shader_type canvas_item;

// Lunar/crater backdrop for the "Big Stable Asteroid" atmosphere. Reacts
// to camera position/zoom exactly like starfield.gdshader (world-space
// sampling from camera uniforms), so it parallaxes correctly as the fleet
// flies around instead of being a static picture.
//
// Structured as a Worley / cellular distance field, following the design
// of the "Floating Bubbles" shader on godotshaders.com: a neighbor scan
// reduces everything to ONE scalar - the normalized distance to the
// nearest crater - and every visual (floor darkness, rim light, rim
// shadow) is then derived from that single value.
//
// This single-field structure matters, and is the whole reason for the
// rewrite. The previous version accumulated three fields independently
// (floor via max() over all neighbors, rim light/shadow via a separate
// "strongest crater wins" test). Those could disagree: one crater's dark
// rim band could be composited over a different, nearer crater's floor,
// cutting a hard wrongly-shaped bite into it. Deriving everything from
// the nearest crater makes that class of bug structurally impossible -
// the floor and the rim always describe the same crater.
//
// Where two craters overlap, the nearest-crater field has a crease (normal
// for Worley noise) and the lighting direction flips across it, which drew
// a thin bright rim wedge through the middle of the merged basin. The
// second-nearest distance is tracked purely to detect that case and fade
// the rim out there - see the note further down.
//
// This runs on EVERY pixel of a fullscreen rect on a phone, so it is the
// single most expensive thing in the surface scene and the subject of
// Forgejo #68 ("the moon game is a bit slow"). The three optimizations
// that brought it down - a bounded 3x3 scan, one hash per cell instead of
// three, and one sqrt per pixel instead of one per sample - are each
// justified at the point they appear below. Time it with
//   bash tests/profile_shader.sh res://lunar_surface.gdshader

uniform vec2 camera_cell = vec2(0.0, 0.0);
uniform vec2 camera_frac = vec2(0.0, 0.0);
uniform float camera_zoom = 1.0;
uniform vec2 viewport_size = vec2(720.0, 1280.0);
uniform float cell_size = 220.0;
// Sampled from a NASA Artemis lunar photograph: the regolith is a
// bright neutral grey with a very faint cool cast (blue a touch above
// red), craters read as mid-grey floors rather than black, and the lit
// rims are only slightly brighter than the surface.
uniform vec4 base_color : source_color = vec4(0.75, 0.75, 0.76, 1.0);
uniform vec4 crater_color : source_color = vec4(0.44, 0.44, 0.45, 1.0);
uniform vec4 highlight_color : source_color = vec4(0.89, 0.89, 0.90, 1.0);
// DIAGNOSTIC ONLY (set in PlanetSurface.tscn for a debug build; 0 = off).
//   1 = tint alternate cells, to show where cell boundaries fall.
//   2 = output the raw normalized distance field as grayscale, with NO
//       rim, lighting, dither or tint. This bisects the shader: the field
//       and the shading built on top of it are independent halves, so if
//       cuts appear in this grayscale output the FIELD is at fault, and if
//       it is smooth the field is fine and the SHADING is at fault.
uniform float debug_mode = 0.0;
// Width of the repeating tile, in cells. The crater field is made PERIODIC
// with this period (see the wrap in the loop), which is what lets
// LunarBackground.gd bake one square of it into a texture and then scroll
// that texture forever with repeat wrapping, instead of re-deriving the
// field per pixel per frame. Set from LunarBackground.TILE_CELLS, which is
// the single source of truth shared with lunar_tiled.gdshader.
uniform float tile_cells = 32.0;
// BAKE MODE. 0 = normal camera-relative sampling for direct display.
// Above zero, UV is mapped straight onto [0, tile_cells] instead, so
// rendering this shader to a square SubViewport produces exactly one period
// of the field - the texture the runtime shader then samples.
uniform float bake_cells = 0.0;

