Added: Initial world generation tool

Tools/Terrain/internal/check/margin_test.go

@@ -0,0 +1,96 @@
+package check
+
+import (
+	"testing"
+
+	"salty/terrain/internal/manifest"
+	"salty/terrain/internal/uplift"
+)
+
+// TestBorderIsAlwaysOcean is the boundary condition the whole solve rests on, asserted rather than assumed.
+//
+// A cell on the map border is an outlet: `fluvial.isOutlet` returns true for it whatever else is true, so it
+// takes no uplift and is never eroded, and the repose clamp will not lower it either. Land that reaches the
+// border therefore freezes at whatever the initial relief gave it while the interior erodes away beneath it,
+// and what that looks like is a rim of untouched terrain standing a hundred metres over its neighbour — it is
+// what once reported a 66 degree slope on a map whose angle of repose was 22. `continentMask` imposes a
+// margin to prevent it, and this test is what says the margin is still doing its job.
+//
+// It is worth a test rather than a comment because the margin is easy to break from a distance. The uplift
+// rate used to be multiplied by the continent mask, so the margin's taper was suppressing uplift at the border
+// as a side effect; decoupling the two (D-52) removed that, and nothing in the change was anywhere near this
+// file. The invariant is the border cells themselves, so that is what is asserted; the distances and rates are
+// reported alongside because they are what a person would want to see when it does fail.
+func TestBorderIsAlwaysOcean(t *testing.T) {
+	const size = 1400
+	const sideM = 14280.0
+	cell := sideM / (size - 1)
+
+	for _, seed := range []int64{7, 9342, 67914} {
+		m := manifest.Defaults()
+		m.Source.Seed = seed
+		up := uplift.Build(size, cell, m)
+
+		for x := 0; x < size; x++ {
+			for _, i := range [2]int{x, (size-1)*size + x} {
+				if !up.Base[i] {
+					t.Errorf("seed %d: cell (%d,%d) on the top or bottom border is land", seed, i%size, i/size)
+				}
+			}
+		}
+		for y := 0; y < size; y++ {
+			for _, i := range [2]int{y * size, y*size + size - 1} {
+				if !up.Base[i] {
+					t.Errorf("seed %d: cell (%d,%d) on the left or right border is land", seed, i%size, i/size)
+				}
+			}
+		}
+
+		// How hard the margin is pressing. The margin does not only keep land off the border; where the
+		// continent would have run past the edge it *cuts* it, and it cuts along a contour of distance-to-edge,
+		// which is a straight line parallel to that edge. So a coastline inside the margin band is a coastline
+		// the margin drew rather than one the noise did, and the count is the measure of how much of one there
+		// is. It is logged, not asserted: some is unavoidable and the right threshold is a judgement.
+		band := int(0.04 * size)
+		nearest := size
+		var peakRate, peakHeight float64
+		landInBand, shoreInBand, shore := 0, 0, 0
+		for i := range up.Base {
+			x, y := i%size, i/size
+			d := min(min(x, size-1-x), min(y, size-1-y))
+			if up.Base[i] {
+				// A waterline cell: ocean with land in the four-neighbourhood.
+				for _, n := range [4]int{i - 1, i + 1, i - size, i + size} {
+					if n >= 0 && n < len(up.Base) && !up.Base[n] {
+						shore++
+						if d < band {
+							shoreInBand++
+						}
+						break
+					}
+				}
+				continue
+			}
+			if d < nearest {
+				nearest = d
+			}
+			if d < band {
+				landInBand++
+				if r := float64(up.Rate.Data[i]) * 1000; r > peakRate {
+					peakRate = r
+				}
+				if h := float64(up.Height.Data[i]); h > peakHeight {
+					peakHeight = h
+				}
+			}
+		}
+		pct := 0.0
+		if shore > 0 {
+			pct = float64(shoreInBand) / float64(shore) * 100
+		}
+		t.Logf("seed %6d: nearest land %d cells (%.0f m) from the border; inside the %d-cell margin, "+
+			"%d land cells, peak uplift %.2f mm/yr, peak initial relief %.0f m; %.1f%% of the shoreline is "+
+			"inside the margin band (a coast the margin drew, not the noise)",
+			seed, nearest, float64(nearest)*cell, band, landInBand, peakRate, peakHeight, pct)
+	}
+}