SSZ impl fixes (#960)

* fix serialization mixup of array types, fix variable size vector serialization, document, structure and de-deduplicate code
* more cleanup + minor fixes in earlier improvements
* Make type-inference stable on empty lists/vectors
* Add get_zero_value

test_libs/pyspec/eth2spec/utils/minimal_ssz.py

@@ -1,5 +1,6 @@
 from .hash_function import hash
 
+from typing import Any
 
 BYTES_PER_CHUNK = 32
 BYTES_PER_LENGTH_PREFIX = 4
@@ -9,15 +10,16 @@
 def SSZType(fields):
     class SSZObject():
         def __init__(self, **kwargs):
-            for f in fields:
+            for f, t in fields.items():
                 if f not in kwargs:
-                    raise Exception("Missing constructor argument: %s" % f)
-                setattr(self, f, kwargs[f])
+                    setattr(self, f, get_zero_value(t))
+                else:
+                    setattr(self, f, kwargs[f])
 
         def __eq__(self, other):
             return (
-                self.fields == other.fields and
-                self.serialize() == other.serialize()
+                    self.fields == other.fields and
+                    self.serialize() == other.serialize()
             )
 
         def __hash__(self):
@@ -58,18 +60,40 @@ def __len__(self):
 
 
 def is_basic(typ):
-    return isinstance(typ, str) and (typ[:4] in ('uint', 'bool') or typ == 'byte')
+    # if not a string, it is a complex, and cannot be basic
+    if not isinstance(typ, str):
+        return False
+    # "uintN": N-bit unsigned integer (where N in [8, 16, 32, 64, 128, 256])
+    elif typ[:4] == 'uint' and typ[4:] in ['8', '16', '32', '64', '128', '256']:
+        return True
+    # "bool": True or False
+    elif typ == 'bool':
+        return True
+    # alias: "byte" -> "uint8"
+    elif typ == 'byte':
+        return True
+    # default
+    else:
+        return False
 
 
 def is_constant_sized(typ):
+    # basic objects are fixed size by definition
     if is_basic(typ):
         return True
+    # dynamic size array type, "list": [elem_type].
+    # Not constant size by definition.
     elif isinstance(typ, list) and len(typ) == 1:
-        return is_constant_sized(typ[0])
-    elif isinstance(typ, list) and len(typ) == 2:
         return False
+    # fixed size array type, "vector": [elem_type, length]
+    # Constant size, but only if the elements are.
+    elif isinstance(typ, list) and len(typ) == 2:
+        return is_constant_sized(typ[0])
+    # bytes array (fixed or dynamic size)
     elif isinstance(typ, str) and typ[:5] == 'bytes':
-        return len(typ) > 5
+        # if no length suffix, it has a dynamic size
+        return typ != 'bytes'
+    # containers are only constant-size if all of the fields are constant size.
     elif hasattr(typ, 'fields'):
         for subtype in typ.fields.values():
             if not is_constant_sized(subtype):
@@ -90,40 +114,98 @@ def coerce_to_bytes(x):
         raise Exception("Expecting bytes")
 
