The random number generation for hive-engine is defined here:
https://github.com/hive-engine/steemsmartcontracts/blob/hive-engine/libs/SmartContracts.js#L134

const rng = seedrandom(`${prevRefHiveBlockId}${refHiveBlockId}${transactionId}`);

the random number function is using rng:
https://github.com/hive-engine/steemsmartcontracts/blob/hive-engine/libs/SmartContracts.js#L180

The mining source code can be found here:
https://github.com/hive-engine/steemsmartcontracts/blob/hive-engine/contracts/mining.js