package check

import (
	"math"
	"os"
	"strconv"
	"testing"

	"salty/terrain/internal/fluvial"
	"salty/terrain/internal/manifest"
	"salty/terrain/internal/thermal"
	"salty/terrain/internal/uplift"
)

// TestTalusHoldsInThePipeline is the gap between a unit test and a system. thermal.Apply demonstrably cuts a
// cone to the angle of repose, and yet a full run with the angle set to 22 degrees reports a median land slope
// of 37. One of those two things is wrong and the difference matters: the angle of repose is the only direct
// lever on how steep hill country looks.
func TestTalusHoldsInThePipeline(t *testing.T) {
	const (
		size  = 300
		deg   = 22.0
		steps = 300
	)
	passesUnderTest := 3
	if v := os.Getenv("TALUS_PASSES"); v != "" {
		passesUnderTest, _ = strconv.Atoi(v)
	}
	m := manifest.Defaults()
	m.Source.Seed = 7
	cellM := m.SideM() / float64(size-1)
	up := uplift.Build(size, cellM, m)
	h := up.Height.Clone()

	g := fluvial.NewGrid(size, size, cellM, up.Base)
	g.SetElevationRange(-2000, 4000)
	talus := thermal.TalusFromDegrees(deg)
	g.Run(h.Data, up.Rate.Data, up.K.Data, fluvial.Params{
		K: m.Pipeline.Fluvial.K, M: 0.5, N: 1, DtYr: 1500, Steps: steps,
		Diffusion: 0.02, FillEvery: 1,
		TalusSlope: talus, ThermalEvery: 1, ThermalPasses: passesUnderTest,
	}, nil)

	if os.Getenv("CLAMP_AFTER") != "" {
		t.Logf("removed %.2f m mean by a final clamp", g.ClampToRepose(h.Data, talus))
	}

	// The steepest neighbour drop anywhere on land, in the same units the angle of repose is written in.
	worst, count := 0.0, 0
	wx, wy := 0, 0
	worstOther := float32(0)
	worstOtherBase := false
	for y := 1; y < size-1; y++ {
		for x := 1; x < size-1; x++ {
			i := y*size + x
			if up.Base[i] {
				continue
			}
			for _, d := range [][2]int{{1, 0}, {0, 1}, {1, 1}, {1, -1}} {
				ni := (y+d[1])*size + (x + d[0])
				if up.Base[ni] {
					continue
				}
				dist := cellM
				if d[0] != 0 && d[1] != 0 {
					dist = cellM * math.Sqrt2
				}
				if s := math.Abs(float64(h.Data[i]-h.Data[ni])) / dist; s > worst {
					worst = s
					wx, wy = x, y
					worstOther = h.Data[ni]
					worstOtherBase = up.Base[ni]
				}
			}
			if count++; false {
				_ = count
			}
		}
	}
	t.Logf("worst pair at (%d,%d): h=%.1f vs h=%.1f, base=%v/%v", wx, wy, h.Data[wy*size+wx], worstOther, up.Base[wy*size+wx], worstOtherBase)
	gotDeg := math.Atan(worst) * 180 / math.Pi
	t.Logf("steepest land slope after %d steps: %.1f deg (repose %.1f, cell %.0f m)", steps, gotDeg, deg, cellM)
	if gotDeg > deg+5 {
		t.Errorf("steepest land slope is %.1f deg with the angle of repose at %.1f: landsliding is not binding", gotDeg, deg)
	}
}