[BugFix] Fix `MinPLogitsProcessor.update_states()` (#23401)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/v1/sample/logits_processor/builtin.py

@@ -53,29 +53,37 @@ class MinPLogitsProcessor(LogitsProcessor):
         # Process added requests.
         for index, params, _, _ in batch_update.added:
             min_p = params.min_p
-            if self.min_p_cpu[index] != min_p:
+            min_p_before = self.min_p_cpu[index]
+            if min_p_before != min_p:
                 needs_update = True
                 self.min_p_cpu[index] = min_p
-            if min_p:
-                self.min_p_count += 1
+                if min_p and not min_p_before:
+                    self.min_p_count += 1
+                elif not min_p and min_p_before:
+                    self.min_p_count -= 1
 
         if self.min_p_count:
             # Process removed requests.
-            needs_update |= bool(batch_update.removed)
-            for index in batch_update.removed:
-                if self.min_p_cpu[index]:
-                    self.min_p_count -= 1
+            if batch_update.removed:
+                needs_update = True
+                for index in batch_update.removed:
+                    if self.min_p_cpu[index]:
+                        self.min_p_cpu[index] = 0
+                        self.min_p_count -= 1
 
-            # Process moved requests, unidirectional (a->b) and swap (a<->b)
+            # Process moved requests, unidirectional (a->b) and swap (a<->b).
             for adx, bdx, direct in batch_update.moved:
-                change = (min_p_a :=
-                          self.min_p_cpu[adx]) != (min_p_b :=
-                                                   self.min_p_cpu[bdx])
-                needs_update |= change
-                if change:
+                min_p_a, min_p_b = self.min_p_cpu[adx], self.min_p_cpu[bdx]
+                if min_p_a != min_p_b:
+                    needs_update = True
                     self.min_p_cpu[bdx] = min_p_a
                     if direct == MoveDirectionality.SWAP:
                         self.min_p_cpu[adx] = min_p_b
+                if direct == MoveDirectionality.UNIDIRECTIONAL:
+                    if min_p_a:
+                        self.min_p_cpu[adx] = 0
+                    if min_p_b:
+                        self.min_p_count -= 1
 
         # Update tensors if needed.
         size = batch_update.batch_size