Implemented the program

.gitmodules

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+[submodule "submodules/utilitiec"]
+	path = submodules/utilitiec
+	url = https://git.jossco.de/VegOwOtenks/utilitiec.git

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.5.0)
+
+project(spiral-galaxy C)
+
+find_package(SDL3 REQUIRED CONFIG REQUIRED COMPONENTS SDL3-shared)
+
+add_subdirectory(submodules/)
+
+add_subdirectory(src/)

src/CMakeLists.txt

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+add_executable(spiral-galaxy WIN32 spiral-galaxy.c)
+
+target_link_libraries(spiral-galaxy PRIVATE SDL3::SDL3 rand)

src/spiral-galaxy.c

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+#include <SDL3/SDL_events.h>
+#include <SDL3/SDL_init.h>
+#include <SDL3/SDL_pixels.h>
+#include <SDL3/SDL_render.h>
+#include <SDL3/SDL_stdinc.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#define SDL_MAIN_USE_CALLBACKS
+#include <SDL3/SDL_main.h>
+
+#include <SDL3/SDL.h>
+
+#define WINDOW_SIZE 600
+#define SWIRLINESS 35
+#define ARM_COUNT 3
+#define STAR_COUNT 2000
+
+#include "../submodules/utilitiec/src/Vector/Vec2DF64.h"
+#include "../submodules/utilitiec/src/rand/xoshiro256.h"
+
+static SDL_Window *window = NULL;
+static SDL_Renderer *renderer = NULL;
+
+static SDL_Color star_colors[] = {
+	{ 188,  66,  73, 255},
+	{ 252, 182,  79, 255},
+	{ 248, 215,  84, 255},
+	{ 213, 221, 234, 255},
+	{  89, 159, 195, 255},
+};
+
+static uint64_t star_color_weights[] = {
+	3,
+	5,
+	8,
+	13,
+	21
+};
+
+typedef struct Star {
+	Vec2DF64 position;
+	SDL_Color color;
+} Star;
+
+static Star stars[STAR_COUNT];
+
+double SDL_norm2(double x, double y)
+{
+	return SDL_sqrt(SDL_pow(x, 2) + SDL_pow(y, 2));
+}
+
+double Galaxy_ArmDistance(double x, double y, double swirliness, size_t arm_count)
+{
+	double angle = SDL_atan2(0.5 - y, 0.5 - x);
+	double center_distance = SDL_norm2(x - 0.5, y - 0.5);
+	double arm_distance = SDL_sin(swirliness * center_distance + angle * arm_count) + 1;
+
+	return arm_distance / 2;
+}
+
+SDL_Color SDL_ColorMix(SDL_Color self, SDL_Color other)
+{
+	return (SDL_Color) {
+		.r = (self.r + other.r) / 2,
+		.g = (self.g + other.g) / 2,
+		.b = (self.b + other.b) / 2,
+		.a = (self.a + other.a) / 2,
+	};
+}
+
+SDL_Color SDL_ColorLerp(SDL_Color self, SDL_Color other, uint progress)
+{
+	uint inverse_progress = 100 - progress;
+	return (SDL_Color) {
+		.r = (self.r * inverse_progress + other.r * progress) / 100,
+		.g = (self.g * inverse_progress + other.g * progress) / 100,
+		.b = (self.b * inverse_progress + other.b * progress) / 100,
+		.a = (self.a * inverse_progress + other.a * progress) / 100,
+	};
+}
+
+
+size_t _SelectWeighed(uint64_t weights[], size_t weight_count, Xoshiro256State* state)
+{
+	uint64_t sum = 0;
+	for (size_t i = 0; i < weight_count; i++) sum += weights[i];
+
+	uint64_t chosen = xoshiro256_next(state) % sum;
+
+	for (size_t i = 0; i < weight_count; i++) {
+		if (chosen < weights[i]) return i;
+		else chosen -= weights[i];
+	}
+
+	return SIZE_MAX;
+}
+
+void GenerateStars()
+{
+	Xoshiro256State random_state = { 1, 2, 3, 4 };
+	for (size_t i = 0; i < SDL_arraysize(stars); i++) {
