package fluvial

import (
	"math"
	"testing"
)

// TestClampToReposeCutsACone is the smallest possible check on the constraint: a cone far steeper than the
// repose angle must come back at or under it.
func TestClampToReposeCutsACone(t *testing.T) {
	const (
		w, h  = 81, 81
		cellM = 10.0
		talus = 0.4 // about 22 degrees
	)
	base := make([]bool, w*h)
	field := make([]float32, w*h)
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			d := math.Hypot(float64(x-w/2), float64(y-h/2)) * cellM
			field[y*w+x] = float32(math.Max(0, 800-2.0*d)) // slope 2.0, five times repose
		}
	}
	g := NewGrid(w, h, cellM, base)
	g.SetElevationRange(-100, 2000)
	removed := g.ClampToRepose(field, talus)

	worst := 0.0
	for y := 0; y < h; y++ {
		for x := 0; x < 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(field[y*w+x]-field[ny*w+nx]) / d; s > worst {
					worst = s
				}
			}
		}
	}
	t.Logf("removed %.2f m mean; steepest slope now %.3f (repose %.3f)", removed, worst, talus)
	if worst > talus*1.02 {
		t.Errorf("steepest slope %.3f exceeds repose %.3f", worst, talus)
	}
}