// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract BurialChainReceipts {
    bytes32 public latestHash;
    uint256 public totalCount;
    uint256 public immutable BURIAL_DEPTH;

    event ReceiptAppended(
        uint256 indexed index,
        bytes32 indexed receiptHash,
        bytes32 previousHead,
        bytes32 newHead
    );

    constructor(uint256 burialDepth) {
        require(burialDepth > 0, "depth > 0");
        BURIAL_DEPTH = burialDepth;
        latestHash = bytes32(0);
        totalCount = 0;
    }

    function append(bytes32 receiptHash) external returns (uint256 index, bytes32 newHead) {
        bytes32 prevHead = latestHash;
        newHead = keccak256(abi.encodePacked(prevHead, receiptHash));
        latestHash = newHead;
        index = totalCount;
        totalCount += 1;
        emit ReceiptAppended(index, receiptHash, prevHead, newHead);
    }

    function verify(
        bytes32 receiptHash,
        uint256 index,
        bytes32 prevHead,
        bytes32[] calldata subsequentReceiptHashes
    ) external view returns (bool) {
        require(index < totalCount, "index out of range");
        uint256 remaining = totalCount - index - 1;
        require(subsequentReceiptHashes.length == remaining, "bad proof length");
        require(remaining >= BURIAL_DEPTH, "not buried");

        bytes32 current = keccak256(abi.encodePacked(prevHead, receiptHash));
        uint256 len = subsequentReceiptHashes.length;
        for (uint256 i = 0; i < len; ++i) {
            current = keccak256(abi.encodePacked(current, subsequentReceiptHashes[i]));
        }
        return current == latestHash;
    }
}