 
+def encode_bytes(value):
+    serialized_bytes = coerce_to_bytes(value)
+    assert len(serialized_bytes) < 2 ** (8 * BYTES_PER_LENGTH_PREFIX)
+    serialized_length = len(serialized_bytes).to_bytes(BYTES_PER_LENGTH_PREFIX, 'little')
+    return serialized_length + serialized_bytes
+
+
+def encode_variable_size_container(values, types):
+    return encode_bytes(encode_fixed_size_container(values, types))
+
+
+def encode_fixed_size_container(values, types):
+    return b''.join([serialize_value(v, typ) for (v, typ) in zip(values, types)])
+
+
 def serialize_value(value, typ=None):
     if typ is None:
         typ = infer_type(value)
+    # "uintN"
     if isinstance(typ, str) and typ[:4] == 'uint':
         length = int(typ[4:])
         assert length in (8, 16, 32, 64, 128, 256)
         return value.to_bytes(length // 8, 'little')
-    elif typ == 'bool':
+    # "bool"
+    elif isinstance(typ, str) and typ == 'bool':
         assert value in (True, False)
         return b'\x01' if value is True else b'\x00'
-    elif (isinstance(typ, list) and len(typ) == 1) or typ == 'bytes':
-        serialized_bytes = coerce_to_bytes(value) if typ == 'bytes' else b''.join([serialize_value(element, typ[0]) for element in value])
-        assert len(serialized_bytes) < 2**(8 * BYTES_PER_LENGTH_PREFIX)
-        serialized_length = len(serialized_bytes).to_bytes(BYTES_PER_LENGTH_PREFIX, 'little')
-        return serialized_length + serialized_bytes
+    # Vector
     elif isinstance(typ, list) and len(typ) == 2:
+        # (regardless of element type, sanity-check if the length reported in the vector type matches the value length)
         assert len(value) == typ[1]
-        return b''.join([serialize_value(element, typ[0]) for element in value])
+        # If value is fixed-size (i.e. element type is fixed-size):
+        if is_constant_sized(typ):
+            return encode_fixed_size_container(value, [typ[0]] * len(value))
+        # If value is variable-size (i.e. element type is variable-size)
+        else:
+            return encode_variable_size_container(value, [typ[0]] * len(value))
+    # "bytes" (variable size)
+    elif isinstance(typ, str) and typ == 'bytes':
+        return encode_bytes(value)
+    # List
+    elif isinstance(typ, list) and len(typ) == 1:
+        return encode_variable_size_container(value, [typ[0]] * len(value))
+    # "bytesN" (fixed size)
     elif isinstance(typ, str) and len(typ) > 5 and typ[:5] == 'bytes':
         assert len(value) == int(typ[5:]), (value, int(typ[5:]))
         return coerce_to_bytes(value)
+    # containers
     elif hasattr(typ, 'fields'):
-        serialized_bytes = b''.join([serialize_value(getattr(value, field), subtype) for field, subtype in typ.fields.items()])
+        values = [getattr(value, field) for field in typ.fields.keys()]
+        types = list(typ.fields.values())
         if is_constant_sized(typ):
-            return serialized_bytes
+            return encode_fixed_size_container(values, types)
         else:
-            assert len(serialized_bytes) < 2**(8 * BYTES_PER_LENGTH_PREFIX)
-            serialized_length = len(serialized_bytes).to_bytes(BYTES_PER_LENGTH_PREFIX, 'little')
-            return serialized_length + serialized_bytes
+            return encode_variable_size_container(values, types)
     else:
         print(value, typ)
         raise Exception("Type not recognized")
 
 
+def get_zero_value(typ: Any) -> Any:
+    if isinstance(typ, str):
+        # Bytes array
+        if typ == 'bytes':
+            return b''
+        # bytesN
+        elif typ[:5] == 'bytes' and len(typ) > 5:
+            length = int(typ[5:])
+            return b'\x00' * length
+        # Basic types
+        elif typ == 'bool':
+            return False
+        elif typ[:4] == 'uint':
+            return 0
+        elif typ == 'byte':
+            return 0x00
+        else:
+            raise ValueError("Type not recognized")
+    # Vector:
+    elif isinstance(typ, list) and len(typ) == 2:
+        return [get_zero_value(typ[0]) for _ in range(typ[1])]
+    # List:
+    elif isinstance(typ, list) and len(typ) == 1:
+        return []
+    # Container:
+    elif hasattr(typ, 'fields'):
+        return typ(**{field: get_zero_value(subtype) for field, subtype in typ.fields.items()})
+    else:
+        print(typ)
+        raise Exception("Type not recognized")
+
+
 def chunkify(bytez):
     bytez += b'\x00' * (-len(bytez) % BYTES_PER_CHUNK)
     return [bytez[i:i + 32] for i in range(0, len(bytez), 32)]
@@ -152,12 +234,27 @@ def mix_in_length(root, length):
 
