Tooling
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Rainer Leit
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package uplift
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import (
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"math"
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"testing"
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"salty/terrain/internal/manifest"
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"salty/terrain/internal/world"
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)
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// The defect D-62 caused and D-63 answers: a sub-parallel fault set stacked.
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//
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// Widening a fault's reach from 600 m to 6 km made overlap the ordinary case rather than a rarity, and a
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// fault set is sub-parallel by construction - traces within one cell of the orientation grain share a
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// strike. On Bake_018's region 11, 75 % of the faulted ground had two or more faults on it and the sum was
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// a median 1.77 and up to 4.46 times the largest single contribution, which took the landmass from 75 m to
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// 221 m and fired the repose clamp on 4.1 % of it.
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//
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// Five parallel traces two kilometres apart is that in miniature: at a 6 km footwall every one of them
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// reaches every other, so an unbounded sum would be several throws deep.
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func TestParallelFaultsDoNotStack(t *testing.T) {
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const w, h, cellM = 512, 1600, 16.0
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const throw, runYears = 300.0, 1.5e6
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p := testPlanet(t, w, h, cellM)
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f := world.Whole(p)
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trace := func(yM float64) FaultTrace {
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pts := make([][2]float64, 17)
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for i := range pts {
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pts[i] = [2]float64{float64(i) * float64(w) * cellM / 16, yM}
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}
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return FaultTrace{PointsM: pts, ThrowM: throw, LengthM: float64(w) * cellM}
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}
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midM := float64(h) * cellM / 2
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var many []FaultTrace
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for i := -2; i <= 2; i++ {
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many = append(many, trace(midM+float64(i)*2000))
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}
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one := []FaultTrace{trace(midM)}
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peakOf := func(faults []FaultTrace) (hi, lo float64) {
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d := FaultDelta(f, faults, runYears)
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if d == nil {
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t.Fatal("the traces reached nothing")
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}
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col := w / 2
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for y := 0; y < h; y++ {
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v := float64(d[y*w+col]) * runYears
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hi = math.Max(hi, v)
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lo = math.Min(lo, v)
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}
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return hi, lo
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}
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hi1, lo1 := peakOf(one)
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hiN, loN := peakOf(many)
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// A belt still stands higher than one fault does, which is the point of a belt.
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if hiN <= hi1 {
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t.Errorf("five faults build %.0f m against one fault's %.0f m; the stack is suppressed entirely",
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hiN, hi1)
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}
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// But not five times higher. The asymptote is 1+faultStackBonus times the largest single contribution
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// at a cell, and the crest of the stack sits near the crest of its strongest member.
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limit := (1 + faultStackBonus) * 1.05
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if hiN > hi1*limit {
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t.Errorf("five parallel faults build %.0f m against one fault's %.0f m, a factor of %.2f; the "+
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"bound is %.2f", hiN, hi1, hiN/hi1, limit)
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}
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// The hanging walls are bounded on the same terms, or a fault set digs a hole instead of building one.
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if loN < lo1*limit {
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t.Errorf("five parallel faults drop %.0f m against one fault's %.0f m, a factor of %.2f",
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loN, lo1, loN/lo1)
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}
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t.Logf("one fault %+.0f/%+.0f m, five at 2 km spacing %+.0f/%+.0f m (x%.2f)",
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hi1, lo1, hiN, loN, hiN/hi1)
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// And it is still solvable ground: bending the stack must not put a step back into the rate field.
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d := FaultDelta(f, many, runYears)
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var steepest float64
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col := w / 2
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for y := 1; y < h; y++ {
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g := math.Abs(float64(d[y*w+col]-d[(y-1)*w+col])) * runYears / cellM
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steepest = math.Max(steepest, g)
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}
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if deg := math.Atan(steepest) * 180 / math.Pi; deg > 25 {
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t.Errorf("the steepest cell-to-cell step across the stack is %.1f degrees", deg)
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}
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}
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// One fault is untouched by the bound, so D-62's calibration still holds: the step across a fault is the
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// throw its author asked for. softStack is the identity below its knee and the knee is the largest single
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// contribution, so this is the property that makes the two changes compose rather than fight.
