yeah boy this sand be fallin

.gitignore

@@ -1,2 +1 @@
-.vscode
 *.o

.vscode/settings.json

@@ -0,0 +1,13 @@
+{
+  "makefile.launchConfigurations": [
+    {
+      "name": "sand_mangohud",
+      "cwd": "${workspaceFolder}",
+      "binaryPath": "/usr/bin/mangohud",            // use full path to be safe
+      "binaryArgs": [
+        "--dlsym",
+        "${workspaceFolder}/fallingCand"           
+      ]
+    }
+  ]
+}

Makefile

@@ -2,7 +2,7 @@ CC      = gcc
 CFLAGS  = -Wall -Wextra -O2 $(shell pkg-config --cflags glfw3)
 LDFLAGS = $(shell pkg-config --libs glfw3) -ldl
 
-SRC = glad.c gl_utils.c main.c
+SRC = glad.c gl_utils.c main.c sand_sim.c
 OBJ = $(SRC:.c=.o)
 BIN = fallingCand
 

gl_utils.c

@@ -110,6 +110,34 @@ GLuint create_program_from_files(const char* vsPath, const char* fsPath) {
 
     return prog;
 }
+GLuint create_compute_program_from_file(const char* path) {
+    char* source = load_text_file(path);
+    if (!source) {
+        fprintf(stderr, "Could not load compute shader from '%s'\n", path);
+        exit(EXIT_FAILURE);
+    }
+
+    // Reuse compile_shader; type is GL_COMPUTE_SHADER
+    GLuint cs = compile_shader(GL_COMPUTE_SHADER, source, path);
+    free(source);
+
+    GLuint prog = glCreateProgram();
+    glAttachShader(prog, cs);
+    glLinkProgram(prog);
+    glDeleteShader(cs);
+
+    GLint linked = 0;
+    glGetProgramiv(prog, GL_LINK_STATUS, &linked);
+    if (!linked) {
+        char log[1024];
+        glGetProgramInfoLog(prog, sizeof(log), NULL, log);
+        fprintf(stderr, "Compute program link failed (%s):\n%s\n", path, log);
+        glDeleteProgram(prog);
+        exit(EXIT_FAILURE);
+    }
+
+    return prog;
+}
 
 GLFWwindow* init_glfw_glad(const char* title, int width, int height) {
     glfwSetErrorCallback(glfw_error_callback);

gl_utils.h

@@ -18,7 +18,7 @@ GLuint compile_shader(GLenum type, const char* source, const char* debugName);
 
 // Compile & link a vertex+fragment program from files.
 GLuint create_program_from_files(const char* vsPath, const char* fsPath);
-
+GLuint create_compute_program_from_file(const char* path);
 // Minimal GLFW+GLAD init: sets error callback, (optionally) hints Wayland,
 // calls glfwInit, creates window, makes context current, loads GLAD.
 // Returns the created window or NULL on fatal error.

main.c

@@ -3,8 +3,40 @@
 #include <stdlib.h>
 
 #include "gl_utils.h"
+#include "sand_sim.h"
+#define GRID_W 1024
+#define GRID_H 1024
 static int g_fbWidth = 800;
 static int g_fbHeight = 800;
+SandSim sim;
+
+static void render_sand(SandSim* sim,
+                        GLuint program,
+                        GLuint vao,
+                        int fbW,
+                        int fbH) {
+    glUseProgram(program);
+
+    GLint uResLoc = glGetUniformLocation(program, "u_resolution");
+    GLint uGridLoc = glGetUniformLocation(program, "u_gridSize");
+    GLint uStateLoc = glGetUniformLocation(program, "u_state");
+
+    if (uResLoc >= 0) {
+        glUniform2f(uResLoc, (float)fbW, (float)fbH);
+    }
+    if (uGridLoc >= 0) {
+        glUniform2i(uGridLoc, sim->gridW, sim->gridH);
+    }
+
+    if (uStateLoc >= 0) {
+        glActiveTexture(GL_TEXTURE0);
+        glBindTexture(GL_TEXTURE_2D, sim->tex_curr);
+        glUniform1i(uStateLoc, 0);
+    }
+
+    glBindVertexArray(vao);
+    glDrawArrays(GL_TRIANGLES, 0, 3);
+}
 
 int main(void) {
     GLFWwindow* window = init_glfw_glad("Falling Sand - Fullscreen Quad", g_fbWidth, g_fbHeight);
@@ -44,22 +76,37 @@ int main(void) {
         (void*)0);
 
     glBindVertexArray(0);
-
+    if (!sand_init(&sim, GRID_W, GRID_H, "shaders/sand_step.comp")) {
+        fprintf(stderr, "Failed to initialize sand sim\n");
+        return EXIT_FAILURE;
+    }
     // --- Create shader program ---
     GLuint program = create_program_from_files(
         "shaders/fullscreen.vert",
-        "shaders/gradient.frag");
-    GLint uResLoc = glGetUniformLocation(program, "u_resolution");
-    if (uResLoc == -1) {
-        fprintf(stderr, "[warn] u_resolution uniform not found (maybe optimized out?)\n");
-    }
+        "shaders/sand_display.frag");
+
     glUseProgram(program);
 
     glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
+    double sim_dt = 1.0 / 120.0;
+    double currentTime = glfwGetTime();
+    double accumulator = 0.0;
     
