add shortcircuit of all-zero for collection shrinker

Author: tybug

hypothesis-python/RELEASE.rst

@@ -0,0 +1,3 @@
+RELEASE_TYPE: patch
+
+This patch improves shrinking involving long strings or byte sequences whose value is not relevant to the failure.

hypothesis-python/src/hypothesis/internal/conjecture/shrinker.py

@@ -1362,12 +1362,14 @@ def minimize_nodes(self, nodes):
             Bytes.shrink(
                 value,
                 lambda val: self.try_shrinking_nodes(nodes, val),
+                min_size=kwargs["min_size"],
             )
         elif ir_type == "string":
             String.shrink(
                 value,
                 lambda val: self.try_shrinking_nodes(nodes, val),
                 intervals=kwargs["intervals"],
+                min_size=kwargs["min_size"],
             )
         else:
             raise NotImplementedError

hypothesis-python/src/hypothesis/internal/conjecture/shrinking/collection.py

@@ -14,14 +14,23 @@
 
 
 class Collection(Shrinker):
-    def setup(self, *, ElementShrinker, to_order=identity, from_order=identity):
+    def setup(
+        self, *, ElementShrinker, to_order=identity, from_order=identity, min_size
+    ):
         self.ElementShrinker = ElementShrinker
         self.to_order = to_order
         self.from_order = from_order
+        self.min_size = min_size
 
     def make_immutable(self, value):
         return tuple(value)
 
+    def short_circuit(self):
+        zero = self.from_order(0)
+        success = self.consider([zero] * len(self.current))
+        # we could still simplify by deleting elements (unless we're minimal size).
+        return success and len(self.current) == self.min_size
+
     def left_is_better(self, left, right):
         if len(left) < len(right):
             return True

hypothesis-python/tests/conjecture/test_engine.py

@@ -200,13 +200,6 @@ def nodes(data):
 
 
 def test_draw_to_overrun(monkeypatch):
-    # TODO_BETTER_SHRINK: sometimes we can get unlucky and fail to shrink the
-    # initial size draw d to 2 before shrinking the 128 * d bytes, but I'm not
-    # sure why.
-    #
-    # If we do get unlucky in such a way then we need more than 500 shrinks to finish.
-    monkeypatch.setattr(engine_module, "MAX_SHRINKS", 1000)
-
     @run_to_nodes
     def nodes(data):
         d = (data.draw_bytes(1, 1)[0] - 8) & 0xFF

hypothesis-python/tests/conjecture/test_minimizer.py

@@ -72,7 +72,9 @@ def test_can_sort_bytes_by_reordering_partially_not_cross_stationary_element():
     ],
 )
 def test_shrink_strings(initial, predicate, intervals, expected):
-    assert String.shrink(initial, predicate, intervals=intervals) == tuple(expected)
+    assert String.shrink(
+        initial, predicate, intervals=intervals, min_size=len(expected)
+    ) == tuple(expected)
 
 
 @pytest.mark.parametrize(
@@ -85,9 +87,11 @@ def test_shrink_strings(initial, predicate, intervals, expected):
     ],
 )
 def test_shrink_bytes(initial, predicate, expected):
-    assert bytes(Bytes.shrink(initial, predicate)) == expected
+    assert bytes(Bytes.shrink(initial, predicate, min_size=len(expected))) == expected
 
 
 def test_collection_left_is_better():
-    shrinker = Collection([1, 2, 3], lambda v: True, ElementShrinker=Integer)
+    shrinker = Collection(
+        [1, 2, 3], lambda v: True, ElementShrinker=Integer, min_size=3
+    )
     assert not shrinker.left_is_better([1, 2, 3], [1, 2, 3])

hypothesis-python/tests/conjecture/test_shrinker.py

@@ -518,3 +518,16 @@ def shrinker(data: ConjectureData):
     # shrinking. Since the second draw is forced, this isn't possible to shrink
     # with just this pass.
     assert shrinker.choices == (15, 10)
+
+
+@pytest.mark.parametrize("n", [10, 50, 100, 200])
+def test_can_quickly_shrink_to_trivial_collection(n):
+    @shrinking_from(ir(b"\x01" * n))
+    def shrinker(data: ConjectureData):
+        b = data.draw_bytes()
+        if len(b) >= n:
+            data.mark_interesting()
+
+    shrinker.fixate_shrink_passes(["minimize_individual_nodes"])
+    assert shrinker.choices == (b"\x00" * n,)
+    assert shrinker.calls < 10