1"""
2Pure functions for converting JSON strings to Python values.
3
4Because I expect some of this JSON to be written by me, and I can
5make copy-paste mistakes, there are a couple of ways it tries
6to catch errors.
7"""
8
9import json
10import re
11import typing
12
13
14def decode_from_js(js_string: str, *, varname: str) -> typing.Any:
15    """
16    Parse a string as a JavaScript value.
17    """
18    # Matches 'const varname = ' or 'var varname = ' at the start
19    # of a string.
20    m = re.compile(r"^(?:const |var )?%s = " % varname)
21
22    if not m.match(js_string):
23        raise ValueError("Does not start with JavaScript `const` declaration!")
24
25    json_string = m.sub(repl="", string=js_string).rstrip().rstrip(";")
26
27    return decode_from_json(json_string)
28
29
30def dict_with_unique_names(
31    pairs: list[tuple[str, typing.Any]],
32) -> dict[str, typing.Any]:
33    """
34    Convert a list of name/value pairs to a dict, but only if the
35    names are unique.
36
37    This is similar to the builtin parser, but it will look for
38    duplicate names and throw a ValueError if they're found; this is
39    a protection against me making a copy/paste error in my JavaScript.
40    """
41    # First try to parse the object as a dictionary; if it's the same
42    # length as the pairs, then we know all the keys were unique and
43    # we can return.
44    pairs_as_dict = dict(pairs)
45
46    if len(pairs_as_dict) == len(pairs):
47        return pairs_as_dict
48
49    # Otherwise, let's work out what the duplicate name(s) were, so we
50    # can throw an appropriate error message for the user.
51    import collections
52
53    name_tally = collections.Counter(k for k, _ in pairs)
54
55    duplicate_names = [k for k, count in name_tally.items() if count > 1]
56    assert len(duplicate_names) > 0
57
58    if len(duplicate_names) == 1:
59        raise ValueError(f"Found duplicate name in JSON object: {duplicate_names[0]}")
60    else:
61        raise ValueError(
62            f"Found duplicate names in JSON object: {', '.join(duplicate_names)}"
63        )
64
65
66def decode_from_json(json_string: str) -> typing.Any:
67    """
68    Parse a string as a JSON value.
69    """
70    return json.loads(json_string, object_pairs_hook=dict_with_unique_names)