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Fix Bitmap offset bug
wearekickback
kickback
## Bounty
- 1 ETh to the first person who raises PR to fix the bug with a complete test.
## Issue summary
At Kickback, we send a list of participants who were checked in as a bitmap to save gas cost.
eg: when 300 people RSVPed and i1, i2, i256, i257, i298, i299 attended, we send bitmap of 2 bytes containing the following.
`finalise([110, 11000000000000000000000000000000000000000011])`
NOTE: Bitmap starts from right to left so 110 is 0, 1, 1
For more info, please refer to [our test](https://github.com/wearekickback/contracts/blob/master/test/behaviors/conferenceFinalize.behavior.js#L50).
At the last [EthLondon hackathon](https://kickback.events/event/0x741909cab7ab8efce27761237d3463fc779ca539), we had a dozen of reports from participants who could not withdraw despite marked as "checked in" because [their transactions](https://etherscan.io/tx/0xba5b98dbfaf8649de0597bc26ced743404bd03a737368e04e1971acdd151c6b3) got reverted with message 'Fail with error 'event still active or you did not attend''
After [our analysis](https://docs.google.com/spreadsheets/d/14xaRx6vP_VklsHeuQflv17lQS6paF0Qxey1G8-I0sIw/edit#gid=0), it turned out `isAttended(address)` that the logic to return whether the user has attended or not does not match by one row after 123rd participant.
The column C(bitmap) is derived from the bitmap info sent at finalisation at [etherscan](https://etherscan.io/tx/0x0f84a80131d1b2c840259445988d58d358c3f293a2ff04f78df6c906d99b255e#eventlog)
Input data
```
0xe17e082b0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000100000000000f7fdffffdadbfaffefffb57ff87dbddd2f797ef7dfbfcfceff4df
```
Hex `f7fdffffdadbfaffefffb57ff87dbddd2f797ef7dfbfcfceff4df` => Bin `11110111111111011111111111111111110110101101101111111010111111111110111111111111101101010111111111111000011111011011110111011101001011110111100101111110111101111101111110111111110011111100111011111111010011011111` then you reverse the order.
The column D(`isAttended`) is derived by calling the function for each address
```
function isAttended(address _addr) public view returns (bool){
if (!isRegistered(_addr) || !ended) {
return false;
}
// check the attendance maps
else {
Participant storage p = participants[_addr];
uint256 pIndex = p.index - 1;
uint256 map = attendanceMaps[uint256(pIndex / 256)];
return (0 < (map & (2 ** (pIndex % 256))));
}
}
```
## Submission criteria
We suspect that the bug is either in [our smart contract](https://github.com/wearekickback/contracts) or [js library](https://github.com/wearekickback/shared/blob/master/src/participants.js) to encode/decode the bitmap.
- Write a failing test to replicate the error.
- Write a fix to make the test pass.
- Make sure that other tests do not fail.
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