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)
	}
}