+		Star* star = stars + i;
+
+		double arm_distance;
+		uint64_t discard_probability;
+		do {
+			star->position.x = xoshiro256_next(&random_state) % WINDOW_SIZE;
+			star->position.y = xoshiro256_next(&random_state) % WINDOW_SIZE;
+			size_t color_index = _SelectWeighed(
+				star_color_weights, 
+				SDL_arraysize(star_color_weights), 
+				&random_state
+			);
+			star->color = star_colors[color_index];
+
+			arm_distance = Galaxy_ArmDistance(
+				star->position.x / WINDOW_SIZE,
+				star->position.y / WINDOW_SIZE,
+				SWIRLINESS,
+				ARM_COUNT
+			);
+			discard_probability = xoshiro256_next(&random_state) % 1000;
+		} while (arm_distance * 1000 > discard_probability);
+	}
+}
+
+void SDL_RenderStar(Star* star)
+{
+	SDL_SetRenderDrawColor(renderer, star->color.r, star->color.g, star->color.b, star->color.a);
+	SDL_RenderPoint(renderer, star->position.y, star->position.x);
+
+	SDL_Color white = { 255 };
+	SDL_Color adjacent_color = SDL_ColorLerp(star->color, white, 5); 
+	SDL_SetRenderDrawColor(
+		renderer,
+		adjacent_color.r,
+		adjacent_color.g,
+		adjacent_color.b,
+		adjacent_color.a
+	);
+	SDL_RenderPoint(renderer, star->position.y - 1, star->position.x);
+	SDL_RenderPoint(renderer, star->position.y + 1, star->position.x);
+	SDL_RenderPoint(renderer, star->position.y, star->position.x - 1);
+	SDL_RenderPoint(renderer, star->position.y, star->position.x + 1);
+}
+
+void SDL_RenderArms()
+{
+	for (size_t y = 0; y < WINDOW_SIZE; y++) {
+		double norm_y = (double) y / WINDOW_SIZE;
+		for (size_t x = 0; x < WINDOW_SIZE; x++) {
+			double norm_x = (double) x / WINDOW_SIZE;
+			double distance = Galaxy_ArmDistance(norm_x, norm_y, SWIRLINESS, ARM_COUNT);
+			uint progress = distance * 100;
+			SDL_Color black = { 0, 0, 0, 255 };
+			SDL_Color grey = { 31, 31, 31, 255 };
+			SDL_Color arm_color = SDL_ColorLerp(black, grey, 101 - progress);
+			SDL_SetRenderDrawColor(renderer, arm_color.r, arm_color.g, arm_color.b, arm_color.a);
+			SDL_RenderPoint(renderer, y, x);
+		}
+	}
+}
+
+SDL_AppResult SDL_AppInit(void **appstate, int argc, char **argv)
+{
+	GenerateStars();
+
+	if (!SDL_CreateWindowAndRenderer("Spiral Galaxy", WINDOW_SIZE, WINDOW_SIZE, 0, &window, &renderer)) {
+		SDL_Log("Couldn't create window and renderer: %s\n", SDL_GetError());
+		return SDL_APP_FAILURE;
+	}
+
+	SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0);
+	SDL_RenderClear(renderer);
+
+	SDL_RenderArms();
+	for (size_t i = 0; i < SDL_arraysize(stars); i++) {
+		SDL_RenderStar(stars + i);
+	}
+
+	SDL_RenderPresent(renderer);
+
+
+	return SDL_APP_CONTINUE;
+}
+
+SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
+{
+	switch (event->type) {
+		case SDL_EVENT_QUIT:
+			return SDL_APP_SUCCESS;
+
+		default:
+			break;
+	}
+
+	return SDL_APP_CONTINUE;
+}
+
+SDL_AppResult SDL_AppIterate(void *appstate)
+{
+	return SDL_APP_CONTINUE;
+}
+
+void SDL_AppQuit(void *appstate, SDL_AppResult result)
+{
+}

submodules/CMakeLists.txt

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+add_subdirectory(utilitiec/)

submodules/utilitiec

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+Subproject commit e8bea8924bf48dd025ae2968b3885b9d6a1bc523