contract Shareable {
  // TYPES

  // struct for the status of a pending operation.
  struct PendingState {
    uint yetNeeded;
    uint ownersDone;
    uint index;
  }


  // FIELDS

  // the number of owners that must confirm the same operation before it is run.
  uint public required;

  // list of owners
  address[256] owners;
  uint constant c_maxOwners = 250;
  // index on the list of owners to allow reverse lookup
  mapping(address => uint) ownerIndex;
  // the ongoing operations.
  mapping(bytes32 => PendingState) pendings;
  bytes32[] pendingsIndex;


  // EVENTS

  // this contract only has six types of events: it can accept a confirmation, in which case
  // we record owner and operation (hash) alongside it.
  event Confirmation(address owner, bytes32 operation);
  event Revoke(address owner, bytes32 operation);


  // MODIFIERS

  address thisContract = this;

  // simple single-sig function modifier.
  modifier onlyOwner {
    if (isOwner(msg.sender))
      _;
  }

  // multi-sig function modifier: the operation must have an intrinsic hash in order
  // that later attempts can be realised as the same underlying operation and
  // thus count as confirmations.
  modifier onlyManyOwners(bytes32 _operation) {
    if (confirmAndCheck(_operation))
      _;
  }


  // CONSTRUCTOR

  // constructor is given number of sigs required to do protected "onlymanyowners" transactions
  // as well as the selection of addresses capable of confirming them.
  function Shareable(address[] _owners, uint _required) {
    owners[1] = msg.sender;
    ownerIndex[msg.sender] = 1;
    for (uint i = 0; i < _owners.length; ++i) {
      owners[2 + i] = _owners[i];
      ownerIndex[_owners[i]] = 2 + i;
    }
    if (required > owners.length) throw;
    required = _required;
  }


  // new multisig is given number of sigs required to do protected "onlymanyowners" transactions
  // as well as the selection of addresses capable of confirming them.
  // take all new owners as an array
  function changeShareable(address[] _owners, uint _required) onlyManyOwners(sha3(msg.data)) {
    for (uint i = 0; i < _owners.length; ++i) {
      owners[1 + i] = _owners[i];
      ownerIndex[_owners[i]] = 1 + i;
    }
    if (required > owners.length) throw;
    required = _required;
  }

  // METHODS

  // Revokes a prior confirmation of the given operation
  function revoke(bytes32 _operation) external {
    uint index = ownerIndex[msg.sender];
    // make sure they're an owner
    if (index == 0) return;
    uint ownerIndexBit = 2**index;
    var pending = pendings[_operation];
    if (pending.ownersDone & ownerIndexBit > 0) {
      pending.yetNeeded++;
      pending.ownersDone -= ownerIndexBit;
      Revoke(msg.sender, _operation);
    }
  }

  // Gets an owner by 0-indexed position (using numOwners as the count)
  function getOwner(uint ownerIndex) external constant returns (address) {
    return address(owners[ownerIndex + 1]);
  }

  function isOwner(address _addr) constant returns (bool) {
    return ownerIndex[_addr] > 0;
  }

  function hasConfirmed(bytes32 _operation, address _owner) constant returns (bool) {
    var pending = pendings[_operation];
    uint index = ownerIndex[_owner];

    // make sure they're an owner
    if (index == 0) return false;

    // determine the bit to set for this owner.
    uint ownerIndexBit = 2**index;
    return !(pending.ownersDone & ownerIndexBit == 0);
  }

  // INTERNAL METHODS

  function confirmAndCheck(bytes32 _operation) internal returns (bool) {
    // determine what index the present sender is:
    uint index = ownerIndex[msg.sender];
    // make sure they're an owner
    if (index == 0) return;

    var pending = pendings[_operation];
    // if we're not yet working on this operation, switch over and reset the confirmation status.
    if (pending.yetNeeded == 0) {
      // reset count of confirmations needed.
      pending.yetNeeded = required;
      // reset which owners have confirmed (none) - set our bitmap to 0.
      pending.ownersDone = 0;
      pending.index = pendingsIndex.length++;
      pendingsIndex[pending.index] = _operation;
    }
    // determine the bit to set for this owner.
    uint ownerIndexBit = 2**index;
    // make sure we (the message sender) haven't confirmed this operation previously.
    if (pending.ownersDone & ownerIndexBit == 0) {
      Confirmation(msg.sender, _operation);
      // ok - check if count is enough to go ahead.
      if (pending.yetNeeded <= 1) {
        // enough confirmations: reset and run interior.
        delete pendingsIndex[pendings[_operation].index];
        delete pendings[_operation];
        return true;
      }
      else
        {
          // not enough: record that this owner in particular confirmed.
          pending.yetNeeded--;
          pending.ownersDone |= ownerIndexBit;
        }
    }
  }

  function clearPending() internal {
    uint length = pendingsIndex.length;
    for (uint i = 0; i < length; ++i)
    if (pendingsIndex[i] != 0)
      delete pendings[pendingsIndex[i]];
    delete pendingsIndex;
  }
}
/*
 * Shareable
 * 
 * Effectively our multisig contract
 *
 * Based on https://github.com/ethereum/dapp-bin/blob/master/wallet/wallet.sol
 *
 * inheritable "property" contract that enables methods to be protected by requiring the acquiescence of either a single, or, crucially, each of a number of, designated owners.
 *
 * usage:
 * use modifiers onlyowner (just own owned) or onlymanyowners(hash), whereby the same hash must be provided by some number (specified in constructor) of the set of owners (specified in the constructor) before the interior is executed.
 */
function revoke(bytes32 _operation) external {
  uint index = ownerIndex[msg.sender];
  // make sure they're an owner
  if (index == 0) return;
  uint ownerIndexBit = 2**index;
  var pending = pendings[_operation];
  if (pending.ownersDone & ownerIndexBit > 0) {
    pending.yetNeeded++;
    pending.ownersDone -= ownerIndexBit;
    Revoke(msg.sender, _operation);
  }
}

// METHODS
// Revokes a prior confirmation of the given operation