Compute distinguishing argument index

Source/bindings/scripts/v8_interface.py

 # Copyright (C) 2013 Google Inc. All rights reserved.
 # coding=utf-8
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 311 matching lines @@
     """
     # Filter to only methods that are actually overloaded
     method_counts = Counter(method['name'] for method in methods)
     overloaded_method_names = set(name
                                   for name, count in method_counts.iteritems()
                                   if count > 1)
     overloaded_methods = [method for method in methods
                           if method['name'] in overloaded_method_names]
 
     # Group by name (generally will be defined together, but not necessarily)
-    overloaded_methods.sort(key=itemgetter('name'))
-    method_overloads = dict(
-        (name, list(methods_iterator)) for name, methods_iterator in
-        itertools.groupby(overloaded_methods, itemgetter('name')))
+    method_overloads = sort_and_groupby(overloaded_methods, itemgetter('name'))
 
     # Add overload information only to overloaded methods, so template code can
     # easily verify if a function is overloaded
     for name, overloads in method_overloads.iteritems():
-        effective_overload_set(overloads)  # to test, compute but discard result
+        effective_overloads_by_length = effective_overload_set_by_length(overloads)
+        for effective_overloads in effective_overloads_by_length.itervalues():
+            # To test, compute but discard result
+            distinguishing_argument_index(effective_overloads)
+
         for index, method in enumerate(overloads, 1):
             method.update({
                 'overload_index': index,
                 'overload_resolution_expression':
                     overload_resolution_expression(method),
             })
 
         # Resolution function is generated after last overloaded function;
         # package necessary information into |method.overloads| for that method.
         minimum_number_of_required_arguments = min(
@@ skipping 34 matching lines @@
     Spec: http://heycam.github.io/webidl/#dfn-effective-overload-set
 
     Formally the input and output lists are sets, but methods are stored
     internally as dicts, which can't be stored in a set because they are not
     hashable, so we use lists instead.
 
     Arguments:
         F: list of overloads for a given callable name.
 
     Returns:
-        S: list of tuples of the form <callable, type list, optionality list>.
+        S: list of tuples of the form (callable, type list, optionality list).
     """
     # Code closely follows the algorithm in the spec, for clarity and
     # correctness, and hence is not very Pythonic.
 
     # 1. Initialize S to ∅.
     # (We use a list because we can't use a set, as noted above.)
     S = []
 
     # 2. Let F be a set with elements as follows, according to the kind of
     # effective overload set:
     # (Passed as argument, nothing to do.)
 
     # 3. & 4. (maxarg, m) are only needed for variadics, not used.
 
     # 5. For each operation, extended attribute or callback function X in F:
     for X in F:  # X is the "callable", F is the overloads.
         arguments = X['arguments']
         # 1. Let n be the number of arguments X is declared to take.
         n = len(arguments)
         # 2. Let t0..n−1 be a list of types, where ti is the type of X’s
         # argument at index i.
-        t = [argument['idl_type'] for argument in arguments]  # type list
+        # (“type list”)
+        t = tuple(argument['idl_type_object'] for argument in arguments)
         # 3. Let o0..n−1 be a list of optionality values, where oi is “variadic”
         # if X’s argument at index i is a final, variadic argument, “optional”
         # if the argument is optional, and “required” otherwise.
-        # (We're just using a boolean for optional vs. required.)
-        o = [argument['is_optional'] for argument in arguments]  # optionality list
+        # (“optionality list”)
+        # (We’re just using a boolean for optional vs. required.)
+        o = tuple(argument['is_optional'] for argument in arguments)
         # 4. Add to S the tuple <X, t0..n−1, o0..n−1>.
         S.append((X, t, o))
         # 5. If X is declared to be variadic, then:
         # (Not used, so not implemented.)
         # 6. Initialize i to n−1.
         i = n - 1
         # 7. While i ≥ 0:
         # Spec bug (fencepost error); should be “While i > 0:”
         # https://www.w3.org/Bugs/Public/show_bug.cgi?id=25590
         while i > 0:
             # 1. If argument i of X is not optional, then break this loop.
             if not o[i]:
                 break
             # 2. Otherwise, add to S the tuple <X, t0..i−1, o0..i−1>.
             S.append((X, t[:i], o[:i]))
             # 3. Set i to i−1.
             i = i - 1
         # 8. If n > 0 and all arguments of X are optional, then add to S the
         # tuple <X, (), ()> (where “()” represents the empty list).
         if n > 0 and all(oi for oi in o):
             S.append((X, [], []))
     # 6. The effective overload set is S.
     return S
 
