Test the two-billion-version window in every implementation

test_two_billion_versions asserted nothing about the results, and hash_table
was never exercised across a version jump larger than two billion. Assert the
expected results and add test_two_billion_version_window, which checks the
window boundary (and the count == 0 case) for radix_tree, skip_list and
hash_table.

test_conflict_set.py

@@ -205,8 +205,32 @@ def test_internal_version_zero():
 def test_two_billion_versions():
     with DebugConflictSet() as cs:
         cs.addWrites(int(2e9) + 1)
-        cs.check(read(0, b"\x00", b"\xff"))
-        cs.check(read(1, b"\x00", b"\xff"))
+        assert cs.check(read(0, b"\x00", b"\xff")) == [Result.TOO_OLD]
+        assert cs.check(read(1, b"\x00", b"\xff")) == [Result.COMMIT]
+
+
+def test_two_billion_version_window():
+    # Regression test for issue #98: `addWrites` advances `newestVersion`, so a
+    # read whose readVersion fell more than two billion versions behind it is
+    # TooOld. `hash_table` used to discard writeVersion entirely, so it reported
+    # Conflict/Commit for reads far outside the version window.
+    window = 2 * 10**9
+    newest = 2**32
+
+    def probe(implementation, read_version, *writes):
+        with ConflictSet(0, build_dir=build_dir, implementation=implementation) as cs:
+            cs.addWrites(newest, *writes)
+            return cs.check(read(read_version, b"k"))[0]
+
+    for implementation in ("radix_tree", "skip_list", "hash_table"):
+        # The window advances even when addWrites added no writes at all.
+        assert probe(implementation, 0, write(b"k")) == Result.TOO_OLD
+        assert probe(implementation, 0) == Result.TOO_OLD
+        assert probe(implementation, newest - window - 1, write(b"k")) == Result.TOO_OLD
+        assert probe(implementation, newest - window - 1) == Result.TOO_OLD
+        # Exactly `window` versions behind newestVersion is still evaluable.
+        assert probe(implementation, newest - window, write(b"k")) == Result.CONFLICT
+        assert probe(implementation, newest - window) == Result.COMMIT
 
 
 def test_positive_bytes():