Handle merkleRoot of empty sequence

Co-Authored-By: Balazs Komuves <bkomuves@gmail.com>

poseidon2/merkle.nim

@@ -15,6 +15,9 @@ func merkleRoot(xs: openArray[F], isBottomLayer: static bool) : F =
   let b = high(xs)
   let m = b-a+1
 
+  when isBottomLayer:
+    assert m > 0, "merkle root of empty sequence is not defined"
+
   when not isBottomLayer:
     if m==1:
       return xs[a]

tests/poseidon2/testIo.nim

@@ -32,6 +32,14 @@ suite "conversion to/from bytes":
     let elements = toSeq bytes.elements(F)
     check bool(elements[^1] == expected)
 
+  test "converts empty sequence of bytes to single field element":
+    let bytes = seq[byte].default
+    let marker = @[1'u8]
+    let expected = F.fromBytes(marker.toArray)
+    let elements = toSeq bytes.elements(F)
+    check elements.len == 1
+    check bool(elements[0] == expected)
+
   test "conversion from bytes pads the last chunk when it's less than 31 bytes":
     let bytes = toSeq 1'u8..80'u8
     let marker = @[1'u8]

tests/poseidon2/testMerkle.nim

@@ -65,3 +65,13 @@ suite "merkle root":
     let bytes = toSeq 1'u8..80'u8
     let rootAsBytes = merkleRoot(bytes).toBytes()
     check rootAsBytes.toHex == "0x40989b63104f39e3331767883381085bcfc46e2202679123371f1ffe53521b16"
+
+  test "merkle root of empty sequence of elements":
+    let empty = seq[F].default
+    expect Exception:
+      discard merkleRoot(empty)
+
+  test "merkle root of empty sequency of bytes":
+    # merkle root of empty sequence of bytes is uniquely defined through padding
+    let empty = seq[byte].default
+    check merkleRoot(empty).toBytes.toHex == "0xcc8da1d157900e611b89e258d95450e707f4f9eec169422d7c26aba54f803c08"