     // --- Main loop ---
     while (!glfwWindowShouldClose(window)) {
+        double newTime = glfwGetTime();
+        double frameTime = newTime - currentTime;
+        currentTime = newTime;
+        if (frameTime > 0.25) frameTime = 0.25;
+        accumulator += frameTime;
+        
+        
+        while (accumulator >= sim_dt) {
+            sand_step_gpu(&sim);  // one discrete CA step on the GPU (commented bc unimplemeted)
+            accumulator -= sim_dt;
+        }
         glfwPollEvents();
+
         // Re-query framebuffer size each frame (cheap and simple)
         int fbw, fbh;
         glfwGetFramebufferSize(window, &fbw, &fbh);
@@ -70,13 +117,7 @@ int main(void) {
         }
         glClear(GL_COLOR_BUFFER_BIT);
 
-        glUseProgram(program);
-        if (uResLoc != -1) {
-            glUniform2f(uResLoc, (float)g_fbWidth, (float)g_fbHeight);
-        }
-        glBindVertexArray(vao);
-        glDrawArrays(GL_TRIANGLES, 0, 3);
-
+        render_sand(&sim, program, vao, g_fbWidth, g_fbHeight);
         glfwSwapBuffers(window);
     }
 

sand_sim.c

@@ -0,0 +1,134 @@
+// sand_sim.c
+#include "sand_sim.h"
+#include "gl_utils.h"   // for create_compute_program_from_file
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+static void init_textures(SandSim* sim) {
+    // tex_curr
+    glGenTextures(1, &sim->tex_curr);
+    glBindTexture(GL_TEXTURE_2D, sim->tex_curr);
+    glTexStorage2D(GL_TEXTURE_2D, 1, GL_R8UI, sim->gridW, sim->gridH);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+
+    // tex_next
+    glGenTextures(1, &sim->tex_next);
+    glBindTexture(GL_TEXTURE_2D, sim->tex_next);
+    glTexStorage2D(GL_TEXTURE_2D, 1, GL_R8UI, sim->gridW, sim->gridH);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+
+    // Unbind
+    glBindTexture(GL_TEXTURE_2D, 0);
+}
+
+static void upload_initial_state(SandSim* sim) {
+    size_t count = (size_t)sim->gridW * (size_t)sim->gridH;
+    uint8_t* data = calloc(count, 1);
+    if (!data) {
+        fprintf(stderr, "[sand_init] Out of memory for initial grid\n");
+        exit(EXIT_FAILURE);
+    }
+
+    // Seed RNG once (safe enough for now)
+    static bool seeded = false;
+    if (!seeded) {
+        seeded = true;
+        srand((unsigned int)time(NULL));
+    }
+
+    // Fill top 1/3 of grid with random sand vs air
+    for (int y = 0; y < sim->gridH / 3; ++y) {
+        for (int x = 0; x < sim->gridW; ++x) {
+            data[(size_t)y * sim->gridW + x] = (rand() & 1) ? 1 : 0;  // 0=air,1=sand
+        }
+    }
+
+    // Upload into tex_curr and tex_next
+    glBindTexture(GL_TEXTURE_2D, sim->tex_curr);
+    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, sim->gridW, sim->gridH,
+                    GL_RED_INTEGER, GL_UNSIGNED_BYTE, data);
+
+    glBindTexture(GL_TEXTURE_2D, sim->tex_next);
+    glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, sim->gridW, sim->gridH,
+                    GL_RED_INTEGER, GL_UNSIGNED_BYTE, data);
+
+    glBindTexture(GL_TEXTURE_2D, 0);
+
+    free(data);
+}
+
+bool sand_init(SandSim* sim, int gridW, int gridH, const char* computeShaderPath) {
+    if (!sim || gridW <= 0 || gridH <= 0 || !computeShaderPath) {
+        fprintf(stderr, "[sand_init] Invalid arguments\n");
+        return false;
+    }
+
+    sim->gridW = gridW;
+    sim->gridH = gridH;
+    sim->tex_curr = 0;
+    sim->tex_next = 0;
+    sim->prog_sim = 0;
+
+    init_textures(sim);
+    upload_initial_state(sim);
+
+    sim->prog_sim = create_compute_program_from_file(computeShaderPath);
+    if (!sim->prog_sim) {
+        fprintf(stderr, "[sand_init] Failed to create compute program\n");
+        return false;
+    }
+
+    return true;
+}
+
+void sand_step_gpu(SandSim* sim) {
+    if (!sim || !sim->prog_sim) return;
+
+    glUseProgram(sim->prog_sim);
+
+    // Set grid size uniform
+    GLint uGridLoc = glGetUniformLocation(sim->prog_sim, "u_gridSize");
+    if (uGridLoc >= 0) {
+        glUniform2i(uGridLoc, sim->gridW, sim->gridH);
+    }
+
+    // Bind images (must match bindings in sand_step.comp)
+    glBindImageTexture(0, sim->tex_curr, 0, GL_FALSE, 0, GL_READ_ONLY,  GL_R8UI);
+    glBindImageTexture(1, sim->tex_next, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R8UI);
+
+    GLuint groupsX = (sim->gridW + 15) / 16;
+    GLuint groupsY = (sim->gridH + 15) / 16;
+
+    glDispatchCompute(groupsX, groupsY, 1);
+    glMemoryBarrier(GL_SHADER_IMAGE_ACCESS_BARRIER_BIT);
+
+    // Ping-pong
+    GLuint tmp = sim->tex_curr;
+    sim->tex_curr = sim->tex_next;
+    sim->tex_next = tmp;
+}
+
+void sand_destroy(SandSim* sim) {
+    if (!sim) return;
+
+    if (sim->tex_curr) {
+        glDeleteTextures(1, &sim->tex_curr);
+        sim->tex_curr = 0;
+    }
+    if (sim->tex_next) {
+        glDeleteTextures(1, &sim->tex_next);
+        sim->tex_next = 0;
+    }
+    if (sim->prog_sim) {
+        glDeleteProgram(sim->prog_sim);
+        sim->prog_sim = 0;
+    }
+}

