From 85d710906565753b2cd48e9f580bff05ac819c37 Mon Sep 17 00:00:00 2001
From: Agnish Ghosh <80243668+agnxsh@users.noreply.github.com>
Date: Mon, 23 Sep 2024 03:05:57 +0530
Subject: [PATCH] add: `get_data_column_sidecars` (#6566)

* add: get_data_column_sidecars

* remove redundant type conversions

* remove redundant for loop

* review 2

* review 1
---
 beacon_chain/spec/crypto.nim            |   3 +
 beacon_chain/spec/datatypes/eip7594.nim |   1 +
 beacon_chain/spec/eip7594_helpers.nim   | 127 ++++++++++++++++++++++++
 3 files changed, 131 insertions(+)

diff --git a/beacon_chain/spec/crypto.nim b/beacon_chain/spec/crypto.nim
index 4efac1688..3fac973e4 100644
--- a/beacon_chain/spec/crypto.nim
+++ b/beacon_chain/spec/crypto.nim
@@ -403,6 +403,9 @@ func toHex*(x: CookedPubKey): string =
 func `$`*(x: CookedPubKey): string =
   $(x.toPubKey())
 
+func toValidatorSig*(x: TrustedSig): ValidatorSig =
+  ValidatorSig(blob: x.blob)
+
 func toValidatorSig*(x: CookedSig): ValidatorSig =
   ValidatorSig(blob: blscurve.Signature(x).exportRaw())
 
diff --git a/beacon_chain/spec/datatypes/eip7594.nim b/beacon_chain/spec/datatypes/eip7594.nim
index 0bcef4ef3..5ca9c9644 100644
--- a/beacon_chain/spec/datatypes/eip7594.nim
+++ b/beacon_chain/spec/datatypes/eip7594.nim
@@ -28,6 +28,7 @@ const
 
   # https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.5/specs/_features/eip7594/p2p-interface.md#preset
   KZG_COMMITMENTS_INCLUSION_PROOF_DEPTH* = 4
+  KZG_COMMITMENTS_INCLUSION_PROOF_DEPTH_GINDEX* = 27
 
 type
   # https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.6/specs/_features/eip7594/polynomial-commitments-sampling.md#custom-types
diff --git a/beacon_chain/spec/eip7594_helpers.nim b/beacon_chain/spec/eip7594_helpers.nim
index 0ecaefe85..e1824d610 100644
--- a/beacon_chain/spec/eip7594_helpers.nim
+++ b/beacon_chain/spec/eip7594_helpers.nim
@@ -12,9 +12,18 @@ import
   std/algorithm,
   results,
   eth/p2p/discoveryv5/[node],
+  kzg4844/[kzg],
+  ssz_serialization/[
+    proofs,
+    types],
+  ./crypto,
   ./[helpers, digest],
   ./datatypes/[eip7594]
 
+type
+  CellBytes = array[eip7594.CELLS_PER_EXT_BLOB, Cell]
+  ProofBytes = array[eip7594.CELLS_PER_EXT_BLOB, KzgProof]
+
 func sortedColumnIndices*(columnsPerSubnet: ColumnIndex,
                           subnetIds: HashSet[uint64]):
                           seq[ColumnIndex] =
@@ -155,6 +164,124 @@ proc recover_matrix*(partial_matrix: seq[MatrixEntry],
 
   ok(extended_matrix)
 
