import hashlib
import json
from typing import List

def _hash_bytes(data: bytes) -> str:
    return hashlib.sha256(data).hexdigest()

def merkle_root(digests: List[str]) -> str:
    if not digests:
        return ""
    level = [bytes.fromhex(d) for d in digests]
    while len(level) > 1:
        next_level = []
        for i in range(0, len(level), 2):
            left = level[i]
            right = level[i+1] if i+1 < len(level) else level[i]
            next_level.append(hashlib.sha256(left + right).digest())
        level = next_level
    return level[0].hex()

class DeltaLog:
    def __init__(self):
        self.entries = []  # each entry is a dict with digest and payload

    def add_entry(self, entry: dict) -> str:
        # entry must be serializable, and we store a digest for Merkle
        payload_bytes = json.dumps(entry, sort_keys=True).encode('utf-8')
        digest = hashlib.sha256(payload_bytes).hexdigest()
        self.entries.append({
            "digest": digest,
            "payload": entry,
        })
        return digest

    def delta_from_index(self, index: int) -> List[dict]:
        # return full payloads for simplicity (MVP). In a real system you'd return compact digests with proofs.
        return [e["payload"] for e in self.entries[index:]]

    def root(self) -> str:
        digests = [e["digest"] for e in self.entries]
        return merkle_root(digests)