 
+def effective_overload_set_by_length(overloads):
+    def type_list_length(entry):
+        # Entries in the effective overload set are 3-tuples:
+        # (callable, type list, optionality list)
+        return len(entry[1])
+
+    effective_overloads = effective_overload_set(overloads)
+    return sort_and_groupby(effective_overloads, type_list_length)
+
+
+def distinguishing_argument_index(entries):
+    """Returns the distinguishing argument index for a sequence of entries.
+
+    Entries are elements of the effective overload set with the same number
+    of arguments (formally, same type list length), each a 3-tuple of the form
+    (callable, type list, optionality list).
+
+    Spec: http://heycam.github.io/webidl/#dfn-distinguishing-argument-index
+
+    If there is more than one entry in an effective overload set that has a
+    given type list length, then for those entries there must be an index i
+    such that for each pair of entries the types at index i are
+    distinguishable.
+    The lowest such index is termed the distinguishing argument index for the
+    entries of the effective overload set with the given type list length.
+    """
+    if len(entries) < 2:
+        # Only need to distinguish if two or more entries
+        return
+    type_lists = [tuple(idl_type.name for idl_type in entry[1])
+                  for entry in entries]
+    type_list_length = len(type_lists[0])
+    assert all(len(type_list) == type_list_length for type_list in type_lists)
+    name = entries[0][0]['name']
+
+    # The spec defines the distinguishing argument index by conditions it must
+    # satisfy, but does not give an algorithm.
+    #
+    # We compute the distinguishing argument index by first computing the
+    # minimum index where not all types are the same, and then checking that
+    # all types in this position are distinguishable (and the optionality lists
+    # up to this point are identical), since "minimum index where not all types
+    # are the same" is a *necessary* condition, and more direct to check than
+    # distinguishability.
+    try:
+        # “In addition, for each index j, where j is less than the
+        #  distinguishing argument index for a given type list length, the types
+        #  at index j in all of the entries’ type lists must be the same”
+        index = next(i for i, types in enumerate(zip(*type_lists))
+                     if len(set(types)) > 1)
+    except:

[Jens Widell, 2014/05/09 06:08:05]

Maybe "except StopIteration:" instead, to avoid catching unrelated exceptions
here, and to make it more clear what we expect from the code above?

Catching everything and treating it as a single (expected) type of error,
without checking, is a good way to confuse oneself and hide typos and errors.
:-)

[Nils Barth (inactive), 2014/05/09 06:17:09]

(>.<)
Thanks, lint should have caught this, but guess that's not part of Blink pylint.

+        raise ValueError('No distinguishing index found for %s, length %s:\n'
+                         'All entries have the same type list:\n'
+                         '%s' % (name, type_list_length, type_lists[0]))
+    # Check optionality
+    # “and the booleans in the corresponding list indicating argument
+    #  optionality must be the same.”
+    # FIXME: spec typo: optionality value is no longer a boolean
+    # https://www.w3.org/Bugs/Public/show_bug.cgi?id=25628
+    initial_optionality_lists = set(entry[2][:index] for entry in entries)
+    if len(initial_optionality_lists) > 1:
+        raise ValueError(
+            'Invalid optionality lists for %s, length %s:\n'
+            'Optionality lists differ below distinguishing argument index %s:\n'
+            '%s'
+            % (name, type_list_length, index, set(initial_optionality_lists)))
+
+    # FIXME: check distinguishability
+
+    return index
+
+
 def overload_resolution_expression(method):
     # Expression is an OR of ANDs: each term in the OR corresponds to a
     # possible argument count for a given method, with type checks.
     # FIXME: Blink's overload resolution algorithm is incorrect, per:
     # Implement WebIDL overload resolution algorithm.  http://crbug.com/293561
     #
     # Currently if distinguishing non-primitive type from primitive type,
     # (e.g., sequence<DOMString> from DOMString or Dictionary from double)
     # the method with a non-primitive type argument must appear *first* in the
     # IDL file, since we're not adding a check to primitive types.
@@ skipping 68 matching lines @@
         # change behavior of existing bindings for other types.
         type_check = '%s->IsObject()' % cpp_value
         added_check_template = null_or_optional_check()
         if added_check_template:
             type_check = ' || '.join([added_check_template % cpp_value,
                                       type_check])
         return type_check
     return None
 
 
+################################################################################
+# Utility functions
+################################################################################
+
+def Counter(iterable):
+    # Once using Python 2.7, using collections.Counter
+    counter = defaultdict(lambda: 0)
+    for item in iterable:
+        counter[item] += 1
+    return counter
+
+
 def common_value(dicts, key):
     """Returns common value of a key across an iterable of dicts, or None.
 
     Auxiliary function for overloads, so can consolidate an extended attribute
     that appears with the same value on all items in an overload set.
     """
     values = (d[key] for d in dicts)
     first_value = next(values)
     if all(value == first_value for value in values):
         return first_value
     return None
 
 
-def Counter(iterable):
-    # Once using Python 2.7, using collections.Counter
-    counter = defaultdict(lambda: 0)
-    for item in iterable:
-        counter[item] += 1
-    return counter
+def sort_and_groupby(l, key=None):
+    """Returns a dict of {key: list}, sorting and grouping input list by key."""
+    l.sort(key=key)
+    return dict((k, list(g)) for k, g in itertools.groupby(l, key))
 
 
 ################################################################################
 # Constructors
 ################################################################################
 
 # [Constructor]
 def generate_constructor(interface, constructor):
     return {
         'argument_list': constructor_argument_list(interface, constructor),
@@ skipping 260 matching lines @@
         deleter = next(
             method
             for method in interface.operations
             if ('deleter' in method.specials and
                 len(method.arguments) == 1 and
                 str(method.arguments[0].idl_type) == 'DOMString'))
     except StopIteration:
         return None
 
     return property_deleter(deleter)