Added: Initial world generation tool

Tools/Terrain/internal/check/talus_test.go

@@ -0,0 +1,86 @@
+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)
+	}
+}