"""Validation helpers: ext size/depth, UUID, inp_hash/out_hash format."""

from __future__ import annotations

import base64
import json
import re
from typing import Any

EXT_MAX_SIZE = 4096
EXT_MAX_DEPTH = 5
DEFAULT_MAX_PRED_LENGTH = 100

_UUID_RE = re.compile(
    r"^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
)
def _json_depth(obj: Any, depth: int = 0) -> int:
    if depth > EXT_MAX_DEPTH:
        return depth
    if isinstance(obj, dict):
        return max((_json_depth(v, depth + 1) for v in obj.values()), default=depth + 1)
    if isinstance(obj, list):
        return max((_json_depth(x, depth + 1) for x in obj), default=depth + 1)
    return depth


def validate_ext(ext: dict[str, Any] | None) -> None:
    """Raise ValueError if ext exceeds EXT_MAX_SIZE or nesting depth EXT_MAX_DEPTH."""
    if not ext:
        return
    raw = json.dumps(ext)
    if len(raw.encode("utf-8")) > EXT_MAX_SIZE:
        raise ValueError("ect: ext exceeds max size (4096 bytes)")
    if _json_depth(ext) > EXT_MAX_DEPTH:
        raise ValueError("ect: ext exceeds max nesting depth (5)")


def valid_uuid(s: str) -> bool:
    """Return True if s is a UUID string (RFC 9562)."""
    return bool(_UUID_RE.match(s))


def validate_hash_format(s: str) -> None:
    """Raise ValueError if s is non-empty and not plain base64url per RFC 9449 / ECT spec.

    The ECT spec (draft-nennemann-wimse-ect-01) and RFC 9449 specify
    ``base64url(SHA-256(data))`` — a plain base64url string without any
    algorithm prefix.  This matches how ACT handles hashes.
    """
    if not s:
        return
    # Reject strings containing non-base64url characters.
    # base64url alphabet: A-Z a-z 0-9 - _  (no padding '=' expected)
    if not re.fullmatch(r"[A-Za-z0-9_-]+", s):
        raise ValueError("ect: inp_hash/out_hash must be plain base64url (no prefix)")
    # Verify it actually decodes.
    pad = 4 - len(s) % 4
    padded = s + "=" * pad if pad != 4 else s
    try:
        base64.urlsafe_b64decode(padded)
    except Exception:
        raise ValueError("ect: inp_hash/out_hash must be plain base64url (no prefix)") from None
