Add function to compare the priority of two semaphores

src/threads/synch.c

@@ -265,6 +265,27 @@ struct semaphore_elem
     struct semaphore semaphore;         /* This semaphore. */
   };
 
+/* Function that compares the two *semaphores* associated with the provided
+   list_elem structures. [i.e., takes list_elem of semaphore_elem, and]
+   Returns true if the thread associated with the semaphore associated with a_
+   has a higher priority than that of b_. */
+static bool
+sema_priority_more(const struct list_elem *a_, const struct list_elem *b_,
+                   void *aux UNUSED)
+{
+  struct semaphore_elem *a = list_entry(a_, struct semaphore_elem, elem);
+  struct semaphore_elem *b = list_entry(b_, struct semaphore_elem, elem);
+
+  /* Get the highest priority thread from the waiters list of each semaphore. By
+     design, this is the first element in the list (See sema_down). */
+  struct thread *a_thread =
+    list_entry(list_front(&a->semaphore.waiters), struct thread, elem);
+  struct thread *b_thread =
+    list_entry(list_front(&b->semaphore.waiters), struct thread, elem);
+
+  return a_thread->priority > b_thread->priority;
+}
+
 /* Initializes condition variable COND.  A condition variable
    allows one piece of code to signal a condition and cooperating
    code to receive the signal and act upon it. */