+# https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.6/specs/_features/eip7594/das-core.md#get_data_column_sidecars
+proc get_data_column_sidecars*(signed_beacon_block: electra.TrustedSignedBeaconBlock,
+                               cellsAndProofs: seq[CellsAndProofs]):
+                               seq[DataColumnSidecar] =
+  ## Given a trusted signed beacon block and the cells/proofs associated
+  ## with each data column (thereby blob as well) corresponding to the block,
+  ## this function assembles the sidecars which can be distributed to 
+  ## the peers post data column reconstruction at every slot start.
+  ## 
+  ## Note: this function only accepts `TrustedSignedBeaconBlock` as
+  ## during practice we would be computing cells and proofs from 
+  ## data columns only after retrieving them from the database, where
+  ## they we were already verified and persisted.
+  template blck(): auto = signed_beacon_block.message
+  let
+    beacon_block_header =
+      BeaconBlockHeader(
+        slot: blck.slot,
+        proposer_index: blck.proposer_index,
+        parent_root: blck.parent_root,
+        state_root: blck.state_root,
+        body_root: hash_tree_root(blck.body))
+    
+    signed_beacon_block_header =
+      SignedBeaconBlockHeader(
+        message: beacon_block_header,
+        signature: signed_beacon_block.signature.toValidatorSig)
+  
+  var
+    sidecars =
+      newSeqOfCap[DataColumnSidecar](CELLS_PER_EXT_BLOB)
+
+  for column_index in 0..<NUMBER_OF_COLUMNS:
+    var
+      column_cells: seq[KzgCell]
+      column_proofs: seq[KzgProof]
+    for i in 0..<cellsAndProofs.len:
+      column_cells.add(cellsAndProofs[i].cells)
+      column_proofs.add(cellsAndProofs[i].proofs)
+
+    var sidecar = DataColumnSidecar(
+      index: ColumnIndex(column_index),
+      column: DataColumn.init(column_cells),
+      kzg_commitments: blck.body.blob_kzg_commitments,
+      kzg_proofs: KzgProofs.init(column_proofs),
+      signed_block_header: signed_beacon_block_header)
+    blck.body.build_proof(
+      KZG_COMMITMENTS_INCLUSION_PROOF_DEPTH_GINDEX.GeneralizedIndex,
+      sidecar.kzg_commitments_inclusion_proof).expect("Valid gindex")
+    sidecars.add(sidecar)
+
+  sidecars  
+    
+# Alternative approach to `get_data_column_sidecars` by directly computing
+# blobs from blob bundles
+proc get_data_column_sidecars*(signed_beacon_block: electra.SignedBeaconBlock, 
+                               blobs: seq[KzgBlob]): 
+                               Result[seq[DataColumnSidecar], string] =
+  ## Given a signed beacon block and the blobs corresponding to the block,
+  ## this function assembles the sidecars which can be distributed to 
+  ## the peers post data column reconstruction at every slot start.
+  ## 
+  ## Note: this function only accepts `SignedBeaconBlock` as
+  ## during practice we would be extracting data columns
+  ## before publishing them, all of this happens during block 
+  ## production, hence the blocks are yet untrusted and have not 
+  ## yet been verified.
+  template blck(): auto = signed_beacon_block.message
+  let
+    beacon_block_header =
+      BeaconBlockHeader(
+        slot: blck.slot,
+        proposer_index: blck.proposer_index,
+        parent_root: blck.parent_root,
+        state_root: blck.state_root,
+        body_root: hash_tree_root(blck.body))
+    
+    signed_beacon_block_header =
+      SignedBeaconBlockHeader(
+        message: beacon_block_header,
+        signature: signed_beacon_block.signature)
+  
+  var
+    sidecars =
+      newSeqOfCap[DataColumnSidecar](CELLS_PER_EXT_BLOB)
+    cells = newSeq[CellBytes](blobs.len)
+    proofs = newSeq[ProofBytes](blobs.len)
+
+  for i in 0..<blobs.len:
+    let
+      cell_and_proof = computeCellsAndKzgProofs(blobs[i])
+    if cell_and_proof.isErr():
+      return err("EIP7549: Could not compute cells")
+
+    cells[i] = cell_and_proof.get.cells
+    proofs[i] = cell_and_proof.get.proofs
+
+  for columnIndex in 0..<CELLS_PER_EXT_BLOB:
+    var 
+      column: seq[KzgCell]
+      kzgProofOfColumn: seq[KzgProof]
+    for rowIndex in 0..<blobs.len:
+      column.add(cells[rowIndex][columnIndex])
+      kzgProofOfColumn.add(proofs[rowIndex][columnIndex])
+
+    var sidecar = DataColumnSidecar(
+      index: ColumnIndex(columnIndex),
+      column: DataColumn.init(column),
+      kzg_commitments: blck.body.blob_kzg_commitments,
+      kzg_proofs: KzgProofs.init(kzgProofOfColumn),
+      signed_block_header: signed_beacon_block_header)
+    blck.body.build_proof(
+      KZG_COMMITMENTS_INCLUSION_PROOF_DEPTH_GINDEX.GeneralizedIndex,
+      sidecar.kzg_commitments_inclusion_proof).expect("Valid gindex")
+    sidecars.add(sidecar)
+
+  ok(sidecars)
+
 # https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.6/specs/_features/eip7594/peer-sampling.md#get_extended_sample_count
 func get_extended_sample_count*(samples_per_slot: int,
                                 allowed_failures: int):