random source generatation doc/postconditions

contracts/epochs/FlowEpoch.cdc

@@ -207,7 +207,7 @@ access(all) contract FlowEpoch {
  /// using the same procedure as during a spork.
  /// CAUTION: Validity of the QCs is not explicitly verified during the recovery process. An
  /// invalid cluster QC will prevent the respective cluster from starting its local consensus
- /// and hence prevent if from functioning for the entire epoch. If all cluster QCs are invalid,
+ /// and hence prevent it from functioning for the entire epoch. If all cluster QCs are invalid,
  /// the blockchain cannot ingest transactions, which can only be resolved through a spork!
  clusterQCVoteData: [FlowClusterQC.ClusterQCVoteData],
 
@@ -433,15 +433,16 @@ access(all) contract FlowEpoch {
  }
  }
 
+ /// Generates 128 bits of randomness using system random (derived from Random Beacon).
  access(contract) fun generateRandomSource(): String {
- /// random source must be a hex string of 32 characters (i.e 16 bytes or 128 bits)
- /// `revertibleRandom` returns a UInt64 (8 bytes)
+ post {
+ result.length == 32:
+ "FlowEpoch.generateRandomSource: Critical invariant violated! "
+ .concat("Expected hex random source with length 32 (128 bits) but got length ")
+ .concat(result.length.toString())
+ .concat(" instead.")
+ }
  var randomSource = String.encodeHex(revertibleRandom<UInt128>().toBigEndianBytes())
- assert (
- randomSource.length == 32,
- message: "Random source must be a hex string of 32 characters"
- )
-
  return randomSource
  }