package numerics

import (
	"math"
	"testing"
)

func TestRK4ExponentialDecay(t *testing.T) {
	// dAlt/dt = -Alt → exact: Alt(t) = Alt0 * exp(-t).
	f := func(_ float64, y GeoVec) GeoVec { return GeoVec{Altitude: -y.Altitude} }

	y := GeoVec{Altitude: 1}
	tnow, dt := 0.0, 0.01
	for range 100 {
		y = RK4Step(tnow, y, dt, f)
		tnow += dt
	}
	want := math.Exp(-1.0)
	if math.Abs(y.Altitude-want) > 1e-8 {
		t.Errorf("RK4 exp decay at t=1: got %v, want %v", y.Altitude, want)
	}
}

func TestRK4ReverseTime(t *testing.T) {
	// dAlt/dt = Alt → exact: Alt(t) = Alt0 * exp(t).
	f := func(_ float64, y GeoVec) GeoVec { return GeoVec{Altitude: y.Altitude} }

	y := GeoVec{Altitude: math.E}
	tnow, dt := 1.0, -0.01
	for range 100 {
		y = RK4Step(tnow, y, dt, f)
		tnow += dt
	}
	if math.Abs(y.Altitude-1.0) > 1e-8 {
		t.Errorf("RK4 reverse: got %v, want 1.0", y.Altitude)
	}
}

func TestRefineCrossing(t *testing.T) {
	y1 := GeoVec{Altitude: 1}
	y2 := GeoVec{Altitude: -1.5}
	crossed := func(_ float64, y GeoVec) bool { return y.Altitude <= 0 }

	tr, yr := RefineCrossing(0, y1, 1, y2, crossed, 0.001)
	if math.Abs(tr-0.4) > 0.01 {
		t.Errorf("refined t = %v, want ~0.4", tr)
	}
	if math.Abs(yr.Altitude) > 0.01 {
		t.Errorf("refined alt = %v, want ~0", yr.Altitude)
	}
}

func TestGeoAddWrapsLongitude(t *testing.T) {
	y := GeoAdd(GeoVec{Lng: 350}, 1, GeoVec{Lng: 20})
	if math.Abs(y.Lng-10) > 1e-9 {
		t.Errorf("GeoAdd wrap: lng = %v, want 10", y.Lng)
	}
}

func TestGeoLerpWrap(t *testing.T) {
	mid := GeoLerp(GeoVec{Lng: 350}, GeoVec{Lng: 10}, 0.5)
	if math.Abs(mid.Lng) > 1e-9 && math.Abs(mid.Lng-360) > 1e-9 {
		t.Errorf("GeoLerp lng wrap: %v, want 0 or 360", mid.Lng)
	}
}

func TestPathSoA(t *testing.T) {
	p := NewPath(4)
	p.Append(0, GeoVec{Lat: 1, Lng: 2, Altitude: 3})
	p.Append(60, GeoVec{Lat: 4, Lng: 5, Altitude: 6})
	if p.Len() != 2 {
		t.Fatalf("len = %d, want 2", p.Len())
	}
	tt, last := p.Last()
	if tt != 60 || last.Lat != 4 {
		t.Errorf("last = %v, %+v", tt, last)
	}
}