// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Proofs {

  mapping(bytes32=>bool) private ids;
  mapping(bytes32=>uint) private periods;
  mapping(bytes32=>uint) private timeouts;
  mapping(bytes32=>uint) private markers;
  mapping(bytes32=>uint) private missed;
  mapping(bytes32=>mapping(uint=>bool)) private received;
  mapping(bytes32=>mapping(uint=>bool)) private missing;

  function _period(bytes32 id) internal view returns (uint) {
    return periods[id];
  }

  function _timeout(bytes32 id) internal view returns (uint) {
    return timeouts[id];
  }

  function _missed(bytes32 id) internal view returns (uint) {
    return missed[id];
  }

  function _expectProofs(bytes32 id, uint period, uint timeout) internal {
    require(!ids[id], "Proof id already in use");
    ids[id] = true;
    periods[id] = period;
    timeouts[id] = timeout;
    markers[id] = uint(blockhash(block.number - 1)) % period;
  }

  function _isProofRequired(
    bytes32 id,
    uint blocknumber
  )
    internal view
    returns (bool)
  {
    bytes32 hash = blockhash(blocknumber);
    return hash != 0 && uint(hash) % periods[id] == markers[id];
  }

  function _isProofTimedOut(
    bytes32 id,
    uint blocknumber
  )
    internal view
    returns (bool)
  {
    return block.number >= blocknumber + timeouts[id];
  }

  function _submitProof(
    bytes32 id,
    uint blocknumber,
    bool proof
  )
    internal
  {
    require(proof, "Invalid proof"); // TODO: replace bool by actual proof
    require(
      _isProofRequired(id, blocknumber),
      "No proof required for this block"
    );
    require(
      !_isProofTimedOut(id, blocknumber),
      "Proof not allowed after timeout"
    );
    require(!received[id][blocknumber], "Proof already submitted");
    received[id][blocknumber] = true;
  }

  function _markProofAsMissing(bytes32 id, uint blocknumber) internal {
    require(
      _isProofTimedOut(id, blocknumber),
      "Proof has not timed out yet"
    );
    require(
      !received[id][blocknumber],
      "Proof was submitted, not missing"
    );
    require(
      _isProofRequired(id, blocknumber),
      "Proof was not required"
    );
    require(!missing[id][blocknumber], "Proof already marked as missing");
    missing[id][blocknumber] = true;
    missed[id] += 1;
  }
}