shader_type canvas_item;

// Backdrop for a landed enemy research lab.
//
// The other surfaces are NATURAL: lunar_surface.gdshader erodes craters,
// gas_surface.gdshader bands clouds. A research lab is the one place in the
// game that is BUILT, and it was previously borrowing the crater field with
// a dark red palette - which read as "rocky planet, tinted", not as walking
// onto someone's facility. So this draws a plated floor instead: panel
// seams, service conduits running through them, and lit junctions where two
// conduits cross.
//
// Deliberately loop-free. lunar_surface.gdshader needs a neighbour scan
// because a crater overlaps its cell's borders; nothing here does, so every
// feature is a function of this pixel's own cell and the shader stays a
// handful of hashes per pixel (#68's lesson, applied up front rather than
// after the fact).
//
// The conduits still have to run CONTINUOUSLY across cell borders, which is
// what a neighbour scan would normally buy. The trick is to make a
// horizontal run's existence and height depend only on cell.y, and a
// vertical run's on cell.x: every cell in a row then independently agrees on
// the same answer, so the line joins up without any cell ever looking at
// its neighbours.

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;
// Shared with the lunar background so the same camera feed drives both, and
// so the field repeats on the same period. See LunarBackground.gd.
uniform float tile_cells = 32.0;
uniform vec4 base_color : source_color = vec4(0.16, 0.12, 0.12, 1.0);
uniform vec4 crater_color : source_color = vec4(0.05, 0.03, 0.03, 1.0);
uniform vec4 highlight_color : source_color = vec4(0.75, 0.20, 0.15, 1.0);

// Plates per cell, across and down.
const float SUBPANELS = 3.0;
const float SEAM_WIDTH = 0.05;
const float CONDUIT_WIDTH = 0.055;
// How many rows/columns carry a conduit. Low enough that the floor stays
// mostly plating, high enough that a screen usually shows a junction.
const float CONDUIT_CHANCE = 0.38;

float hash11(float p) {
	vec3 p3 = fract(vec3(p) * vec3(0.1031, 0.1030, 0.0973));
	p3 += dot(p3, p3.yzx + 33.33);
	return fract((p3.x + p3.y) * p3.z);
}

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;

	// Same camera-relative construction as every other background here: an
	// absolute world position reaches the tens of thousands, where a mobile
	// float has already lost the fractional bits this needs. camera_cell
	// arrives pre-wrapped from GDScript's double precision.
	vec2 rel = camera_frac + centered * (camera_zoom / cell_size);
	vec2 cell_rel = floor(rel);
	vec2 local = rel - cell_rel;
	vec2 cell = mod(camera_cell + cell_rel, tile_cells);

	// --- plating ---------------------------------------------------------
	vec2 sub = local * SUBPANELS;
	vec2 sub_cell = floor(sub);
	vec2 sub_local = sub - sub_cell;
	vec2 edge = min(sub_local, 1.0 - sub_local);
	float seam = 1.0 - smoothstep(0.0, SEAM_WIDTH, min(edge.x, edge.y));
	// A little per-plate variation, so a large floor does not read as one
	// flat fill: plates are stamped, not poured.
	float plate = hash21(cell * SUBPANELS + sub_cell);

	vec3 color = mix(base_color.rgb, crater_color.rgb, plate * 0.40);
	color = mix(color, crater_color.rgb, seam * 0.85);

	// --- conduits --------------------------------------------------------
	// Existence and offset keyed to a SINGLE axis of the cell index, which
	// is what makes a run continuous across every cell it passes through.
	float has_h = step(1.0 - CONDUIT_CHANCE, hash11(cell.y + 0.5));
	float has_v = step(1.0 - CONDUIT_CHANCE, hash11(cell.x + 13.7));
	float run_y = 0.25 + 0.5 * hash11(cell.y + 7.3);
	float run_x = 0.25 + 0.5 * hash11(cell.x + 3.1);

	float h_band = has_h * (1.0 - smoothstep(0.0, CONDUIT_WIDTH, abs(local.y - run_y)));
	float v_band = has_v * (1.0 - smoothstep(0.0, CONDUIT_WIDTH, abs(local.x - run_x)));

	// Charge travelling along the run. Phase is built from cell + local, a
	// continuous coordinate, so the pulse crosses cell borders unbroken
	// rather than restarting inside every cell.
	float pulse_h = 0.55 + 0.45 * sin((cell.x + local.x) * 2.4 - TIME * 1.6);
	float pulse_v = 0.55 + 0.45 * sin((cell.y + local.y) * 2.4 - TIME * 1.6);
	float conduit = max(h_band * pulse_h, v_band * pulse_v);
	// A dark channel under the glow, so a conduit reads as recessed into the
	// floor rather than painted on top of it.
	color = mix(color, crater_color.rgb, max(h_band, v_band) * 0.55);
	color = mix(color, highlight_color.rgb, conduit * 0.75);

	// Junction box wherever a horizontal and a vertical run cross.
	float junction = has_h * has_v * (1.0 - smoothstep(0.0, CONDUIT_WIDTH * 2.4,
		length(vec2(local.x - run_x, local.y - run_y))));
	color = mix(color, highlight_color.rgb, junction * 0.9);

	// Same dither as the other backgrounds: the gradients here are shallow
	// and dark, which is exactly where 8-bit quantization bands visibly.
	float dither = (hash21(screen_px + vec2(7.0, 13.0)) - 0.5) / 128.0;
	color += vec3(dither);

	COLOR = vec4(color, 1.0);
}