nim/flatbuffers/src/table.nim

import endian


type
   uoffset* = uint32 ## offset in to the buffer
   soffset* = int32 ## offset from start of table, to a vtable
   voffset* = uint16 ## offset from start of table to value

type Offsets* = uoffset | soffset | voffset

type Vtable* = object
   Bytes*: seq[byte]
   Pos*: uoffset


using this: Vtable


func GetVal*[T](b: ptr seq[byte]): T {.inline.} =
   when T is float64:
      result = cast[T](GetVal[uint64](b))
   elif T is float32:
      result = cast[T](GetVal[uint32](b))
   elif T is string:
      result = cast[T](b[])
   else:
      if b[].len < T.sizeof:
         b[].setLen T.sizeof
      result = cast[ptr T](unsafeAddr b[][0])[]


template Get*[T](this; off: uoffset): T =
   var seq = this.Bytes[off..^1]
   GetVal[T](addr seq)

template Get*[T](this; off: soffset): T =
   var seq = this.Bytes[off..^1]
   GetVal[T](addr seq)

template Get*[T](this; off: voffset): T =
   var seq = this.Bytes[off..^1]
   GetVal[T](addr seq)

func WriteVal*[T: not SomeFloat](b: var openArray[byte]; off: uoffset;
      n: T) {.inline.} =
   when sizeof(T) == 8:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 4:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 2:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 1:
      b[off] = n.uint8
   else:
      discard
      #littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
      #{.error:"shouldnt appear".}

func WriteVal*[T: not SomeFloat](b: var seq[byte]; off: uoffset;
      n: T) {.inline.} =
   when sizeof(T) == 8:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 4:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 2:
      littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
   elif sizeof(T) == 1:
      b[off] = n.uint8
   else:
      discard
      #littleEndianX(addr b[off], unsafeAddr n, T.sizeof)
      #{.error:"shouldnt appear".}

func WriteVal*[T: SomeFloat](b: var openArray[byte]; off: uoffset;
      n: T) {.inline.} =
   when T is float64:
      WriteVal(b, off, cast[uint64](n))
   elif T is float32:
      WriteVal(b, off, cast[uint32](n))

func WriteVal*[T: SomeFloat](b: var seq[byte]; off: uoffset; n: T) {.inline.} =
   when T is float64:
      WriteVal(b, off, cast[uint64](n))
   elif T is float32:
      WriteVal(b, off, cast[uint32](n))

func Offset*(this; off: voffset): voffset =
   let vtable = (this.Pos - this.Get[:uoffset](this.Pos)).voffset
   let vtableEnd = this.Get[:voffset](vtable)
   if off < vtableEnd:
      return this.Get[:voffset](vtable + off)
   return 0


func Indirect*(this; off: uoffset): uoffset =
   result = off + this.Get[:uoffset](off)

func VectorLen*(this; off: uoffset): int =
   var newoff: uoffset = off + this.Pos
   newoff += this.Get[:uoffset](newoff)
   return this.Get[:uoffset](newoff).int

func Vector*(this; off: uoffset): uoffset =
   let newoff: uoffset = off + this.Pos
   var x: uoffset = newoff + this.Get[:uoffset](newoff)
   x += (uoffset.sizeof).uoffset
   result = x

func Union*(this; off: uoffset): Vtable =
   let newoff: uoffset = off + this.Pos
   result.Pos = newoff + this.Get[:uoffset](newoff)
   result.Bytes = this.Bytes

func GetSlot*[T](this; slot: voffset; d: T): T =
   let off = this.Offset(slot)
   if off == 0:
      return d
   return this.Get[T](this.Pos + off)

func GetOffsetSlot*[T: Offsets](this; slot: voffset; d: T): T =
   let off = this.Offset(slot)
   if off == 0:
      return d
   return off

func ByteVector*(this; off: uoffset): seq[byte] =
   let
      newoff: uoffset = off + this.Get[:uoffset](off)
      start = newoff + (uoffset.sizeof).uoffset
   var newseq = this.Bytes[newoff..^1]
   let
      length = GetVal[uoffset](addr newseq)
   result = this.Bytes[start..<start+length]

func String*(this; off: uoffset): string =
   var byte_seq = this.ByteVector(off)
   result = GetVal[string](addr byte_seq)

using this: var Vtable

proc Mutate*[T](this; off: uoffset; n: T): bool =
   WriteVal(this.Bytes, off, n)
   return true

func MutateSlot*[T](this; slot: voffset; n: T): bool =
   let off: voffset = this.Offset(slot)
   if off != 0:
      return this.Mutate(this.Pos + off.uoffset, n)
   return false