const vec2 LIGHT_DIR = vec2(-0.6, -0.8);
const float MIN_RADIUS = 0.16;
// MUST stay below 1.0. The 3x3 scan below is only correct because a crater
// cannot reach out of its own cell's immediate neighbourhood; see the long
// note at the loop. tests/test_lunar_field.gd asserts this bound directly
// against the shader source so it cannot drift unnoticed.
const float MAX_RADIUS = 0.38;
// Sentinel for "this cell holds no crater", in SQUARED normalized units to
// match the loop. Must stay well inside mediump range (max ~65504) so it
// can never become +Inf - see the note in the loop. Only needs to exceed
// the largest real squared normalized distance, which for a 3x3 scan is
// (2.83 cells / 0.16 min radius)^2 ~= 313.
const float MISS_DISTANCE_SQ = 10000.0;
// Loop-carried initial value. Above any real value INCLUDING one carrying
// the miss sentinel (~10313), so a pixel whose whole neighbourhood is
// empty still leaves the "winning" offset populated with something finite.
const float START_DISTANCE_SQ = 20000.0;

// One hash call yields all four values a cell needs (crater centre x/y,
// whether the cell holds a crater at all, and its radius). The previous
// version called two different hashes three times per cell with different
// constant offsets, which was three quarters of the loop body's arithmetic
// for no extra information - a single hash with four independent output
// lanes decorrelates just as well.
//
// Multipliers chosen for precision safety on mobile GPUs: they are all
// < 1, so every intermediate stays small no matter how large the cell
// index gets. An earlier version multiplied by ~123/456 first, which lands
// the intermediate in the thousands where a mobile float loses its
// fractional bits entirely. (Standard "hash without sine" formulation,
// Dave Hoskins.)
vec4 hash42(vec2 p) {
	vec4 p4 = fract(vec4(p.xyxy) * vec4(0.1031, 0.1030, 0.0973, 0.1099));
	p4 += dot(p4, p4.wzxy + 33.33);
	return fract((p4.xxyz + p4.yzzw) * p4.zywx);
}

float hash21(vec2 p) {
	vec3 p3 = fract(vec3(p.xyx) * 0.1031);
	p3 += dot(p3, p3.yzx + 33.33);
	return fract((p3.x + p3.y) * p3.z);
}

void fragment() {
	vec2 screen_px = UV * viewport_size;
	vec2 centered = screen_px - viewport_size * 0.5;

	// Position is expressed RELATIVE TO THE CAMERA'S CELL, never as an
	// absolute world coordinate. camera_frac is the camera's offset inside
	// its own cell (always in [0,1]) and `centered` spans at most a few
	// cells, so every value here stays small and exactly representable even
	// on a mobile GPU - whereas an absolute world position reaches the tens
	// of thousands, where float precision degrades and no `highp` qualifier
	// can help (it widens the type, but the magnitude is the problem).
	// GDScript does the large-number split in double precision; see
	// LunarBackground.gd / Starfield.gd.
	//
	// cell and local also MUST come from one shared value, with local
	// derived as (rel - cell_rel) rather than an independent fract():
	// calling floor(x) and fract(x) separately lets the GPU round the two
	// evaluations differently, which right at a cell boundary makes them
	// disagree by a full cell and snaps the sampled point one cell sideways.
	//
	// In bake mode none of that applies: the target is one tile of the field
	// in isolation, so UV maps directly onto [0, bake_cells] and there is no
	// camera in the picture at all.
	bool baking = bake_cells > 0.0;
	vec2 rel = baking
		? UV * bake_cells
		: camera_frac + centered * (camera_zoom / cell_size);
	vec2 cell_rel = floor(rel);
	vec2 local = rel - cell_rel;
	// Absolute cell index, used only for hashing. Built from exact integers.
	vec2 cell = baking ? cell_rel : camera_cell + cell_rel;

