tests/test-config-parselist.py

encoding: fix trim() to be O(n) instead of O(n^2)

`encoding.trim()` iterated over the possible lengths smaller than the
input and created a slice for each. It then calculated the column
width of the result, which is of course O(n), so the overall algorithm
was O(n). This patch rewrites it to iterate over the unicode
characters, keeping track of the length so far. Also, the old
algorithm started from the end of the string, which made it much worse
when the input is large and the limit is small (such as the typical 72
we pass to it).

You can time it by running something like this:

```
time python3 -c 'from mercurial.utils import stringutil; print(stringutil.ellipsis(b"0123456789" * 1000, 5))'
```

That drops from 4.05 s to 83 ms with this patch (and most of that is
of course startup time).

Differential Revision: https://phab.mercurial-scm.org/D12089

"""
List-valued configuration keys have an ad-hoc microsyntax. From `hg help config`:

> List values are separated by whitespace or comma, except when values are
> placed in double quotation marks:
>
>     allow_read = "John Doe, PhD", brian, betty
>
> Quotation marks can be escaped by prefixing them with a backslash. Only
> quotation marks at the beginning of a word is counted as a quotation
> (e.g., ``foo"bar baz`` is the list of ``foo"bar`` and ``baz``).

That help documentation is fairly light on details, the actual parser has many
other edge cases. This test tries to cover them.
"""

from mercurial.utils import stringutil


def assert_parselist(input, expected):
    result = stringutil.parselist(input)
    if result != expected:
        raise AssertionError(
            "parse_input(%r)\n     got %r\nexpected %r"
            % (input, result, expected)
        )


# Keep these Python tests in sync with the Rust ones in `rust/hg-core/src/config/values.rs`

assert_parselist(b'', [])
assert_parselist(b',', [])
assert_parselist(b'A', [b'A'])
assert_parselist(b'B,B', [b'B', b'B'])
assert_parselist(b', C, ,C,', [b'C', b'C'])
assert_parselist(b'"', [b'"'])
assert_parselist(b'""', [b'', b''])
assert_parselist(b'D,"', [b'D', b'"'])
assert_parselist(b'E,""', [b'E', b'', b''])
assert_parselist(b'"F,F"', [b'F,F'])
assert_parselist(b'"G,G', [b'"G', b'G'])
assert_parselist(b'"H \\",\\"H', [b'"H', b',', b'H'])
assert_parselist(b'I,I"', [b'I', b'I"'])
assert_parselist(b'J,"J', [b'J', b'"J'])
assert_parselist(b'K K', [b'K', b'K'])
assert_parselist(b'"K" K', [b'K', b'K'])
assert_parselist(b'L\tL', [b'L', b'L'])
assert_parselist(b'"L"\tL', [b'L', b'', b'L'])
assert_parselist(b'M\x0bM', [b'M', b'M'])
assert_parselist(b'"M"\x0bM', [b'M', b'', b'M'])
assert_parselist(b'"N"  , ,"', [b'N"'])
assert_parselist(b'" ,O,  ', [b'"', b'O'])