from __future__ import annotations
from dataclasses import dataclass, field
from typing import List, Optional
import hashlib
import time
import json


def _hash(data: str) -> str:
    return hashlib.sha256(data.encode("utf-8")).hexdigest()


@dataclass
class Block:
    index: int
    timestamp: float
    payload: str  # JSON-serialized contract data or attestation
    previous_hash: str
    hash: str = field(init=False)

    def __post_init__(self):
        self.hash = _hash(self._block_string())

    def _block_string(self) -> str:
        return f"{self.index}:{self.timestamp}:{self.payload}:{self.previous_hash}"


class AttestationLedger:
    def __init__(self):
        self._chain: List[Block] = []
        self._current_previous_hash: str = "0" * 64

    def append(self, payload_json: str) -> Block:
        index = len(self._chain)
        block = Block(
            index=index,
            timestamp=time.time(),
            payload=payload_json,
            previous_hash=self._current_previous_hash,
        )
        self._chain.append(block)
        self._current_previous_hash = block.hash
        return block

    def latest(self) -> Optional[Block]:
        return self._chain[-1] if self._chain else None

    def to_json(self) -> str:
        return json.dumps([{
            "index": b.index,
            "timestamp": b.timestamp,
            "payload": b.payload,
            "previous_hash": b.previous_hash,
            "hash": b.hash,
        } for b in self._chain], sort_keys=True)