	// Single scalar field: normalized distance to the nearest crater.
	// < 1.0 means this pixel is inside that crater; 1.0 is exactly on its
	// edge. Normalizing by each crater's own radius is what lets craters
	// vary in size while still collapsing to one comparable number.
	//
	// The SECOND-nearest crater is tracked alongside the nearest, purely so
	// the rim can be suppressed inside overlaps. Where two craters overlap,
	// the nearest-crater test flips along the line between them; the
	// lighting direction flips with it and paints a thin bright rim wedge
	// through the middle of what should read as one merged depression.
	// A pixel in that region is inside BOTH craters, i.e. second_nearest is
	// also < 1.0 - which is precisely the condition to fade the rim out.
	//
	// Tracked SQUARED, and square-rooted once at the end. Ordering is
	// unchanged by squaring (both sides are non-negative), so the same
	// crater wins - but this turns one sqrt per sample into two per pixel,
	// and sqrt is one of the more expensive things a fragment shader can
	// ask for.
	float nearest_sq = START_DISTANCE_SQ;
	float second_nearest_sq = START_DISTANCE_SQ;
	vec2 nearest_offset = vec2(0.0, 1.0);

	// 3x3 is provably sufficient, and provably so only because of the two
	// clamps in the loop body. A crater's centre is clamped into its own
	// cell and its radius is clamped to at most MAX_RADIUS (0.38) cells, so
	// the furthest a crater can reach past its own cell is 0.38 cells. This
	// pixel's `local` lies in [0,1], so a crater two cells out is at least
	// 1.0 cells away - a normalized distance of at least 1/0.38 = 2.6,
	// which is past every threshold used below (the floor fades out by 0.9,
	// the rim band by 1.0, and the overlap suppression saturates at 1.15).
	// Such a crater therefore cannot change a single pixel of the output.
	//
	// This replaces an earlier 5x5 scan. That width was not chosen from the
	// geometry - 3x3 was already the geometric answer - but as insurance
	// against craters that were observed clipping at cell boundaries
	// on-device, which meant some crater's influence was reaching further
	// than its nominal radius allowed, e.g. a hash returning a value outside
	// [0,1] on that GPU and extrapolating mix() into a huge radius. Clamping
	// both the radius AND the centre fixes that at the source and makes the
	// bound above hold no matter what the hash returns, so paying for 25
	// samples per pixel to hide a driver quirk is no longer necessary.
	// tests/test_lunar_field.gd checks the two scans agree exactly.
	for (int dx = -1; dx <= 1; dx++) {
		for (int dy = -1; dy <= 1; dy++) {
			vec2 offset = vec2(float(dx), float(dy));
			// Wrapping the cell index before hashing is the entire trick that
			// makes the field tileable: cell N and cell N + tile_cells hash
			// identically, so the pattern repeats exactly with that period.
			// It works on a Worley field specifically because the scan only
			// ever reads immediate neighbours - wrap those consistently and
			// the seam is not merely hidden, it does not exist. GLSL mod()
			// returns a non-negative result for a positive divisor, so
			// negative cell indices wrap correctly too.
			highp vec2 neighbor_cell = mod(cell + offset, tile_cells);
			vec4 h = hash42(neighbor_cell);

			// Cells without a crater are pushed far away rather than
			// branched around, keeping the loop body uniform. NOTE the
			// sentinel is deliberately a small number, not something like
			// 1e9: mediump floats top out around 65504, so a huge literal
			// becomes +Inf there, and (1.0 - has_crater) * Inf evaluates to
			// 0 * Inf = NaN for every cell that DOES hold a crater - which
			// compares false against everything and would silently discard
			// every crater.
			float has_crater = step(0.35, h.z);
			// clamp() on both of these is what bounds a crater's reach to
			// its own cell plus MAX_RADIUS, which is what lets the scan
			// above be 3x3. Without them a driver returning a hash outside
			// [0,1] could place a centre, or inflate a radius, far enough to
			// reach past the scan - which is exactly the clipping that was
			// seen on-device.
			vec2 crater_center = offset + clamp(h.xy, 0.0, 1.0);
			float radius = clamp(mix(MIN_RADIUS, MAX_RADIUS, h.w), MIN_RADIUS, MAX_RADIUS);

			vec2 to_sample = local - crater_center;
			float normalized_sq = dot(to_sample, to_sample) / (radius * radius)
				+ (1.0 - has_crater) * MISS_DISTANCE_SQ;