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func TestOneFaultIsNotBentByTheStackBound(t *testing.T) {
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const w, h, cellM = 512, 1600, 16.0
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const throw, runYears = 300.0, 1.5e6
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p := testPlanet(t, w, h, cellM)
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f := world.Whole(p)
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midM := float64(h) * cellM / 2
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pts := make([][2]float64, 17)
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for i := range pts {
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pts[i] = [2]float64{float64(i) * float64(w) * cellM / 16, midM}
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}
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d := FaultDelta(f, []FaultTrace{{PointsM: pts, ThrowM: throw, LengthM: float64(w) * cellM}}, runYears)
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if d == nil {
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t.Fatal("the trace reached nothing")
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}
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col := w / 2
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var hi, lo float64
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for y := 0; y < h; y++ {
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v := float64(d[y*w+col]) * runYears
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hi = math.Max(hi, v)
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lo = math.Min(lo, v)
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}
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if step := hi - lo; math.Abs(step-throw) > throw/20 {
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t.Errorf("the step across a lone fault is %.0f m against a throw of %.0f m; the stack bound is "+
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"biting on a single fault", step, throw)
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}
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}
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// The initial relief is scaled by the rate *before* the faults (D-63).
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//
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// It used to be scaled by the finished rate with the fault delta in it and no upper bound, so D-62's six
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// kilometre footwalls raised the stamped noise from about 39 m to about 166 m on a landmass whose whole
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// relief was 221 m - and a thousand steps cannot erase initial relief the size of the landscape, so the
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// ridged fBm stopped breaking the symmetry and became the flank texture instead. The initial relief exists
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// to give the solve something to bite on; how much noise sits on a hillside is not a fault's decision.
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func TestTheInitialReliefIgnoresFaults(t *testing.T) {
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const w, h, cellM = 256, 256, 64.0
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p := testPlanet(t, w, h, cellM)
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f := world.Whole(p)
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class := make([]uint8, w*h)
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land := make([]bool, w*h)
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for y := 0; y < h; y++ {
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for x := 0; x < w; x++ {
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if x >= 16 {
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class[y*w+x], land[y*w+x] = 1, true
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}
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}
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}
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midM := float64(h) * cellM / 2
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pts := make([][2]float64, 17)
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for i := range pts {
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pts[i] = [2]float64{float64(i) * float64(w) * cellM / 16, midM}
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}
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faults := []FaultTrace{{PointsM: pts, ThrowM: 400, LengthM: float64(w) * cellM}}
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m := manifest.Defaults()
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m.Source.Seed = 7
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build := func(fs []FaultTrace) *Result {
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return FromTemplate(Paint{
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Frame: f, Class: class, Land: land,
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Rates: []float32{0, 0.00025}, // 0.25 mm/yr, the shipped highland
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Ks: []float32{1, 1},
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Faults: fs,
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RunYears: 1.5e6,
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Variation: 0,
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// The repose bound off, so the fault delta survives into the rate and the difference this
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// test is looking for is actually there to find.
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ClampCeilM: 0,
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}, m)
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}
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with, without := build(faults), build(nil)
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differs := 0
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for i := range with.Rate.Data {
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if with.Rate.Data[i] != without.Rate.Data[i] {
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differs++
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}
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}
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if differs == 0 {
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t.Fatal("the fault changed no rate at all; this test measured nothing")
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}
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for i := range with.Height.Data {
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if with.Height.Data[i] != without.Height.Data[i] {
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t.Fatalf("the initial relief at cell %d is %.3f m with faults and %.3f m without, over %d "+
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"cells whose rate the fault changed; the amplitude is still reading the finished rate",
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i, with.Height.Data[i], without.Height.Data[i], differs)
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}
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}
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t.Logf("the fault moved %d of %d rates and no initial relief at all", differs, len(with.Rate.Data))
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}
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