Add getCutoffs helper (buggy)

beacon_chain/datatypes.nim

@@ -85,4 +85,4 @@ const
   EpochLength*         = 64                           # blocks
   SlotDuration*        = 8                            # seconds
   MinCommitteeSize*    = 128                          # (rationale: see recommended minimum 111 here https://vitalik.ca/files/Ithaca201807_Sharding.pdf)
-  EndEpochGracePeriod = 8                            # blocks
+  EndEpochGracePeriod* = 8                            # blocks

beacon_chain/private/helpers.nim

@@ -8,8 +8,8 @@
 # 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
+func getShuffling*(seed: Blake2_256_Digest, validatorCount: int): seq[int] {.noInit.}=
+  ## Pseudorandomly shuffles the validator set based on some seed
 
   assert validatorCount <= MaxValidatorCount
 
@@ -31,3 +31,49 @@ func getShuffling(seed: Blake2_256_Digest, validatorCount: int): seq[int] {.noIn
         let replacementPos = m mod remaining + i
         swap result[i], result[replacementPos]
         inc i
+
+func getCutoffs*(validatorCount: int): tuple[height, shard: seq[int]] {.noInit.} =
+  ## Split up validators into groups at the start of every epoch,
+  ## determining at what height they can make attestations and what shard they are making crosslinks for
+  ## Implementation should do the following: http://vitalik.ca/files/ShuffleAndAssign.png
+  ## TODO: It doens't work.
+
+  result.height = @[0]
+  let cofactor = 39 # EpochLength / phi
+  const StandardCommitteeSize = MaxValidatorCount div ShardCount
+
+  var heightCount: int
+  var heights: seq[int]
+
+  if validatorCount < EpochLength * MinCommitteeSize:
+    # If there are not enough validators to fill a minimally
+    # sized committee at every height, skip some heights
+    heightCount = validatorCount div MinCommitteeSize or 1 # TODO div/or precedence ?
+    for i in 0 ..< heightCount:
+      heights.add (i * cofactor) mod EpochLength
+  else:
+    # If there are enough validators, fill all the heights
+    heightCount = EpochLength
+    heights = toSeq(0 ..< EpochLength)
+
+  var filled = 0
+  for i in 0 ..< EpochLength - 1:
+    if i notin heights: # TODO, this will be slow for seq, use intsets instead?
+      result.height.add result.height[^1]
+    else:
+      inc filled
+      result.height.add filled * validatorCount div heightCount
+  result.height.add validatorCount
+
+  # For the validators assigned to each height, split them up
+  # into committees for different shards. Do not assign the
+  # last END_EPOCH_GRACE_PERIOD heights in an epoch to any shards.
+
+  result.shard = @[0]
+  for i in 0 ..< EpochLength - EndEpochGracePeriod:
+    let
+      size = result.height[i+1] - result.height[i]
+      shards = (size + StandardCommitteeSize - 1) div StandardCommitteeSize
+      pre = result.shard[^1]
+    for j in 1 .. shards:
+      result.shard.add pre + size * j div shards