			if (normalized_sq < nearest_sq) {
				second_nearest_sq = nearest_sq;
				nearest_sq = normalized_sq;
				// Offset from the winning crater's centre to this pixel,
				// kept so the rim's light/shadow split can be derived from
				// the SAME crater that won the distance test - the shading
				// can then never describe one crater while the floor
				// describes another.
				nearest_offset = to_sample;
			} else if (normalized_sq < second_nearest_sq) {
				second_nearest_sq = normalized_sq;
			}
		}
	}

	float nearest = sqrt(nearest_sq);
	float second_nearest = sqrt(second_nearest_sq);
	// normalize() would divide by zero for a pixel sitting exactly on a
	// crater centre; the guard keeps that pixel's rim term merely arbitrary
	// rather than NaN. It is deep inside the floor, where the rim band is
	// zero anyway.
	float offset_length = length(nearest_offset);
	vec2 nearest_dir = offset_length > 0.0001 ? nearest_offset / offset_length : vec2(0.0, 1.0);

	vec2 light_dir = normalize(LIGHT_DIR);
	float facing = dot(nearest_dir, light_dir);

	// Everything below is a pure function of `nearest` (plus the winning
	// crater's direction), so all three visuals stay mutually consistent.
	float floor_amount = 1.0 - smoothstep(0.45, 0.9, nearest);

	// Thin raised-rim band right at the crater's edge, split into a lit
	// half (facing LIGHT_DIR) and a shadowed half (facing away), giving a
	// sense of a raised lip catching light.
	float rim_band = smoothstep(0.55, 0.85, nearest) * (1.0 - smoothstep(0.85, 1.0, nearest));
	// Fade the rim out where this pixel also lies inside a second crater:
	// that is the overlap region, where a rim would be an internal crease
	// rather than a real outer lip. Craters there merge into one basin.
	rim_band *= smoothstep(0.85, 1.15, second_nearest);
	float rim_light = rim_band * clamp(facing, 0.0, 1.0);
	float rim_shadow = rim_band * clamp(-facing, 0.0, 1.0);

	vec3 color = base_color.rgb;
	color = mix(color, crater_color.rgb, floor_amount * 0.85);
	color = mix(color, highlight_color.rgb, rim_light * 0.6);
	color = mix(color, crater_color.rgb * 0.7, rim_shadow * 0.5);

	// Tiny per-pixel dither to mask 8-bit colour-quantization banding: the
	// crater shading is a smooth gradient over a fairly large area, which
	// can visibly "step" into faint bands on real hardware. A small random
	// jitter well under one visible step breaks the stepping up without
	// being perceptible as noise.
	// Skipped when baking: dither is a per-SCREEN-pixel jitter, and baking it
	// would freeze the noise into the texture, where magnification smears it
	// into visible blotches and tiling repeats it. lunar_tiled.gdshader
	// applies the identical term live instead, which is where it belongs.
	if (!baking) {
		float dither = (hash21(screen_px + vec2(7.0, 13.0)) - 0.5) / 128.0;
		color += vec3(dither);
	}

	// Diagnostic checkerboard: alternate cells tinted, plus a bright line
	// right at each cell edge. Makes the grid unmistakable in a screenshot.
	if (debug_mode > 0.5 && debug_mode < 1.5) {
		float checker = mod(cell.x + cell.y, 2.0);
		color = mix(color, vec3(0.10, 0.30, 0.55), checker * 0.35);
		vec2 edge = min(local, 1.0 - local);
		float on_edge = 1.0 - smoothstep(0.0, 0.012, min(edge.x, edge.y));
		color = mix(color, vec3(1.0, 0.25, 0.25), on_edge * 0.9);
	}

	// Bisection output: the field alone, with none of the shading above
	// applied. Godot forbids `return` inside fragment(), so this overrides
	// the computed colour rather than exiting early.
	if (debug_mode > 1.5) {
		color = vec3(clamp(nearest / 2.0, 0.0, 1.0));
	}

	COLOR = vec4(color, 1.0);
}