SampleReducer.sol

import "../MerkluxReducer.sol";

contract TransferReducer is MerkluxReducer {
    function reduce(
        IStateTree _tree,
        address _from,
        bytes memory _encodedParams // encoded params
    ) public returns (
        bytes memory _encodedPairs // encoded key value pairs
    ) {
        // 1. Decode data
        (uint _amount, address _to) = abi.decode(_encodedParams, (uint, address));

        // 2. Calculate
        bytes memory _senderKey = abi.encodePacked(_from);
        bytes memory _receiverKey = abi.encodePacked(_to);
        uint _senderBalance = _tree.read(_senderKey).toRlpItem().toUint();
        uint _receiverBalance = _tree.read(_to).toRlpItem().toUint();
        require(_senderBalance >= _amount);

        // 3. Return pairs to update
        ReducerUtil.RlpData memory pairs;
        pairs = pairs.addUint(_senderKey, _senderBalance - amount);
        pairs = pairs.addUint(_receiverKey, _receiverBalance + _amount);
        return pairs.encode();
    }
}