// dt may be negative to integrate backwards in time. Longitude wrapping is
// applied at every intermediate add via GeoAdd, matching the reference
// integrator. The function performs no heap allocation.
func RK4Step(t float64, y GeoVec, dt float64, f Field) GeoVec {
half := dt / 2
k1 := f(t, y)
k2 := f(t+half, GeoAdd(y, half, k1))
k3 := f(t+half, GeoAdd(y, half, k2))
k4 := f(t+dt, GeoAdd(y, dt, k3))
y2 := GeoAdd(y, dt/6, k1)
y2 = GeoAdd(y2, dt/3, k2)
y2 = GeoAdd(y2, dt/3, k3)
y2 = GeoAdd(y2, dt/6, k4)
return y2
}
// RefineCrossing locates a crossing between (t1, y1) (not crossed) and
// (t2, y2) (crossed) by binary search in the linear-interpolation parameter
// space, stopping when the parameter interval is narrower than tol.