+further multithreading

I split the grid into 4 quadrants which are managed by their own
thread.  Improves speed and uses more resources correctly.

lib.h

@@ -12,9 +12,10 @@ typedef struct State
   size_t dwidth;
 
   size_t window_len;
-  int multiplier;
+  double multiplier;
 
   bool thread_alive;
+  uint64_t payload;
 } state_t;
 
 bool load_from_file(state_t *state, const char *filepath);

main.c

@@ -17,12 +17,22 @@
 
 #include "./lib.h"
 
-void step(state_t *state)
+struct StepArg
 {
-  for (size_t i = 0; i < state->dwidth; ++i)
-    for (size_t j = 0; j < state->dwidth; ++j)
+  state_t *state;
+  size_t x_min, x_max, y_min, y_max;
+};
+
+pthread_mutex_t mutex;
+
+void step(struct StepArg arg)
+{
+  state_t *state = arg.state;
+  for (size_t i = arg.x_min; i < arg.x_max; ++i)
+    for (size_t j = arg.y_min; j < arg.y_max; ++j)
       if (state->data[(i * state->dwidth) + j] >= 4)
       {
+        pthread_mutex_lock(&mutex);
         uint64_t *references[] = {
             (j == 0) ? NULL : &state->data[((i)*state->dwidth) + j - 1],
             (i == state->dwidth - 1)
@@ -36,29 +46,43 @@ void step(state_t *state)
           if (references[k])
             *references[k] += state->data[(i * state->dwidth) + j] / 4;
         state->data[(i * state->dwidth) + j] %= 4;
+        pthread_mutex_unlock(&mutex);
       }
 }
 
 void *compute_thread(void *input)
 {
-  state_t *state = input;
-  while (state->thread_alive)
-    step(state);
+  struct StepArg *arg = input;
+  while (arg->state->thread_alive)
+    step(*arg);
   return NULL;
 }
 
 int main(void)
 {
-  state_t state    = {NULL, 512, 512, 0, true};
+  state_t state    = {NULL, 512, 512, 0, true, pow(2, 20)};
   state.data       = calloc(state.dwidth * state.dwidth, sizeof(*state.data));
   state.multiplier = state.window_len / state.dwidth;
+  state.data[(state.dwidth * state.dwidth / 2) + (state.dwidth / 2)] =
+      state.payload;
 
   const float zoom = 0.125f;
   Camera2D camera  = {0};
   camera.zoom      = 1.0f;
 
-  pthread_t step_thread;
-  pthread_create(&step_thread, NULL, &compute_thread, &state);
+  pthread_mutex_init(&mutex, NULL);
+  struct StepArg a = {&state, 0, state.dwidth / 2, 0, state.dwidth / 2};
+  struct StepArg b = {&state, 0, state.dwidth / 2, state.dwidth / 2,
+                      state.dwidth};
+  struct StepArg c = {&state, state.dwidth / 2, state.dwidth, 0,
+                      state.dwidth / 2};
+  struct StepArg d = {&state, state.dwidth / 2, state.dwidth, state.dwidth / 2,
+                      state.dwidth};
+  pthread_t thread_a, thread_b, thread_c, thread_d;
+  pthread_create(&thread_a, NULL, &compute_thread, &a);
+  pthread_create(&thread_b, NULL, &compute_thread, &b);
+  pthread_create(&thread_c, NULL, &compute_thread, &c);
+  pthread_create(&thread_d, NULL, &compute_thread, &d);
 
   InitWindow(state.window_len, state.window_len, "Abelian sand pile");
   SetTargetFPS(60);
@@ -119,10 +143,14 @@ int main(void)
   if (state.thread_alive)
   {
     state.thread_alive = false;
-    pthread_join(step_thread, NULL);
+    pthread_join(thread_a, NULL);
+    pthread_join(thread_b, NULL);
+    pthread_join(thread_c, NULL);
+    pthread_join(thread_d, NULL);
   }
 
   CloseWindow();
+
   unsigned char *image_data =
       calloc(3 * state.dwidth * state.dwidth, sizeof(*image_data));