(main)+state data structure and iteration logic

Per iteration, increment the centre of the grid, and perform any
collapsing necessary (if >= 4 then sand pile "collapses" onto its
cardinal directions).

main.c

@@ -3,22 +3,67 @@
  * Author: Aryadev Chavali
  * Description: Entry point of program
  */
+#include <malloc.h>
 #include <stdio.h>
 
 #include <raylib.h>
 
-#define WIDTH  512
+struct State
-#define HEIGHT 512
+{
+  // Sandpiles
+  unsigned char *data;
+  size_t dwidth;
+
+  size_t window_len;
+  int multiplier;
+};
+
+typedef struct State state_t;
+
+void step(state_t *state)
+{
+  state->data[((state->dwidth / 2) * state->dwidth) + (state->dwidth / 2)]++;
+  for (size_t i = 0; i < state->dwidth; ++i)
+    for (size_t j = 0; j < state->dwidth; ++j)
+      if (state->data[(i * state->dwidth) + j] >= 4)
+      {
+        unsigned char *references[] = {
+            (j == 0) ? NULL : &state->data[((i)*state->dwidth) + j - 1],
+            (i == state->dwidth - 1)
+                ? NULL
+                : &state->data[((i + 1) * state->dwidth) + j],
+            (j == state->dwidth - 1)
+                ? NULL
+                : &state->data[(i * state->dwidth) + j + 1],
+            (i == 0) ? NULL : &state->data[((i - 1) * state->dwidth) + j]};
+        for (size_t k = 0; k < 4; ++k)
+          if (references[k])
+            *references[k] += 1;
+        state->data[(i * state->dwidth) + j] = 0;
+      }
+}
 
 int main(void)
 {
-  InitWindow(WIDTH, HEIGHT, "Abelian sand pile");
+  state_t state    = {NULL, 256, 512, 0};
+  state.data       = calloc(state.dwidth * state.dwidth, sizeof(*state.data));
+  state.multiplier = state.window_len / state.dwidth;
+
+  InitWindow(state.window_len, state.window_len, "Abelian sand pile");
   SetTargetFPS(60);
   while (!WindowShouldClose())
   {
+    step(&state);
     BeginDrawing();
     ClearBackground(BLACK);
     DrawText("Hello, world!", 100, 100, 25, RAYWHITE);
+
+    for (size_t i = 0; i < state.dwidth; ++i)
+      for (size_t j = 0; j < state.dwidth; ++j)
+      {
+        DrawRectangle(i * state.multiplier, j * state.multiplier,
+                      state.multiplier, state.multiplier, BLACK);
+      }
     EndDrawing();
   }
   CloseWindow();