nim-stint/src/private/conversion.nim

# Mpint
# Copyright 2018 Status Research & Development GmbH
# Licensed under either of
#
#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
#
# at your option. This file may not be copied, modified, or distributed except according to those terms.

import  ./datatypes,
        macros, typetraits

func initUintImpl*[InType, OutType](x: InType, _: typedesc[OutType]): OutType {.inline.} =

  const
    size_in = getSize(x)
    size_out = getSize(result)

  static:
    assert size_out >= size_in, "The result type size (" & $size_out &
      " for " & $OutType.name &
      ") should be equal or bigger than the input type size (" & $size_in &
      " for " & $InType.name & ")."

  when OutType is SomeUnsignedInt:
    result = x.OutType
  elif size_in == size_out:
    result = cast[type result](x)
  else:
    result.lo = initUintImpl(x, type result.lo)

func toSubtype*[T: SomeInteger](b: bool, _: typedesc[T]): T {.inline.}=
  b.T

func toSubtype*[T: UintImpl](b: bool, _: typedesc[T]): T {.inline.}=
  type SubTy = type result.lo
  result.lo = toSubtype(b, SubTy)

func zero*[T: BaseUint](_: typedesc[T]): T {.inline.}=
  discard

func one*[T: BaseUint](_: typedesc[T]): T {.inline.}=
  when T is SomeUnsignedInt:
    result = T(1)
  else:
    let r_ptr = cast[ptr array[getSize(result) div 8, byte]](result.addr)
    when system.cpuEndian == bigEndian:
      r_ptr[0] = 1
    else:
      r_ptr[r_ptr[].len - 1] = 1

func toUint*(n: UintImpl): auto {.inline.}=
  ## Casts a multiprecision integer to an uint of the same size

  # TODO: uint128 support
  when n.sizeof > 8:
    raise newException("Unreachable. You are trying to cast a StUint with more than 64-bit of precision")
  elif n.sizeof == 8:
    cast[uint64](n)
  elif n.sizeof == 4:
    cast[uint32](n)
  elif n.sizeof == 2:
    cast[uint16](n)
  else:
    raise newException("Unreachable. StUint must be 16-bit minimum and a power of 2")

func toUint*(n: SomeUnsignedInt): SomeUnsignedInt {.inline.}=
  ## No-op overload of multi-precision int casting
  n

func asDoubleUint*(n: BaseUint): auto {.inline.} =
  ## Convert an integer or StUint to an uint with double the size

  type Double = (
    when n.sizeof == 4: uint64
    elif n.sizeof == 2: uint32
    else: uint16
  )

  n.toUint.Double


func toUintImpl*(n: uint16|uint32|uint64): auto {.inline.} =
  ## Cast an integer to the corresponding size UintImpl
  # Sometimes direct casting doesn't work and we must cast through a pointer

  when n is uint64:
    return (cast[ptr [UintImpl[uint32]]](unsafeAddr n))[]
  elif n is uint32:
    return (cast[ptr [UintImpl[uint16]]](unsafeAddr n))[]
  elif n is uint16:
    return (cast[ptr [UintImpl[uint8]]](unsafeAddr n))[]

func toUintImpl*(n: UintImpl): UintImpl {.inline.} =
  ## No op
  n