sand_sim.h

@@ -0,0 +1,27 @@
+// sand_sim.h
+#ifndef SAND_SIM_H
+#define SAND_SIM_H
+
+#include <stdbool.h>
+#include "glad/glad.h"
+
+// Simple binary sand vs air simulation
+typedef struct {
+    GLuint tex_curr;   // R8UI; 0 = air, 1 = sand
+    GLuint tex_next;   // R8UI; ping-pong target
+    GLuint prog_sim;   // compute shader program
+    int gridW;
+    int gridH;
+} SandSim;
+
+// Initialize sim, allocate textures, upload initial state, load compute shader.
+// Returns true on success, false on failure.
+bool sand_init(SandSim* sim, int gridW, int gridH, const char* computeShaderPath);
+
+// Advance simulation by 1 discrete tick (one CA step).
+void sand_step_gpu(SandSim* sim);
+
+// Destroy all GL objects owned by the sim.
+void sand_destroy(SandSim* sim);
+
+#endif // SAND_SIM_H

shaders/sand_display.frag

@@ -0,0 +1,26 @@
+#version 430 core
+
+out vec4 FragColor;
+
+uniform vec2  u_resolution;   // framebuffer size in pixels
+uniform ivec2 u_gridSize;     // sand grid size
+uniform usampler2D u_state;   // GL_R8UI texture
+
+void main() {
+    vec2 fragCoord = gl_FragCoord.xy;
+
+    // Normalized coordinates
+    vec2 uv = fragCoord / u_resolution;
+    uv.y = 1.0 - uv.y;
+    // Map to grid coordinates
+    ivec2 cell = ivec2(uv * vec2(u_gridSize));
+    cell = clamp(cell, ivec2(0), u_gridSize - ivec2(1));
+
+    uint v = texelFetch(u_state, cell, 0).r;
+
+    vec3 color = (v == 1u)
+        ? vec3(0.9, 0.8, 0.1)        // sand
+        : vec3(0.05, 0.05, 0.10);    // background
+
+    FragColor = vec4(color, 1.0);
+}

shaders/sand_step.comp

@@ -0,0 +1,42 @@
+#version 430 core
+
+layout(local_size_x = 16, local_size_y = 16) in;
+
+layout(r8ui, binding = 0) readonly  uniform uimage2D state_curr;
+layout(r8ui, binding = 1) writeonly uniform uimage2D state_next;
+
+uniform ivec2 u_gridSize;
+
+void main() {
+    ivec2 pos = ivec2(gl_GlobalInvocationID.xy);
+    if (pos.x >= u_gridSize.x || pos.y >= u_gridSize.y) {
+        return;
+    }
+
+    const uint AIR  = 0u;
+    const uint SAND = 1u;
+
+    uint self = imageLoad(state_curr, pos).r;
+
+    bool canFallDown =
+        (self == SAND) &&
+        (pos.y + 1 < u_gridSize.y) &&
+        (imageLoad(state_curr, pos + ivec2(0, 1)).r == AIR);
+
+    bool filledFromAbove =
+        (self == AIR) &&
+        (pos.y > 0) &&
+        (imageLoad(state_curr, pos + ivec2(0, -1)).r == SAND);
+
+    uint next;
+
+    if (canFallDown) {
+        next = AIR;
+    } else if (filledFromAbove) {
+        next = SAND;
+    } else {
+        next = self;
+    }
+
+    imageStore(state_next, pos, uvec4(next, 0u, 0u, 0u));
+}