"""What a page title may be, stated as pure functions.

A title becomes a file name one to one, so the rule is the intersection of what
Windows, macOS and Linux accept - checked on every platform, because a corpus
written on Linux is checked out on the others. The normative statement is
`kb/CONTRACT.md` § "Titles are identifiers"; this module implements it and holds
no `fail()`, so `lint_core` can use it without going through `commands/`.
"""
from __future__ import annotations

import unicodedata

FORBIDDEN_CHARS = frozenset('<>:"/\\|?*')

# Windows MAX_PATH is 259 usable characters for the whole path, install folder
# included, and long paths are off on the target system. A file's path below the
# instance root may use 160 of them; the folder limit that `doctor` and preflight
# enforce is the other half of the same sum, so change one only together with it.
PATH_BUDGET = 160

# Windows device names, matched on the part before the first dot. The superscript
# forms are reserved by some Windows versions and not others; refusing all of
# them costs nothing.
_WINDOWS_RESERVED = frozenset(
    {"CON", "PRN", "AUX", "NUL"}
    | {f"{device}{digit}" for device in ("COM", "LPT") for digit in "0123456789¹²³"}
)

# Names the stack itself owns next to a page: the generated catalog shard and
# the per-collection authoring contract (`kb_scan.GENERATED_INDEX`,
# `kb_scan._COLLECTION_CONTRACT`, both matched here by the part before the dot).
_STACK_RESERVED = frozenset({"INDEX", "COLLECTION"})


def collision_key(name: str) -> str:
    """The key under which two names count as the same file on a case-insensitive,
    normalizing file system (NTFS, APFS): NFC, then `casefold`."""
    return unicodedata.normalize("NFC", name).casefold()


def title_problems(title: str) -> list[str]:
    """Every reason `title` cannot be a page title; empty when it can."""
    if not title:
        return ["the title is empty"]

    problems: list[str] = []

    forbidden = sorted({c for c in title if c in FORBIDDEN_CHARS})
    if forbidden:
        shown = " ".join(f"'{c}'" for c in forbidden)
        problems.append(f"contains {shown}, which Windows does not allow in a file name")

    control = sorted({c for c in title if unicodedata.category(c) == "Cc"})
    if control:
        shown = " ".join(f"U+{ord(c):04X}" for c in control)
        problems.append(f"contains the control character(s) {shown}")

    # Windows ignores trailing spaces and dots on the name before the extension,
    # so `CON .x` opens the console device just as `CON.x` does.
    base = title.split(".", 1)[0].rstrip(" ").upper()
    if base in _WINDOWS_RESERVED:
        problems.append(f"'{base}' is a reserved device name on Windows")
    elif base in _STACK_RESERVED:
        problems.append(f"'{base}' is reserved for a file the stack generates or owns")

    if title.endswith((".", " ")):
        problems.append("ends with a dot or a space, which Windows strips from file names")

    return problems


def path_length(rel_path: str) -> int:
    """`rel_path`'s length in UTF-16 code units, which is how Windows counts
    MAX_PATH - a character outside the BMP (an emoji) counts twice."""
    return len(rel_path.encode("utf-16-le")) // 2


def path_budget_problem(rel_path: str) -> str | None:
    """Why the path of a file, relative to the instance root and written with
    `/`, is over `PATH_BUDGET`; None when it fits."""
    length = path_length(rel_path)
    if length <= PATH_BUDGET:
        return None
    excess = length - PATH_BUDGET
    return (
        f"{rel_path} is {length} characters long (counted in UTF-16 code units, as Windows "
        f"counts MAX_PATH) and the budget is {PATH_BUDGET}: the path has to get {excess} shorter"
    )