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Concurrent Systems/notes/4c - Dining Philosophers.md

@@ -67,4 +67,32 @@ In this case we break the symmetry by letting an odd number of philosophers sit
 
 #### Solution 4 - Monitors!
 Pick up 2 chopsticks only if both are free
-a philosopher moves to his/her eating state only if both neighbors are not in their eating states à need to define a state for each philosopher • if one of my neighbors is eating, and I’m hungry, ask them to signal me when they’re done à thus, states of each philosopher are: thinking, hungry, eating à need condition variables to signal waiting hungry philosopher(s) This solutoin very well fits with the features of monitors!
+- a philosopher moves to his/her eating state only if both neighbors are not in their eating states
+	- need to define a state for each philosopher
+- if one of my neighbors is eating, and I’m hungry, ask them to signal me when they’re done
+- thus, states of each philosopher are: thinking, hungry, eating
+	- need condition variables to signal waiting hungry philosopher(s)
+	
+```
+monitor DP
+	status state[N] all initialized at thinking;
+	condition self[N];
+
+Pickup(i) :=
+	state[i] = hungry;
+	test(i);
+	if (state[i] != eating) then
+		self[i].wait;
+
+Putdown(i) :=
+	state[i] = thinking;
+	test((i+1)%N);
+	test((i-1)%N);
+
+test(i) :=
+	if (state[(i+1)%N] != eating
+		&& state[(i-1)%N] != eating
+		&& state[i] == hungry) then
+			state[i] = eating;
+			self[i].signal();
+```