beam: Q94 bullets 2/3/4 -- three cells that each kill a named mutant

Rebuild test_no_path_compression_find_stable on 4 nodes so the node it asks
about (3) sits two levels down under union-by-size; the old cell built
union(0,1);union(1,2) -> parent [0,0,0,0], so find(2) rewrote nothing and a
path-compressing find passed the cell that names the mechanism.

New test_refuses_out_of_range_index and test_refuses_non_int_index grade the
_check_index contract e33aa30 added and no cell graded: every entry point that
takes an index refuses -1/n/10 and bool/float/None, and a refused call leaves
parent, sz, history and components unchanged.

Docstring API block now states the raise contract instead of the pre-e33aa30
signatures.

Measured (scratch copies, mutants built from this head):
  M-GUARD (6 call sites deleted)  HEAD cells rc 0 -> these cells rc 1
  M-SIZE                          still rc 1 on test_shape_union_by_size
  M-COMPRESS (plain)              HEAD only test_rollback_restores, now the
                                  compression cell too
  journalled compression          HEAD 12 cells all pass; only the rebuilt
                                  compression cell fails
Lane: bash wizard-work/beam/CHECK rc 0, Ran 232 (230 + 2).

wizard-work/beam/dsu_rollback.py

@@ -6,13 +6,16 @@ cheaply reversed. This implementation uses union by size and records enough
 state to undo each union in O(1).
 
 API:
-  DSU(n)
-  find(x) -> root (no path compression)
-  union(a, b) -> bool  (True if merged)
+  DSU(n)                  n < 0 -> ValueError
+  find(x) -> root         no path compression; TypeError if x is not an int or
+                          is a bool, ValueError if x is outside [0, n)
+  union(a, b) -> bool     True if merged; both arguments are checked as above
+                          and a refused call mutates nothing
   snapshot() -> int (marker)
-  rollback(marker) -> None (revert to marker)
-  size(x) -> component size
-  connected(a, b) -> bool
+  rollback(marker) -> None  revert to marker; TypeError if marker is not an int
+                          or is a bool, ValueError if marker outside [0, len(history)]
+  size(x) -> component size     index checked as find
+  connected(a, b) -> bool      both indexes checked as find
 """
 from typing import List, Tuple
 

wizard-work/beam/test_dsu_rollback.py

@@ -61,12 +61,17 @@ class TestDSURollback(unittest.TestCase):
             d.rollback(-1)
 
     def test_no_path_compression_find_stable(self):
+        # Q94 bullet 2: the node asked about must sit TWO levels down, else a
+        # compressing find rewrites nothing and the cell is vacuous.
+        # union-by-size: union(0,1)==(1<-0), union(2,3)==(3<-2), union(1,2)==(2<-0)
         d = DSU(4)
-        d.union(0, 1); d.union(1, 2)
+        d.union(0, 1); d.union(2, 3); d.union(1, 2)
+        self.assertEqual(d.parent, [0, 0, 0, 2])
+        self.assertEqual(d.parent[3], 2)          # precondition: depth 2
         before = list(d.parent)
-        r = d.find(2)
-        self.assertEqual(r, d.find(0))
-        self.assertEqual(d.parent[2], before[2])
+        self.assertEqual(d.find(3), 0)            # root via parent[3] -> 2 -> 0
+        self.assertEqual(d.parent, before)        # a compressing find writes parent[3]=0
+        self.assertEqual(d.size(3), 4)
 
     def test_size_after_rollback(self):
         d = DSU(3)
@@ -112,6 +117,48 @@ class TestDSURollback(unittest.TestCase):
             with self.assertRaises((TypeError, ValueError)):
                 d.rollback(bad)
 
+    def _fingerprint(self, d):
+        return (list(d.parent), list(d.sz), list(d.history), d.components())
+
+    def test_refuses_out_of_range_index(self):
+        # Q94 bullet 3: every entry point that takes an index refuses a range
+        # violation, and a refused call mutates nothing.
+        d = DSU(3)
+        d.union(0, 1)
+        before = self._fingerprint(d)
+        for bad in (-1, 3, 10):
+            for name in ("find", "size"):
+                with self.assertRaises(ValueError, msg="%s(%r)" % (name, bad)):
+                    getattr(d, name)(bad)
+                self.assertEqual(self._fingerprint(d), before)
+            for args in ((bad, 0), (0, bad)):
+                with self.assertRaises(ValueError, msg="union%r" % (args,)):
+                    d.union(*args)
+                self.assertEqual(self._fingerprint(d), before)
+                with self.assertRaises(ValueError, msg="connected%r" % (args,)):
+                    d.connected(*args)
+                self.assertEqual(self._fingerprint(d), before)
+
+    def test_refuses_non_int_index(self):
+        # Q94 bullet 4: a bool is an int subclass in Python; True must not alias
+        # index 1 and False must not merge index 0.
+        d = DSU(3)
+        before = self._fingerprint(d)
+        for bad in (True, False):
+            with self.assertRaises(TypeError, msg="find(%r)" % (bad,)):
+                d.find(bad)
+            with self.assertRaises(TypeError, msg="size(%r)" % (bad,)):
+                d.size(bad)
+            with self.assertRaises(TypeError, msg="union(%r, 1)" % (bad,)):
+                d.union(bad, 1)
+            with self.assertRaises(TypeError, msg="connected(%r, 0)" % (bad,)):
+                d.connected(bad, 0)
+            self.assertEqual(self._fingerprint(d), before)
+        with self.assertRaises(TypeError):
+            d.find(1.5)
+        with self.assertRaises(TypeError):
+            d.find(None)
+
 
 if __name__ == "__main__":
     unittest.main()