Allow difficulty to be changed sooner

pool/difficulty_adjustment.py

@@ -23,7 +23,17 @@ def get_new_difficulty(
 
     # If we recently updated difficulty, don't update again
     if any(difficulty != current_difficulty for timestamp, difficulty in recent_partials):
-        return current_difficulty
+
+        before_last_time = last_time = recent_partials[0][0]
+        last_diff = recent_partials[0][1]
+        for partial in recent_partials:
+            if partial[1] != last_diff:
+                before_last_time = partial[0]
+                break
+
+        # Do not allow difficulty changes in less than 2 hours
+        if last_time - before_last_time < 2 * 3600:
+            return current_difficulty
 
     # Lower the difficulty if we are really slow since our last partial
     last_timestamp = recent_partials[0][0]

tests/test_difficulty.py

@@ -27,6 +27,20 @@ def test_recently_updated(self):
 
         assert get_new_difficulty(partials, num_partials, time_target, 50, 'MEDIUM', current_time, 1) == 50
 
+    def test_recently_updated_2h(self):
+        num_partials = 300
+        time_target = 24 * 3600
+        partials = []
+        current_time = uint64(time.time())
+        for i in range(num_partials):
+            if i < 50:
+                diff = 30
+            else:
+                diff = 20
+            partials.append((current_time - i * 200, diff))
+
+        assert get_new_difficulty(partials, num_partials, time_target, 20, 'MEDIUM', current_time, 1) == 28
+
     def test_really_slow(self):
         num_partials = 300
         time_target = 24 * 3600
@@ -58,9 +72,10 @@ def test_not_enough_partials_yet(self):
         partials = []
         current_time = uint64(time.time())
         for i in range(num_partials):
-            partials.append((uint64(current_time - (i) * 200), 20))
+            partials.append((uint64(current_time - i * 200), 20))
 
-        partials[-1] = (partials[-1][0], 15)
+        # Difficulty changed less than 2 hours ago, so do not change again
+        partials[20] = (partials[20][0], 15)
 
         # Doesn't change diff
         assert get_new_difficulty(partials, num_partials, time_target, 20, 'MEDIUM', current_time, 1) == 20