Array of Structures to Structure of Arrays

This library allows to generate a SoA (Structure of Arrays) type from a Nim object, and use syntax like a[i].field on it as if it were an AoS (Array of Structures).

Installation

With nimble:

nimble install aossoa

Or in a .nimble file put the line:

requires "aossoa"

Example

We start from an object MyObj

type
  MyObj = object
    field1: int
    field2: float

var aos = @[MyObj(field1: 1, field2: 1.2), MyObj(field1: 2, field2: 2.1)]
echo aos[1].field2 # 2.1

and define an SoA

import aossoa
declareSoA(MyObj, MyObjSoA)

var soa: MyObjSoA
soa.setLen(2)
soa[1].field1 = 2
echo soa[1].field2, " ", soa.field2[1] # 2 2

The macro declareSoA expands conceptually to

type
  MyObjSoA = object
    field1: seq[int]
    field2: seq[float]

template `[]`(s: MyObjSoA, i: int): IntermediateType

macro field1(temp: IntermediateType): int
macro field1(temp: IntermediateType): float

proc setLen(s: MyObjSoA, len: int)
  ## (re)sets all internal arrays

proc len(s: MyObjSoA, len: int)
  ## shared len of internal arrays

proc toMyObjSoA(s: MyObjSoA, a: sink openArray[MyObj])
  ## converts an array of MyObj to its SoA representation

proc toMyObj(temp: IntermediateType, a: sink openArray[MyObj])
  ## converts an array of MyObj to its SoA representation

Note that s[i].field is transformed at compile-time to s.field[i] `ÌntermediateType`` is “consumed” by the macros.

One can converts from Aos to SoA with the generated proc to${SoAType}:

let soa = toMyObjSoA(aos)

Due to the current implementation, “naked” indexing soa[i] is meaningless and gives a compile-time error on purpose. To get a value of the type of the original object as one could expect wrap it like this:

echo soa[i].toMyObj

Note also that declareSoA exports the generated type to the world, one can declareSoAPrivate to limit its scope to the current module

Currently, only plain non-generic objects are supported.

License

MIT