Adds method for converting 32 bytes to field element

poseidon2/io.nim

@@ -1,8 +1,9 @@
-import ./types
+import std/options
 import constantine/math/arithmetic
 import constantine/math/io/io_bigints
 import constantine/math/io/io_fields
 import constantine/math/config/curves
+import ./types
 
 export curves
 
@@ -14,6 +15,13 @@ func fromBytes*(_: type F, bytes: array[31, byte]): F =
   ## canonical little-endian big integer.
   F.fromOpenArray(bytes)
 
+func fromBytes*(_: type F, bytes: array[32, byte]): Option[F] =
+  ## Converts bytes into a field element. The byte array is interpreted as a
+  ## canonical little-endian big integer.
+  let big = B.unmarshal(bytes, littleEndian)
+  if big < F.fieldMod():
+    return some(F.fromBig(big))
+
 func toBytes*(element: F): array[32, byte] =
   ## Converts a field element into its canonical representation in little-endian
   ## byte order. Uses at most 254 bits, the remaining most-significant bits are

tests/poseidon2/testIo.nim

@@ -18,6 +18,18 @@ suite "conversion to/from bytes":
     let unmarshalled = F.fromBytes(bytes)
     check bool(unmarshalled == expected)
 
+  test "converts 32 little endian bytes into a field elements":
+    let bytes = toArray toSeq 1'u8..32'u8
+    let expected = F.fromBig(B.unmarshal(bytes, littleEndian))
+    let unmarshalled = F.fromBytes(bytes).get()
+    check bool(unmarshalled == expected)
+
+  test "convert fails for 32 little endian bytes larger than the prime field":
+    let bytes = toArray toSeq(255'u8).repeat(32)
+    let expected = F.fromBig(B.unmarshal(bytes, littleEndian))
+    let unmarshalled = F.fromBytes(bytes)
+    check not unmarshalled.isSome()
+
   test "converts every 31 bytes into a field element":
     let bytes = toSeq 1'u8..62'u8
     let expected1 = F.fromBytes(bytes[0..<31].toArray)