package thermal

import (
	"math"
	"testing"
)

// TestConeIsCutToRepose is the property the whole hillslope story rests on: after enough passes nothing may
// stand steeper than the angle of repose. A cone is the worst case, because every cell on it is steep.
func TestConeIsCutToRepose(t *testing.T) {
	const (
		w, h  = 101, 101
		cellM = 10.0
		deg   = 33.0
	)
	talus := TalusFromDegrees(deg)
	field := make([]float32, w*h)
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			// A steep cone: 60 m of fall per 10 m cell at the flanks, far beyond repose.
			d := math.Hypot(float64(x-w/2), float64(y-h/2))
			field[y*w+x] = float32(math.Max(0, 600-6*d*cellM))
		}
	}
	before := maxSlope(field, w, h, cellM)
	scratch := make([]float32, w*h)

	volBefore := sum(field)
	Apply(field, w, h, cellM, talus, 400, nil, scratch)
	volAfter := sum(field)

	after := maxSlope(field, w, h, cellM)
	t.Logf("max slope %.1f deg -> %.1f deg (repose %.1f)", degOf(before), degOf(after), deg)
	if degOf(after) > deg+2 {
		t.Errorf("after 400 passes the steepest slope is %.1f deg, want no more than %.1f", degOf(after), deg+2)
	}
	if rel := math.Abs(volAfter-volBefore) / volBefore; rel > 1e-4 {
		t.Errorf("mass changed by %.4f%%; thermal weathering must conserve it", rel*100)
	}
}

func sum(a []float32) float64 {
	var s float64
	for _, v := range a {
		s += float64(v)
	}
	return s
}

func degOf(slope float64) float64 { return math.Atan(slope) * 180 / math.Pi }

func maxSlope(a []float32, w, h int, cellM float64) float64 {
	worst := 0.0
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*w + x
			for k := 0; k < 8; k++ {
				nx, ny := x+dx8[k], y+dy8[k]
				if nx < 0 || ny < 0 || nx >= w || ny >= h {
					continue
				}
				d := cellM
				if dx8[k] != 0 && dy8[k] != 0 {
					d = cellM * math.Sqrt2
				}
				if s := float64(a[i]-a[ny*w+nx]) / d; s > worst {
					worst = s
				}
			}
		}
	}
	return worst
}