Add the pseudo renadom shuffle

beacon_chain/datatypes.nim

@@ -14,8 +14,8 @@ type
   AttestationVote* = object
     # Implementation pending https://ethresear.ch/t/implementations-of-proof-of-concept-beacon-chains/2509/5?u=mratsim
 
-  Keccak256_Digest* = distinct Hash256  # TODO, previously Keccak256 fields used the "bytes" type
-  Blake2_256_Digest* = distinct Hash256 # while Blake2 used hash32, but latest spec changed everything to hash32
+  Keccak256_Digest* =  Hash256  # TODO, previously Keccak256 fields used the "bytes" type
+  Blake2_256_Digest* = Hash256 # while Blake2 used hash32, but latest spec changed everything to hash32
 
   BeaconBlock* = object
     parentHash*: Keccak256_Digest             # Hash of the parent block
@@ -53,8 +53,8 @@ type
     currentEpochShuffling: seq[int32] #int24   # The permutation of validators used to determine who cross-links what shard in this epoch
     lastJustifiedEpoch: int64                  # The last justified epoch
     lastFinalizedEpoch: int64                  # The last finalized epoch
-    current_dynasty: int64                     # The current dynasty
-    next_shard: int16                          # The next shard that cross-linking assignment will start from
+    currentDynasty: int64                      # The current dynasty
+    nextShard: int16                           # The next shard that cross-linking assignment will start from
     currentCheckpoint: Keccak256_Digest        # The current FFG checkpoint
     crosslinkRecords: seq[CrosslinkRecord]     # Records about the most recent crosslink `for each shard
     totalDeposits: Int256                      # Total balance of deposits

beacon_chain/private/helpers.nim

@@ -0,0 +1,33 @@
+# beacon_chain
+# Copyright (c) 2018 Status Research & Development GmbH
+# Licensed and distributed under either of
+#   * MIT license (license terms in the root directory or at http://opensource.org/licenses/MIT).
+#   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+# Helper functions
+import ../datatypes, sequtils, nimcrypto
+
+func getShuffling(seed: Blake2_256_Digest, validatorCount: int): seq[int] {.noInit.}=
+  # Pseudorandomly shuffles the validator set based on some seed
+
+  assert validatorCount <= MaxValidatorCount
+
+  let randMax = MaxValidatorCount - MaxValidatorCount mod validatorCount
+  result = toSeq(0 ..< validatorCount)
+  var source = seed
+
+  var i = 0
+  while i < validatorCount:
+    source = blake2_256.digest source.data
+    for pos in countup(0, 29, 3):
+      let remaining = validatorCount - i
+      if remaining == 0:
+        break
+
+      let m = source.data[pos].int shl 16 or source.data[pos+1].int shl 8 or source.data[pos+2].int
+
+      if validatorCount < randMax:
+        let replacementPos = m mod remaining + i
+        swap result[i], result[replacementPos]
+        inc i