nimbus-eth2/beacon_chain/ssz/bytes_reader.nim

import
  endians, typetraits, options,
  stew/[objects, bitseqs], serialization/testing/tracing,
  ../spec/[digest, datatypes], ./types

template setLen[R, T](a: var array[R, T], length: int) =
  if length != a.len:
    raise newException(MalformedSszError, "SSZ input of insufficient size")

template assignNullValue(loc: untyped, T: type): auto =
  when T is ref|ptr:
    loc = nil
  elif T is Option:
    loc = T()
  else:
    raise newException(MalformedSszError, "SSZ list element of zero size")

# fromSszBytes copies the wire representation to a Nim variable,
# assuming there's enough data in the buffer
func fromSszBytes*(T: type SomeInteger, data: openarray[byte]): T =
  ## Convert directly to bytes the size of the int. (e.g. ``uint16 = 2 bytes``)
  ## All integers are serialized as **little endian**.
  ## TODO: Assumes data points to a sufficiently large buffer
  doAssert data.len == sizeof(result)
  # TODO: any better way to get a suitably aligned buffer in nim???
  # see also: https://github.com/nim-lang/Nim/issues/9206
  var tmp: uint64
  var alignedBuf = cast[ptr byte](tmp.addr)
  copyMem(alignedBuf, unsafeAddr data[0], result.sizeof)

  when result.sizeof == 8: littleEndian64(result.addr, alignedBuf)
  elif result.sizeof == 4: littleEndian32(result.addr, alignedBuf)
  elif result.sizeof == 2: littleEndian16(result.addr, alignedBuf)
  elif result.sizeof == 1: copyMem(result.addr, alignedBuf, sizeof(result))
  else: {.fatal: "Unsupported type deserialization: " & $(type(result)).name.}

func fromSszBytes*(T: type bool, data: openarray[byte]): T =
  # TODO: spec doesn't say what to do if the value is >1 - we'll use the C
  #       definition for now, but maybe this should be a parse error instead?
  fromSszBytes(uint8, data) != 0

func fromSszBytes*(T: type Eth2Digest, data: openarray[byte]): T =
  doAssert data.len == sizeof(result.data)
  copyMem(result.data.addr, unsafeAddr data[0], sizeof(result.data))

template fromSszBytes*(T: type Slot, bytes: openarray[byte]): Slot =
  Slot fromSszBytes(uint64, bytes)

template fromSszBytes*(T: type Epoch, bytes: openarray[byte]): Epoch =
  Epoch fromSszBytes(uint64, bytes)

template fromSszBytes*(T: type enum, bytes: openarray[byte]): auto =
  T fromSszBytes(uint64, bytes)

template fromSszBytes*(T: type BitSeq, bytes: openarray[byte]): auto =
  BitSeq @bytes

proc fromSszBytes*[N](T: type BitList[N], bytes: openarray[byte]): auto =
  BitList[N] @bytes

proc readSszValue*(input: openarray[byte], T: type): T =
  mixin fromSszBytes, toSszType

  type T = type(result)

  template readOffset(n: int): int =
    int fromSszBytes(uint32, input[n ..< n + offsetSize])

  when useListType and result is List:
    type ElemType = type result[0]
    result = T readSszValue(input, seq[ElemType])
  elif result is ptr|ref:
    if input.len > 0:
      new result
      result[] = readSszValue(input, type(result[]))
  elif result is Option:
    if input.len > 0:
      result = some readSszValue(input, result.T)
  elif result is string|seq|openarray|array:
    type ElemType = type result[0]
    when ElemType is byte|char:
      result.setLen input.len
      if input.len > 0:
        copyMem(addr result[0], unsafeAddr input[0], input.len)

    elif isFixedSize(ElemType):
      const elemSize = fixedPortionSize(ElemType)
      if input.len mod elemSize != 0:
        var ex = new SszSizeMismatchError
        ex.deserializedType = cstring typetraits.name(T)
        ex.actualSszSize = input.len
        ex.elementSize = elemSize
        raise ex
      result.setLen input.len div elemSize
      trs "READING LIST WITH LEN ", result.len
      for i in 0 ..< result.len:
        trs "TRYING TO READ LIST ELEM ", i
        let offset = i * elemSize
        result[i] = readSszValue(input[offset ..< offset+elemSize], ElemType)
      trs "LIST READING COMPLETE"

    else:
      if input.len == 0:
        # This is an empty list.
        # The default initialization of the return value is fine.
        return

      var offset = readOffset 0
      trs "GOT OFFSET ", offset
      let resultLen = offset div offsetSize
      trs "LEN ", resultLen
      result.setLen resultLen
      for i in 1 ..< resultLen:
        let nextOffset = readOffset(i * offsetSize)
        if nextOffset == offset:
          assignNullValue result[i - 1], ElemType
        else:
          result[i - 1] = readSszValue(input[offset ..< nextOffset], ElemType)
        offset = nextOffset

      result[resultLen - 1] = readSszValue(input[offset ..< input.len], ElemType)

  elif result is object|tuple:
    enumInstanceSerializedFields(result, fieldName, field):
      const boundingOffsets = T.getFieldBoundingOffsets(fieldName)
      trs "BOUNDING OFFSET FOR FIELD ", fieldName, " = ", boundingOffsets

      type FieldType = type field
      type SszType = type toSszType(default(FieldType))

      when isFixedSize(SszType):
        const
          startOffset = boundingOffsets[0]
          endOffset = boundingOffsets[1]
        trs "FIXED FIELD ", startOffset, "-", endOffset
      else:
        let
          startOffset = readOffset(boundingOffsets[0])
          endOffset = if boundingOffsets[1] == -1: input.len
                      else: readOffset(boundingOffsets[1])
        trs "VAR FIELD ", startOffset, "-", endOffset

      # TODO The extra type escaping here is a work-around for a Nim issue:
      when type(FieldType) is type(SszType):
        trs "READING NATIVE ", fieldName, ": ", name(SszType)
        field = readSszValue(input[startOffset ..< endOffset], SszType)
        trs "READING COMPLETE ", fieldName
      elif useListType and FieldType is List:
        field = readSszValue(input[startOffset ..< endOffset], FieldType)
      else:
        trs "READING FOREIGN ", fieldName, ": ", name(SszType)
        field = fromSszBytes(FieldType, input[startOffset ..< endOffset])

  elif result is SomeInteger|bool|enum:
    trs "READING BASIC TYPE ", type(result).name, "  input=", input.len
    result = fromSszBytes(type(result), input)
    trs "RESULT WAS ", repr(result)

  else:
    unsupported T