public static DifficultyCalculator calculator(final GenesisConfigOptions config) {
    long difficulty = config.getEthashConfigOptions().getFixedDifficulty().getAsLong();
    return (time, parent) -> BigInteger.valueOf(difficulty);
  }
This provides a difficulty calculator that can be used during development efforts; given
development (typically) uses CPU based mining, a negligible difficulty ensures tests etc. execute
quickly.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/difficulty/fixed/FixedDifficultyCalculators.java