Batch Serialization (#283)

* feat: batchSerialization banderwagon elements

* tests for Batch Serialize

* fix: removed failure point in batchOperation

* fix: changed to allocStackArray

* Update constantine/math/elliptic/ec_twistededwards_batch_ops.nim

Co-authored-by: Mamy Ratsimbazafy <mamy_github@numforge.co>

* fix: removed return to bool & changed to debug

---------

Co-authored-by: Mamy Ratsimbazafy <mamy_github@numforge.co>

constantine/math/elliptic/ec_twistededwards_batch_ops.nim

@@ -115,9 +115,9 @@ func batchInvert*[F](
     dst[i] *= accumulator
     accumulator *= elements[i]
 
-func batchInvert*[F](dst: openArray[F], source: openArray[F]): bool {.inline.} =
-  if dst.len != source.len:
-    return false
-  let N = dst.len
-  batchInvert(dst.asUnchecked(), source.asUnchecked(), N)
-  return true
+func batchInvert*[F](dst: var openArray[F], source: openArray[F]) {.inline.} =
+  debug: doAssert dst.len == source.len
+  batchInvert(dst.asUnchecked(), source.asUnchecked(), dst.len)
+
+func batchInvert*[N: static int, F](dst: var array[N, F], src: array[N, F]) =
+  batchInvert(dst.asUnchecked(), src.asUnchecked(), N)

constantine/serialization/codecs_banderwagon.nim

@@ -17,7 +17,8 @@ import
   ../math/config/curves,
   ../math/elliptic/[
     ec_twistededwards_affine,
-    ec_twistededwards_projective
+    ec_twistededwards_projective,
+    ec_twistededwards_batch_ops
   ],
   ../math/[
     extension_fields,
@@ -111,3 +112,43 @@ func deserialize*(dst: var EC_Prj, src: array[32, byte]): CttCodecEccStatus =
     return cttCodecEcc_PointNotInSubgroup
 
   return cttCodecEcc_Success
+
+## ############################################################
+##
+##              Banderwagon Batch Serialization
+##
+## ############################################################
+
+func serializeBatch*(
+    dst: ptr UncheckedArray[array[32, byte]],
+    points: ptr UncheckedArray[EC_Prj],
+    N: int,
+  ) : CttCodecEccStatus {.noInline.} =
+
+  # collect all the z coordinates
+  var zs = allocStackArray(Fp[Banderwagon], N)
+  var zs_inv = allocStackArray(Fp[Banderwagon], N)
+  for i in 0 ..< N:
+    zs[i] = points[i].z
+
+  zs_inv.batchInvert(zs, N)
+  
+  for i in 0 ..< N:
+    var X: Fp[Banderwagon]
+    var Y: Fp[Banderwagon]
+
+    X.prod(points[i].x, zs_inv[i])
+    Y.prod(points[i].y, zs_inv[i])
+
+    let lexicographicallyLargest = Y.toBig() >= Fp[Banderwagon].getPrimeMinus1div2()
+    if not lexicographicallyLargest.bool():
+      X.neg()
+
+    dst[i].marshal(X, bigEndian)
+
+  return cttCodecEcc_Success
+
+func serializeBatch*[N: static int](
+        dst: var array[N, array[32, byte]],
+        points: array[N, EC_Prj]): CttCodecEccStatus {.inline.} =
+  return serializeBatch(dst.asUnchecked(), points.asUnchecked(), N)

tests/t_ethereum_verkle_primitives.nim

@@ -303,7 +303,7 @@ suite "Batch Operations on Banderwagon":
         one.double()
 
       var arr_fp_inv: array[n, Fp[Banderwagon]]
-      doAssert arr_fp_inv.batchInvert(arr_fp) == true
+      arr_fp_inv.batchInvert(arr_fp)
 
       # Checking the correspondence with singular element inversion
       for i in 0 ..< n:
@@ -341,3 +341,27 @@ suite "Batch Operations on Banderwagon":
       doAssert (expected_b == scalars[1]).bool(), "expected scalar for point `B` is incorrect"
 
     testBatchMapToBaseField()
+
+  ## Check encoding if it is as expected or not
+  test "Test Batch Encoding from Fixed Vectors":
+    proc testBatchSerialize(len: static int) =
+      # First the point is set to generator P
+      # then with each iteration 2P, 4P, . . . doubling
+      var points: array[len, EC]
+      var point {.noInit.}: EC
+      point.fromAffine(generator)
+      
+      for i in 0 ..< len:
+        points[i] = point
+        point.double() #doubling the point
+
+      var arr: array[len, Bytes]
+      let stat = arr.serializeBatch(points)
+
+      # Check if the serialization took place and in expected way
+      doAssert stat == cttCodecEcc_Success, "Serialization Failed"
+
+      for i in 0 ..< len:
+        doAssert expected_bit_strings[i] == arr[i].toHex(), "bit string does not match expected"
+
+    testBatchSerialize(expected_bit_strings.len)