"""RSA key generation and HTTP Signature signing/verification.

Keys are stored in DB (ActorProfile), not the filesystem.
Ported from ~/art-dag/activity-pub/keys.py.
"""
from __future__ import annotations

import base64
import hashlib
import json
from datetime import datetime, timezone

from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import rsa, padding


def generate_rsa_keypair() -> tuple[str, str]:
    """Generate an RSA-2048 keypair.

    Returns:
        (private_pem, public_pem) as UTF-8 strings.
    """
    private_key = rsa.generate_private_key(
        public_exponent=65537,
        key_size=2048,
    )

    private_pem = private_key.private_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PrivateFormat.PKCS8,
        encryption_algorithm=serialization.NoEncryption(),
    ).decode()

    public_pem = private_key.public_key().public_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PublicFormat.SubjectPublicKeyInfo,
    ).decode()

    return private_pem, public_pem


def sign_request(
    private_key_pem: str,
    key_id: str,
    method: str,
    path: str,
    host: str,
    body: bytes | None = None,
    date: str | None = None,
) -> dict[str, str]:
    """Build HTTP Signature headers for an outgoing request.

    Returns a dict of headers to merge into the request:
    ``{"Signature": ..., "Date": ..., "Digest": ..., "Host": ...}``
    """
    if date is None:
        date = datetime.now(timezone.utc).strftime("%a, %d %b %Y %H:%M:%S GMT")

    headers_to_sign = [
        f"(request-target): {method.lower()} {path}",
        f"host: {host}",
        f"date: {date}",
    ]

    out_headers: dict[str, str] = {
        "Host": host,
        "Date": date,
    }

    if body is not None:
        digest = base64.b64encode(hashlib.sha256(body).digest()).decode()
        digest_header = f"SHA-256={digest}"
        headers_to_sign.append(f"digest: {digest_header}")
        out_headers["Digest"] = digest_header

    signed_string = "\n".join(headers_to_sign)
    header_names = " ".join(
        h.split(":")[0] for h in headers_to_sign
    )

    private_key = serialization.load_pem_private_key(
        private_key_pem.encode(), password=None,
    )
    signature_bytes = private_key.sign(
        signed_string.encode(),
        padding.PKCS1v15(),
        hashes.SHA256(),
    )
    signature_b64 = base64.b64encode(signature_bytes).decode()

    out_headers["Signature"] = (
        f'keyId="{key_id}",'
        f'headers="{header_names}",'
        f'signature="{signature_b64}",'
        f'algorithm="rsa-sha256"'
    )

    return out_headers


def verify_request_signature(
    public_key_pem: str,
    signature_header: str,
    method: str,
    path: str,
    headers: dict[str, str],
) -> bool:
    """Verify an incoming HTTP Signature.

    Args:
        public_key_pem: PEM-encoded public key of the sender.
        signature_header: Value of the ``Signature`` header.
        method: HTTP method (GET, POST, etc.).
        path: Request path (e.g. ``/users/alice/inbox``).
        headers: All request headers (case-insensitive keys).

    Returns:
        True if the signature is valid.
    """
    # Parse Signature header
    parts: dict[str, str] = {}
    for part in signature_header.split(","):
        part = part.strip()
        eq = part.index("=")
        key = part[:eq]
        val = part[eq + 1:].strip('"')
        parts[key] = val

    signed_headers = parts.get("headers", "date").split()
    signature_b64 = parts.get("signature", "")

    # Reconstruct the signed string
    lines: list[str] = []
    # Normalize header lookup to lowercase
    lc_headers = {k.lower(): v for k, v in headers.items()}
    for h in signed_headers:
        if h == "(request-target)":
            lines.append(f"(request-target): {method.lower()} {path}")
        else:
            val = lc_headers.get(h, "")
            lines.append(f"{h}: {val}")

    signed_string = "\n".join(lines)

    public_key = serialization.load_pem_public_key(public_key_pem.encode())
    try:
        public_key.verify(
            base64.b64decode(signature_b64),
            signed_string.encode(),
            padding.PKCS1v15(),
            hashes.SHA256(),
        )
        return True
    except Exception:
        return False


def create_ld_signature(
    private_key_pem: str,
    key_id: str,
    activity: dict,
) -> dict:
    """Create an RsaSignature2017 Linked Data signature for an activity."""
    canonical = json.dumps(activity, sort_keys=True, separators=(",", ":"))

    private_key = serialization.load_pem_private_key(
        private_key_pem.encode(), password=None,
    )
    signature_bytes = private_key.sign(
        canonical.encode(),
        padding.PKCS1v15(),
        hashes.SHA256(),
    )
    signature_b64 = base64.b64encode(signature_bytes).decode()

    return {
        "type": "RsaSignature2017",
        "creator": key_id,
        "created": datetime.now(timezone.utc).isoformat(),
        "signatureValue": signature_b64,
    }