Use composite strategy in trie.tools.strategies

This is a cleanup for the backfill tests, using the shared strategy to
try to expand the surface area of the hypothesis tests.

Note:

- The trie_from_keys() builder with a minimum_value_length=0 would try
to "insert" at key b'' the value b'', which is of course a delete.  So
the smallest permissable size must be a byte length of 1.

- An empty trie is not handled well by these backfill tests, but
backfill is entirely uninteresting for an empty trie, so we can ignore
the case.

tests/test_hexary_trie_walk.py

@@ -18,13 +18,10 @@
 
 
 @given(
-    st.lists(
-        st.binary(min_size=3, max_size=3),
-        unique=True,
-        max_size=1024,
-    ),
+    # starting trie keys
+    trie_keys_with_extensions(allow_empty_trie=False),
     # minimum value length (to help force trie nodes to stop embedding)
-    st.integers(min_value=3, max_value=32),
+    st.integers(min_value=1, max_value=32),
     st.lists(
         st.integers(min_value=0, max_value=0xf),
         max_size=4 * 2,  # one byte (two nibbles) deeper than the longest key above
@@ -81,13 +78,10 @@ def test_trie_walk_backfilling(trie_keys, minimum_value_length, index_nibbles):
 
 
 @given(
-    st.lists(
-        st.binary(min_size=3, max_size=3),
-        unique=True,
-        max_size=1024,
-    ),
+    # starting trie keys
+    trie_keys_with_extensions(allow_empty_trie=False),
     # minimum value length (to help force trie nodes to stop embedding)
-    st.integers(min_value=3, max_value=32),
+    st.integers(min_value=1, max_value=32),
     st.lists(
         st.integers(min_value=0, max_value=0xf),
         max_size=4 * 2,  # one byte (two nibbles) deeper than the longest key above

trie/tools/strategies.py

@@ -25,7 +25,7 @@ def random_trie_strategy(draw):
 
 
 @st.composite
-def trie_keys_with_extensions(draw):
+def trie_keys_with_extensions(draw, allow_empty_trie=True):
     """
     Build trie keys that tend to have lots of extension/branch/leaf nodes.
     Anecdotally, this was more likely to produce examples like the one
@@ -34,7 +34,7 @@ def trie_keys_with_extensions(draw):
     # Simplest possible trie: an empty trie
     # Test it about once, on average, per run of 200 tests (the default example count)
     # Also, this will shrink down to the empty trie as you shrink these integers.
-    if draw(st.integers(min_value=0, max_value=200)) == 0:
+    if allow_empty_trie and draw(st.integers(min_value=0, max_value=200)) == 0:
         return ()
 
     def build_up_from_children(children):