 
 def infer_type(value):
+    """
+    Note: defaults to uint64 for integer type inference due to lack of information.
+    Other integer sizes are still supported, see spec.
+    :param value: The value to infer a SSZ type for.
+    :return: The SSZ type.
+    """
     if hasattr(value.__class__, 'fields'):
         return value.__class__
     elif isinstance(value, Vector):
-        return [infer_type(value[0]) if len(value) > 0 else 'uint64', len(value)]
+        if len(value) > 0:
+            return [infer_type(value[0]), len(value)]
+        else:
+            # Element type does not matter too much,
+            # assumed to be a basic type for size-encoding purposes, vector is empty.
+            return ['uint64']
     elif isinstance(value, list):
-        return [infer_type(value[0])] if len(value) > 0 else ['uint64']
+        if len(value) > 0:
+            return [infer_type(value[0])]
+        else:
+            # Element type does not matter, list-content size will be encoded regardless, list is empty.
+            return ['uint64']
     elif isinstance(value, (bytes, str)):
         return 'bytes'
     elif isinstance(value, int):
@@ -169,24 +266,41 @@ def infer_type(value):
 def hash_tree_root(value, typ=None):
     if typ is None:
         typ = infer_type(value)
+    # -------------------------------------
+    # merkleize(pack(value))
+    # basic object: merkleize packed version (merkleization pads it to 32 bytes if it is not already)
     if is_basic(typ):
         return merkleize(pack([value], typ))
-    elif isinstance(typ, list) and len(typ) == 1 and is_basic(typ[0]):
-        return mix_in_length(merkleize(pack(value, typ[0])), len(value))
-    elif isinstance(typ, list) and len(typ) == 1 and not is_basic(typ[0]):
-        return mix_in_length(merkleize([hash_tree_root(element, typ[0]) for element in value]), len(value))
+    # or a vector of basic objects
     elif isinstance(typ, list) and len(typ) == 2 and is_basic(typ[0]):
         assert len(value) == typ[1]
         return merkleize(pack(value, typ[0]))
+    # -------------------------------------
+    # mix_in_length(merkleize(pack(value)), len(value))
+    # if value is a list of basic objects
+    elif isinstance(typ, list) and len(typ) == 1 and is_basic(typ[0]):
+        return mix_in_length(merkleize(pack(value, typ[0])), len(value))
+    # (needs some extra work for non-fixed-sized bytes array)
     elif typ == 'bytes':
         return mix_in_length(merkleize(chunkify(coerce_to_bytes(value))), len(value))
+    # -------------------------------------
+    # merkleize([hash_tree_root(element) for element in value])
+    # if value is a vector of composite objects
+    elif isinstance(typ, list) and len(typ) == 2 and not is_basic(typ[0]):
+        return merkleize([hash_tree_root(element, typ[0]) for element in value])
+    # (needs some extra work for fixed-sized bytes array)
     elif isinstance(typ, str) and typ[:5] == 'bytes' and len(typ) > 5:
         assert len(value) == int(typ[5:])
         return merkleize(chunkify(coerce_to_bytes(value)))
-    elif isinstance(typ, list) and len(typ) == 2 and not is_basic(typ[0]):
-        return merkleize([hash_tree_root(element, typ[0]) for element in value])
+    # or a container
     elif hasattr(typ, 'fields'):
         return merkleize([hash_tree_root(getattr(value, field), subtype) for field, subtype in typ.fields.items()])
+    # -------------------------------------
+    # mix_in_length(merkleize([hash_tree_root(element) for element in value]), len(value))
+    # if value is a list of composite objects
+    elif isinstance(typ, list) and len(typ) == 1 and not is_basic(typ[0]):
+        return mix_in_length(merkleize([hash_tree_root(element, typ[0]) for element in value]), len(value))
+    # -------------------------------------
     else:
         raise Exception("Type not recognized")