Optimized Slasher Docs and Helpers (#9578)

* bring over helpers

* slasher helpers pass tests

* fix dead link

* rem eth2

* gaz

* params

* gaz

* builds

Co-authored-by: Preston Van Loon <preston@prysmaticlabs.com>

beacon-chain/slasher/doc.go

@@ -0,0 +1,43 @@
+// Package slasher defines an optimized implementation of Ethereum proof-of-stake slashing
+// detection, namely focused on catching "surround vote" slashable
+// offenses as explained here: https://blog.ethereum.org/2020/01/13/validated-staking-on-eth2-1-incentives/.
+//
+// Surround vote detection is a difficult problem if done naively, as slasher
+// needs to keep track of every single attestation by every single validator
+// in the network and be ready to efficiently detect whether incoming attestations
+// are slashable with respect to older ones. To do this, the Sigma Prime team
+// created an elaborate design document: https://hackmd.io/@sproul/min-max-slasher
+// offering an optimal solution.
+//
+// Attesting histories are kept for each validator in two separate arrays known
+// as min and max spans, which are explained in our design document:
+// https://hackmd.io/@prysmaticlabs/slasher.
+//
+// A regular pair of min and max spans for a validator look as follows
+// with length = H where H is the amount of epochs worth of history
+// we want to persist for slashing detection.
+//
+//  validator_1_min_span = [2, 2, 2, ..., 2]
+//  validator_1_max_span = [0, 0, 0, ..., 0]
+//
+// Instead of always dealing with length H arrays, which can be prohibitively
+// expensive to handle in memory, we split these arrays into chunks of length C.
+// For C = 3, for example, the 0th chunk of validator 1's min and max spans would look
+// as follows:
+//
+//  validator_1_min_span_chunk_0 = [2, 2, 2]
+//  validator_1_max_span_chunk_0 = [2, 2, 2]
+//
+// Next, on disk, we take chunks for K validators, and store them as flat slices.
+// For example, if H = 3, C = 3, and K = 3, then we can store 3 validators' chunks as a flat
+// slice as follows:
+//
+//     val0     val1     val2
+//      |        |        |
+//   {     }  {     }  {     }
+//  [2, 2, 2, 2, 2, 2, 2, 2, 2]
+//
+// This is known as 2D chunking, pioneered by the Sigma Prime team here:
+// https://hackmd.io/@sproul/min-max-slasher. The parameters H, C, and K will be
+// used extensively throughout this package.
+package slasher