import itertools
import os
import re
import sys


def get_characters():
    """Find every Unicode character that is valid in a Python `identifier`_ but
    is not matched by the regex ``\\w`` group.

    ``\\w`` matches some characters that aren't valid in identifiers, but
    :meth:`str.isidentifier` will catch that later in lexing.

    All start characters are valid continue characters, so we only test for
    continue characters.

    _identifier: https://docs.python.org/3/reference/lexical_analysis.html#identifiers
    """
    for cp in range(sys.maxunicode + 1):
        s = chr(cp)

        if ("a" + s).isidentifier() and not re.match(r"\w", s):
            yield s


def collapse_ranges(data):
    """Given a sorted list of unique characters, generate ranges representing
    sequential code points.

    Source: https://stackoverflow.com/a/4629241/400617
    """
    for _, g in itertools.groupby(enumerate(data), lambda x: ord(x[1]) - x[0]):
        lb = list(g)
        yield lb[0][1], lb[-1][1]


def build_pattern(ranges):
    """Output the regex pattern for ranges of characters.

    One and two character ranges output the individual characters.
    """
    out = []

    for a, b in ranges:
        if a == b:  # single char
            out.append(a)
        elif ord(b) - ord(a) == 1:  # two chars, range is redundant
            out.append(a)
            out.append(b)
        else:
            out.append(f"{a}-{b}")

    return "".join(out)


def main():
    """Build the regex pattern and write it to
    ``jinja2/_identifier.py``.
    """
    pattern = build_pattern(collapse_ranges(get_characters()))
    filename = os.path.abspath(
        os.path.join(os.path.dirname(__file__), "..", "src", "jinja2", "_identifier.py")
    )

    with open(filename, "w", encoding="utf8") as f:
        f.write("import re\n\n")
        f.write("# generated by scripts/generate_identifier_pattern.py\n")
        f.write("pattern = re.compile(\n")
        f.write(f'    r"[\\w{pattern}]+"  # noqa: B950\n')
        f.write(")\n")


if __name__ == "__main__":
    main()
