import hashlib
import json
from typing import Optional


class SimpleLedger:
    """A minimal tamper-evident hash chain for blocks.

    Each anchor is SHA256(prev_anchor || block_json). Stored externally (here we return the anchor).
    """

    def __init__(self):
        self.last_anchor = None

    def anchor(self, block_json: str) -> str:
        m = hashlib.sha256()
        if self.last_anchor:
            m.update(self.last_anchor.encode())
        m.update(block_json.encode())
        anchor = m.hexdigest()
        self.last_anchor = anchor
        return anchor

    def verify_chain(self, block_jsons: list, anchors: list) -> bool:
        """Verify that anchors are consistent with the provided block_jsons sequence."""
        prev = None
        for bj, a in zip(block_jsons, anchors):
            m = hashlib.sha256()
            if prev:
                m.update(prev.encode())
            m.update(bj.encode())
            if m.hexdigest() != a:
                return False
            prev = a
        return True