mews.page/mews/css.py

"""Comparing a site's stylesheet against the default one.

SPEC.md 5.2 lets a site host its own copy of the default stylesheet and change
nothing in it, except to add @font-face rules that load fonts from the site
itself. Checking that means comparing text, since a site may legitimately
differ in line endings and trailing whitespace after a copy-paste.
"""

from dataclasses import dataclass, field
import difflib
from functools import cache
from importlib import resources
import re

FONT_FACE = re.compile(r"@font-face\b", re.IGNORECASE)
URL_IN_SRC = re.compile(r"url\(\s*['\"]?([^'\")]+)", re.IGNORECASE)
IMPORT_RULE = re.compile(r"@import\b", re.IGNORECASE)


@dataclass
class Comparison:
    """What a candidate stylesheet differs from the default by."""

    equal: bool
    diff_line: int | None = None
    diff: str = ""
    font_faces: list[str] = field(default_factory=list)
    has_import: bool = False

    @property
    def font_urls(self) -> list[str]:
        """Every url() the added @font-face rules load."""
        return [
            match.group(1).strip()
            for block in self.font_faces
            for match in URL_IN_SRC.finditer(block)
        ]


@cache
def canonical() -> str:
    """Return the normalised default stylesheet shipped with this package."""
    data = resources.files("mews.data").joinpath("mews-0.1.css").read_text("utf-8")
    return normalise(data)


def normalise(text: str) -> str:
    """Strip the differences a copy may pick up without changing any rule."""
    text = text.lstrip("").replace("\r\n", "\n").replace("\r", "\n")
    lines = [line.rstrip() for line in text.split("\n")]
    out: list[str] = []
    for line in lines:
        if line or (out and out[-1]):
            out.append(line)
    return "\n".join(out).strip("\n")


def strip_font_faces(text: str) -> tuple[str, list[str]]:
    """Remove top-level @font-face blocks, returning the rest and the blocks.

    Only brace depth zero counts, so an @font-face inside a media query is left
    in place and shows up as a difference.
    """
    blocks: list[str] = []
    out: list[str] = []
    index = 0
    depth = 0
    while index < len(text):
        if depth == 0 and FONT_FACE.match(text, index):
            end = _block_end(text, index)
            if end is None:
                break
            blocks.append(text[index:end])
            index = end
            continue
        character = text[index]
        if character == "{":
            depth += 1
        elif character == "}":
            depth = max(0, depth - 1)
        out.append(character)
        index += 1
    out.append(text[index:])
    return "".join(out), blocks


def _block_end(text: str, start: int) -> int | None:
    """Index just past the brace-balanced block beginning at start."""
    opened = text.find("{", start)
    if opened == -1:
        return None
    depth = 0
    for index in range(opened, len(text)):
        if text[index] == "{":
            depth += 1
        elif text[index] == "}":
            depth -= 1
            if depth == 0:
                return index + 1
    return None


def compare(candidate: str) -> Comparison:
    """Compare a stylesheet against the default, allowing added @font-face rules."""
    expected = canonical()
    if normalise(candidate) == expected:
        return Comparison(equal=True)

    remainder, blocks = strip_font_faces(candidate)
    found = normalise(remainder)
    has_import = bool(IMPORT_RULE.search(candidate))
    if found == expected:
        return Comparison(equal=True, font_faces=blocks, has_import=has_import)

    diff = list(
        difflib.unified_diff(
            expected.split("\n"), found.split("\n"), "mews-0.1.css", "your copy", n=1
        )
    )
    line = next(
        (
            index + 1
            for index, (left, right) in enumerate(
                zip(expected.split("\n"), found.split("\n"), strict=False)
            )
            if left != right
        ),
        min(len(expected.split("\n")), len(found.split("\n"))) + 1,
    )
    return Comparison(
        equal=False,
        diff_line=line,
        diff="\n".join(diff[:8]),
        font_faces=blocks,
        has_import=has_import,
    )